Note: Descriptions are shown in the official language in which they were submitted.
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LASER ADDITIVE REPAIRING OF NICKEL BASE SUPERALLOY COMPONENTS
[0001]
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the repair, reshaping and cladding
of
superalloy components, and more particularly, to the weld build up of nickel
base superalloy
components containing relatively large amounts of aluminum and/or titanium,
employing a
hold and cool process so as to reduce susceptibility to cracking, and to the
materials so
produced.
[0004] 2. Background and Related Art
[0005] Nickel base superalloys (also known as nickel based or nickel-
based) are high-
temperature materials which display excellent resistance to mechanical and
chemical
degradation of properties even as temperatures approach the melting points of
the materials.
Ni base superalloys are based upon nickel(Ni) and typically contain numerous
other elements
such as chromium (Cr), aluminum (Al), titanium (Ti), tungsten (W), cobalt
(Co), tantalum
(Ta), carbon (C), among others. Such high-temperature superalloys found early
application in
aircraft turbine engines. A higher operating temperature typically leads to
increased fuel
efficiency and lower carbon emissions, causing superalloys to find increasing
uses in ground-
bases turbine systems as well. For example, see The Superalloys, by Roger C.
Reed,
(Cambridge University Press, 2006, particularly Chapter 1).
[0006] The Al and Ti content of Ni base superalloys is typically
increased in order to
improve the high temperature strength, but at the expense of introducing
challenges in
welding or weld buildup of such materials. Generally, increased Al and/or Ti
content of a Ni
base superalloy increases the susceptibility of the material to cracking
during welding or weld
build up. Our previous work in this field cited above addressed the
improvement in the weld
repair of such superalloys. The present work addresses the related problem of
weld build up of
material while reducing the susceptibility to cracking of the materials so
constructed.
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[0007] Thus a need exists in the art for improved methods to build up
Ni base
superalloy materials by a weld build up process, typically a laser additive
repair process,
particularly for those superalloys including relatively large amounts of Al
and/or Ti.
BRIEF SUMMARY OF THE INVENTION
[0008] It is one objective of the processes described herein to provide a
process for
build up of Ni based superalloy materials from powder by heating and
controlled stepwise
cooling of the material so as to produce no more than about 20% 7' phase in
any single cool
and hold portion of the process, as well as no more than about 20% y' phase in
the final room
temperature material.
[0009] Nickel base superalloys with large amounts of Al and Ti contents are
known to
be difficult to weld build up. As the Al and Ti content of a superalloy is
increased to improve
the component high temperature strength, weldability of the component is
drastically reduced.
Some embodiments of this invention employ elemental partitioning of Al and Ti
to y and y'
through a controlled step cool and hold process. The time-temperature protocol
of the step
1 5 cool and hold process is chosen so as to deplete the 7 from Al and Ti
in order to improve
weldability. When the Al and Ti content of the y in the weld build up is
reduced to a weldable
region of no more than about 20% 7', the controlled step cool and hold process
is replaced
with regular weld argon cooling.
[0010] The processes described herein provide for elemental
partitioning of Al and Ti
during the hold and cool process so as to deplete y from Al and Ti and to
reduce susceptibility
to cracking in the material so produced.
[0011] According to one aspect of the present invention, there is
provided a method of
repair additive build up of a nickel (Ni) base superalloy with significant
titanium (Ti) and
aluminum (Al) content comprising: a) preplacing a Ni base superalloy powder
having
substantial Ti and Al content onto a substrate of substantially the same or
similar composition
as the Ni base superalloy powder; b) preheating the preplaced powder to a
temperature above
1200 C with a first heat source; c) melting the preplaced powder with one or
more passes of a
directed energy beam second heat source so as to produce a heat affected zone
having a width
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less than 100 microns, removing the second heat source when melting is
completed; d)
performing a controlled step and hold cooling of the melted preplaced powder
with the first
heat source such that a known fraction of a 7' phase is formed during the step
and hold
cooling; and, e) adjusting the step and hold cooling such that a partition of
Al and Ti between
a 7 phase and the 7' phase in the additive build up causes a reduction in
cracking
susceptibility, wherein the step and hold cooling is from an initial
temperature T1 of
1200 C or above and to room temperature and consists of a plurality of steps:
a. Holding at T1
for a time in the range from 1 min to 3 min; b. Cooling to a temperature T2
lower than T1 and
hold for a time in the range from 2 min to 15 min so as to produce less than
20 weight percent
7' phase; c. Cooling to a temperature T3 lower than T2 and hold for a time in
the range from
2 min to 30 min so as to produce less than 20 weight percent 7' phase; d.
Cooling to a
temperature T4 lower than T3 and hold for a time in the range from 6 min to
120 min so as to
produce less than 20 weight percent 7' phase; and e. Cooling to room
temperature in a
plurality of cooling steps from T4 to room temperature wherein the temperature
at each step is
maintained from 1 hour to 20 hours, so as to produce less than 20 weight
percent 7'.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a graphical depiction of the weldability of some
superalloys as a
function of Ti and Al content.
[0013] FIG. 2 are graphical depictions of details of hold and cool
processes pursuant
to some embodiments of the present invention:
(2A) Elemental partitioning at full phase equilibrium from Ni-Al pseudo binary
phase diagram.
(2B) Shift in TTT (time-temperature-transformation) diagram due to step cool
and hold process.
(2C) Anticipated stress relief of a welded interface at each hold temperature
during partitioning of Al and Ti.
(2D) Shift of composition of alloy 247 to crack free region due to
partitioning.
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[0014] FIG. 3 is a schematic depiction of typical apparatus for
performing a hold and
cool process pursuant to some embodiments of the present invention with
preplaced
powder.
[0015] FIG. 4 is a schematic depiction of typical apparatus for
performing a hold and
cool process pursuant to some embodiments of the present invention with
concurrently
placed powder.
DETAILED DESCRIPTION
[0016] All percentages given herein are weight percent unless
otherwise specified.
[0017] Ti and Al are typically added to Ni base superalloys to
increase the high
temperature strength of the component, but with the disadvantage of
drastically
increasing the difficulty of producing satisfactory welds or weld build ups.
For economy
of language we refer hereinafter to Ni base superalloy with relatively high
Al, Ti content
as simply "Ni base superalloys" or "Ni superalloys." The welds or weld build
ups
typically produced with such Ni superalloys are susceptible to cracking either
during the
weld or build up process or in subsequent repair steps involving these
materials.
Previous work by the present inventors (cited above) involving a detailed
study of factors
affecting weldability of Ni base superalloys and their susceptibility to
cracking has led
the present inventors to conclude that a 7' phase present in an amount
generally less than
about 20 weight percent is indicative of weldability without unacceptable
susceptibility to
cracking. A 7' content greater than about 60% is generally indicative of
nonweldability
(that is, susceptibility to strain age cracking) while intermediate 7' values
typically
indicate difficult and expensive welding. Substantially the same conclusions
can be
drawn for additive or weld build up processes. That is, a 7' phase present in
an amount
less than about 20 weight percent is indicative of weld build up without
unacceptable
susceptibility to cracking. 7' greater than about 60% is generally indicative
of weld build
up having an unacceptable susceptibility to cracking.
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[0018] Additive manufacturing by laser beam weld build up (also called
build-up
welding or build up welding) is comparable to plasma build up welding as well
as plasma
spraying. To be concrete in our description, we discuss herein the important
practical
case in which a laser beam provides the directed energy to heat the material
as desired.
This is not to exclude other sources of directed energy such as plasma, second
laser,
electron beam, among others as would be apparent to those having ordinary
skills in the
art. However, for economy of language we refer to all such additive build up
processes
as laser additive processes or laser weld build up or equivalent language.
[0019] Previous work by this inventor (cited above) focused on
reducing cracking in
the welding of Ni base superalloys. The work described herein relates to the
additive
build up of a layer of material (typically from about 1 millimeter (mm) to
about 50 mm in
thickness). The present discussion is directed to the repair build up
(typically 1-50mm
thick) that has favorable welding characteristics, that is, reduced
susceptibility to
cracking. Thus, the present description relates to the fabrication of a Ni
base superalloy
material or component having favorable welding properties. It is anticipated
that such
materials or components can be fabricated for use in myriad applications
apparent to one
having ordinary skill in the art.
[0020] FIG. 1 is a graphical depiction of the weldability of typical
Ni base superalloys
as functions of the Al and Ti content. Those alloys lying above line 100 in
FIG. 1 are
generally considered not to be weldable, and hence, not amenable to weld build
up. In
practice, this typically means that materials having compositions above line
100 produce
materials susceptible to strain age cracking in the fusion zone (FZ). Thus,
whenever such
components in commercial equipment require repair, they are typically replaced
rather
than repaired since the susceptibility to cracking will result in a large
fraction of failed
repairs.
[0021] Conversion of the compositions given in FIG. 1 to the fraction of
various
phases present shows that nonweldable alloys generally have more than about
60% 7'
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phase in their final structure. In contrast, the weldable Ni base superalloys
depicted in
FIG. 1 have generally less than about 20% 7' phase in their final structure,
below line 101
in FIG. 1. Therefore, it is anticipated that Ni base superalloys with less
than about 20% 7'
will be weldable without detrimental amounts of strain age cracking in the FZ.
[0022] Heat affected zone (HAZ) cracking in high strength Ni base
superalloys occurs
due to the presence of grain boundaries containing low melting point elements.
A large
heat input during laser build up thus creates a large HAZ and results in a
large amount of
HAZ cracking due to melting at the grain boundaries. This is a common problem
in
previous build up processes in which a laser beam typically interacts with the
base metal
during powder deposition. Thus, an important problem in the field of additive
build up of
superalloys is to produce crack free, near 100% base metal laser build up,
particularly on
the important commercial use of superalloys for gas turbine components. As
described in
detail herein, one advantage of the present processes relates to the creation
of a relatively
small HAZ, typically no more than about 100 pm HAZ (pm = micron = 10-6 meter).
[0023] Typical embodiments of the welding apparatus 200 present
invention include
placing a component substrate 201 into a chamber 210 containing an inert
atmosphere 220 and predepositing the powder 230 to be melted in front of the
moving
laser 240 onto the substrate 201 having substantially the same composition as
the
powder 230, as depicted schematically in FIG. 3. Other embodiments include
concurrent
deposition of the powder 230 before and/or after the application of the
laser 240 energy dQ, as depicted in FIG. 4. To be concrete in our description,
we
describe in most detail the example of preplaced powder depicted in FIG. 3,
understanding thereby that modifications to handle concurrent powder placement
(FIG. 4), are modifications of the techniques described for preplaced powder,
apparent to
one having ordinary skill in the art.
[0024] In contrast to typical weld build up processes, the processes
described herein
include some or all of the following steps:
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a: Preplacing the powder 230 onto a component substrate or
substrate 201
where a buildup of the same composition is desired and both powder and
substrate have
substantially the same composition (FIG. 3), or laying the powder in front and
behind the
moving laser beam 242 (or other directed energy beam, as shown in FIG. 4.
b: Heating the preplaced powder 230 to above about 1200 deg. C.
c: Laser 240 melting the preplaced powder 230 so as to produce a heat
affected zone (HAZ less than about 100 microns in extent).
d: Producing a known fraction of y' during each cooling step in the cooling
of
the solidified powder 250, resulting in;
e: Partitioning Al and Ti between y and y' to reduce fusion zone (FZ)
cracking susceptibility.
[0025] These procedures represent an improvement over conventional
build up
techniques for superalloys that are generally not completely successful in
eliminating
strain age cracking and incipient melting. To avoid cracking, some previous
techniques
employ a lower-temperature method such as brazing but this typically has the
disadvantage of lowering strength.
[0026] Nearly all commonly used laser build up processes include
interaction of the
laser beam with the base material. This process causes the HAZ to be large and
increases
the susceptibility of the base metal to grain boundary cracking. Some
embodiments of
the present invention preplaces powder of the same or similar composition as
the
component onto the surface of the component where build up is needed to a
thickness of
about 1.0 mm to 50 mm (millimeter). Powder size is typically in the range from
about 10
microns to about 100 microns.
[0027] This preplaced powder is heated under an inert atmosphere with
a first heat
source to about 1200 deg. C and held at that temperature for a minimum of 5
minutes to
dissolve substantially all y' phase. An induction coil 260 may advantageously
be used as
this first heat source as depicted in FIG. 3. This is by way of illustration
and not
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limitation as other first heat sources could also be employed as would be
apparent to one
having ordinary skill in the art.
[0028] A second heat source such as a laser 240 generating a laser
beam 242 or other
directed energy source, scans the preplaced powder 230 and heats the powder.
The
powder 230 thus melts and solidifies to a certain first depth 250 as depicted
in FIG. 3A.
To avoid producing an overly large HAZ, the laser power dQ is advantageously
adjusted
so that more than one laser scan is typically required to melt and solidify
the preplaced
powder. That is, the laser power dQ is adjusted so that a relatively small
amount of
dilution with the base metal occurs when the melting process of the preplaced
powder 230 is finished, understanding that small dilution means a small HAZ.
[0029] FIG. 3 is a schematic depiction of typical melting apparatus
201 for preplaced
powder 230 (FIG. 3) and for concurrently placed powder 230 (FIG. 4). Preplaced
powder 230 prevents the fully intense laser beam 242 from interacting with the
substrate 201 base metal and causing a large HAZ. Preheating the powder 230 is
aimed
at reducing the FZ cracks. Preplacing the powder 230 is aimed at preventing
the laser
beam 242 from interacting with the substrate 201 base metal and reducing grain
boundary
cracking.
[0030] In FIG. 4, a thin layer of preplaced powder 230 is heated to
approximately
1200 deg. C by the induction preheater and, once the laser begins scanning,
further
heating of the preplaced powder occurs. A laser beam 242 moves over and melts
this
preplaced powder 230, fusing it to the base material while additional powder
232 is
continuously preplaced in front of and behind the moving laser. The process is
repeated
for as many layers of powder as desired. This embodiment depicted
schematically in
FIG. 4 likewise employs the concept of the laser beam 242 striking the powder
230 and
not the substrate.
[0031] In the first pass over the preplaced powder 230 (FIG. 3), only the
top portion of
the preplaced powder is melted 250, typically only a few microns per pass.
Following
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passes melt layers 252 having similar thicknesses until the final layer in
contact with the
substrate 201 base metal is melted and fusion is accomplished. This method
significantly
reduces the HAZ thickness since direct contact of the laser beam 242 with the
substrate 201 base metal is substantially reduced.
[0032] Once the melting process for the preplaced powder 230 is
finished and the
molten powder solidifies to a temperature of no less than 1200 deg. C, the
solidified
powder 250 is held at that temperature a minimum of one minute followed by a
hold and
cool process.
[0033] The hold and cool process employs the elemental partitioning of
Al and Ti to 7
and 7' in full thermodynamic phase equilibrium to accomplish no more than 20%
7'
formation at any time during the joining and build up process. The process
depletes the 7
from Al and Ti. The final 7 composition is moved to the weldable region at the
end of
the SCH (stepwise hold and cool) process as shown in FIG. 2 to prevent strain
age
cracking (FZ cracking)
[0034] FIG. 2 is a schematic depiction of metallurgical reactions for
high strength Ni
base superalloys which are laser built up with processes described herein.
When the laser
melting operation is finished the heat source (e.g., the induction coil 260 in
FIG. 3, or
similar heat source) is operational and the following process is utilized.
a. Hold at T1 for 1-3 minutes.
b. Cool to T2 and hold for 2-15 min: Produce less than 20% 7'
c. Cool to T3 and hold for 2-30 min: Produce less than 20% 7'
d. Cool to T4 and hold for 0.1-2 hrs: Produce less than 20% 7'
..
..
..
..
Cool to Tii and hold for 1-20 hrs (n=1-20): Produce less than 20% 7'
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Cool to room temperature to produce final 7' content, which is less than
about 20%.
[0035] It is anticipated that some embodiments of the present
invention can be used
for laser build up of high temperature nickel base superalloys typically
having more than
about 20% 7' in their room temperature structure but, pursuant to some
embodiments of
the present invention, result in 7' less than about 20% at each hold and cool
step. These
nonweldable superalloys include each superalloy listed above the nonweldable
line 100
in FIG. 1, but improved pursuant to some embodiments of the present invention
to lie
closer to the weldable zone.
[0036] Some embodiments of this invention advantageously employ two
heat sources.
First heat source is used to melt the preplaced powder, typically a laser beam
240 or other
directed energy beam, as depicted schematically in FIG. 3. A second heat
source is used
for pre-heating deposited powder and for the controlled cool and hold portion
of the
process. This second heat source is conveniently taken to be an induction coil
260 as
depicted in FIGs. 3 and 4 but other heating sources are not excluded. This
induction
coil 260 or other second heat source adjusts the temperature of the weld build
up in order
to produce 20% or less 7' from the 7 at any hold temperature. Elemental
partitioning of
Al and Ti into 7 and 7' is calculated from the processing conditions employed
making use
of available thermodynamic data. Conditions are chosen so as to produce a
maximum 20% 7' formation at any hold temperature. Hold times to reach 20% 7'
are
calculated from known phase transformation kinetics of the 7-7' system such as
those
available through the JMatPro thermodynamic software available through Sente
Software, Inc., Pittsburgh, PA.
[0037] Thus, briefly stated, some embodiments of the present invention
relate to the
laser build up of Ni base superalloy materials generally considered to be non-
weldable as
would be useful, for example, in the build up of substantially crack free,
near 100% base
metal build up on gas turbine components.
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[0038] The welding process described herein makes use of elemental
partitioning of
Al and Ti into y and y' phases through a step cool and hold process. This
depletes the y
phase from Al in a controlled fashion and Ti and improves weldability,
typically be
achieving a low weight % of y'. When the Al and Ti content of the stress
relieved y in the
joint is reduced to weldable values, the step cool and hold process is
terminated and
replaced with conventional weld argon cooling.
[0039] Typical embodiments of this invention use two heat sources. One
heat source
(the first) is used for melting/joining as in a conventional welding process.
A laser heat
source 240 is advantageously used as this first heat source but other heat
sources are not
inherently excluded such as arcs, discharges, electron beams, particle beams,
among
others.
[0040] The other (second) heat source is used for an initial heating
of prepositioned
powder and for the hold and cool portion of the process. This second heat
source adjusts
the isothermal hold temperature of the joint to produce no more than about 20%
y' at any
isothermal hold temperature. An induction heat source 260 is advantageously
used as
this second heat source but other heat sources are not inherently excluded. Of
course,
when the powder 230 is deposited concurrently with the melting step, as
depicted in
FIG. 4 for example, preheating temperature is not known precisely. However,
even with
concurrent deposition of powder, the preplaced powder quickly reaches the
temperature
of the bare metal of the substrate 201.
[0041] One important goal of the concurrent deposition of powder as
depicted in
FIG. 4 is to prevent the laser beam 242 from interacting directly with the
bare metal
substrate 201. However, it is important for success of the step, hold and cool
process to
start at a temperature of about 1200 deg. C or above. The concurrent
deposition process
achieves this start temperature by the use of an induction heater 260 as well
as the heating
of the powder by the adjacent laser-generated melt pool 250.
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[0042] Elemental partitioning of Al and Ti is calculated from
available
thermodynamic data that allows a maximum 20% y' formation at any isothermal
hold
temperature. Hold times needed to achieve 20% y' are calculated from the known
phase
transformation kinetics of the y- y' system. Alloys of particular interest
here include
those noted on FIG. 1.
[0043] Cooling of a Ni base y' superalloy from its melting temperature
results in such
superalloys going through a transition from y phase to y + y' phases. The hold
and cool
process described herein employs the elemental partitioning of Al and Ti into
y and y'
phases in full thermodynamic phase equilibrium to produce no more than 20% y'
at any
hold time during the hold and cool process. This depletes the y phase from Al
and Ti and
moves the final y composition into the weldable region as depicted in FIG. 1.
[0044] In this process as depicted in FIG. 2, y is depleted from Al
and Ti through
elemental partitioning until its final composition is reduced below the
weldable line 100
in FIG. 2D. Compositional change of y is shown with spots T1- 'Li in FIG. 2A.
FIGs. 2B
and 2C show the anticipated shift in the cooling curves and the stress vs.
time curve after
each hold step due to stress relief of the weld at each hold step.
[0045] It is expected that the general hold and cool process as described
herein can be
used on almost any superalloy that experiences strain age cracking. Elemental
partitioning of Al and Ti during the hold portions of the process reduces the
likelihood of
strain age cracking and hot cracking. Such partitioning also significantly
reduces the
tendency towards strain age cracking during post weld heat treatment since y
is
substantially depleted from Al and Ti, and stress relieved, at each step of
the hold and
cool process.
[0046] Although various embodiments which incorporate the teachings of
the present
invention have been shown and described in detail herein, those skilled in the
art can
readily devise many other varied embodiments that still incorporate these
teachings.
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