U.S. patent application number 10/493803 was filed with the patent office on 2005-07-28 for method of forming articles from alloys of tin and/or titanium.
Invention is credited to Choy, Chee Mun, Hu, Banghong, Li, Qingfa, Zhang, Suxia.
Application Number | 20050163646 10/493803 |
Document ID | / |
Family ID | 20429002 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050163646 |
Kind Code |
A1 |
Li, Qingfa ; et al. |
July 28, 2005 |
Method of forming articles from alloys of tin and/or titanium
Abstract
The invention relates to a method of forming an article from an
alloy, such as a tin-containing alloy or titanium-containing alloy.
Elemental metal powders of metal constituents of the alloy are
injected into a pre-heated die and pressure is applied to form a
green part. The green part is then alloyed at a predetermined
temperature for a pre-determined time period to form the article.
The invention also relates to an article formed by such a
method.
Inventors: |
Li, Qingfa; (Singapore,
SG) ; Hu, Banghong; (Singapore, SG) ; Choy,
Chee Mun; (Singapore, SG) ; Zhang, Suxia;
(Singapore, SG) |
Correspondence
Address: |
Daniel B Schein
P O Box 28403
San Jose
CA
95159
US
|
Family ID: |
20429002 |
Appl. No.: |
10/493803 |
Filed: |
March 29, 2005 |
PCT Filed: |
October 31, 2001 |
PCT NO: |
PCT/SG01/00226 |
Current U.S.
Class: |
419/32 ;
419/66 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2999/00 20130101; B22F 3/14 20130101; B22F 3/225 20130101;
B22F 3/22 20130101; B22F 3/14 20130101; B22F 3/1035 20130101; B22F
3/225 20130101; B22F 3/14 20130101; C22C 1/0458 20130101; B22F
2999/00 20130101; B22F 2998/10 20130101; C22C 14/00 20130101; B22F
2998/00 20130101; B22F 2998/10 20130101 |
Class at
Publication: |
419/032 ;
419/066 |
International
Class: |
B22F 003/02 |
Claims
1. A method of forming an article from a tin-containing alloy, the
method comprising the steps of: (a) injecting elemental metal
powders of metal constituents of the alloy into a preheated die at
a temperature of above about 240.degree. C.; (b) applying a
predetermined pressure to the elemental powders for a predetermined
time period to produce a green part; and (c) alloying the green
part at a predetermined temperature for a predetermined time period
to form said article; wherein said elemental metal powders include
Sn powder in an amount of at least 2 wt % based on the total weight
of the elemental metal powders.
2. A method according to claim 1, wherein said elemental metal
powders include at least Ti and Sn powders.
3. A method according to claim 1, wherein said elemental metal
powders include at least Ti, Sn and Al powders.
4. A method according to claim 1, wherein said elemental metal
powders include Sn powder in an amount of from 2-12 wt % based on
the total weight of the elemental metal powders.
5. A method according to claim 1, wherein pressurising step (b)
includes applying a pressure of from 1000 to 2800 psi for a time
period of from 0.5 to 3 minutes.
6. A method according to claim 1, wherein alloying step (c)
includes alloying the green part at a temperature of from 1250 to
1350.degree. C. for a time period of from 30 to 150 minutes.
7. A method of forming an article from a titanium-containing alloy,
the method comprising the steps of: (a) injecting elemental metal
powders of metal constituents of the alloy into a preheated die at
a predetermined temperature, the temperature being determined based
on the constituents of the alloy; (b) applying a predetermined
pressure to the elemental powders for a predetermined time period
to produce a green part; and (c) alloying the green part at a
predetermined temperature for a predetermined time period to form
said article; wherein said predetermined temperature of injecting
step (a) is greater than about 100.degree. C. if said elemental
metal powders include Sn powder, and is greater than about
350.degree. C. if said elemental metal powders include Al
powder.
8. A method according to claim 7, wherein said elemental metal
powders include Ti and Al powders and said predetermined
temperature of injecting step (a) is between 450 and 550.degree.
C.
9. A method according to claim 8, wherein the predetermined
pressure in the pressurising step (b) is from 2000 to 3000 psi and
the predetermined time period in step (b) is from 120 to 480
minutes.
10. A method of forming an article from an alloy, the method
comprising the steps of: (a) introducing elemental metal powders of
metal constituents of the alloy into a die and applying a pressure
of from 2000 to 3000 psi for a time period of from 1 to 5 minutes
to form a preform; (b) applying a predetermined pressure and
temperature to the preform for a predetermined time period to
produce a green part; and (c) alloying the green part at a
predetermined temperature for a predetermined time period to form
said article.
11. An article formed by the method of claim 1.
12. An article formed by the method of claim 2.
13. An article formed by the method of claim 3.
14. An article formed by the method of claim 4.
15. An article formed by the method of claim 5.
16. An article formed by the method of claim 6.
17. An article formed by the method of claim 7.
18. An article formed by the method of claim 8.
19. An article formed by the method of claim 9.
20. An article formed by the method of claim 10.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods for the production
of articles, particularly shaped articles, from alloys which
contain Sn and/or Ti. In particular, the invention provides, in
various aspects, methods which include semi-solid metal powder
forming, solid state metal powder diffusion and metal powder
preform forming followed by hot pressing.
BACKGROUND TO THE INVENTION
[0002] Titanium is the fourth most abundant structural metal in the
crust of the earth after aluminum, iron and magnesium. Due to their
superior strength to weight ratios, excellent corrosion and erosion
resistance and high heat transfer efficiency, titanium and its
alloys have proven to be some of the most appropriate material
choices for a wide variety of critical applications in the
aerospace, marine and automotive industries. With improvements in
titanium and titanium alloy production, the cost for titanium and
its alloys in some regions of the world has been dramatically
reduced. The cost of Ti powders is equivalent to the cost of the
stainless steel powders which are commonly used in the powder
metallurgy (PM) and powder injection molding (PIM) industries world
wide. As such, the use of titanium and its alloys has rapidly
expanded to include applications in the pharmaceutical and chemical
areas as well as in nuclear power plants, food, and medical
prostheses. Commercial applications are also seen in sporting
equipment, fashion and apparel, such as golf clubs, bicycle frames,
watch cases, jewellery, eyeglasses and pens.
[0003] Generally, casting is the most widely used technology to
produce products of titanium and its alloys. However, titanium,
particularly in the liquid state, has a very high chemical
activity. It reacts strongly with oxygen, nitrogen, hydrogen, water
and carbon monoxide/dioxide and also reacts with almost all the
refectory crucible materials at high temperatures. Therefore,
melting and casting must be carried out in special crucibles under
a very high vacuum. After casting, expensive post-machining is
often required to achieve the desired final dimensions. In
addition, for high performance applications, hot isotropic pressing
(HIP) of the castings is normally required in order to completely
eliminate casting porosity. All of these barriers result in a very
high fabrication cost, which limits the widespread application of
titanium alloys to a great extent.
[0004] Powder metallurgy has been evaluated as another means of
producing articles of titanium alloys. However, this process
generally involves complicated HIP compaction methods which makes
it difficult for general applications.
[0005] Semi-solid metal forming (SSMF) is a newly developed process
for forming alloys under semi-solid state conditions, rather than
in the liquid state such as in casting or solid state such as in
sintering (conventional PM and PIM). The process relies on the
thixotropic behavior of the semi-solid slurries containing
non-dendritic solid particles which are able to flow like viscous
liquids when a shear force is applied. This peculiar flow behavior
has led to the development of some novel forming processes, such as
so called thixo-casting and thixo-molding, for fabrication of
near-net shaped components with high performance. Mass production
is being carried out in the United States of America and Europe
with a growing trend. Due to the process characteristics, the
current applications are only limited to certain Al and Mg
alloys.
[0006] The principle of semi-solid processing has also been applied
to the fabrication of metallic slurries based on pre-blending and
compaction of powders with different melting points in a process
termed as COMPASS (consolidation of mixed powders as synthetic
slurry).
[0007] Recently, Yasue, et al in the National Industrial Research
Institute of NaGaya (NIRIN), Japan, successfully formed a Ti-6 wt.
% Al alloy via semi-solid metal powder forming methods. The process
showed the feasibility in processing the above alloys at a
temperature around 700.degree. C. (die set temperature). However,
the relatively high process temperature (die set temperature)
required makes this process unattractive to commercial
producers.
[0008] The principle of the solid state diffusion process is to
press the alloy powder below the melting points of the powder for a
predetermined pressure and time. The microstructures produced in
this method are similar to those produced by semi-solid forming
processes, but using a relatively long process time. There has been
little discussion in the public literature related to this forming
method which can be used for producing prototyping components.
[0009] Objectives
[0010] The invention advantageously provides a semi-solid metal
powder forming technology for fabricating net-shaped and miniature
titanium alloy components with low cost and high dimensional
accuracy. As such, industry may advantageously benefit in being
able to produce titanium alloy components cost effectively with
enhanced productivity.
SUMMARY OF THE INVENTION
[0011] According to a first aspect of the present invention there
is provided a method of forming an article from a tin-containing
alloy, the method comprising the steps of:
[0012] (a) injecting elemental metal powders of metal constituents
of the alloy into a preheated die at a temperature of above about
240.degree. C.;
[0013] (b) applying a predetermined pressure to the elemental
powders for a predetermined time period to produce a green part;
and
[0014] (c) alloying the green part at a predetermined temperature
for a predetermined time period to form the article;
[0015] wherein the elemental metal powders include Sn powder in an
amount of at least 2 wt % based on the total weight of the
elemental metal powders.
[0016] In this aspect, the elemental metal powders may include at
least Ti and Sn powders, optionally with other metal powders. In
one embodiment the elemental metal powders include at least Ti, Sn
and Al powders.
[0017] According to this aspect, the elemental metal powders
include Sn powder in an amount of at least 2 wt % based on the
total weight of the elemental metal powders. Preferably, the
elemental metal powders include Sn powder in an amount of from 2-12
wt % based on the total weight of the elemental metal powders.
[0018] In this aspect, the alloy include any alloy of Sn, but is
preferably directed to alloys of Sn with Al and/or Ti. As such, the
invention of this aspect may be considered to be a semi-solid metal
powder forming process for Sn containing alloys given the
temperature of injecting step (a) of 240.degree. C.
[0019] Preferably, in this aspect, the pressurising step (b)
includes applying a pressure of from 1000 to 2800 psi for a time
period of from 0.5 to 3 minutes. Further, it is preferred that
alloying step (c) includes alloying the green part at a temperature
of from 1250 to 1350.degree. C. for a time period of from 30 to 150
minutes.
[0020] According to a second aspect of the invention there is
provided a method of forming an article from a titanium-containing
alloy, the method comprising the steps of:
[0021] (a) injecting elemental metal powders of metal constituents
of the alloy into a preheated die at a predetermined temperature,
the temperature being determined based on the constituents of the
alloy;
[0022] (b) applying a predetermined pressure to the elemental
powders for a predetermined time period to produce a green part;
and
[0023] (c) alloying the green part at a predetermined temperature
for a predetermined time period to form the article;
[0024] wherein the predetermined temperature of injecting step (a)
is greater than about 100.degree. C. if the elemental metal powders
include Sn powder, and is greater than about 350.degree. C. if the
elemental metal powders include Al powder.
[0025] In one embodiment, according to this aspect, the elemental
metal powders include Ti and Al powders and the predetermined
temperature of injecting step (a) is between 450 and 550.degree. C.
In this embodiment, it is preferred that the predetermined pressure
in the pressurizing step (b) is from 2000 to 3000 psi and the
predetermined time period in step (b) is from 120 to 480 minutes.
This embodiment may therefore be considered a solid state diffusion
process for Ti--Al alloys.
[0026] According to a third aspect the invention provides a method
of forming an article from an alloy, the method comprising the
steps of:
[0027] (a) introducing elemental metal powders of metal
constituents of the alloy into a die and applying a pressure of
from 2000 to 3000 psi for a time period of from 1 to 5 minutes to
form a preform;
[0028] (b) applying a predetermined pressure and temperature to the
preform for a predetermined time period to produce a green part;
and
[0029] (c) alloying the green part at a predetermined temperature
for a predetermined time period to form the article.
[0030] In this regard, this aspect of the invention includes a
process which involves an initial step of forming a preform
followed by, generally, a semi-solid state metal powder forming
process or a solid state metal powder diffusion process. For
example, pressurizing step (b) and alloying step (c) may include
either of the processes described for the first and second aspects
of the invention
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] Embodiments of the various aspects of the invention will now
be described in detail, in some instances with reference to the
accompanying drawings in which:
[0032] FIG. 1 illustrates graphically a Ti--Al binary phase
diagram.
[0033] FIG. 2 illustrates graphically a Ti--Sn binary phase
diagram.
[0034] FIG. 3 illustrates a Ti-6Sn elemental blended alloy formed
by semi-solid forming followed by alloying treatment at
1350.degree. C. v 1 hr.
[0035] FIG. 4 illustrates a Ti-6Sn elemental blended alloy formed
by semi-solid forming followed by alloying treatment at
1400.degree. C. v 1 hr.
[0036] FIG. 5 illustrates Ti-6Sn elemental blended alloy formed by
semi-solid forming followed by alloying treatment at 1450.degree.
C. v 1 hr.
[0037] FIG. 6 illustrates Ti-9Sn elemental blended alloy formed by
semi-solid forming followed by alloying treatment at 1350.degree.
C. v 1 hr.
[0038] FIG. 7 illustrates Ti-9Sn elemental blended alloy formed by
semi-solid forming followed by alloying treatment at 1400.degree.
C. 2.5 hr (spherical Ti powders).
[0039] FIG. 8 illustrates a Ti-12Sn elemental blended alloy formed
by semi-solid forming followed by alloying treatment at
1400.degree. C. v 1 hr.
[0040] FIG. 9 illustrates Tii-5Al-2.5Sn elemental blended alloy
formed by semi-solid forming followed by alloying treatment at
1350.degree. C. v 1 hr.
[0041] FIG. 10 illustrates Ti-8Al fabricated by semi-solid state
diffusion followed by alloying treatment 1350.degree. C. for 1 hr.
(predominated alpha-Ti structure).
[0042] FIG. 11 XRD analysis results for Ti-9Sn elemental blended
alloy formed by semi-solid metal powder forming process prior to
alloying.
[0043] FIG. 12 XRD analysis results for Ti-9Sn elemental blended
alloy formed by semi-solid metal powder forming process followed by
alloying at a temperature of 1350.degree. C.
[0044] FIG. 13 XRD analysis results for Ti-5Al-2.5.Sn elemental
blended alloy formed by semi-solid state metal powder forming
process followed by alloying at a temperature of 1450.degree.
C.
[0045] FIG. 14 XRD analysis results for Ti-8Al elemental blended
alloy formed by solid state diffusion process followed by alloying
at a temperature of 1250.degree. C.
[0046] Raw Materials-Metal Powders
[0047] The elemental metal powders used in each of the aspects of
the invention are Ti, Al and Sn. Many other elemental metal powders
may be added, but for simplicity and clarification, only the above
elemental metal powders have been the subject of experimental
analysis.
[0048] Metal Alloys and Their Blending
[0049] The above metal powders were carefully weighed and mixed
using an experimental blender. The duration of the mixing was
approximately 4 hours minimum. Various mixtures of the elemental
metal powders were formed as indicated below.
[0050] Ti--Al Alloys
[0051] Ti-6Al
[0052] Ti-8Al
[0053] Ti-20Al
[0054] Ti--Sn Alloys
[0055] Ti-6Sn
[0056] Ti-9Sn
[0057] Ti-12Sn
[0058] Ti--Al--Sn Alloys
[0059] Ti-5Al-2.5Sn
[0060] Ti-6Al--Sn
[0061] Ti-6Al-8Sn
[0062] It is impossible to conduct experiments in respect of all
possible combinations of Ti, Al and/or Sn alloys. However, the
first aspect of the invention provides advantages using semi-solid
metal forming techniques With the incorporation of elemental Sn
powder. In principle, any alloys containing more than 2% elemental
Sn powder can be formed in this method. Other identified commercial
and semi-commercial grades of Ti alloys, which can be processed in
this way are given below by way of example only:
[0063] Ti-6Al-2Sn-4Zr-2Mo
[0064] Ti-5Al-2.5Sn-ELI (clarification required)
[0065] Ti-2.25Al-11 Sn-5Zr-1Mo
[0066] Ti-5Al-5Sn-2Zr-2Mo
[0067] Ti-6Al-2V-2Sn
[0068] Ti-6Al-2Sn-4Zr-6Mo
[0069] Ti-5Al-2Sn-2Zr-4Mo-4Cr
[0070] Ti-6Al-2Sn-2Zr-2Mo-2Cr
[0071] Ti-1 1.SMo-6Zr-4.5Sn
[0072] Ti-1 5V-3Al-3Cr-3Sn
[0073] Ti-5Al-2.5Sn
[0074] Ti-5Al-6Sn-2Zr-1Mo-2.5Si
[0075] The Die Set Design
[0076] Two die sets have been designed: one a Ti-watch case which
is used to verify the formability of the materials using the newly
developed metal powder semi-solid forming technology of the fist
aspect of the invention, and the other a tensile bar which is used
to verify the mechanical properties.
[0077] The die set used for the tensile bars was very similar to a
conventional PM die set design, but was combined with heating
facilities generally adopted in conventional PIM, plastic injection
molding or die casting die set designs.
[0078] This die set used for the watchcase components was similar
to a conventional PM die set design, but a full profile ejector was
applied. In order to minimize friction and possible damage to the
component, an upper part for accommodating the extra powders was
designed so as to be movable such that the parts formed could be
easily ejected without damage.
[0079] Semi-solid metal powder forming according to the first
aspect of the invention was carried out using a hot plate press and
a hydraulic press specifically designed and installed for this
project.
[0080] The die set on the hot plate press could be heated to a
maximum temperature of 600.degree. C. A maximum pressure of 3000
psi could be applied to the die set and held at a predetermined
temperature for up to at least 10 hours. All of the Ti watchcase
samples were produced with this press using the designed watchcase
die set. Initially, tensile bars were also produced using this
small hot plate press. When the fusibilities were shown on this
machine, a tensile bar die set for large press and semi-auto
operation was then designed.
[0081] As stated above, a hydraulic press was used to produce the
required tensile bars for tensile property verification after
initial testing on the small hot plate press. The die set was
heated up to 280.degree. C. and held for about 1-5 minutes. Most of
the tensile bars were produced using this machine as it is very
fast and easy to operate (semi-auto) whereas the small press was
manual and very slow.
[0082] Forming Methods
[0083] The present invention considers a number of different
forming methods. These, which include powder metallurgy, solid
state metal powder diffusion, semi-solid metal powder forming and
metal powder forming followed by hot pressing, will be dealt with
in turn below.
[0084] Powder Metallurgy
[0085] For the tensile bars, predetermined metal powder
constituents of the alloy were poured into the cavity of the die
set and pressed under a pressure of 2500 to 2800 psi for about 3 to
5 minutes. Tensile bars were successfully produced.
[0086] For watchcase components, predetermined metal powder
constituents of the alloy were poured into the cavity of the die
set and pressed under a pressure of 2500 to 2800 psi for about 3 to
10 minutes. It was found that it was very difficult to produce
complete watchcase components using this method. The watchcases
produced exhibited defects such as cracks.
[0087] From this, it can be seen that conventional powder
metallurgy processing can only be used to produce some simple
shaped tensile bars and is not appropriate for the production of
more complex shaped components like watchcases without the use of
binders.
[0088] Solid State Metal Powder Diffusion Process
[0089] For tensile bars, solid state diffusion processing was used
to produce articles of Ti-6Al, Ti-8Al and Ti-20Al elementally mixed
metal powder alloys. The powders were poured into the preheated die
at 450 to 600.degree. C. and held for about 3 to 6 hours at a
pressure of 2500 to 3500 psi. It was found that once the
temperature was over 550.degree. C. the die set became jammed and
various surface defects, such as scratches and distortion occurred.
At temperatures lower than 500.degree. C., the die set exhibited no
obvious problems.
[0090] For the watchcase components, solid state diffusion
processing was used to produce articles of Ti-6Al, Ti-8Al and
Ti-20Al elementally mixed metal powder alloys. These were produced
under the same parameters as used for the tensile bars. Some
watchcases have been successfully produced in this way.
[0091] For Ti--Sn alloys, die set temperatures of greater than
100.degree. C. at applied pressure of from 2500 to 3000 psi for
periods of from 1 to 8 hours have successfully produced both
tensile bar and watchcase samples.
[0092] Semi-Solid Metal Powder Forming Process
[0093] For tensile bars, a group of Ti alloys containing 2 to 12%
elemental-Sn-metal powder was processed using a semi-solid metal
powder forming process. The Sn-containing Ti alloys were put into
the die set cavity which was preheated to 250 to 300.degree. C. and
held under a pressure of 1000 to 2500 psi for about 1 to 3 minutes.
Tensile bars were successfully produced in this way.
[0094] For watchcase components, the group of Sn-containing Ti
alloys were put into the watchcase die cavity which was preheated
temperature of 250 to 300.degree. C. and held under a pressure of
2500 to 2800 psi for about 1 to 3 minutes. Watchcase samples were
successfully produced in this way.
[0095] Metal Powder Preform Forming Followed by Hot Pressing
[0096] In this process, the metal powder alloys were firstly formed
into a simple shape, similar to the final geometry of the
components to be made, by conventional PM process. The preforms
were then processed using either a semi-solid forming or solid
state diffusion process. This process is advantageously tidy and
surface finish can be further improved.
[0097] The preforms were then processed using either a semi-solid
forming or solid state diffusion process. For more complicated
geometry components, a simple shape can be formed first and then
followed by progressive forming (several die set together) using
either semi-solid or solid state diffusion process.
[0098] Comparison of the forming methods
[0099] Conventional powder metallurgy can be used to produce simple
tensile bars, but is not easily employed to produce sound watchcase
samples. Solid state diffusion processes can be used to produce
articles of almost all Sn containing alloys including Ti-xSn-x, but
with a long holding/processing time. This time may be from 30
minutes to 8 hours in order to achieve reasonably high compacted
density. However, the semi-solid metal powder forming process makes
it possible to form alloys of Ti--Sn, Ti--Sn-x, Ti--Sn-x-x etc. at
relatively low temperature and very short cycling time. Table 1
attached summarizes the characteristics of the above three
processes.
[0100] Alloy Treatment/Sintering
[0101] The above-formed tensile bars and watchcases were sintered
at a temperature of from 1200 to 1450.degree. C. under vacuum and
Argon. The holding time was about 1 to 3 hours. The sintering
profiles are shown in Table 2.
[0102] Property Evaluation
[0103] Density
[0104] Density was measured using a pycnometer. The elemental
powder density is given in Table 3, the densities of the elemental
alloy mixture, green parts and sintered parts are given in Table 4
and the dimensional characteristics are shown in Table 5.
[0105] As can be seen from Table 4, the green part density is very
close to the sintered density, the sintered density being about 98%
of the theoretical density, assuming the mixed powder density is
the theoretical density. It is also noted that for some alloys
containing Al elemental powder, the sintered density is lower than
the green part density. This may be due to the relaxation of the Al
powder during sintering. It is also confirmed that for Ti-20% Al,
the sintered density is much lower than the green part density and
the size of the sintered components are much larger than those of
the green parts.
[0106] Shrinkage Factor
[0107] From Table 5, it can be concluded that:
[0108] The sintering temperature does not affect the shrinkage once
the temperature is over 1300.degree. C.
[0109] For semi-solid metal powder forming, the shrinkage factor is
in the range of OA to 2.1% for Ti--Sn alloys and 0.25 to 0.7% for
Ti--Al--Sn alloys.
[0110] The shrinkage in both the length and width directions are
isotropic for semi-sold forming, but anisotropic for the dimensions
of the component made by PM.
[0111] Mechanical Testing
[0112] Mechanical testing was carried out using an Instron tensile
machine. The parameters used were: speed: 3 mm/min. Max. 10 tons
and the tensile results are given in Table 6.
[0113] Microstructure and phase diagram
[0114] As shown from Ti--Al binary phase diagram illustrated in
FIG. 1, for Al compositions containing 8 wt % Al or less, the final
composition will be alpha-Ti provided equilibrium conditions are
met. In practice, there may be a small quantity of beta or delta
phase Ti existing as the equilibrium conditions may not be reached
or phase transformation may be incomplete.
[0115] The binary phase diagram for Ti--Sn is given in FIG. 2. As
can be seen in the diagram, the final phase will be alpha-Ti, where
the original composition contains less than 20 wt. % Sn, under
equilibrium conditions. Again, in practice there may be some beta
and other compounds present due to the sintering conditions
applied.
[0116] Microstructure
[0117] The selective microstructures for Ti-6Sn, Ti-9Sn and Ti-12Sn
are given in FIGS. 3 to 5, FIGS. 6 to 7 and FIG. 8 respectively. As
can be seen, the microstructures mainly consist of the alpha-Ti
phase with some minor compounds, which are identified by the
subsequent XRD analysis. The basic microstructures following
sintering at temperatures in the range of 1300 to 1450.degree. C.
are similar. Based on the microstructure, there are still voids
present at a level of 1 to 2% which may be eliminated by optimizing
the process parameters.
[0118] The selective microstructure for Ti-5Al-2.5Sn is shown in
FIG. 9. It can be seen that the microstructures are uniform and
predominated by .A-inverted.-Ti and minor compounds identified by
the subsequent XRD analysis.
[0119] The selective microstructures for Ti-8Al are given in FIG.
10. It can be seen that the grain size is very similar to the
original particle size, which indicates that no abnormal grain size
growth has taken place. The percentage voids for this alloy formed
by semi-solid state processing followed by sintering is relative
large compared to the other alloys outlined above. The reason is
that this process is very similar to the conventional PM process
but only the processing temperature is increased from room to a
temperature which is below the melting point of Al.
[0120] XRD Analysis
[0121] The XRD analysis of selective compositions is given in FIGS.
11 to 14. As can seen from the FIG. 11, there are some indications
of oxidation during forming which was conducted at open atmosphere.
However, after alloying or sintering, no oxides were found in the
samples. It is also indicated that there are some compounds present
in the microstructures as indicated in FIGS. 12 to 14, which is
probably due to insufficient holding time for the phase
transformation to take place completely. It should be pointed out
that the sintering step has not yet been optimized for this
process.
[0122] Those skilled in the art will appreciate that the invention
described herein is susceptible to variations and modifications
other than those specifically described. It is to be understood
that the invention includes all such variations and modifications
which fall within its spirit and scope. The invention also includes
all the steps, features, compositions and compounds referred to or
indicated in this specification, individually or collectively, and
any and all combinations of any two or more of said steps or
features.
* * * * *