Note: Descriptions are shown in the official language in which they were submitted.
1
METHOD FOR THE POWDER-METALLURGICAL PRODUCTION OF
COMPONENTS FROM TITANIUM OR TITANIUM ALLOYS
The present invention relates to a method for the powder-metallurgical
production of a
component from titanium or a titanium alloy, wherein first, using metal powder
produced from
titanium or the titanium alloy, a green part is fonned and this is densified
and compacted in a
subsequent sintering step.
BACKGROUND
Various powder-metallurgical methods for producing true-to-size titanium
components
(here and in the following, "components from titanium" will be used here and
in the following
as a simplified teiiii for components made from titanium (pure titanium) or
one or more
titanium alloys) are known, where in all methods, first a green part is
produced, and this is
densified and compacted in a subsequent sintering step. The green part can be
produced in
various ways, especially using additive production methods, metal powder
injection molding,
extrusion methods and non-pressurized powder-metallurgical production methods.
Because of the excellent properties of the material, titanium, plus the
efficient and
economical production method, the powder-metallurgical production of titanium
is becoming
increasingly more widely used. The good biocompatibility and the high specific
strength of the
material, titanium, play an important role especially in applications in
medical engineering and
air and space technology. The economically most significant alloy with sales
figures
accounting for more than 50% of the total titanium market 1 Ti6A14V.
As a rule, the following steps must be executed to produce a powder-
metallurgically processed titanium component:
a) &Inning
b) debinding
c) sintering
The objective of {bulling is to bring the titanium powder particles into the
tightest
possible packing in a form close to the final contour. In this step, depending
on the method
employed, additives are used which must be removed in one or more subsequent
debinding
step(s). In the subsequent process step, frequently also the final one,
sintering, the powder
particles are consolidated by material transport.
Because of the high reactivity of titanium, all processing steps must take
place under
special process conditions. In patent EP 1 119 429 B1 [1], Gerling et al.
describe necessary
process conditions for sintering titanium. The combined implementation of
debinding and
Date Recue/Date Received 2021-03-01
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sintering in a combined furnace design is described by Bliim in EP 1 496 325
A2 [2].
Titanium has two crystal modifications. The hexagonal a phase, which with pure
titanium and noiiiial pressure is present up to a temperature of 882.5 C, and
the cubic
space-centered 13 phase, which with pure titanium and noiiiial pressure occurs
above the
aforementioned temperature. The presence of the different phases at room
temperature is
used to classify titanium alloys into a-Ti, (a+13)-Ti and 13-Ti alloys.
Ti6A14V, for example,
is an (a+13)-alloy, i.e., both phases are present in the grain structure at
room temperature. To
produce components with a generally desired density >97% in the process of
sintering
titanium and titanium-alloy components, sintering temperatures of about 1100-
1 400 C at a
sintering duration of about 2-5 h are needed. For pure titanium and Ti6A14V
this means
that the materials are processed in the 13-phase region, which leads to a
massive 13-grain
growth.
In EP 1 119 429 B1 [11, Gerling et al. describe that the grain structure that
becomes
established has a 13-grain growth of about 150 gm. Here, the nomenclature
according to
Sieniawski et al. [3], shown in Fig. 1, is used to describe the sizes of the
various structures in
the lamellar (a+13) alloys. The following designations are used:
D: grain size of the primary 13 phase
d: the size of a parallel a-lamella colony
t: the width of an a-lamella
In contrast to refoiiiiing processes, foiiiiing takes place as the first step
on the powder
metallurgy route. In the next process step, sintering, the compacted titanium
alloy, previously
brought into shape, is produced. In contrast to standard processing
approaches, because of the
reverse sequence of the process steps (1. foiiiiing, 2. material
consolidation) in the powder
metallurgy approach, the possibility of refining or optimizing the grain
structure of the metal
and thus its material properties by theimal/mechanical working before the
foiiiiing step does not
exist. For powder-metallurgical methods for producing components from titanium
and/or
titanium alloys, precisely the process-deteiiiiined inverse sequence, combined
with the very
limited influence on the grain structure that develops during the familiar
sintering process, is a
limiting factor. As an example: the grain structure of a Ti6A14V sample
produced in the
standard manner from titanium powders commonly used in the prior art (with
powder grain
sizes <45 gm) and sintered under sintering conditions typically used in the
prior art is shown in
Fig. 2. Here it is possible to recognize the typical lamellar mixed grain
structure for titanium
components produced in the known way by powder metallurgy and sintered, made
up of a
phases and 13 phases, the (a+13) grain structure, with a mean primary 13-phase
grain size (D) of
about 190 gm.
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The production of powder-metallurgically processed titanium and titanium
alloys with
small grain sizes is described in US 4,601,874 [4] by Marty et al. Through the
targeted
admixtures of S, P, B, As, Se, Te, Y and lanthanoids, during the consolidation
process a
material is produced with grain sizes smaller by two orders of magnitude than
the titanium
powder particles used. The drawback of this approach is that the use of
titanium and titanium
alloys is widespread precisely in strictly regulated market segments. For
these application
purposes, the chemical compositions of the material and its mechanical
properties are regulated
by standards. For example, the material compositions and mechanical properties
of Ti6A14V
and pure titanium are regulated in the standards ASTM F2885 and ASTM F2889
respectively.
An additional procedure for producing fine-grained titanium and such titanium
alloys
by powder metallurgy is described in WO 2012/148471 Al. Here a green part made
from
titanium (alloy) powder with grain sizes of less than 325 mesh (less than 44
gm) is produced
and then subjected to a multistep process of compaction and forming. In a
first step the green
part is sintered in a hydrogen atmosphere at temperatures of 1100 to 1500 C;
in the
embodiments, the processing temperature is always given as 1200 C. In this
process titanium
material in the 13-phase {buns. In a subsequent step of controlled cooling, a
phase
transformation occurs, in which restructuring occurs in the p-grains,
resulting in a phase
mixture of fine a-grains, 13-grains and 6-phases. Then in a final step, the
hydrogen must be
expelled from the component obtained, which is done by applying a vacuum. With
this
procedure especially the use of hydrogen is especially problematic, since this
gas can only be
expelled from the component with great effort and often not completely
expelled. Negative
effects on the material properties and the stability of the material have been
blamed on
hydrogen remaining in the grain structure of the material. Outgassing from
residual hydrogen
from the finished component in various applications is also anything but
desirable.
SUMMARY
One goal generally pursued with the invention is that of creating the
possibility, in the
case of powder-metallurgically produced and sintered titanium components, of
manipulating
the grain structure and optimizing the material properties. In particular the
intention was to
make it possible to adapt the material properties to the specific use case
directly in the sintering
process and/or to create, during the sintering process, an optimal starting
point for further
thermal treatment steps after sintering. For example, it should be possible,
by modifying the
sintering conditions, to create a primarily globular grain structure with high
ductility.
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To solve the problem, a process is suggested that comprises a method for the
powder-
metallurgical production of a component from titanium or a titanium alloy,
wherein first, using
metal powder from titanium or the titanium alloy, a green part is fonned and
this is densified
and compacted in a subsequent sintering step, characterized in that for
producing the green
part, metal powder from titanium or titanium alloy with a mean grain size of
<25 am,
measured using laser diffraction according to ASTM B822-10 is used and that
the sintering
step is perfonned at a sintering temperature up to a maximum of 1100 C., at a
sintering
duration of <5 h in an atmosphere under a reduced pressure in comparison with
nomial
pressure. Advantageous embodiments of the invention are that the maximum grain
size of the
metal powder from titanium or the titanium alloy is <30 am; that the sintering
step is
performed under a vacuum with a pressure of 10-3 mbar, especially at a
pressure of 10-5
mbar; and that the sintering step is performed in an inert gas atmosphere,
especially an argon
atmosphere, at a pressure of <300 mbar. For producing the green part, metal
powder from
titanium or the titanium alloy with a mean grain size of <20 [tm, in
particular of <10 am,
preferably of <5 am, is used. The sintering duration is 3.5 h, in particular
of s 3 h, preferably of
<2.5 h. Furthermore, the sintering duration is at least 1 h, preferably at
least <2 h. The sintering
temperature is up to a maximum of 1050 C., preferably up to a maximum of
temperature up to
a maximum of 1000 C., especially up to a maximum of 950 C. and the sintering
temperature
amounts to at least 860 C. The method is further characterized in that in the
sintering step, the
sintering temperature is adjusted in the range below a 0-transition
temperature of the titanium
or titanium alloy material. The component after the sintering step has a
material density of
>97%, in particular >98%, preferably 99%. In the sintering step, a sintering
temperature of
below 950 C. is selected and that to achieve a material density in the
component of >97%,
after the sintering step this is exposed to an additional step with pressure
and optionally a
temperature, e.g., a step of cold isostatic pressing (CIP) and/or hot
isostatic pressing (HIP).
The component, following the sintering step, is subjected to a thential
aftertreatment that is
conducted in the fonn of one or more of the following treatment procedures:
hot isostatic
pressing (HIP), quench, unifoim rapid quench (URQ). An additional aspect to
solving this
problem lies in a titanium component that exhibits the properties that it has
a globular a-
structure with a grain size of <30 tun; that it has a grain structure with
globular a-structure
with mean grain size of <30 [tm and lamellar (a+13) grain structure with a
mean primary 13-
phase grain size of <90 am; and/or that it has a lamellar (a+13) grain
structure with a mean
primary to f3-phase grain size of <120 VIM.
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BRIEF DESCRIPTION OF THE FIGURES
Fig. 1 a representation of a lamellar (a+fl) grain structure of a Ti6A14V
sample with
description of the gran structure fractions according to Sieniawski et al.
[3];
Fig. 2 an enlarged photomicrograph of a standard sintered Ti6A14V sample,
produced
by powder-metallurgically using powder particles <45 gm and standard-
sintered and confirms a lame liar (a+fl) grain structure for this;
Fig. 3 a schematic representation of the effect of reducing the grain size by
half (using the
example of spherical particles) on the number of particles required to fill a
defined
volume;
Fig. 4 a schematic representation of the reduction in size of the hollow space
between
adjacent particles due to reducing the grain size by half (using the example
of spherical
particles);
Fig. 5 an enlarged polished micrograph section of a powder-metallurgically
produced and
sintered Ti6A14V sample made from powder particles <20 gm, confiiiiiing the
foimation of a distinct globular a-structure; and
Fig. 6 an enlarged polished micrograph section of a powder-metallurgically
produced and
sintered Ti6A14V sample made from powder particles <20 gm, confiiiiiing the
foimation of a bimodal grain structure with a globular a-structure and
distinct lamellar
(a+13) grain structure.
DETAILED DESCRIPTION
An essential prerequisite for implementing the process according to the
invention and
creating the possibility of influencing the material properties in the
sintering process is the use
of metal powder, produced from titanium or a titanium alloy, with a mean grain
size of <25
gm, so-called fine powder. In such fine powder used for the process according
to the
invention, the maximum grain size may in particular be <30 gm. The maximum
grain size is
specified as a limit value by the manufacturers of such fine powders. At the
same time, a small
fraction of particles in such batches can always have grain sizes above this
limit. Such a
fraction, as a rule, is generally specified as a maximum of 1 to a maximum of
5 wt.-%.
The mean grain size may advantageously even be lower, especially <20 gm,
advantageously <10 gm and particularly preferably even <5 gm. The smaller the
grain size of
the metal powder is, the more readily high final densities can be achieved
even at sintering
temperatures markedly reduced compared to the relatively high sintering
temperatures
previously used.
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The measurement of the grain sizes essential for the invention and the
distribution
thereof is perfonned by grain size testing using laser diffraction according
to ASTM B822-10
(published 2010), valid at the time of this application. The grain size
distribution is
detegnined by wt.-% and according to DI0/D50/D90, wherein D50 is the mean
grain size.
Specifically, the grain sizes given here in comparison tests were measured
using the
COULTER LS grain size analyzer made by Beckman Coulter and evaluated using the
Fraunhofer theory according to ASTM B822-10.
For spherical particles, the grain size in the sense of the invention is
specified as the
particle diameter. For nonspherical particles, the grain size corresponds to
the projected
maximum particle dimension.
As a result of the reduced grain size, the surface area in the nonconsolidated
component available for the sintering process increases, and thus so does the
stored surface
energy. Since the reduction of this energy is the driving force in the
sintering process, the
sintering process can then take place using little theigial energy.
An additional advantage of using fine powders of the sizes indicated above for
limning the green part is that more powder particles can be introduced per
unit volume. In
addition to the enlarged surface, this leads to a higher number of contact
points per unit
volume, as shown in Fig. 3. There, in a schematic representation, the effect
of reducing the
grain size by half (using the example of spherical particles) on the particle
count to fill a
defined volume is shown.
The contact points of the particles in turn are the starting point and a
necessary
condition for the sintering process, which is driven by diffusion processes.
The increased
number of such contact points per unit volume therefore improves the starting
conditions for
the sintering process.
Through the use according to the invention of fine powders with mean grain
sizes
<25 gm, when considering the ideal packing density in addition to the
aforementioned
advantages, the result also occurs that the volume enclosed by the powder
particles, as shown
in an idealized representation in Fig. 4, is decreased. In Fig. 4, in a
schematic representation
the decrease in size of the hollow space between adjacent particles is
illustrated by reducing
the grain size by half (using the example of spherical particles). Since this
hollow space must
be closed to achieve the - high - material density desired for the component
following the
sintering process must be closed by material transport during the sintering
process, a smaller
volume to be covered is an additional decisive reason for an improvement in
the process
result.
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The sintering step typically takes place in a reduced-pressure atmosphere.
This
can be a vacuum with a pressure of < 10-3 mbar, especially < 10-5 mbar.
However, it may
also be a reduced-pressure inert gas atmosphere with a pressure of, e.g., <
300 mbar.
Argon gas in particular is considered as the inert gas here.
The sintering temperatures according to the invention are below 1100 C. They
can in
particular be a maximum of 1050 C, a maximum of 1000 C, and even a maximum of
only
950 C. Preferably, however, to achieve a good sintering result, the sintering
temperature
selected advantageously should not be below 860 C. The sintering temperature
may be kept
uniform. In particular, however, it is also possible and falls within the
meaning of the invention
to vary the temperature during the sintering process. The sintering
temperature is defined here
as the temperature that the workpiece to be sintered has undergone. Depending
on the sintering
unit, in the unit control, an adapted process temperature is to be selected,
which distinguishes
the process temperature measured at a distance remote from the workpiece from
the sintering
temperature undergone by the workpiece.
The duration of sintering may especially be < 3.5 h, often also < 3 h or even
< 2.5 h.
However, it was found that as a rule, for achieving good results, the
sintering time should
amount to at least 1 hour, preferably at least 2 hours.
After the sintering step, components from titanium or titanium alloys produced
with the
method of the invention generally have a final density of > 97%. However,
final densities above
98% may also be reached, even >99%.
To achieve a globular grain structure, the titanium components are sintered at
less than
the 13-transition temperature (e.g., at a temperature 30 C below the 13-
transition temperature.
For example, in initial experiments at a sintering temperature of 950 C, which
is below
the 13-transition temperature, and with a sintering duration of less than
three hours, components
with a final density of >97% were produced. These had a globular grain
structure with an a-
grain size on average of 10.1 am and a max. size of 29 am. The grain structure
of this material
is shown in Fig. 5. These grain sizes fall in the order of magnitude of the
powder particles used.
According to the literature, the 13-transition temperature of Ti6A14V falls in
the range
of 985 C to 1015 C [3; 51. This relatively wide range given in the literature
is attributable, on
one hand, to the distribution of the alloying elements in the titanium alloys.
On the other hand,
the ambient pressure is an additional influential factor. For example, Huang
et al. describe that
as a result of elevated process pressures (1500 bar), a reduction of the a-
transition temperature
can be observed in the alloy Ti4A18Nb [6].
Date Recue/Date Received 2021-03-01
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The inventors now believe that depending on the process conditions, shifts in
the 13-
transition temperature of only a maximum of 20 C will be observable due to
pressure
variations.
For creating a bimodal structure, the components were sintered close to the 13-
transition
temperature, but still below this.
For example, in order also to produce the lamellar grain structure with
reduced primary
13-phase grain size of the Ti6A14V alloy, which is also advantageous for many
use cases,
initially samples were produced in which the titanium components were sintered
at a sintering
temperature of 1000 C (Fig. 6). As shown by studies of the samples obtained
with respect to
the grain structure formed, this sintering temperature was still below the 13-
transition
temperature, although only slightly. The bimodal grain structure fanned is
composed of
globular a-structure and small portions of lamellar (a+13) structures, wherein
the mean f3-grain
size is 81 am.
The density measurement was perfonned according to the specifications of ASTM
8962 and ASTM B311. The grain size determination was perfonned according to
the
provisions of ASTM E112.
For creating a lamellar grain structure with the smallest possible grain size
of the
primary 13-phase grains, the components were largely sintered, i.e., for the
greatest part of the
time, below the 13-transition temperature, but with a minimal hold time that
remained below 30
min, preferably below 20 min, especially below 10 min, and also above the 13-
transition
temperature in phases, so that the 13-phase is entirely present, in order thus
to create the
lamellar grain structure, but also the primary 13-phase grain does not exceed
the size range
given in claim 16. The sintering above the 13-transition temperature always
took place at a
temperature in excess of 1015 C. This temperature was always kept below 1080
C, but
advantageously was below 1040 C and especially < 1020 c was selected.
The possibilities mentioned above for influencing the phase composition in the
sintered material by systematic adjustment of the sintering conditions at
sintering temperatures
below 1100 C, especially primarily below the 13-transition temperature,
present a particular
advantage of the process according to the invention. The prerequisite for this
variability is that
sufficiently compact titanium components can be produced below the 13-
transition temperature,
which is possible, as the inventors recognized, based on the use of the fine
powder, essential to
the invention, with grain sizes below <30 am.
Thus it has been shown that according to the method of the invention, powder-
metallurgical moldings from titanium and titanium alloys can be sintered at
sintering
temperatures below the usual mark of beyond 1100 C, generally 1200 C or more,
Date Recue/Date Received 2021-03-01
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advantageously below the 13-transition temperature, and thereby components
with good
structural and other material properties can be obtained. It was possible to
show that at
distinctly lower set sintering temperatures compared with the sintering
temperatures customary
in the prior art - unexpectedly - components with high final densities of >97%
can be obtained.
In particular it was shown that the method according to the invention makes it
possible to vary
the grain structure of the titanium component in the sintering process and
drastically reduce the
grain size, which makes it possible to optimize the mechanical properties of
the components,
e.g., the tensile strength, ductility and fatigue strength.
For example, within the scope of the invention, a particularly low temperature
may
also be selected for sintering, e.g., a temperature below 950 C, can be
selected, and if the
desired material density in the finished component (generally >97%) is not yet
achieved in
such a sintering step, further compaction of the material can be performed in
the subsequently
performed pressing step, in which the material is subjected to pressure and
optionally a
temperature, especially by cold isostatic pressing (CIP) or hot isostatic
pressing (HIP). Here,
for example, the material density after sintering may be at <97%, and it may
be compacted to
>97% by the pressing step after sintering.
In addition, following the sintering step, components produced according to
the
method of the invention may be subjected to additional thermal aftertreatments
to further
modify the properties of the materials. Such additional theimal
aftertreatments can, for
example, be one or more of the following methods: hot isostatic pressing
(HIP), quench,
uniform rapid quench (URQ).
The lower sintering temperature compared to the sintering temperatures from
the prior
art also result in additional environmental/financial and process technology
advantages. On
one hand, less theimal energy is required in the sintering process, leading to
lower costs but
also to shorter processing times. On the other hand, the method in accordance
with the
invention perfoimed with reduced sintering temperature also allows the use of
how-wall
furnace designs which are once again more economical than furnaces designed
for process
temperatures >1100 C, where cold-wall furnaces are typically used.
The selective combination of fine powders with mean grain size <25 gm,
preferably
also with maximum grain sizes <30 gm, and reduced sintering temperatures
compared with the
prior art, to be classified as low, allows the unrivaled manipulation of the
grain structure and
thus of the material properties.
Date Recue/Date Received 2021-03-01
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References:
[1] R. Gerling, T. Ebel, T. Hartwig: Method for producing components by
metal powder
injection molding. European patent EP1119429B 1, 2003.
[2] H.-J. Blum: Method for combined debinding and sintering of glass-
ceramic,
ceramic and metal molded parts. European Patent EP1496325A2, 2004.
[3] J. Sieniawski, W. Ziaja, K. Kubiak, M. Motyka: Microstructure and
Mechanical
Properties of High Strength Two-Phase Titanium Alloys. Materials Science/
Metals
and Nonmetals "Titanium Alloys - Advances in Properties Control," 2013, ISBN
978-
953-51-1110-8.
[4] M. Marty, H. Octor, A. Walder: Process for forming a titanium base
alloy with small
grain size by powder metallurgy. United States Patent U.S. 4,601,874, 1986.
[5] J. Lindemann: Titanium alloys, Laboratory Course on Lightweight
Construction
Materials, Department of Metals Science and Materials Technology Brandenburg
Technical University Cottbus, 2012.
[6] A. Huang, D. Hu, M.H. Loretto, J. Mei, X, Wu: The influence of pressure
on solid-state
transformations in Ti-46A1-8Nb. Scripta Materialia, Vol. 56, 4th Ed., 2007, p.
253-324.
Date Recue/Date Received 2021-03-01
