Note: Descriptions are shown in the official language in which they were submitted.
CA 02597248 2012-11-05
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Method for casting titanium alloy
The invention relates to a process for casting objects
from a 13-titanium alloy, more specifically a titanium-
molybdenum alloy.
Titanium alloys are becoming more and more popular on
account of their numerous advantageous properties.
Titanium alloys are used in all fields in which high
demands are imposed on the material, in particular on
account of their good chemical stability, even at high
temperature, and their low weight combined with
excellent mechanical properties. On account of their
excellent biocompatibility, titanium alloys are also
preferentially used in the medical sector, in
particular for implants and prostheses.
Various methods for shaping titanium alloys are known.
In addition to cutting processes, these primarily
include casting and forging processes. In principle,
titanium alloys are forging alloys, for which reason
forging processes are generally used, since it has been
found that titanium alloys are difficult to cast. This
approach is generally taken for complicated shapes but
leads to restrictions in terms of the choice of
suitable alloys. In particular, it has been found that
only unsatisfactory results are achieved when casting
P-titanium alloys 2004/013 6859)
The invention is based on the object of providing an
improved casting process for 13-titanium alloys which
allows even complex shapes to be produced with good
material properties.
CA 02597248 2015-08-17
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According to the invention, in a process for casting
objects from a P-titanium alloy comprising titanium-
molybdenum with a molybdenum content of from 7.5 to
25%, it is provided that the alloy is melted at a
temperature of over 1770 C, the molten alloy is
investment-cast into a casting mold corresponding to
the object to be produced, is hot-isostatically
pressed, solution-annealed and then quenched.
According to one aspect of the invention, there is
provided a process for casting an object from a
13-titanium alloy being free of bismuth comprising
titanium-molybdenum with a molybdenum content of 15 wt%,
wherein the process comprises melting the alloy at a
temperature of over 1770 C, investment-casting the
molten alloy into a casting mold corresponding to the
object to be produced, hot isostatic pressing, solution
annealing at a temperature from 760 C to 800 C and
subsequent quenching.
In the present context, an object is to be understood
as meaning a product which has been shaped for final
use. The object may, for example in the aeronautical
industry, be parts used for jet engines, rotor
bearings, wing boxes or other supporting structure
parts, or in the field of medicine may be
endoprostheses, such as hip prostheses, or implants,
such as plates or pins or dental implants. The term
object in the context of the present application does
not encompass billets which are intended for further
processing by shaping processes, i.e in particular
does not include ingots produced by permanent mold
casting for further processing by forging.
CA 02597248 2014-05-28
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The process according to the invention achieves
economical production of objects made from P-titanium
alloys using the investment-casting process. The
invention therefore provides the possibility of
combining the advantageous properties of 13-titanium
alloys, in particular their excellent mechanical
properties, with the advantages of production of
objects using the investment-casting process. The
invention allows even objects of complex shapes, which
CA 02597248 2007-08-08
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it has been impossible to produce (economically) using
conventional forging processes, to be produced from a
13-titanium alloy. Therefore, the invention also opens
up the application area of complex-shaped objects to
13-titanium alloys, which are known to have favorable
mechanical properties and biocompatibility.
The molybdenum content in the alloy or its molybdenum
equivalent is in the range from 7.5 to 25%. The result
of this is that, in particular for a molybdenum content
of at least 10%, the 13-phase is sufficiently stabilized
even as far as the room temperature range. It is
preferable for the content to be between 12 and 16%.
This allows a meta-stable 3-phase to be achieved by
rapid cooling following the investment casting. There
is generally no need to add further alloy-forming
elements. In particular, there is no need to add
vanadium or aluminum. Dispensing with these has the
advantage mentioned above that the toxicity resulting
from these alloy-forming elements can be avoided. The
same correspondingly applies to bismuth, which also
does not have the same biocompatibility as titanium.
It has been found that the invention, using the
13-titanium alloys which have hitherto been almost
impossible to use for investment casting, allows the
production of even more complex shapes than the
a/13-titanium alloys which have hitherto been used for
investment casting, such as for example TiA16V4. The
process according to the invention achieves improved
mold filling properties. This means that as a result of
the invention, in particular sharp edges can be
produced with a higher quality during investment
casting. The susceptibility to the formation of voids
in investment casting is also reduced as a result of
the improved mold filling properties.
CA 02597248 2007-08-08
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It is expedient for a cold-wall crucible vacuum
induction installation to be used to melt the
13-titanium alloy. An installation of this type makes it
possible to reach the high temperatures which are
required for reliable melting of titanium-molybdenum
alloys for investment casting. For example, the melting
point of T1M015 is 1770 C. A supplement of approx. 60 C
should expediently be added to this to effect reliable
investment casting. In particular, therefore, a
temperature of 1830 C has to be reached for TiMo15.
It is preferable for the hot isostatic pressing to take
place at a temperature which is at most equal to a beta
transus temperature of the titanium-molybdenum alloy
and is no more than 100 C below the beta transus
temperature.
The hot isostatic pressing counteracts undesirable
effects of concentrating the molybdenum in dendrites
while depleting the remaining melts by dissolving
inter-dendritic precipitations. A temperature below the
beta-transus temperature, specifically at most 100 C
below it, is favorable. Temperatures in the range from
710 C to 760 C, preferably of approx. 740 C, at an
argon pressure of approximately 1100 to 1200 bar have
proven suitable for a titanium-molybdenum alloy with a
molybdenum content of 15%.
Temperatures of at least 700 C to 880 , preferably in
the range from 800 C to 860 C, have proven suitable for
solution annealing. Argon is preferably used to produce
a shielding gas atmosphere. This improves the ductility
of the alloy.
It is expedient for quenching of the object by water to
be carried out after the solution annealing. It is
preferable to use cold water. In this context, the term
CA 02597248 2007-08-08
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"cold" is to be understood as meaning the temperature
of unheated tap water. It has been found that the
quenching has a considerable influence on the
mechanical properties of the object which are
ultimately achieved. Alternatively, quenching may also
take place in shielding gas, for example by argon
cooling. The results achieved, however, are not as good
as those achieved with cold water.
It may be expedient for the object finally also to be
hardened. This may allow the modulus of elasticity to
be increased slightly, if required. For this purpose,
it is preferable for the hardening to be carried out in
a temperature range from approx. 600 C to approx.
700 C.
The invention is explained in more detail below with
reference to the drawing, which illustrates an
advantageous exemplary embodiment. In the drawing:
Fig. 1 shows a table which gives mechanical properties
of the investment-cast titanium alloy according
to the invention;
Fig. 2 shows an image of the microstructure in a cast
state immediately after casting;
Fig. 3 shows an image of the microstructure after hot
isostatic pressing;
Fig. 4 shows an image of the microstructure after
solution annealing with a subsequent quench;
and
Fig. 5 illustrates liquidus and solidus temperatures
for a titanium-molybdenum alloy.
CA 02597248 2007-08-08
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The text which follows describes a way of carrying out
the method according to the invention.
The starting material is a p-titanium alloy with a
molybdenum content of 15% (TiMol5). This alloy can be
obtained commercially in the form of small billets
(ingots).
A first step involves investment casting of the objects
that are to be cast. A casting installation is provided
for melting and casting the TiMol5. This is preferably
a cold-wall crucible vacuum induction melting and
casting installation. An installation of this type can
reach the high temperatures which are required for
reliable melting of TiMol5 for investment casting. The
melting point of TiMol5 is 1770 C, plus a supplement of
approx. 60 C for reliable investment casting. Overall,
therefore, a temperature of 1830 C has to be reached.
The investment casting of the melt then takes place
using processes which are known per se, for example,
with wax cores and ceramic molds as lost molds.
Investment casting techniques of this type are known
for the investment casting of TiA16V4.
As can be seen from the figure (1000 times
magnification) in Fig. 2, dendrites are formed, and
considerable precipitations are evident in inter-
dendritic zones. This is a consequence of what is known
as the negative segregation of titanium-molybdenum
alloys. This effect is based on the specific profile of
the liquidus and solidus temperatures of titanium-
molybdenum alloys, as illustrated in Fig. 5. On account
of the profile of the melting temperatures of the
liquid phase (TL) and the solid phase (Ts) illustrated,
it is firstly the regions with a high molybdenum
content which solidify in the melt, during which
process the dendrites that can be seen in the figure
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are formed. This leads to depletion of the residual
melt, i.e. its molybdenum content drops. The inter-
dendritic zones in the cast microstructure have a
molybdenum content of less than 15%, and it is even
possible for the molybdenum content to drop to approx.
10%. As a result of the molybdenum depletion, the
inter-dendritic zones lack a sufficient quantity of p-
stabilizers. The result of this is that an increased
a/P transformation temperature is locally established,
resulting in the formation of the precipitations shown
in Fig. 2.
It is expedient for a surface zone which may form
during casting as a hard, brittle layer, known as the
a-case, to be removed by pickling. The thickness of
this layer is usually approx. 0.03 mm.
To counteract the unfavorable effect of the negative
segregation with the precipitations in the inter-
dendritic zones, according to the invention the
castings, after the casting molds have been removed
following the investment casting, are subjected to a
heat treatment. This involves hot isostatic pressing
(HIP) specifically at a temperature just below the
P-transus temperature. It may be in the range from
710 C to 760 C and is preferably approximately 740 C.
This causes the undesirable precipitations in the
inter-dendritic zones to be dissolved again. There is
no need for any preliminary age-hardening before or
after the hot isostatic pressing. However, fine
secondary phases precipitate again during the cooling
following hot isostatic pressing, preferentially in the
original inter-dendritic zones (cf. Fig. 3, 1000 times
magnification). This leads to undesirable embrittlement
of the material.
ak 02597248 2007-08-08
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The objects have only a low ductility following the hot
isostatic pressing.
To eliminate the disruptive precipitations, the
castings are annealed in a chamber furnace under a
shielding gas atmosphere (e.g. argon). A temperature
range from approx. 700 C to 860 C with a duration of
several hours, generally two hours, is selected for
this purpose. In this context, there is a reciprocal
relationship between the temperature and duration; at
higher temperature, a shorter time is sufficient, and
vice versa. Following the solution annealing, the
castings are quenched with cold water. Fig. 4 (1000
times magnification) illustrates the microstructure
following the solution annealing. Primary 3-grains and,
within the grains, very fine inter-dendritic
precipitations (cf. cloud-like accumulation in the top
left of the figure) can be seen. The objects which have
been investment-cast using the process according to the
invention have 3-grains with a mean size of more than
0.3 mm in their crystal structure. This size is typical
of the crystal structure achieved by the process
according to the invention.
The mechanical properties achieved following the
solution annealing are given in the table in Fig. 1.
It can be seen that the modulus of elasticity drops
with an increasing temperature during the solution
annealing, specifically as far as levels of
60,000 N/mm2. The
ductility values improve with
decreasing strength and hardness. For example, after
solution annealing for two hours at 800 C, a modulus of
elasticity of 60,000 N/mm2 combined with an elongation
at break of approx. 40% and a fracture strength Rm of
approx. 730 N/mm2 are achieved.
