Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SOLIDIFICATION REFINEMENT AND GENERAL PHASE
TRANSFORMATION CONTROL THROUGH APPLICATION OF IN SITU
GAS JET IMPINGEMENT IN METAL ADDITIVE MANUFACTURING
RELATED APPLICATIONS
[0001] Benefit of priority is claimed to U.S. Patent Application Ser. No.
16/019,460, titled "SOLIDIFICATION REFINEMENT AND GENERAL PHASE
TRANSFORMATION CONTROL THROUGH APPLICATION OF IN SITU GAS
JET IMPINGEMENT IN METAL ADDITIVE MANUFACTURING," filed June 26,
2018, and to U.S. Provisional Application No. 62/527,656, titled "REFINEMENT
OF
SOLIDIFICATION STRUCTURES IN ADDITIVE MANUFACTURING BY MELT
POOL GAS JET IMPINGEMENT," filed June 30, 2017.
[0002] Where permitted, the subject matter of each of the above-referenced
applications is incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The present invention relates to devices and methods for manufacturing
objects by solid freeform fabrication, especially titanium and titanium alloy
objects.
Discussion of the Related Art
[0004] Structured metal parts made of titanium or titanium alloys, or other
metal alloys, are conventionally made by casting, forging or machining from a
billet.
These techniques have a number of disadvantages, such as high material use of
the
expensive titanium metal and large lead times in the fabrication of the metal
object.
Casting, which often can be used for production of a potentially near-net-
shape object,
typically has a reduced material quality due to lack of control of
solidification and
cooling rates. Tooling costs and the inability to prepare objects with complex
shapes
are additional disadvantages of the conventional methods.
[0005] Fully dense physical objects can be made by a manufacturing
technology known as rapid prototyping, rapid manufacturing, layered
manufacturing
or additive manufacturing. Additive manufacturing offers great fabrication
freedom
and potential cost-savings due to the layered build-up of near-net-shape
products. It is
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desirable to match the material properties of conventional thermo-mechanical
processing methods such as forging while utilizing the same established metal
alloys.
[0006] In thermo-mechanical processing, the material properties are in most
cases a result of the refined grain structures achieved by recrystallization
induced by
the plastic deformation of the mechanical forming steps. This mechanism is not
available in a typical additive manufacturing process, where molten material
is added
in layers, solidifies and cools down without any mechanical forming. This
typically
results in coarse as-solidified grain structures. In many alloys the resulting
structures
will also be elongated with a high aspect ratio. This is due to the
directional heat
extraction provided by the relatively colder workpiece as superheated molten
metal is
added. Solidification initiates from the previously deposited layer(s), and
propagates
up into the deposited material as it cools down. The solidification structures
will in
many cases extend across several layers, up to several centimeters in size.
These
characteristics are typically not optimal to mechanical properties, giving
rise to
reduced and/or anisotropic strength, elongation and fatigue performance. Upon
further
cooling after solidification, allotropic phase transformations (transformation
from one
crystal structure to another), precipitation and other solid state
thermochemical
reactions occur. The nature of these depend on the alloy system in question.
Of
primary concern is the cooling rate in key temperature ranges where these
transformations happen. The layered additive manufacturing process generates
complex cyclic heating, cooling and reheating conditions where control over
all
relevant phase transformations in every deposited layer is crucial to achieve
a
consistent product. Achieving thermal control despite changing workpiece
geometry,
heat sink properties and accumulated heat is therefore a challenge faced in
additive
manufacturing. In addition to the effect of the cooling rate on the just
deposited and
solidified area, the cooling applied post deposition also contributes to the
overall
cooling of the work piece, allowing start of deposition of a new string or
layer to occur
without any significant waiting time. This is especially beneficial for
compact
geometries with less cycle time between strings or layers. In situ gas jet
impingement
in targeted phase transformation regions can increase cooling rate and result
in
solidification refinement and general phase transformation modulation and/or
control.
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[0007] Prior techniques include, for example, utilization of a hybrid process
where each deposited layer is plastically deformed to achieve a recrystallized
grain
structure has been applied to reduce distortion and improve mechanical
properties (see
U.S. Pat. Ap. Pub. No. US2015/0360289, Liou et at. (2015)). Such intermediate
forming steps, however, give a reduced effective deposition rate (negatively
impacting
productivity), and can limit the freedom of fabrication in terms of the
ability to form
complex shapes. Other techniques include inter-layer laser peening and
ultrasonic
impact treatment, such as described in International Pat. Appl. WO 2013140147
Al
(Wescott et at. (2013)) and inter-layer cold rolling, such as described in
European Pat.
App. Pub. EP2962788 Al (Liou et at. (2016)).
[0008] Forced cooling has been applied on the as-solidified layer during
cooling of the solidified metal in preparation for laser or ultrasonic impact
treatment to
reduce thermal distortion and refine grain structures as a result of
recrystallization (see
U.S. Pat. App. Pub. No. U52015/0041025, Wescott et at. (2015)). This helps
reduce
waiting times between layers, but still requires waiting for the right
workpiece
temperature followed by conditioning of the as-deposited layer which will
negatively
affect productivity and potentially limit fabrication freedom. None of the
prior art
mentions applying any cooling during deposition, and definitely not applying
cooling
on a melt pool or to an area adjacent to a melt pool during deposition (in
situ).
Instead, Wescott et at. describes cooling the as-solidified layer of a string
of a
workpiece in-between string depositions in order to prepare for the
deformation step.
For the methods that physically work the deposited layer, contaminations from
tooling
also will be a concern since any contaminations can get enclosed between
layers of the
final product in an additive process. Wescott et at. does not mention
refinement of
solidification structures in additive manufacturing by melt pool gas jet
impingement.
[0009] Other techniques that have been used to refine metals to achieve grain
refinements include transmission of high frequency vibrations to a body of
molten
material, such as through application of mechanical vibrations (e.g., see U.S.
Pat. No.
3,363,668, Petit et at. (1968)), acoustic energy (U.S. Pat. App. Pub. No.
2014/0255620, Shuck et at. (2014)), or an oscillating electromagnetic field
(International Pat. App. W02015028065 Al, Jarvis et at. (2015)). In addition
to
potentially prohibitive costs and lack of practical methods of implementation,
the
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effectiveness of the general principle of melt pool agitation is very limited
on many of
the relevant metal alloys. Specifically, it requires a zone of partially
solidified material
at the propagating solidification front to be able to disrupt that front
through
fragmentation. The nature of many alloys applicable to additive manufacturing,
such
as many of the titanium alloys, and particularly the major titanium alloy Ti-
6A1-4V, is
a narrow freezing range which makes it very resistant to fragmentation of the
solidification front through the techniques that utilize a vibration
mechanism, such as
an acoustic, electromagnetic or a mechanical vibration mechanism.
[0010] Accordingly, there exists a need in this art for an economical method
of
performing metal additive manufacturing at an increased rate of metal
deposition in an
additive manufacturing system that yields metal products having a finer grain
structure, particularly having more equiaxed grains, and a more consistent
microstructure after additional cooling below any relevant phase
transformation
temperatures, compared to what is achieved in traditional additive
manufacturing
processes.
SUMMARY OF THE INVENTION
[0011] Accordingly, the present invention is directed to refinement of
solidification structures in additive manufacturing by melt pool gas jet
impingement
that substantially obviates one or more of the problems due to limitations and
disadvantages of the related art. An extension of the device, or a separate
gas jet
device can be used to achieve further in-situ thermal control of the as-
deposited and
solidified material. Provided are devices, systems and methods to refine the
solidification structures and control microstructures during metal additive
manufacturing to achieve products with improved material quality, particularly
having
more equiaxed as-solidified grain structure. Manufactured products having
these
refined grain structures demonstrate increased strength, fatigue resistance,
and
ductility. There further exists a need in this art for a method of increasing
throughput
and yield of metal products produced by metal additive manufacturing methods.
[0012] An advantage of the present invention is to provide grain refinement in
metal articles produced by additive manufacturing, wherein the resulting grain
structure has comparable aspect ratio and homogeneity to that typically
present in
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mechanically worked metals and a significantly reduced average grain size
compared
to typical cast or additively manufactured materials.
[0013] The devices and methods provided herein result in solidification
structure refinement and microstructural control through gas jet impingement
on the
free surface of the melt pool, or the boundary between liquid and solid, or on
the
solidified metal in the vicinity of the liquid-solid boundary, or on the
solidified metal,
or any combination thereof, during formation of a layered metal deposit using
additive
manufacturing. The gas used can be inert or non-inert, elemental or mixed,
depending
on whether the metal alloy in question is sensitive to atmospheric
contaminations.
[0014] Grading of microstructures and optimization of material properties
in different sections of a deposit also are made possible with the use of the
devices and
methods provided herein in additive manufacturing. The devices and methods
provide
a practical way to achieve significant refinement of metal structure,
resulting in grains
that in most cases will be somewhat coarser than typical mechanically worked
metals,
but of comparable aspect ratio and homogeneity. Directed cooling gas jets at
the
liquid surface and liquid-solid boundary of the melt pool can induce and
accelerate
opposing solidification front at the free melt pool surface. Blocking of
epitaxy can be
achieved as consecutive layers nucleate and solidify from the top-layer
grains. Forced
cooling through concentrated turbulent gas flow provided by the devices
provided
herein when applied on the as-solidified material can enhance, modulate or
control
solid-state phase transformation.
[0015] Another advantage of the present invention is that the
device and
methods allow manipulation of solidification conditions and significant
refinement
potential in many metal alloys without requiring time consuming conditioning
between
layers, limitations on shape processing, or significant reductions in
deposition rate or
deposition productivity. Use of the cooling jet device to force cool the
deposited
material during additive manufacturing by in situ application of jets of
cooling gas at
targeted areas, alone or in combination with a cooling jet device directed to
the melt
pool, can significantly increase deposition productivity. High cooling gas
flows from
the jet device directed to the as-deposited material can significantly remove
thermal
energy, resulting in improved bulk cooling rate of the deposited material. The
cooling
jet devices provided herein can be configured to work with most melting tools,
and can
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be adjusted, activated or deactivated at any time while deposition is taking
place in an
additive manufacturing process. This flexibility provides the ability to
modify the
underlying grain structure of the manufactured product during the
manufacturing
process. The methods can be used with any metal additive manufacturing
process,
including plasma and wire-based processes, and laser system, and is
particularly
suitable to high deposition rate processes. While Ti and Ti alloy workpiece
product
are mentioned as examples throughout, the methods likely are equally suitable
to many
other alloy systems based on metallurgical theory. For example, Inconel
superalloys
also are predisposed to achieving the refinement effect achieved using the
devices,
methods and systems provided herein.
[0016] The jet gas flow from the jet device provided herein
directed at the
melt pool, such as the melt pool free surface, can increase crystallographic
diversity,
and the extent of grain boundary alignment can be reduced. The directed jetted
gas
can yield a more homogeneous and finely distributed presence of different
microstructural elements. Typically additively manufactured metal products can
include the presence of columnar solidification structures extending several
centimeters across the deposit layers. They can be broken up by finer grains
at
irregular intervals due to minor fluctuations in thermal gradients and melt
pool
convection etc. The jet tool provided herein when directed at the melt pool
can induce
or promote nucleation at the melt pool free surface, along with a reduced
temperature
gradient, can result in break-up of the columnar structures traditionally
present in
additively manufactured materials and yield improved repeatable material
properties.
[0017] Another advantage of the present invention is that the
devices and
methods allow for modulation of cooling rates during the additive
manufacturing
process. In additive manufacturing, multiple elements most commonly referred
to as
strings, beads, or tracks, typically can be stitched and stacked to form what
is quite
often very complex shapes. The strings are formed by feeding metal material,
typically
in wire or powder form, into a travelling heat source, where the metal
material is
melted and fused by the supplied energy of the heat source. The heat source
can be a
high-energy laser beam, electron beam or plasma arc, or any combination
thereof
This layered deposition can generate complex, cyclic and transient thermal
conditions.
Cyclic, because previously deposited material typically is reheated by
deposition of
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consecutive layers, and transient due to change of boundary conditions like
heat sink
characteristics as the build progresses.
[0018] Most metal alloys are sensitive to their thermal
history. Typically,
the cooling rates from the high temperatures of string deposition to the bulk
workpiece
temperature have a profound effect on the final material properties. In
addition, the
effects of heat input from consecutive layers can alter material
characteristics through
in-process annealing and aging effects. It is therefore critical to control
the local
thermal conditions to produce consistent material properties throughout a
complex
additively manufactured product. The invention disclosed herein relates to
devices,
systems and methods that improve the capability to modulate or control thermal
conditions in additive manufacturing by application of in-process temperature
measurement and application of forced convective cooling using the jet devices
provided herein.
[0019] Additional features and advantages of the invention will be set forth
in
the description which follows, and in part will be apparent from the
description, or
may be learned by practice of the invention. The objectives and other
advantages of
the invention will be realized and attained by the structure particularly
pointed out in
the written description and claims hereof as well as the appended drawings.
[0020] To achieve these and other advantages and in accordance with the
purpose of the present invention, as embodied and broadly described, provided
are jet
devices, that include a first conduit including an inlet for accepting a
cooling gas and
an aperture connected to a nozzle for dispensing a cooling gas; a second
conduit
including an inlet for accepting a cooling gas and an aperture connected to a
nozzle for
dispensing a cooling gas; where the first conduit is attached to a melting
tool
producing a thermal energy source on one side of the thermal energy source and
the
second conduit is attached to the melting tool on an opposite second side of
the
thermal energy source; at least one nozzle is configured to produce a
turbulent flow of
the cooling gas as the cooling gas exits the nozzle; and the nozzles are
configured and
positioned to prevent blowing the cooling gas toward the thermal energy
source.
[0021] Also provided are jet devices that include at least one conduit that
includes an inlet for accepting a cooling gas and one or more apertures each
of which is
connected to one or a plurality of nozzles for dispensing a cooling gas in
situ to an as-
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deposited material. The jet device can be configured to include a plurality of
conduits
each of which includes an inlet for accepting a cooling gas. The conduits can
be
configured to deliver jets of cooling gas in situ to one surface or multiple
surfaces of
the as-deposited material. As an example, a single conduit can be configured
to include
a plurality of nozzles, where some nozzles can be configured to direct cooling
gas jets
to one side surface of the as-deposited material, other nozzles can be
configured to
direct cooling gas jets to the other side surface of the as-deposited
material, and other
nozzles can be configured to direct cooling gas jets to the upper surface of
the as-
deposited material. As another example, the jet device can include multiple
conduits,
where one conduit can be configured to include nozzles that direct cooling gas
jets to
one side surface of the as-deposited material, a second conduit can be
configured to
include nozzles that direct cooling gas jets to the other side surface of the
as-deposited
material, and a third conduit can be configured to include nozzles that direct
cooling
gas jets to the upper surface of the as-deposited material. The jet device can
be
connected to a portion of the system at a location that allows the nozzles to
be directed
to a surface of solidified as-deposited material. In some configurations, the
jet device
can be connected to a wire or powder feed device. The jet device can be
connected to a
bracket or support and be independent from a wire or powder feed device.
[0022] The systems provided herein can include a jet device
that directs
cooling gas jets to an as-deposited material in situ and at least two
temperature sensors
to monitor temperature in the region of application of the cooling gas jets
during the
additive manufacturing process. A first temperature sensor can monitor the
temperature at the surface of the as-deposited material ahead of the
application of a
cooling gas, and a second temperature sensor located after the jet device can
be
included to measure the temperature of a surface of the workpiece after
application of
the cooling gas to the as-deposited string of the workpiece is applied by the
jet device.
The temperature data from the first and second temperature sensors can allow
the user
to control the cooling rate by adjusting the flow rate of cooling gas applied
by jet
device, or the duration of the flow of the cooling gas towards the workpiece,
or both.
[0023] In another aspect of the invention, provided herein are systems for
building a metallic object by additive manufacturing, comprising: a first
melting tool
to preheat a base material prior to deposition of a molten metal; a second
melting tool
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to melt a source of metal into droplets of metallic molten material that are
deposited on
the preheated base material or into a liquid molten pool on the base material;
a jet
device provided herein to direct a cooling gas across the liquid molten pool,
or to
impinge on the liquid molten pool, or to impinge upon a solidified material
adjacent to
a liquid-solid boundary of the liquid molten pool, or any combination thereof;
a supply
of the cooling gas; a system for positioning and moving the base material
relative to
the heating device and jet device; and a control system able to read a design
model of
the metallic object to be formed and employ a design model to regulate the
position
and movement of the system for positioning and moving the base material and to
operate the heating device and jet device such that a physical object is built
by fusing
successive deposits of the metallic material onto the base material.
[0024] In another aspect of the present invention, provided are methods for
manufacturing a three-dimensional object of a metallic material by additive
manufacturing, where the object is made by fusing together successive deposits
of the
metallic material onto a base material, the method comprising: using a first
melting
tool to preheat at least a portion of a surface of the base material; using a
second
melting tool to heat and melt a metallic material such that molten metallic
material is
deposited onto the preheated area of the base material forming a liquid molten
pool;
using a jet device provided herein to direct a cooling gas across the liquid
molten pool,
or to impinge on the liquid molten pool, or to impinge upon a solidified
material
adjacent to a liquid-solid boundary of the liquid molten pool, or to impinge
on as-
solidified material or any combination thereof; and moving the base material
relative
to the position of the first and second heating devices in a predetermined
pattern such
that the successive deposits of molten metallic material solidifies and forms
the three-
dimensional object. In the methods, a jet device can direct cooling gas jets
at the melt
pool, or a jet device can direct cooling gas jets to solidified deposited
metal region, or
one jet device can be direct cooling gas jets at the melt pool, and a second
jet device
can direct cooling gas jets to a solidified deposited metal region.
[0025] It is to be understood that both the foregoing general description and
the
following detailed description are exemplary and explanatory and are intended
to
provide further explanation of the invention as claimed.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The accompanying drawings, which are included to provide a further
understanding of the invention and are incorporated in and constitute a part
of this
specification, illustrate embodiments of the invention and together with the
description
serve to explain the principles of the invention.
[0027] In the drawings:
[0028] FIG. 1 is a schematic skewed front view depiction of an exemplary jet
device that provides directed gas jets to the melt pool free surface and the
boundary
between liquid and solid as molten material is deposited to form a string. Not
represented in the figure is the melting tool situated above the melt pool, or
wire or
powder feedstock supplied to the melt pool or into the melting arc or laser
beam.
[0029] FIG. 2 is a partial cutaway side view of an exemplary configuration of
the jet device.
[0030] FIG. 3 is a schematic illustration of the cross-section of a single row
wall deposit as layer upon layer is fused. The illustration shows typical
unrefined
grain growth in the first 3 layers, followed by the refinement mechanism of
columnar
grain growth blocking by application of gas jet impingement using the jet
device
provided herein.
[0031] FIGS. 4A and 4B show electron back scatter diffraction (EBSD)
photographs of the crystallography of typical material made by conventional
additive
manufacturing processes (FIG. 4A) versus that achieved using the methods
provided
herein, where gas jet impingement results in a material having a more refined
grain
(FIG. 4B).
[0032] FIGS. 5A and 5B are micrographs comparing the typical structure of a
deposited Ti-6A1-4V sample (FIG. 5A) to the resulting refined structure with
application of melt pool gas jet impingement in a multi-row, multilayered Ti-
6A1-4V
deposit using the jet device provided herein (FIG. 5B).
[0033] FIG. 6 is a photograph showing the result of application of gas jet
impingement on one half of the melt pool in a single row Ti-6A1-4V deposit
using the
jet device provided herein. Dotted lines outline typical grain size and shape
on either
side of the wall.
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[0034] FIG. 7 is a schematic side view depiction of an exemplary jet device
that provides directed gas jets to a solidified metal region as molten
material in the
melt pool cools to form a string in order to affect additional phase
transformations
occurring after solidification and further cooling. A melting tool situated
above the
melt pool provides energy to melt a metal wire or powder feedstock into molten
metal
that drops into the melt pool. A temperature sensor can be located in front of
the jet
device to measure the temperature of the as-forming string and a temperature
sensor
can be located trailing the jet device to measure the temperature of the
solidified metal
of the string during or after application of the gas jet.
[0035] FIG. 8 is schematic side view depiction of an exemplary system that
can be used with the methods provided herein. In the embodiment depicted, a
single
melting tool is used to form molten material that is deposited to form a
deposited sting,
a first jet device directs cooling gas jets to the melt pool free surface and
the boundary
between liquid and solid as the molten material is deposited to form the
string, and a
second jet device directs cooling gas jets to a solidified metal region as the
molten
material cools, such as an area that can undergo allotropic transformation or
precipitation.
[0036] FIG. 9 is schematic side view depiction of an exemplary system that
can be used with the methods provided herein employing two melting tools. In
the
embodiment depicted, one melting tool is used to pre-heat the substrate
surface to form
a preheated area, and a second melting tool is used to heat and melt a metal
onto the
preheated area of the base material to form a deposited sting, a first jet
device directs
cooling gas jets to the melt pool free surface and the boundary between liquid
and
solid as the molten material is deposited to form the string, and a second jet
device
directs cooling gas jets to a solidified metal region as the molten material
cools, such
as an area that can undergo allotropic transformation or precipitation.
[0037] FIGS. 10A and 10B are photomicrographs showing the correlation
between differences in bulk cooling rate and microstructure in Ti-6A104V
material.
DETAILED DESCRIPTION
[0038] Reference will now be made in detail to an embodiment of the present
invention, example of which is illustrated in the accompanying drawings.
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[0039] A. DEFINITIONS
[0040] Unless defined otherwise, all technical and scientific terms used
herein
have the same meaning as is commonly understood by one of skill in the art to
which
the inventions belong. All patents, patent applications, published
applications and
publications, websites and other published materials referred to throughout
the entire
disclosure herein, unless noted otherwise, are incorporated by reference in
their
entirety. In the event that there are a plurality of definitions for terms
herein, those in
this section prevail.
[0041] As used herein, the singular forms "a," "an" and "the" include plural
referents unless the context clearly dictates otherwise.
[0042] As used herein, ranges and amounts can be expressed as "about" a
particular value or range. "About" also includes the exact amount. Hence
"about 5
percent" means "about 5 percent" and also "5 percent." "About" means within
typical
experimental error for the application or purpose intended.
[0043] As used herein, "optional" or "optionally" means that the subsequently
described event or circumstance does or does not occur, and that the
description
includes instances where the event or circumstance occurs and instances where
it does
not. For example, an optional component in a system means that the component
may
be present or may not be present in the system.
[0044] As used herein, a "combination" refers to any association between two
items or among more than two items. The association can be spatial or refer to
the use
of the two or more items for a common purpose.
[0045] As used herein, "additive manufacturing" is also known as "additive
fabrication" and "additive layer manufacturing" and refers to an additive
process
implementing the manufacturing, layer after layer, of an object from a 3D
model data,
a metal source, such as wire or powder, and an energy source (such as a plasma
arc,
laser or electron beam) to melt the metal source.
[0046] As used herein, "additive manufacturing system" refers to the machine
used for additive manufacturing.
[0047] The term "plasma transferred arc torch" or "PTA torch" as used
interchangeably herein refers to any device able to heat and excite a stream
of inert gas
to plasma by an electric arc discharge and then transfer the flow of plasma
gas
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including the electric arc out through an orifice (such as a nozzle) to form a
constricted
plume that extends out of the orifice and transfers the intense heat of the
arc to a target
region.
[0048] The term "metallic material" as used herein refers to any known or
conceivable metal or metal alloy which can be employed in a solid freeform
fabrication
process to form a three-dimensional object. Examples of suitable materials
include, but
are not limited to; titanium and titanium alloys such as i.e. Ti-6A1-4V alloy.
[0049] As used herein, a "Plasma Arc Welding torch" or "PAW torch" refers to
a welding torch that can be used in plasma arc welding. The torch is designed
so that a
gas can be heated to a high temperature to form plasma and becomes
electrically
conductive, the plasma then transfers an electric arc to a workpiece, and the
intense
heat of the arc can melt metal and/or fuse two pieces of metal together. A PAW
torch
can include a nozzle for constricting the arc thereby increasing the power
density of
the arc. The plasma gas typically is argon. Plasma gas can be fed along an
electrode
and ionized and accelerated in the vicinity of a cathode. The arc can be
directed
towards the workpiece and is more stable than a free burning arc (such as in a
TIG
torch). The PAW torch also typically has an outer nozzle for providing a
shielding
gas. The shielding gas can be argon, helium or combinations thereof, and the
shielding
gas assists minimizing oxidation of the molten metal. Current typically is up
to 400
A, and voltage typically is in the range of about 25 ¨ 35 V (but can be up to
about 14
kW). PAW torches include plasma transferred arc torches.
[0050] The term "base material" as used herein refers to the target material
for
the heat from a melting tool and on which a molten pool can be formed. The
melting
tool can be a PAW torch, a PTA torch, a laser device, or any combination
thereof
This will be the holding substrate when depositing the first layer of metallic
material.
When one or more layers of metallic material have been deposited onto the
holding
substrate, the base material will be the upper layer of deposited metallic
material that is
to have deposited a new layer of metallic material.
[0051] As used herein, the term "workpiece" refers to a metal body being
produced using solid free form fabrication.
[0052] The term "design model" or "computer assisted design model" or
"CAD-model" as used interchangeably herein refers to any known or conceivable
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virtual three-dimensional representation of the object that is to be formed
which can be
employed in the control system of the arrangement according to the second
aspect of
the invention: to regulate the position and movement of the holding substrate
and to
operate the welding torch with integrated wire feeder such that a physical
object is
built by fusing successive deposits of the metallic material onto the holding
substrate
in a pattern which results in building a physical object according to the
virtual three-
dimensional model of the object. This may, for instance, be obtained by
forming a
virtual vectorized layered model of the three-dimensional model by first
dividing the
virtual three-dimensional model into a set of virtual parallel layers and then
dividing
each of the parallel layers into a set of virtual quasi one-dimensional
pieces. Then, the
physical object can be formed by engaging the control system to deposit and
fuse a
series of quasi one-dimensional pieces of the metallic material feed onto the
supporting substrate in a pattern according to the first layer of the virtual
vectorized
layered model of the object.
[0053] Then, repeating the sequence for the second layer of the object by
depositing and fusing a series of quasi one-dimensional pieces of the weldable
material
onto the previous deposited layer in a pattern according to the second layer
of the
virtual vectorized layered model of the object. Repetition continues the
deposition and
fusing process layer by layer for each successive layer of the virtual
vectorized layered
model of the object until the entire object is formed. However, the invention
is not tied
to any specific CAD-model and/or computer software for running the control
system
of the arrangement according to the invention, and nor is the invention tied
to any
specific type of control system. Any known or conceivable control system (CAD-
model, computer software, computer hardware and actuators etc.) able to build
metallic three-dimensional objects by solid freeform fabrication can be used
as long as
the control system is adjusted to operate one or more melting tools, such as a
PAW
torch, a PTA torch, a laser heat source, or any combination thereof The jet
device
provided herein can be used with these melting tools to achieve the grain
refinement
described herein.
[0054] As used herein, a "high heat resistant material" refers to a material
that
is not prone to deformation and exhibits low thermal expansion when exposed to
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temperatures greater than 400 C. Exemplary materials include titanium and
titanium
alloys.
[0055] As used herein, a "jet device" refers to manufactured product that
includes one or a plurality of nozzles that direct streams or jets of a
cooling gas at the
melt pool surface, or across the melt pool, or across the liquid-solid
boundary, or on
the solidified metal in the vicinity of the liquid-solid boundary, or in situ
on a solid as-
deposited string, or any combination thereof, to directly influence
solidification,
refinement, to block the growth of grains across deposited layers, general
phase
transformation or any combination thereof
[0056] As used herein, "in situ" means that the manufactured product has not
been moved outside of the deposition chamber, and refers to the application of
a
turbulent gas jet during the additive manufacturing process.
[0057] As used herein, "jet" refers to the stream of cooling gas ejected by a
nozzle.
[0058] As used herein, a "nozzle" refers to a projecting part with an opening
that can regulate or direct a flow of cooling gas.
[0059] As used herein, a "cooling gas" is a gas directed at a melt pool
surface,
or across the melt pool, or across the liquid-solid boundary, or on the
solidified metal
in the vicinity of the liquid-solid boundary, or any combination thereof, to
directly
influence solidification and block the growth of grains across deposited
layers. The
temperature of the gas can be any temperature that cools the surface with
which it
interacts. The temperature can be less than 100 C, or less than 50 C, or less
than
C, or less than 25 C, or less than 10 C, or less than 5 C, or less than 0 C.
Gas at a
cryogenic temperature also can be used. It has been determined that the effect
of gas
25 colder than room temperature has not been found to have a significantly
different
effect than achieved with room temperature gas.
[0060] B. JET DEVICE
[0061] Provided herein is a jet device. The jet device is configured to direct
jets or streams of gas at the melt pool surface, or across the melt pool, or
across the
30 liquid-solid boundary, or on the solidified metal in the vicinity of the
liquid-solid
boundary, or on a solidified metal, or any combination thereof, in order to
directly
influence solidification of the molten metal and to block the growth of grains
across
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deposited layers. The jet device and systems and methods that include using
the jet
device to direct gas jets at the melt pool surface, or across the melt pool,
or across the
liquid-solid boundary, or on the solidified metal in the vicinity of the
liquid-solid
boundary, or on a solidified metal, or any combination thereof, can minimize
or
prevent the directional solidification that form coarse, elongated grain
structures
typical in conventional metal additive manufacturing processes. The
directional
solidification in typical additive manufacturing processes is a result of the
steep
thermal gradients associated with the typical additive manufacturing process.
[0062] This invention involves providing a jet device or a
combination of
jet devices and utilizing a jet device or a combination of jet devices, each
jet device
comprising a plurality of jet nozzles, that direct streams of a cooling gas at
the melt
pool surface, or across the melt pool, or across the liquid-solid boundary, or
on the
solidified metal in the vicinity of the liquid-solid boundary, or on a
solidified metal, or
any combination thereof, to directly influence solidification and block the
growth of
grains across deposited layers, or to improve the capability to control
thermal
conditions in additive manufacturing by application forced convective cooling.
The jet
device includes two separate conduits. The conduits can be connected by a
cross-piece
to form a unitary body. A unitary body configuration can be helpful in the
placement
of the jet device in relationship to the melting tool. Notwithstanding this,
the jet
device can be provided as two separate segments. The separate segments can be
attached to a melting tool, or to a metal material feed, such as a wire feed
or metal
powder feed by means of any attachment that provides the right position and
angle so
that the gas jets from the device(s) impinge on a target area, as described
herein.
[0063] Each conduit, either separately or when joined as a unitary body, is
attached via one side to a portion of the equipment comprising the melting
tool when
the jet device is to deliver cooling gas to a melt pool or an area in the
vicinity of the
melt pool, or by one side to the metal material feed when the jet device is to
deliver a
cooling gas to a solidified metal downstream of the melt pool. The opposite
side of
each conduit comprises one or a plurality of jet nozzles directed toward the
workpiece
and away from the melting tool. Each jet nozzle is connected to an aperture in
the
conduit that allows fluid communication between the nozzle and the conduit so
that
cooling gas can be delivered through the conduit, pass through the aperture of
each
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nozzle, and each nozzle separately can be directed to a location, such as at
the melt
pool surface, or across the melt pool, or across the liquid-solid boundary, or
on the
solidified metal in the vicinity of the liquid-solid boundary, or on a
solidified metal
past the liquid-solid boundary, such as in an allotropic transformation zone
or a region
where precipitation reactions can occur, ordering the constituents of the
alloy to form
particles of a secondary phase. In some configurations, the nozzle can direct
a stream
of cooling gas to two or more locations selected from among the melt pool
surface,
across the melt pool, across the liquid-solid boundary, on the solidified
metal in the
vicinity of the liquid-solid boundary and on a solidified metal past the
liquid-solid
boundary. Each conduit has a fluid connector at one end. The fluid connector
allows
the conduit to be connected to a source of cooling gas. The opposite end of
the
conduit is sealed. The diameter of the conduit is larger than the aperture to
which each
nozzle is attached. For example, the diameter of the nozzle can be in the
range of from
about 1 to about 10 mm, while the diameter of the opening or the aperture
attached to a
nozzle can be in the range of from about 0.5 to about 5 mm. In some
configurations,
the diameter of the nozzle and the aperture is the same, and can be in a range
of from
about 0.5 to about 5 mm, or from about 1 to about 3 mm. The total number of
nozzles
is limited only by space constraints on where the jet device is attached. In
some
configurations, the number of nozzles can be from about 4 to about 24. Instead
of
individual nozzles, a continuous gas diffuser or grid designed to produce a
directed,
turbulent flow of cooling gas also can be used as a gas outlet of the jet
device.
[0064] Each conduit provides a cooling gas to a nozzle or a set of nozzles
attached to the conduit. Each conduit can be divided or can include channels,
or can
contain pipes, tubes or lines, to deliver a separate stream of cooling gas to
each nozzle
individually. The nozzles on each conduit can be configured to be in rows,
with each
row containing one, two, three or four nozzles. The nozzles can be configured
to allow
individual adjustment of the gas flow to each nozzle, or separate gas flows in
different
sets of nozzles.
[0065] One or both conduits can include one or a plurality of sensors. A
conduit can include a flow meter, which allows the rate of flow of gas through
the
conduit to be measured. Any flow meter known in the art can be used in the
system.
The flow meter can include a paddle wheel flow meter, a turbine flow meter, a
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magnetic flow meter, optical sensors, electromagnetic velocity sensors,
coriolis force
flow meters, thermal flow meters, ultrasonic flow meters or any other type of
flow
meter known in the art. Examples of flow meters known in the art are described
U.S.
Pat. Nos. 4,422,338 (Smith, 1983); 4,838,127 (Herremans et at., 1989);
5,594,181
(Strange, 1997); 7,707,898 (Oddie, 2010); and 7,730,777 (Anzal et at, 2010).
In some
configurations, the conduit can include a notch, indentation or protrusion for
the
placement or attachment of a flow meter.
[0066] A conduit can include a temperature sensor, which allows the
temperature of the conduit or the cooling gas within the conduit or both to be
measured. Any temperature sensor known in the art can be used. Exemplary
temperature sensors include thermocouples, resistance temperature detectors,
thermistors, infrared thermometers, bimetallic devices, liquid expansion
devices, and
combinations thereof. In some configurations, the conduit can include a notch,
indentation or protrusion for the placement or attachment of a temperature
sensor.
[0067] The jet device also can include one or a plurality of temperature
sensors
to measure the temperature of the workpiece. In some configurations, a jet
device
configured to direct cooling gas jets at the melt pool or in the immediate
vicinity of the
melt pool can include a temperature sensor directed to a surface of the
workpiece or
the melt pool or a combination thereof. A jet device configured to direct
cooling gas
jets toward a solidified metal region of the workpiece, such as an allotropic
transformation zone, can include a first temperature sensor directed to a
surface of the
workpiece before the area impinged by or exposed to the cooling gas jets, and
a second
temperature sensor directed to a surface of the workpiece after the area
impinged by or
exposed to the cooling gas jets in order to measure and/or control cooling
rates across
the relevant temperature region. The device can include a temperature sensor
directed
to a post-solidification zone following solidification of the melt pool. The
device can
include a temperature sensor directed to a post-transformation zone, where
cooling
deposited solidified metal can undergo an allotropic transformation or other
thermochemical reactions. Any temperature sensor known in the art can be used,
particularly non-contact temperature sensors. Exemplary temperature sensors
include
infrared thermometers and infrared pyrometers. In some configurations, the
conduit
can include one or more notches, indentations or protrusions for the placement
or
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attachment of a temperature sensor. The conduit can be made of or comprise a
high
heat resistant material. Exemplary high heat resistant materials include
titanium and
alloys thereof, tungsten and alloys thereof, stainless steel, alloys
comprising chromium
and nickel, such as Inconel alloys and hastelloy alloys, and an alloy
comprising two or
more of nickel, iron, cobalt, copper, molybdenum, tantalum, tungsten and
titanium. In
some configurations, the conduit is made of titanium or a titanium alloy
containing Ti
in combination with one or a combination of Al, V, Sn, Zr, Mo, Nb, Cr, W, Si,
and
Mn. In some configurations, the conduit is made of Ti-6A1-4V alloy.
[0068] Each conduit can include a plurality of jet nozzles on the ventral side
of
the conduit, configured to be angled opposite to the direction of travel,
towards the
trailing edge of the melt pool produced by the melting device and added
feedstock
material. The nozzles direct a turbulent flow of cooling gas to a location,
such as at
the melt pool surface, or across the melt pool, or across the liquid-solid
boundary, or
on the solidified metal in the vicinity of the liquid-solid boundary, or a
solidified metal
past the liquid-solid boundary. Each nozzle can be positioned at any angle
relative to
the conduit so that the angle formed between the nozzle and the conduit is 90
or less,
such as less than 80 , or less than 70 , or less than 60 , or less than 50 ,
or less than
40 , or less than 30 . A preferred range of angles is from about 70 to about
30 from
horizontal. The nozzles can be configured and positioned to prevent blowing
cooling
gas toward the melting tool, such as a torch, which would disrupt the arc, or
which can
decrease the efficiency of the melting tool's ability to melt the consumable
electrode or
metal wire.
[0069] The jet nozzle can be of any shape. In some configurations, the nozzle
is configured to be tube-like, having a cylindrical shape. The nozzle can have
a
rectangular, hexagonal, octagonal, oval or asymmetric shape. The cross-section
of the
nozzle can be any shape. Exemplary shapes of the cross-sectional opening of
the
nozzle include circular, oval, ovoid, square, rectangular, rhomboidal,
hexagonal, and
octagonal. Non-uniform or an asymmetrical cross-section can be selected to
promote
turbulent flow of the gas out of the nozzle.
[0070] The thickness of the walls of the nozzle are sufficient to withstand
the
pressure of the cooling gas flowing therethrough. The thickness of the walls
also can
be selected to minimize any thermal deformation at the temperatures to which
the jet
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device can be subjected during the additive manufacturing process. For
example, the
wall thickness of the nozzle can be in the range of from about 0.25 to about 5
mm, or
from about 0.5 to about 3 mm.
[0071] The nozzle includes an orifice through which cooling gas flows toward
the workpiece. The orifice of the gas nozzle can have any geometry or shape.
The
orifice can be circular, oval, square, rectangular, rhomboidal, hexagonal, or
octagonal.
Non-uniform or an asymmetrical cross-section of the orifice can be selected to
promote turbulent flow of the gas out of the nozzle. The orifice of the nozzle
can have
a diameter of from about 0.5 to about 5 mm, or from about 1 to about 3 mm. The
diameter of the orifice can be the same as the inner diameter of the nozzle or
less.
When the diameter of the orifice of the nozzle is less than the inner diameter
of the
nozzle, the velocity of the gas exiting the orifice can be higher than the
velocity of the
gas in the conduit. A nozzle can include a plurality of orifices.
[0072] Cooling gas enters the jet device via the inlet in each conduit and
exits
the jet device through each of the nozzles. Each nozzle can deliver a source
of cooling
gas to a set of nozzles. Each conduit can be divided or can include channels
to deliver
a separate stream of cooling gas to each nozzle individually. Maximum flow
rate of
gas delivered to the jet device typically can be about 500 L/min, or 400
L/min, or 300
L/min, or 200 L/min, depending on the configuration and placement of the
cooling jet
device. For example, for a jet device delivering a cooling gas jet that
impinges on a
surface of a melt pool a cooling gas flow rate can be selected so that the
turbulent gas
flow does not deform the molten metal being applied via the melting tool or
the path of
its application, or cause spattering and instability of the molten metal
applied to the
string, or detrimentally affect the stability or shape of the melt pool. A
range of flow
rate of cooling gas can be from about 1 L/min to about 150 L/min, and
typically from
about 5 L/min to about 100 L/min. A minimum flow to effectively achieve the
grain
refinement effect is typically 10 L/min, depending on the material to be
processed and
jet device design. In configurations where the flow of cooling gas to each
nozzle can
be separately controlled, higher cooling gas flow rates can be directed to the
as-
solidified metal compared to the metal of the melt pool. The flow rate of
cooling gas
applied in situ to as-deposited solidified material can be significantly
higher than the
gas flow directed to the melt pool. In these cooling jet devices, the flow
rate of the
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cooling gas directed to a surface of the as-deposited solidified material in
situ can bu
up to 500 L/min. Separate gas supplies can be connected to each cooling jet
device to
allow for separate control of the gas flow rates from the nozzles of the jet
devices. For
example, a first gas supply provides cooling gas to a jet device directed to a
melt pool
or in the vicinity thereof, and a second gas supply is connected to a jet
device directed
to an as-deposited solidified material. Each gas supply can include a
regulator that can
be adjusted manually or automatically, such as via computer control, to adjust
the flow
rate of gas supplied to the cooling jet device connected to conduit connected
to the
regulator. In configurations were a jet device includes a plurality of
separate conduits,
each conduit of the device can be connected to a separate regulator so that
the flow of
cooling gas to each conduit can be separately controlled.
[0073] The cooling gas can be provided as a steady stream out of the nozzles.
The cooling gas can be provided intermittently or in pulses out of the
nozzles. The
intermittent or pulsed flow of the cooling gas can help to disperse thermal
energy away
from the area of impingement of the cooling gas. The provision of gas
intermittently
can be achieved by using valve switches. Pulsed flow refers to time-varying
gas flow
rates, with no limitation as to the amplitudes, phases and other
characteristics of time-
varying phenomena. Pulsed flow typically includes a sequential, repetitive use
of a
plurality of different time-varying gas flow rates. The pulsing of the gas
takes place for
a time such that the time-varying high flow and low flow conditions are
exhibited.
The pulsed flow of the gas can be provided using any method or device known in
the
art (e.g., see U.S. Pat. Nos. 5,954,092 (Kroutil et at., 1999); 6,679,278
(Lemoine et at.,
2004); and 9,566,554 (Wu et at., 2017)).
[0074] Each conduit can include at least one nozzle, so that a minimum of two
nozzles direct a cooling gas at the melt pool surface, or across the melt
pool, or across
the liquid-solid boundary, or on the solidified metal in the vicinity of the
liquid-solid
boundary, or on a solidified metal past the liquid-solid boundary, or any
combination
thereof The total number of nozzles present in the jet device can vary
depending on
the desired configuration. In some configurations, the jet device has a total
number of
nozzles from 2 to 24. The number of nozzles on each conduit can be the same or
different. For example, each conduit can include 10 nozzles, yielding a jet
device with
20 nozzles. In another example, one conduit can have 8 nozzles and the other
conduit
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can have 12 nozzles, yielding a jet device with 20 nozzles but having a
configuration
different from the first jet device that has 10 nozzles on each conduit.
[0075] The number, configuration and spacing of the nozzles can be selected
so that the coverage by the cooling gas jetted from the nozzles covers a
desired length
of the workpiece being formed. For example, in a high deposition rate process,
such
as a plasma and wire-based system, the number of nozzles and their
configuration can
be selected to result in a delivered cooling gas that covers a length from
about 5 mm to
about 50 mm, or from about 10 mm to about 40 mm, or from about 15 mm to about
30
mm, along the direction of travel. The nozzles can be configured to deliver a
cooling
gas that covers a length of about 20 mm along the direction of travel.
[0076] The length of each nozzle can be the same, or different nozzles can
have different lengths. Typically, each nozzle can have a length sufficient to
produce
a directional flow out the orifice. For example, the length can be in a range
of from
about 2.5 mm to about 25 mm, or from about 5 mm to 20 mm. The length of each
nozzle and its position can be selected so that a flow of cooling gas can be
applied
across the deposited molten material. The nozzles can be provided in pairs or
groups,
where the length of each nozzle and its position are selected to result in a
configuration
in which one member of the pair directs or some members of the group direct
cooling
gas to impinge on one location, and the other member of the pair directs or
the other
members of the group direct cooling gas to another location. For example, one
group
of nozzles can be directed on a melt pool surface, while another group of
nozzles can
be directed to solidified material
[0077] The number, configuration and spacing of the nozzles can be selected to
promote a turbulent gas flow in the vicinity of the melt pool surface, or the
liquid-solid
boundary, or the solidified metal in the vicinity of the liquid-solid
boundary, or any
combination thereof For example, the nozzles can be positioned so that the
jets of
cooling gas from at least two nozzles impinge on each other, creating
turbulent flow.
One or more of the nozzles can include a protrusion or an indentation or a
combination
thereof in the orifice of the nozzle or within the body of the nozzle to
interfere with
laminar flow of the cooling gas to promote turbulent flow. The velocity of the
cooling
gas flowing through the nozzle also can be monitored and adjusted so that the
cooling
fluid exiting the nozzles exhibits turbulent flow instead of laminar flow.
Turbulent
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flow can be created during the interaction of the impinging gas jets with
laminar
boundary layers near the workpiece. The cooling effect is increased with
turbulent
flow of the cooling gas. The conduits can include one or more baffles in the
cooling
gas flow path. Gas hitting a baffle can transfer the directed kinetic energy
induced by
the shock upon impact with the baffle into rotational energy resulting in
turbulent
mixing or turbulent flow.
[0078] A thermal insulating material can be used to thermally isolate the jet
device from the melting tool or the molten pool or the metal material feed or
any
combination thereof The thermal insulating material can be positioned between
the
jet device and the melting tool, or between the jet device and the metal
material feed,
or on a surface of the jet device facing the molten pool of the workpiece.
[0079] The thermally insulating material can include any material suitable for
use at the temperatures near the plasma arc, the laser device or the molten
pool. The
thermally insulating material can be or contain a thermally insulative
ceramic. Such
ceramics are known in the art and can include the oxides or nitrides of Al, B,
Zr, Mg,
Y, Ca, Si, Ce, In and Sn and combinations thereof (e.g., see U.S. Pat. Nos.
6,344,287
(Celik et at., 2002); 4,540,879 (Haerther et at., 1985); and 7,892,597 (Hooker
et at.,
2011)). The thermally insulating material can be or contain aluminum nitride,
aluminum oxide, magnesium nitride, magnesium oxide, quartz, silicon nitride,
boron
nitride, or zirconium dioxide, or a mixture or a combination thereof
[0080] A skewed front view drawing of an exemplary embodiment of the jet
device configured for delivery of a gas jet to a melt pool is shown in FIG. 1.
The
direction of travel of the workpiece is indicated by the arrow (in this
instance, the
depicted direction of travel is toward the observer). The jet device 100
depicted in the
figure includes on one side a first conduit 10 containing five pairs of
nozzles 25 that
direct gas jets 30 toward the deposited string 95 and melt pool 90 of the
workpiece.
The jet device shown also includes a second conduit 60 containing five pairs
of
nozzles 75 that direct gas jets 80 toward the deposited string 95 and melt
pool 90 of the
workpiece. The jet device 100 directs cooling gas jets to the melt pool free
surface and
the boundary between liquid and solid, as molten material is deposited to form
a string
95. A cooling gas supply 40 provides cooling gas to the first conduit inlet
15. A
cooling gas supply 50 provides cooling gas to the second conduit inlet 65. A
similar
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configuration of conduits and nozzles is present on the opposite side of the
melting
tool 200, although only the gas jets are visible in the figure.
[0081] In typical configurations, the melting tool can be situated above the
melt pool, and wire feedstock or powder feedstock are supplied to the melt
pool or into
the melting arc or beam. The jet device also can be positioned so that each
conduit of
the jet device is mounted on either side of the melting tool and the nozzles
can be
directed in such a way as to direct a jet of cooling gas to the melt pool free
surface or
the boundary between liquid and solid molten material.
[0082] A partial cutaway side view of an exemplary configuration of the jet
device configured for delivery of a gas jet to a melt pool is shown in FIG. 2.
The
depicted jet device includes a first conduit 10 containing a group of nozzles
25 and a
second conduit 60 containing a group of nozzles 75. A similar configuration
occurs on
the other side of the melting tool 200 to which the jet device is attached.
The
illustrated jet devices shows that the conduits on either side of the melting
tool are
connected by a cross-piece 85 to form a unitary body. Also shown in FIG. 2 is
an
internal diffuser 20 within the conduit 10 and a diffuser 70 within the
conduit 60,
which can help to even gas pressure and flow out of the nozzles. The grey
lines 30 and
80 indicate gas jet direction from the nozzles 25 and 75, respectively.
Cooling gas is
provided to conduit 10 via inlet 15, and cooling gas is delivered to conduit
60 via inlet
65. Also shown in FIG. 2 is a wire feed 300 that delivers metal wire 350 to a
position
above the melt pool 90.
[0083] Application of the cooling gas from the jet device as gas jets 30 and
80
to the melt pool 90, or the boundary between liquid and solid molten material,
or both,
can help to nucleate and propagate an opposing solidification front from the
melt pool
free surface, forming a top cap of finer grains that will block the continued
growth of
directional grains across layers. The effect can be more pronounced in high
deposition
rate processes where solidification rates typically are lower and the
directional
solidification front moves slow enough to allow the top cap to form and
propagate
further than the depth that will be re-melted by consecutive layers. The
mechanism is
illustrated in FIG. 3.
[0084] As illustrated in FIG. 3, at the far left, deposition of metal during
traditional additive manufacturing results in coarse, as-solidified grain
structures and
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can exhibit columnar grain growth. Depending on the alloy, the resulting grain
structures also can be elongated with a high aspect ratio. This typically is
the result of
the directional heat extraction provided by the relatively colder workpiece as
superheated molten metal is added to it in the string. In these conventional
processes,
initiation of solidification begins from the previously deposited metal
layer(s), and
propagates up into the deposited material as the deposited layer cools. The as-
solidified grain structures can in many cases extend across several layers,
and can
grow to be to several centimeters in size. These characteristics are typically
detrimental to mechanical properties, giving rise to reduced and/or
anisotropic
strength, elongation and fatigue performance.
[0085] The jet devices provided herein delivers a cooling gas. The cooling gas
delivered by the cooling jet devices can be any gas that does not interfere
with the
welding process used for deposition of molten metal to form the string during
additive
manufacturing. Exemplary cooling gases include argon, helium, neon, xenon,
krypton
and blends thereof. Typically, the cooling gas comprises argon, alone or in
combination with another gas. The temperature of the cooling gas delivered to
the
inlet of the jet device typically is less than 100 C, or less than 80 C, or
less than 60 C,
or less than 40 C, or less than 25 C. The cooling gas can be delivered to the
inlet of
the jet device at a temperature of about room temperature or below, such as
about
25 C or less, or about 20 C or less, or about 15 C or less, or about 10 C or
less. The
cooling gas can be delivered to the inlet of the jet device at a temperature
of from
about -195 C to about 25 C. The application of the cooling gas by the jet
device to the
melt pool, or the boundary between liquid and solid as molten material cools,
or both,
results in efficient refinement of the metal grains, producing finer grains
than achieved
in the absence of the application of the cooling gas.
[0086] Application of the cooling gas from the jet device to the melt pool, or
the boundary between liquid and solid as molten material cools, or both, also
can help
reduce temperature gradients at the directional solidification front typically
present
using conventional additive manufacturing techniques. The reduction in
temperature
gradients in the directional solidification front can destabilize continued
propagation
because of the cooling effect of the applied cooling gas on the free melt pool
surface.
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[0087] Application of the cooling gas from the jet device to the melt pool, or
the boundary between liquid and solid as molten material cools, or both, also
can help
redirect solidification through the cooling effect on as-solidified material
adjacent to
the liquid-solid boundary. The application of the cooling gas can alter heat
extraction
from the trailing edge of the melt pool. The application of the cooling gas
also can
increase overall solidification rates. Formation of columnar grain structure
is
minimized or prevented as a result of the mechanisms detailed above. Grain
refinement is the effect promoted by the application of cooling gas by the jet
device
provided herein. As a result of the application of cooling gas by the jet
device
provided herein, grain refinement, such as the formation of approximately
equiaxed
grain structure, is induced, thereby improving the mechanical properties of
the
manufactured product.
[0088] To maximize effect of the jet device, other process parameters are
typically set such that they are conducive to break-up of the solidification
front by
managing processing temperature and energy input such that a certain length of
the
melt pool is maintained for the gas jets to impinge on, and temperature
gradients in the
workpiece are minimized. For example, the processing temperature will depend
on
which alloy is being utilized, but typically is maintained within a range of
about 300 C
to about 750 C. Energy input also will depend on which alloy is being
utilized.
Effective energy input for Ti-6A1-4V in a high deposition rate plasma and wire-
based
process typically can be from about 300 Emm to about 1000 Priam. Thermal
gradients
in the workpiece can be minimized by processing at higher workpiece
temperatures
(interpass temperature) and with lower energy input per length unit.
[0089] Elimination of coarse columnar solidification structures
that
characteristically occur during additive manufacturing is expected to be
beneficial in
order to achieve an optimal balance of strength, ductility and fatigue
properties in
additively manufactured products, including titanium-based products, such as
Ti-6A1-
4V products. By manipulating the melt pool conditions, such as by using the
jet
device provided herein to direct cooling gas jets at the liquid-solid boundary
of the
melt pool, induces and accelerates opposing solidification front at the free
melt pool
surface. This can reduce or significantly eliminate formation of elongated
columnar
structures that can impose restriction in the number of favorable grain
variations that
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can form, thereby increasing the diversity of crystallographic orientations in
the
deposited material.
[0090] During additive manufacturing, the deposited material
experiences
changes in temperature from the melt pool through an area of solidifying
crystals to a
solidified metal area and an area of microstructure transitions. Thus, by
manipulating
the conditions throughout the deposition process in addition to the melt pool,
such as
by controlling or modulating the cooling rate in the metal solidification or
transition
areas, or both, formation of desired microstructure can be promoted.
Crystallography
and morphology of microstructures formed by allotropic transformation or other
mechanisms, depending on the alloy, can be affected by as-solidified grain
structure
through orientation relationships, grain boundary nucleation and alignment
caused by
differences in interfacial energy, diffusion rates and thermal conductivity
between the
different crystallographic directions in an alloy lattice as the deposited
material cools
and undergoes solidification and solid-state transformation. Differences in
thermal
history can result in a pronounced differences in strain response across
different grain
boundaries in many alloys
[0091] The jet device provided herein can be used to control or
modulate
cooling rate throughout the deposition process, and thereby influence the
thermal
history of the piece produced by additive manufacturing. Forced cooling
through
concentrated turbulent flow of jets of cooling gas can be applied on the as-
solidified
material using the jet device to control heat transfer, thermal conductivity,
thermal
energy dissipation and solid-state phase transformation. The jet device can
achieve
localized cooling and temperature measurement on targeted areas of the deposit
between string depositions, to precondition and even out workpiece temperature
in
preparation for consecutive layers.
[0092] A side view of an exemplary configuration of the jet
device
configured for delivery of a gas jet to an area of solidified metal is shown
in FIG.7.
The depicted embodiment of the cooling jet device 500 includes a plurality of
nozzles
525 that produce cooling gas jets 530 and that are attached on one side of a
wire feed
300. A similar configuration can occur on the other side of the wire feed 300
to which
the jet device is attached. In alternate embodiments, one or more rows of
nozzles can
be present on the underside of wire feed to which the jet device is attached.
In an
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alternate embodiment, the jet device can include a U-shaped conduit parallel
or nearly
parallel to the workpiece, the arms of which can be located on either side of
the
forming string of the workpiece and including nozzles directed downward toward
the
workpiece. The nozzles can be directed so that the cooling gas jets 530
impinge on an
upper surface of the workpiece, or on a side surface of the workpiece, or on
both an
upper surface and at least one side surface of the workpiece. In an alternate
embodiment, the jet device can include a trident- or v-shaped conduit (U-
shaped
conduit bisected by a separate conduit parallel to the arms of the U) parallel
or nearly
parallel to the workpiece, where the side arms of which are located on either
side of
the forming string of the workpiece and include nozzles directed downward
towards an
upper surface of the forming string or a side surface of the forming string,
and the
central conduit includes nozzles directed downward toward an upper surface of
the
forming string of the workpiece. In an alternate embodiment, the jet device
can
include three separate conduits in parallel, each with its own gas supply. One
outer
conduit can include nozzles directed to one side surface of the deposited
string, the
other outer conduit can include nozzles directed to the other side surface of
the
deposited string, and the central conduit can include nozzles directed to an
upper
surface of the deposited string. The positioning of the sensors and the jet
device can
be adjusted depending on the targeted temperature region deemed to be critical
for
determining and effecting the cooling rate. Therefore, the positioning can be
adjusted
based on the metal alloy to be deposited.
[0093] Also shown in FIG. 7 is a temperature sensor 550 attached to allow a
temperature reading to be taken on a workpiece surface in front of the zone of
application of the jets of cooling gas. Also shown in FIG. 7 is a temperature
sensor
560 attached behind the jet device 500 to allow a temperature reading to be
taken in a
zone of the workpiece after application of the jets of cooling gas. The
direction of
travel of the workpiece is indicated by the D arrow (in this instance, the
depicted
direction of travel of the workpiece is from left to right). In the embodiment
depicted
in shown in FIG. 7, the cooling jet device 500 and temperature sensors 550 and
560
are shown connected to the wire feed 300, but such attachments are
illustrative only.
Brackets or mounting arms separately can be used to attach any of the cooling
jet
device 500 and temperature sensors 550 and 560 to one or more components of
the
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system that allow movement with the melting tool 200, application of the
cooling gas
to the desired surfaces of the workpiece, and appropriate temperature
measurement of
the workpiece. The gas jets are directed in such a way as to not disturb the
melt pool
or metal transfer, and to provide cooling through a backwards gas flow along
the
deposited string. The application of the cooling gas to the solidified metal
can achieve
suitable local cooling for a period determined by the temperature readings
from
temperature sensors that thereby can achieve continuous cooling rate control
during
material addition, and local preconditioning between string depositions. Flow
rates of
the cooling gas jets from the jet device direct to a region of the workpiece
of solidified
metal after the melt pool can be adjusted based on thermal conditions in the
workpiece
during processing, either by taking in situ measurements or following a pre-
programmed computerized schedule based on readings received from the
temperature
sensors before and after the area of impingement of the cooling gas jets.
Suitable
cooling can be achieved by applying a flow of cooling gas for a period of
time, which
can be determined by the data received from the temperature sensors before and
after
the area of impingement of the cooling gas. The positioning of the temperature
sensors and jet device can depend on which temperature region of the workpiece
is
most critical to capture and affect the cooling rate. The positioning can be
adjusted
based on the metal alloy to be deposited.
[0094] The jet device allows for continuous cooling rate control during
material addition, and preconditioning between string deposits without
terminating the
deposition process. Flow rates can be adjusted based on changing thermal
conditions
in the workpiece during processing, either manually by monitoring of the data
from the
temperature sensors, or automatically using a computer that receives
temperature data
from the temperatures and adjusts the flow rate or duration or both to achieve
a
targeted cooling rate. Infrared temperature sensors can be selected and
calibrated for
the relevant temperature ranges experienced in the workpiece and deposition
process.
Sensor data can be measured and stored at a rate of 1 Hz or higher. The
temperature
data can be captured by a computer in a process control system to allow in-
process
feedback control of the deposition process, or viewed post-process and
manually
adjusted as part of an iterative deposit development phase to produce a
deposition
schedule, or a combination of these techniques. Flow can be zero or near zero
in the
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first deposited layer, then increased as residual heat has built up. Flow
rates can range
from zero or near zero to up to about 500 L/min. Flow rates can range from
zero or
near zero to up to about 400 L/min. Flow rates can range from zero or near
zero to up
to about 300 L/min. In some applications, the cooling gas flow rate can be at
least 10
L/min, or at least 25 L/min, or at least 50 L/min, or at least 100 L/min, or
at least 150
L/min, or at least 200 L/min, or at least 250 L/min, or at least 300 L/min, or
at least
350 L/min, or at least 400 L/min, or 500 L/min or less, or 450 L/min or less,
or 400
L/min or less, or 350 L/min or less, or 300 L/min or less, or 250 L/min or
less, or 200
L/min or less, or 250 L/min or less, or 200 L/min or less, or 150 L/min or
less, or 100
L/min or less, or 50 L/min or less. The cooling has can be inert or non-inert,
depending on the requirements of the alloy to be processed. The cooling gas
can be
elemental, or a mix of different gases.
[0095] The jet device can apply suitable cooling for a period of time in these
areas to effectively remove the excess thermal energy applied during
deposition. The
jet device allows application of cooling gas directly on the deposited metal
while
deposition is taking place to achieved localized cooling rate control, as well
as local
cooling rate and temperature measurement on areas of the deposit string
deposition,
allowing preconditioning or evening out of workpiece temperature or both in
preparation of consecutive layers. High velocity cooling gas can be delivered
by the
jet device to areas of consecutive layers of deposited material.
[0096] Conventional welding processes can apply a shielding gas device
trailing the welding torch to direct a laminar flow gas curtain towards the
solidified
material in order to protect the deposited material from the surrounding
atmosphere
and avoid contamination of the weld metal. This laminar flow of gas is
insufficient to
affect or control temperature dissipation or cooling rate. The jet devices
provided
herein apply jets of cooling gas at a flow rate sufficient to result in
turbulent flow of
the gas. Turbulent flow of the cooling gas from the nozzles of the jet device
typically
can be achieved through a high velocity of the cooling gas through the
nozzles.
[0097] C. Systems
[0098] Typical additive manufacturing technology, especially high deposition
rate processes, often can exhibit significant variation in processing
conditions due to
variations in deposit geometry. Local workpiece temperature in a large deposit
with
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greater time between repetitions (i.e., longer time per layer) will have very
different
temperature conditions compared to a smaller deposit where strings can come in
quick
succession and heat is allowed to accumulate. Similarly, local mass input can
determine or effect the cross-section of heat extraction from the deposited
material,
and adjacent mass affect the capacity of the heat sink to handle the added
thermal
energy.
[0099] These factors can result in non-optimal and variable material
properties.
In many cases, post-process heat treatment beyond a basic stress relief is
either
impractical or ineffective for a multitude of metal alloys. Crucially, the
fully formed
deposit may have a section thickness that does not allow a bulk heat treatment
to
achieve the desired cooling rates. The systems for building a metallic object
by
additive manufacturing provided herein overcome these shortcoming of prior art
systems. The piece-by-piece method of additive manufacturing utilizing the jet
devices provided herein can allow for cooling rate control in the smaller
volumes of
material of the individual strings that the final part is comprised of during
the
deposition process. The systems are flexible and highly controllable and
provide a
way to improve consistency of metal additive manufacturing products,
particularly for
large scale, high deposition rate processes. The systems can include a
computer,
which can be used to automate a part or all of the system. The computer can be
in
communication with a control system and can be used to read a design model.
The
computer can collect data, store and/or manipulate data, such as flow rates
and
temperatures, or other parameters of the manufacturing process. The computer
can use
the collected data as to operate or modify the manufacturing process. The
computer
can include a computer processor that can be in communication with one or more
of
the components of the system.
[00100] As a deposited string solidifies and cools down, most relevant alloys
undergo significant solid state transformations that can have a profound
effect on
material properties. One example includes allotropic transformations where the
crystal
structure arrangement changes to another crystal structure arrangement. Many
titanium alloys exhibit allotropic transformations in a temperature range
during
cooling from 10500 to 800 C. For many steels, the temperature range during
cooling
for transformation typically is from 800 to 400 C. Another example of a solid
state
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transformation in the deposited metal during cooling is a precipitation
reaction, where
the ordering of constituents of the alloy form particles of a secondary phase.
As an
example, nickel-based superalloys can exhibit precipitation reactions during
cooling
from about 10000 to 700C, and for prolonged residence above 600 C. Grain
growth
during prolonged residence at higher temperatures also affects the properties
of most
alloys. The jet devices provided herein can affect or control cooling rate
thereby
allowing modification of the properties of the deposited material, resulting
in
improved consistency of the metal additive manufactured products. The systems
provided herein allow for continuous cooling rate control during material
addition,
and local preconditioning between string depositions. The systems provided
herein
allow for control of processing conditions for the manageable volume of an
individual
string segment. The systems allow for temperature control during deposition,
achieving results not possible using a post-process heat treatment, where
controlling
cooling rate in the thicker sections of a full additive-manufactured deposit
are more
difficult, and the high cooling rates achievable using the jet devices
provided herein
are not achievable using a post-process heat treatment without the use of less
practical
methods such as quenching in water or oil.
[00101] The systems provided herein can include a melting tool to melt a
source of metal into droplets of metallic molten material that are deposited
into a
liquid molten pool on a base material; a jet device as provided herein to
direct a
cooling gas across the liquid molten pool, or to impinge on the liquid molten
pool, or
to impinge upon a solidified material adjacent to a liquid-solid boundary of
the liquid
molten pool, or any combination thereof; a supply of the cooling gas; a system
for
positioning and moving the base material relative to the heating device and
jet device;
and a control system able to read a design model, such as a computer assisted
design
(CAD) model, of the metallic object to be formed, and employ the design model
to
regulate the position and movement of the system for positioning and moving
the base
material and to operate the heating device and jet device such that a physical
object is
built by fusing successive deposits of the metallic material onto the base
material.
[00102] A single melting tool can be used, or a two gun system comprising
two melting tools can be used. It has been determined that the deposition rate
of
molten metal to a forming workpiece can be increased using a two gun system in
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which a first gun preheats the base material to form a preheated area, and a
second gun
is used to heat and melt a metal onto the preheated area of the base material.
The first
gun can ensure fusion between the base material or workpiece and the molten
metal
produced by the action of the second gun on a metal, such as a metal wire or
metal
powder. The first gun can deepen the melt-in of the molten metal into the
preheated
area of the base material. The superheat from the droplets of molten metal can
maintain a melt pool in the vicinity of the preheated area of the base
material. The
pre-heating of the base material can lead to better wetting, better deposition
profile and
increased deposition rate. Regarding deposition profile, by pre-heating the
substrate, it
is possible to obtain a rounder and wider deposit profile. The improved
profile can
result in a profile with a beneficial angle towards the base material, which
can promote
fusion to the base material and previous metal depositions. Improved fusion
yields a
manufactured product with improved integrity.
[00103] Each of the guns includes a melting tool. Each gun can be
separately controlled, and each gun can be modulated to produce a separate
temperature effect. An advantage of this arrangement is that the amount of
thermal
energy applied to the metallic feed stock to be melted onto the preheated area
of the
base material can be greater than that applied to the base material, avoiding
over-
heating of the base material.
[00104] In an embodiment of the two gun additive manufacturing system
provided herein, the system can include a torch (PAW, PTA, GMAW or MIG-type)
or
a laser device or any combination thereof as a melting tool. In some
configurations, a
first torch pre-heats a target deposition area on the base material to form a
preheated
area, and a second torch heats and melts a consumable electrode, resulting in
drops of
molten metal that fall into the preheated area of the target deposition area.
In some
configurations, the laser device pre-heats a target deposition area on the
base material
to form a preheated area, and a torch heats and melts a consumable electrode,
resulting
in drops of molten metal that fall into the preheated area of the target
deposition area.
In some configurations, the torch pre-heats a target deposition area on the
base
material to form a preheated area, and a laser device heats and melts a metal
wire,
resulting in drops of molten metal that fall into the preheated area of the
target
deposition area.
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[00105] A laser device or a torch can be arranged to direct thermal energy
(e.g., laser energy or a plasma transferred arc, respectively) to a target
area of the base
material to form a preheated area, and a torch or a laser device can be
arranged to
direct thermal energy onto an end of a consumable electrode or metal wire
positioned
above the preheated area of the base material. The thermal energy melts the
end of the
consumable electrode or metal wire, forming droplets of molten metal that drop
onto
the preheated area of the base material beneath the end of the consumable
electrode or
metal wire. The melting tool that directs thermal energy to a target
deposition area can
promote fusion between the base material and the molten metal material being
deposited thereon by deepening the melt-in of the droplets of molten metal
into the
base material. The melting tool used to melt the consumable electrode or metal
wire
also can contribute thermal energy in the vicinity of the preheated area of
the target
deposition area, contributing to the thermal energy provided by the melting
tool
directed to the base material. The superheat from the droplets of molten metal
can
help maintain a melt pool in the vicinity of the preheated area of the base
material.
[00106] The consumable electrode or metal wire can be or contain Al, Cr,
Cu, Fe, Hf, Sn, Mn, Mo, Ni, Nb, Si, Ta, Ti, V, W, or Zr, or composites or
alloys
thereof In some embodiments, the consumable electrode is a wire that contains
Ti or
a Ti alloy. The consumable electrode or metal wire can be or contain a
titanium alloy
containing Ti in combination with one or a combination of Al, V, Sn, Zr, Mo,
Nb, Cr,
W, Si, and Mn. For example, exemplary titanium alloys include Ti-6A1-4V, Ti-
6A1-
6V-25n, Ti-6A1-25n-4Zr-6Mo, Ti-45A1-2Nb-2Cr, Ti-47A1-2Nb-2Cr, Ti-47A1-2W-
0.55i, Ti-47A1-2Nb-1Mn-0.5W-0.5Mo-0.25i, and Ti-48A1-2Nb-0.7Cr-0.35i. The
consumable electrode or metal wire can contain aluminum, iron, cobalt, copper,
nickel,
carbon, titanium, tantalum, tungsten, niobium, gold, silver, palladium,
platinum,
zirconium, alloys thereof, and combinations thereof.
[00107] A typical cross section of the consumable electrode or metal wire is
a circular cross section. The diameter of the consumable electrode or metal
wire can
be up to about 10 mm, and can be in the range of from about 0.8 mm to about 5
mm.
The consumable electrode or metal wire can have any practically implementable
cross-
sectional dimension, e.g., 1.0 mm, 1.6 mm, and 2.4 mm, or from about 0.5 to
about 3
mm. The feed rate and positioning of the consumable electrode or metal wire
can be
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controlled and modulated in accord with the effect of the power supply to the
PTA
torch or laser device or both in order to ensure that the consumable electrode
or metal
wire is being continuously heated and is melted when it reaches the intended
position
above the molten pool in the base material.
[00108] The laser device can generate a laser beam of sufficient energy to
transfer thermal energy to the base material to preheat an area of the base
material, or
to melt a metal wire. The preheating of the base material via energy from the
laser
beam promotes fusion between the base material and the melted metallic
material by
deepening the melt-in in the base material. In some embodiments, at least a
portion of
the base material can be melted by the energy from the laser beam of the laser
device.
In some embodiments, sufficient heat is applied by the laser beam of the laser
device
to form a molten pool in the base material at the position at which the
metallic material
produced by the PTA torch or another laser is to be deposited.
[00109] Examples of suitable laser devices include a ytterbium (Yb) laser, a
Yb fiber laser, a Yb fiber coupled diode laser, a Yb:glass laser, a diode-
pumped
Yb:YAG laser, a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, a CO2
laser, a CO laser, a Nd:glass laser, a neodymium-doped yttrium orthovanadate
(Nd:YVO) laser, a Cr:ruby laser, a diode laser, a diode pumped laser, an
excimer laser,
a gas laser, a semiconductor laser, a solid-state laser, a dye laser, an X-ray
laser, a free-
electron laser, an ion laser, a gas mixture laser, a chemical laser, and
combinations
thereof Preferred lasers include Yb lasers, particularly Yb fiber lasers. In
many
applications, the wavelength used in a Yb fiber laser can be less reflective
compared to
other laser wavelengths.
[00110] The torch can be of any configuration capable of creating an electric
arc to heat and melt the consumable electrode, or to heat a target area on the
base
material, such as gas metal arc welding (GMAW), particularly using non-
reactive
gases to make the arc (metal inert gas welding or MIG-welding). Thus, the
torch
can be a PAW torch, a PTA torch, a GMAW torch or a MIG-type torch. The
consumable electrode is made to melt in the plasma produced by the torch using
an electric arc, and the melting consumable electrode is deposited into the
molten
pool on the workpiece to add to and to form the near net shape metal bodies.
The
preheating of the base material via energy from the torch promotes fusion
between the
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base material and the melted metallic material by deepening the melt-in in the
base
material. In some embodiments, at least a portion of the base material can be
melted
by the energy from the plasma of the torch. In some embodiments, sufficient
heat is
applied by the plasma of the torch to form a molten pool in the base material
at the
position at which the metallic material melted by a different torch or laser
device is to
be deposited.
[00111] The use of a first melting tool to preheat the base material and form
a preheated area, and a second melting tool to melt the consumable electrode
or metal
wire provides the advantage that it becomes possible to increase the thermal
energy
directed to the consumable electrode or metal wire independently of the heat
supply to
the substrate. The melting power applied to the consumable electrode or metal
wire
can be selected to match the mass input (the amount of molten metal droplets
of
consumable electrode or metal wire to be added to the workpiece) in order to
secure a
stable melting of the consumable electrode or metal wire and/or burn-off
point. Thus,
it is possible to increase the deposition rate of the molten metal without
simultaneously
over-heating the substrate and without risk of spatter or forming an excessive
molten
pool and thus, losing control of the consolidation of the deposited material.
[00112] The systems for manufacture of near net shape metal bodies using
additive manufacturing provided herein utilize a jet device that significantly
alleviates
the problems related to metal grain columnarity and large grain size evident
in many
traditional additive manufactured products. The grain structure in the
manufactured
product using the systems provided herein that include a jet device for
delivery of a
cooling gas jet to a melt pool or to a vicinity of a melt pool, or a jet
device for delivery
of a cooling gas jet to a solidified metal, or a first jet device for delivery
of a cooling
gas jet to a melt pool or to a vicinity of a melt pool and a second jet device
for delivery
of a cooling gas jet to a solidified metal, produce a manufactured metal
product having
metal grains that are approximately equiaxed and that exhibit a refined
structure.
Using one or more jet devices provided herein to apply a cooling gas during
additive
manufacture to create gas jet impingement on the free surface of the melt
pool, or
across the molten pool, or at the boundary between liquid and solid as the
molten
metal cools, or to a solidified metal beyond the liquid-solid boundary, or any
combination thereof, results in a manufactured products having a refined grain
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structure, and the products produced using these systems demonstrate increased
strength, fatigue resistance, and durability.
[00113] A jet device directing jets of inert gas at the liquid-solid boundary
of the melt pool can induce or accelerate opposing solidification front at the
free melt
pool surface. Blocking of epitaxy can be achieved as consecutive layers
nucleate and
solidify from the top-layer grains. Forced cooling through concentrated
turbulent flow
via a jet device directed to a region of as-solidified material can control or
modulate
solid-state phase transformation, precipitation reactions and other secondary
phase
phenomenon that can influence final crystal structure and localized ordering.
[00114] A depiction of an exemplary system that includes a jet device
directing a turbulent flow of cooling gas in situ on the as-solidified
material of a
deposited layer 480 to increase cooling rate by applying jets of cooling gas
in situ to an
as-deposited solidified material is depicted in FIG. 7. The depicted system
includes a
single melting tool 200 that is a main melting tool that produces a main PTA
arc 330
that heats and melts a metal wire 350 from a wire feed 300, forming droplets
of molten
metal 375 that drop into and form a melt pool 425 on the workpiece 400. Forced
cooling of as-deposited material during the deposition process by jets of
cooling gas
530 provided by the jet device 500 can achieve refinement of the
microstructure of the
additively manufactured product.
[00115] As shown in FIG. 7, the system can include the jet device 500
connected to wire feed 300, and temperature sensors 550 and 560 separately
attached
to the wire feed 500, either directly (as in the depiction for an embodiment
of
temperature sensor 550) or via a bracket 570 (as in the depiction of an
embodiment of
temperature sensor 560). Although the embodiment of the system depicted in
FIG. 7
shows temperature sensor 550 and temperature sensor 560 connected to the wire
feed
300, such attachments are illustrative only.
[00116] As illustrated for example in FIGs. 8 and 9, brackets or mounting
arms separately can be used to separately and individually attach each of the
cooling
jet device 500, temperature sensor 550 and temperature sensor 560 to one or
more
components of the system that allow application of the cooling gas to the
desired
surfaces of the workpiece, and appropriate temperature measurement of the
workpiece
to which in situ cooling gas jets are directed. In some configurations, as
illustrated in
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FIG. 9, temperature sensor 550 can be attached to melting tool 200 either
directly or
via a bracket 575. In other configurations, as illustrated in FIG. 8,
temperature sensor
550 can be attached to a bracket 250. Bracket 250 can be attached to or hold
wire
feed 300, or it can be attached to or hold melting tool 200, or it can be
attached to or
hold one or more other components of the system, or any combination thereof.
[00117] Similarly, in some configurations, temperature sensor 560 can be
attached to wire feed 300 either directly or via bracket 570, or to a bracket
that can be
the same as or different from bracket 250, but that like bracket 250 can be
attached to
or hold one or more components of the system. For illustrative purposes, FIG.
8
shows temperature sensor 560 as connected to bracket 250 like temperature
sensor
550, where bracket 250, as described earlier, can be attached to or hold wire
feed 300,
one or more other components of the system, or a combination thereof
[00118] In some configurations, the temperature sensors can include an
infrared fiber optic sensor or detector to allow non-contact measurements of
the
surface of a deposited layer 480 to which cooling gas jets 530 are directed,
while
allowing the bulk of the temperature sensor to by attached to another
component of the
system at a location away from the infrared fiber optic sensor or detector.
Temperature sensor 550 is positioned to allow a temperature reading to be
taken on a
workpiece surface in front of the zone of application of the jets of cooling
gas.
Temperature sensor 560 is position to allow a temperature reading to be taken
on a
workpiece surface behind the zone of the application fo the jets of cooling
gas. The
positioning of the temperature sensors and jet device can depend on which
temperature
region of the workpiece is most critical to capture and affect the cooling
rate. The
positioning can be adjusted based on the metal alloy to be deposited.
[00119] A depiction of an exemplary system that includes a first jet device
directing a turbulent flow of cooling gas at the liquid-solid boundary of the
melt pool
and a second jet device that provides forced convective cooling by directing a
turbulent flow of cooling gas on the as-solidified material of a deposited
layer 480 is
depicted in FIG. 8. The depicted system includes a single melting tool 200
that is a
main melting tool that produces a main PTA arc 330 that heats and melts a
metal wire
350 from a wire feed 300, forming droplets of molten metal 375 that drop into
and
form a melt pool 425 on the workpiece 400. Without application of the cooling
gas via
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the cooling jet device 100, columnar structures typical of additive
manufacture
processes can occur as solidifying crystals 435 in the deposited layer 480.
For
example, in Ti-6A1-4V alloys, solidification is directional and epitaxial with
spatial
and crystallographic I3-grains in a first region or solidification zone 430
dictated by the
steep thermal gradient from the heat source/melt pool to the workpiece. As
cooling
continues, the crystals solidify in a second zone containing solidified
material 450,
which can be followed by transition where there are changes in the
crystallography and
morphology of the a-I3 microstructures upon allotropic transformation. These
are
directly affected by the prior I3-grain structure through orientation
relationships, grain
boundary nucleation and alignment caused by differences in interfacial energy,
diffusion rates and thermal conductivity between the different
crystallographic
directions in the lattice.
[00120] In the system depicted, cooling gas jets 30 from nozzles 25 of jet
device 100 are directed at the liquid-solid boundary of the melt pool.
Impingement of
the gas jets 30 at the liquid-solid boundary of the melt pool 425 induces and
accelerates an opposing solidification front 440 at the melt pool surface.
Blocking of
epitaxy is achieved as consecutive layers nucleate and solidify from the top-
layer
grains. The forced cooling caused by the gas jets 30 of the jet device 100 is
accentuated by concentrated turbulent flow applied by the jet device, across
the melt
pool, at the melt pool surface, at the liquid-solid boundary of the melt pool,
or any
combination thereof
[00121] Forced cooling through concentrated turbulent flow can applied on
the as-solidified material of the deposited layer 480 to control solid-state
phase
transformation via extension of the cooling jet device 100, or as depicted,
via a second
jet device 500 to direct cooling gas jets 525 to the as-solidified material in
zone 450 to
control solid state phase transitions, such as I3-a solid-state phase
transformation in
titanium alloys, or precipitation reactions in nickel-based superalloys.
[00122] As shown in the figure, the system includes a second jet device 500
and at least two temperature sensors to monitor temperature throughout the
additive
manufacturing process. In the embodiment depicted, a first temperature sensor
550
attached to a bracket 250 can monitor the temperature at the surface of the as-
deposited material ahead of the application of a cooling gas, such as in the
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solidification region 440. A second temperature sensor 560 located after the
jet device
can be included to measure the temperature of a surface 565 of the workpiece
after
application of the cooling gas to the string of the workpiece by the second
jet device.
Temperature monitoring by using the temperature data from the first and second
temperature sensors, for example, can allow the user to control the cooling
rate by
adjusting the flow rate of cooling gas applied using the second jet device
500, or the
duration of the flow of the cooling gas towards the workpiece, or both. When
two
separate cooling jet devices are used, a single cooling gas supply can be used
to
provide cooling gas to each jet device. Alternatively, each cooling jet device
can be
attached to a separate cooling gas supply.
[00123] The exemplified system is shown using a one torch system, but the
methods are not limited to such systems. A two torch system also can be used.
[00124] An exemplary two torch system is shown in FIG. 9. In the depicted
system, a melting tool 600 preheats a workpiece 400, forming a pre-heated area
415,
which makes the workpiece 400 more receptive to molten metal. A second melting
tool 200 that is a main melting tool that produces a main PTA arc 330 heats
and melts
a metal wire 350 from a wire feed 300, forming droplets of molten metal 375
that drop
into and form a melt pool 425. Without application of the cooling gas via the
jet
device 100, columnar structures typical of additive manufacture processes can
occur as
solidifying crystals 435 in the deposited layer 480. For example, in Ti-6A1-4V
alloys,
solidification is directional and epitaxial with spatial and crystallographic
13-grains in a
first region or solidification zone 430 dictated by the steep thermal gradient
from the
heat source/melt pool to the workpiece. As cooling continues, the crystals
solidify in a
second zone 450 to form a solidified material.
[00125] In the system depicted, cooling gas jets 30 from nozzles 25 of jet
device 100 are directed at the liquid-solid boundary of the melt pool.
Impingement of
the gas jets 30 at the liquid-solid boundary of the melt pool 425 induces and
accelerates an opposing solidification front 440 at the melt pool surface.
Blocking of
epitaxy is achieved as consecutive layers nucleate and solidify from the top-
layer
grains. The forced cooling caused by the gas jets 30 of the jet device 100 is
accentuated by concentrated turbulent flow applied by the jet device, across
the melt
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pool, at the melt pool surface, at the liquid-solid boundary of the melt pool,
or any
combination thereof
[00126] Forced cooling through concentrated turbulent flow can applied on
the as-solidified material to control solid-state phase transformation via a
second jet
device 500 to direct cooling gas jets 525 to the as-solidified material in
zone 450 to
control solid state phase transitions, such as I3-a solid-state phase
transformation in
titanium alloys, or precipitation reactions in nickel-based superalloys.
[00127] As shown in the figure, the system includes a second jet device 500
and at least two temperature sensors to monitor temperature throughout the
additive
manufacturing process. In the embodiment depicted, a first temperature sensor
550
can monitor the temperature at the surface of the as-deposited material ahead
of the
application of a cooling gas, such as in a post-solidification temperature
monitoring
area 555. A second temperature sensor located after the jet device can be
included to
measure the temperature of a surface 565 of the workpiece after application of
the
cooling gas to the string of the workpiece by the second jet device 500, such
as at a
post transformation temperature monitoring area 565. Temperature monitoring by
using the temperature data from the first and second temperature sensors, for
example,
can allow the user to control the cooling rate by adjusting the flow rate of
cooling gas
applied using the second jet device 500, or the duration of the flow of the
cooling gas
towards the workpiece, or both.
[00128] D. Methods
[00129] Also provided herein are methods for manufacturing a three-
dimensional object of a metallic material by additive manufacture, where the
object is
made by fusing together successive deposits of the metallic material onto a
base
material, the methods including using a first heating device to preheat at
least a portion
of the surface of the base material, e.g., at the position at which the
metallic material is
to be deposited; using a second heating device to heat and melt a metallic
material
such that molten metallic material is deposited onto the preheated area of the
base
material; using a jet device provided herein to direct a cooling gas across
the liquid
molten pool, or to impinge on the liquid molten pool, or to impinge upon a
solidified
material adjacent to a liquid-solid boundary of the liquid molten pool, or any
combination thereof and moving the base material relative to the position of
the first
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and second heating devices and the jet device in a predetermined pattern such
that the
successive deposits of molten metallic material solidifies and forms the three-
dimensional object.
[00130] In one method, a jet device provided herein directs a cooling gas
having a turbulent flow across the melt pool, at the melt pool surface, at the
liquid-
solid boundary of the melt pool, or any combination thereof In another method,
a jet
device provided herein directs a cooling gas having a turbulent flow to an as-
solidified
material, such as in a solid state transformation zone, e.g., an allotropic
transformation
area or an area in which precipitation reactions could occur. In another
method, a first
jet device provided herein directs a cooling gas having a turbulent flow
across the melt
pool, at the melt pool surface, at the liquid-solid boundary of the melt pool,
or any
combination thereof, and a second jet device provided herein directs a cooling
gas
having a turbulent flow to an as-solidified material, such as in a solid state
transformation zone.
[00131] In the methods provided herein, the cooling gas can comprise an
inert gas, such as argon, helium, neon, xenon, krypton and combinations
thereof. The
cooling gas can be a non-inert gas. The cooling gas can be a mixture of
different
elemental gases. The cooling gas directed across the melt pool, at the melt
pool
surface, at the liquid-solid boundary of the melt pool, or any combination
thereof can
have a flow rate from about 1 L/min to about 200 L/min. The cooling gas
directed as
an as-solidified material can have a flow rate from about 0.01 L/min to about
300
L/min. The cooling gas can be applied in a constant stream, or can be applied
intermittently, or can be applied in a pulsed flow.
[00132] The temperature of the cooling gas applied can be any temperature.
The cooling gas temperature can be the ambient temperature of the additive
formation
process. Typically, the cooling gas temperature can be about room temperature
or
less, such as about 25 C or less. The temperature of the gas can be any
temperature
that cools the surface with which it interacts. The temperature can be less
than 100 C,
or less than 50 C, or less than 30 C, or less than 25 C, or less than 10 C, or
less than
5 C, or less than 0 C. Gas at a cryogenic temperature also can be used. For
example,
the temperature of the cooling gas delivered to the inlet of the jet device
can be from at
or about -195 C to at or about 25 C.
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[00133] In the methods provided herein, a jet device having at least two
temperature sensors is used to measure and to produce a targeted cooling rate.
The
positioning of the temperature sensors and the jet device can depend on the
temperature region identified as critical to capture and affect the cooling
rate. The
positioning can be adjusted based on the metal alloy to be deposited. The
temperature
sensors can include IR thermometers to capture the temperature of a surface of
the
deposited string material of a workpiece before and after application of the
turbulent
jets of cooling gas. Based on the data, the flow rate or duration or both of
the cooling
gas can be adjusted to increase or decrease the cooling rate. In some methods,
the
temperature data is captured and used to provide in-process feedback control
to allow
partial or full automation of the cooling rate used in the additive
manufacturing
process. The data also can be captured and used to design post-process an
iterative
deposit development program/schedule to automate a deposition of a workpiece.
[00134] The desired cooling rate can be alloy dependent. Different alloys
can exhibit different changes in solid state phase transformation depending on
the
temperature range and time exposed to a specific temperature range. For
example, for
many titanium alloys, the methods provided herein have a targeted cooling
temperature in the range from 1200 C to about 600 C, or from 10500 to about
800 C to
promote allotropic transformations. For steel alloys, a targeted cooling
temperature
can be in the range from 1000 C to about 300 C, or from about 800 -400 C to
promote
desired solid state transformations. For example for the alloy Ti-6A1-4V, the
cooling
effect from the gas jet device directed at as-solidified material in this
temperature
region can be used to enhance cooling rates from something that typically
gives
undesired colony/ lamellar structures to conditions that promote beneficial
fine
basketweave-type structures. Per temperature measurements during testing, this
corresponds to increasing bulk cooling rate in the phase transformation region
from
around 10 C/s to 15 C/s. Due to localized gas jet impingement, the temperature
captured on the workpiece surface in those cases was between 80 and 140 C/s.
The
relationship between measured surface cooling rate and experienced bulk
cooling rate
needs to be established for the alloy in question. The top of the deposited
string
undergoes elevated cooling rates, but will be reheated back to above the
transformation
temperature during consecutive layers and it is therefore only segments
towards the
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bottom of the heat affected zone for each layer that will remain in the
finished deposit.
Steels with key temperature range for transformation typically during cooling
between
800-400 C.
[00135] In the methods provided herein, suitable local cooling for a period
determined by the temperature readings from the temperature sensors measuring
a
surface temperature of the deposited string can be used to dissipate any
higher local
energy input that can be necessary to form junctions or transitions in the
workpiece.
The methods allow for continuous cooling rate control during material
addition, and
can be used to provide local preconditioning between string depositions. In
the
methods provided herein, flow rates of the cooling gas can be adjusted based
on
changing thermal conditions in the workpiece during processing. The flow of
turbulent
cooling gas can be increased as residual heat has built up during additive
manufacturing, or to dissipate heat added in order to form a particular
structure, such
as a junction or a transition.
[00136] In the methods provided herein, turbulent flow from the nozzles of
the jet device typically can be achieved through a high velocity of the
cooling gas
through the nozzles. Other techniques also can be used to produce turbulent
cooling
gas flow. For example, some of the nozzles of the jet device can be positioned
so that
the jets of cooling gas from at least two nozzles impinge on each other,
creating
turbulent flow of the cooling gas in the vicinity of the molten pool. The
nozzles can
include a protrusion or an indentation or a combination thereof in the orifice
of the
nozzle or within the body of the nozzle to interfere with laminar flow to
promote
turbulent flow. Typically, the velocity of the cooling gas flowing through the
nozzle is
selected so that the cooling gas exiting the nozzles exhibits turbulent flow
instead of
laminar flow.
[00137] The number of nozzles and their configuration can be selected to
deliver cooling gas that covers a targeted length of the workpiece, e.g., from
about 5
mm to about 50 mm, or from about 10 mm to about 40 mm, or from about 15 to
about
mm, along the direction of travel.
30 [00138] Typical process conditions traditionally used in additive
manufacturing usually result in directional solidification and growth of
columnar
crystals due to the presence of steep thermal gradients, but this can be
dependent on
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the alloy utilized. For example, for Ti-6A1-4V alloy, solidification is
directional and
epitaxial with spatial and crystallographic orientation of I3-grains dictated
by process
characteristics that include a steep thermal gradient from heat source/melt
pool to
workpiece. Crystallography and morphology of a-I3 microstuctures in Ti-6A1-4V
alloy
upon allotropic transformation are directly affected by the prior I3-grain
structure
through orientation relationships, grain boundary nucleation and alignment
caused by
difference in interfacial energy, diffusion rates and thermal conductivity
between
different crystallographic directions in the lattice. This macro-micro
interaction leads
to long ranging limitations of crystallographic and morphological diversity
within
prior I3-grains, and thus pronounced differences in strain response across I3-
grain
boundaries.
[00139] The methods provided herein allow for a reduction in the size of the
melt pool length. This can be achieved by the increased solidification rate at
the
trailing edge of the melt pool. Application of turbulent cooling gas toward
the melt
pool increases solidification and reduces the time for solidification to
occur.
Depending on the solidification rate achieved by application of the cooling
gas using
the jet device provided herein the total melt pool length can be reduced by
about 10%
to about 50%. For example, compared to conventional additive manufacturing
methods and systems, melt pool length can be 90% or less, or 80% or less, or
70% or
less, or 60% or less, or 50% or less of the melt pool length in conventional
additive
manufacturing techniques.
[00140] The jet devices provided herein induce the grain refinement.
Controlling process parameters can aid the effectiveness. This is especially
true in
alloys that are resistant to solidification refinement, such as Ti-6A1-4V due
to the
narrow freezing range exhibited in that alloy. The solidification
characteristics make
constitutional undercooling unlikely at the typical thermal gradients and
solidification
rates of metal additive manufacturing.
[00141] The jet devices provided herein can be used in configurations of
metal additive manufacturing using a single melting device or one torch
configuration.
The jet devices provided herein can be used in two torch configurations of
metal
additive manufacturing. A preheater torch can be used to achieve a dedicated
workpiece surface temperature control. A separate second torch can be used as
a
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melter torch to melt the feedstock, such as a metal wire. Thermal gradients
can by
modulated by limiting the energy intensity required in the melter torch and to
achieve
desired contact angles of the molten metal by ensuring wetting at the
perimeter of the
melt pool without excessively superheating the melt pool itself. This is
beneficial for
grain refinement, but not required to achieve the effect achieved by using the
jet
device.
[00142] Dedicated energy transfer to wire, also with resistive heating of the
wire allows high deposition rates without excessive energy transfer directly
to the melt
pool as would be the case if the energy source that melts the wire is also
transferred to
the melt pool. Such an arrangement can limit melt pool superheating and
therefore
reduce thermal gradients. It also allows for deposition rates that are
sufficient to
sustain an extended melt pool length and allow for the interaction of gas jets
from the
jet device on the melt pool surface or in the vicinity of the melt pool. While
these
reductions in thermal gradients can be beneficial for grain refinement,
reductions in
thermal gradients are not required to achieve the effect of grain refinement
realized by
application of the cooling gas using the jet devices provided herein.
[00143] Additional aspects of melt pool control and string shape control is
evident from testing of the jet device testing. As discussed above, the
methods
provided herein allow for a reduction in the size of the melt pool length,
which can be
achieved by the increased solidification rate at the trailing edge of the melt
pool. The
methods provide the ability to shape strings for wider single row walls, and
to
eliminate need for filling in at the end of string by melt displacement from
gas jet
pressure towards end. The methods provided herein allow refinement of
solidification
structures in workpieces made by additive manufacturing processes. The methods
can
eliminate or significantly reduce the coarse columnar structure typically
produced by
conventional additive manufacturing systems. Elimination of these coarse
columnar
structures can result in a manufactured product that exhibits higher strength,
ductility
and fatigue resistance than achieved in conventional additive manufacturing
processes.
[00144] Electron back scatter diffraction (EBSD) allows analysis of crystal
structures, including grain size and boundary types, mis-orientations,
deformations,
phase discrimination and distribution, crystallographic orientations and
texture
measurements (micro- and macro-crystallographic texture). For deposited
layers,
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EBSD can be used to look at epitaxy between layers as well as crystal
orientation. The
elongated columnar structures typical of conventional additive manufacturing
processes impose a restriction on the number of favorable a-grain variations
that can
occur in a Ti-6A1-4V sample. This can be seen in FIGS. 4A and 4B, which shows
EBSD characterization of the crystallography of typical material made by
conventional
additive manufacturing processes (FIG. 4A) versus that achieved using the
methods
provided herein, where gas jet impingement results in a material having a more
refined
grain (FIG. 4B). As can be seen in FIG. 4A, long range alignment and
uniformity of
lamellar structures along prior I3-grain boundaries are exhibited in the
typical coarse
grained material from conventional processes. In material produced using the
jet
device and methods provided herein, crystallographic diversity is increased,
the
material exhibiting a multitude of initial I3-grain orientations. As can be
seen in FIG.
4B, the extent of grain boundary alignment is reduced in the grain refined
material
produced using the jet device and methods provided herein.
[00145] Also provided are methods of minimizing or eliminating coarse
columnar solidification structures in an additively manufactured metal
product. The
methods include application of a turbulent cooling gas jet using a jet device
provided
herein on a free surface of a melt pool. The directed cooling gas jets at the
melt pool,
such as at the liquid-solid boundary of the melt pool, induces or accelerates
or both the
growth of an opposing solidification front at the free melt pool surface. This
can result
in blocking of epitaxy, as consecutive layers nucleate and solidify from the
top-layer
grains, and thereby minimizing or eliminating coarse columnar solidification
structure
formation. Nucleation at the melt pool free surface can result in the break-up
of
columnar solidification structures by finer grains at irregular intervals,
which can lead
to improved, repeated material properties achieved during the additive
manufacturing
process. The methods can result in increased crystallographic diversity, such
as the
formation of a multitude of initial I3-grain orientations. The method also can
reduce
the extent of grain boundary alignment. The methods also can result in reduced
strain
segmentation of the additively manufactured metal product. The method can
result in
a finished material exhibiting increased strain hardening, especially when
loaded
parallel to build direction relative to typical material not produced using
the methods
provided herein that include gas jet impingement using the jet devices as
described.
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Additively manufactured products produced using the method also can exhibit an
increase in ductility in the direction of production (along strings).
[00146] Also provided are methods of refining the microstructure of
additively manufactured metal products. The methods include using a cooling
jet
device provided herein to increase cooling rate by applying jets of cooling
gas in situ
to an as-deposited solidified material. Forced cooling of as-deposited
material during
the deposition process can achieve refinement of the microstructure of the
additively
manufactured product. Cooling rate can significantly effect microstructures
formed
during the manufacturing process. In some methods, application of the
turbulent
cooling gas at the as-solidified deposited material in situ can modulate or
control
allotropic transformation. In methods in which the deposited material is a
titanium
alloy, such as Ti-6A1-4V, forced cooling through application of turbulently
flowing
cooling gas on the as-solidified deposited material in situ can control I3-a
solid-state
phase transformation. The methods of grain refinement provided herein can
counter
long range strain mismatch at boundaries caused by duality of microstructures
by
yielding a more homogeneous and finely distributed presence of different
micro structural elements.
[00147] Cooling rate effect on microstructure can be observed in FIGS. 10A
and 10B. A plasma and wire-based high deposition rate additive manufacturing
process utilizing the Ti-6A1-4V alloy was used to form a product. Cooling at a
faster
cooling rate was found to significantly refine microstructure of the deposited
product.
A much finer basketweave-type microstructure was achieved when the temperature
of
the deposited material was decreased from 1000 C to 900 C at a measured bulk
cooling rate of 15 C/sec (Fig. 10B) than when cooled at a measured bulk
cooling rate
of 10 C/sec (Fig. 10A). When tested for hardness, a hardness indent (the dark
pyramid-shaped indentation in the center of the figures) illustrates increased
uniformity of plastic deformation in the refined basketweave-type
microstructure (FIG.
10B) compared to the less refined basketweave-type microstructure (FIG. 10).
As can
be seen in FIG. 10A, there is a localized concentration of plastic deformation
near the
indent. FIG. 10B does not exhibit any localized concentration of plastic
deformation.
Thus, the application of cooling gas jets to force cool the as-deposited
material during
deposition can achieve a finer basketweave-type microstructure, as well as
improve
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allotropic phase transformations (transformation from one crystal structure to
another),
precipitation and other solid state thermochemical reactions.
[00148] Also provided are methods of force cooling an additively
manufactured metal object in situ. The methods include applying jets of
cooling gas in
situ to an as-deposited solidified material to increase the cooling rate of
the material.
The cooling gas jets are applied by the jet devices in a turbulent flow, and
can achieve
a bulk cooling rate from about 10 C/s to about 25 C/s, or a recorded cooling
rate of
from about 80 C/s to150 C/s measured at the surface to which the cooling gas
is
directed.
[00149] Also provided are methods of increasing uniformity of plastic
deformation in an additively manufactured titanium alloy, such as a Ti-6A1-4V
metal
object in situ. The methods include applying jets of cooling gas in situ to an
as-
deposited solidified material to increase the cooling rate of the material and
thereby
promote the formation of a finer basketweave-type microstructure instead of
the
colony/lamellar microstructure typically produced. The cooling gas jets are
applied by
the jet devices in a turbulent flow. Finer basketweave-type microstructures
can be
achieved as the cooling rate is increased, and the finer basketweave-type
microstructures increase uniformity of plastic deformation. For example,
increasing
the bulk cooling rate from about 10 C/s to about 15 C/s when cooling the
object from
1000 C to 900 C can result in a finer basketweave-type microstructure and
increased
uniformity of plastic deformation.
[00150] The methods provided herein can be performed in any additive
manufacturing system. The methods can be performed in a system that includes a
closed chamber filled with an inert gas to provide an inert atmosphere where
the whole
process is performed in an inert atmosphere. The inert atmosphere can be or
contain
argon, xenon, neon, krypton, helium or combinations thereof, allowing inert
atmosphere deposition.
[00151] E. EXAMPLES
[00152] The following examples are included for illustrative purposes only
and are not intended to limit the scope of the embodiments provided herein.
[00153] Example 1
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[00154] A plasma and wire-based high deposition rate additive
manufacturing process utilizing the Ti-6A1-4V alloy was used without (A) and
with
(B) the jet device provided herein jetting cooling gas during additive
manufacturing.
The cooling gas used was room temperature argon gas. The flow rate of the
cooling
gas was 20 L/min applied using the type of jet device illustrated in FIG. 1.
Deposition
rate was 5 kg/h and workpiece surface temperature/interpass temperature was
650 C.
The deposition rate and the temperature were the same whether or not the jet
device
was used to applying cooling gas.
[00155] Micrographs of the results are shown in FIGS. 5A and 5B. FIG. 5A
shows the structure of a metal object produced by typical additive
manufacturing. The
grain structure in the manufactured product in FIG. 5A is coarse and columnar
structures are visible. FIG. 5B shows the beneficial results achieved when the
jet
device is used to apply a cooling gas to the melt pool during additive
manufacturing as
described herein. The grain structure in the manufactured product in FIG. 5B
is
approximately equiaxed and exhibits a refined structure. Accordingly using the
jet
device provided herein to apply a cooling gas during additive manufacture
results in a
product have a refined grain structure. Manufactured products having these
refined
grain structures demonstrate increased strength, fatigue resistance, and
durability.
[00156] Example 2
[00157] A plasma and wire based high deposition rate additive
manufacturing process utilizing the Ti-6A1-4V alloy was used with unilateral
application of cooling gas to one side of a melt pool in a single row Ti-6A1-
4V string
deposit using the jet device provided herein. The cooling gas used was room
temperature argon gas. The flow rate of the cooling gas was 25 L/min applied
using
the type of jet device illustrated in FIG. 1. Deposition rate was 5 kg/h and
deposition
interpass temperature was 500 C. The argon cooling gas was applied to one half
of
the melt pool, and the other half was untreated. This was achieved by engaging
the
nozzles of the jet device on only one side of the melting tool.
[00158] The results are shown in FIG. 6. As can be seen in the figure, the
untreated side (the right portion in the figure) exhibited coarse grain
structure and
columnar structures. The grain structure on the treated side the manufactured
product
in FIG. 6 (the left side) has metal grains that are approximately equiaxed and
have a
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refined structure. The dotted lines in the figure outline typical grain size
and shape on
either side of the product. The grain size of the treated size is
significantly smaller
(maximum grain dimension <2 mm and average grain size < 1 mm2) compared to
that
achieved in traditional additive manufacturing methods, as shown on the right.
The
untreated side (left) shows a slight tilt of the columnar structure,
attributed to the effect
the impinging cooling gas has on the thermal gradient. The micrograph also
illustrates
that manipulation of the nozzles of the jet device can allow the production of
graded
microstructures and tailoring of local material properties using the jet
device provided
herein in additive manufacturing.
[00159] It will be apparent to those skilled in the art that various
modifications and variation can be made in the present invention without
departing
from the spirit or scope of the invention. Thus, it is intended that the
present invention
cover the modifications and variations of this invention provided they come
within the
scope of the appended claims and their equivalents.
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REFERENCE SIGNS LIST
The following is a listing of the reference numerals used in the description
and the
accompanying Drawings.
First conduit
5 15 First conduit inlet
Diffuser
Nozzle
Gas jet
Cooling gas supply
10 50 Cooling gas supply
60 Second conduit
65 Second conduit inlet
70 Diffuser
75 Nozzle
15 80 Gas jet
85 Cross-piece
90 Melt pool
95 Deposited string
100 Jet device
20 200 Melting tool
250 Bracket
300 Wire feed
330 Melting arc or beam
350 Metal Wire
25 375 Molten metal droplets
400 Workpiece
415 Pre-heated area
425 Melt pool
430 Solidification zone
30 435 Solidifying crystals
440 Opposing solidification induced by cool gas jet impingement
450 Solidified material zone
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480 Deposited layer
500 Second jet device
525 Nozzle
530 Cooling gas jet
550 Temperature sensor
555 Post solidification temperature monitoring area
560 Temperature sensor
565 Post transformation temperature monitoring area
570 Bracket
575 Bracket
600 Melting tool
630 Melting arc or beam
D Direction of travel
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