U.S. patent application number 10/293694 was filed with the patent office on 2004-03-25 for semi-solid metal casting process and product.
This patent application is currently assigned to SPX Corporation. Invention is credited to Apelian, Diran, DasGupta, Rathindra, Saha, Deepak.
Application Number | 20040055724 10/293694 |
Document ID | / |
Family ID | 31996870 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055724 |
Kind Code |
A1 |
Saha, Deepak ; et
al. |
March 25, 2004 |
Semi-solid metal casting process and product
Abstract
A method for the refining of primary silicon in hypereutectic
alloys by mixing a hypereutectic alloy and a solid/semi-solid
hypoeutectic alloy is described. The method provides control of the
morphology, size, and distribution of primary Si in a hypereutectic
Al--Si casting by mixing a hypoeutectic Al--Si liquid with one that
is hypereutectic to impart desirable mechanical properties.
Inventors: |
Saha, Deepak; (Worcester,
MA) ; Apelian, Diran; (West Boylston, MA) ;
DasGupta, Rathindra; (Portage, MI) |
Correspondence
Address: |
BAKER & HOSTETLER LLP
Washington Square, Suite 1100
1050 Connecticut Avenue N.W.
WASHINGTON
DC
20036
US
|
Assignee: |
SPX Corporation
|
Family ID: |
31996870 |
Appl. No.: |
10/293694 |
Filed: |
November 14, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60411872 |
Sep 20, 2002 |
|
|
|
Current U.S.
Class: |
164/47 ; 148/437;
148/538; 164/900 |
Current CPC
Class: |
C22C 1/005 20130101;
B22D 17/007 20130101; C22C 21/02 20130101 |
Class at
Publication: |
164/047 ;
148/538; 148/437; 164/900 |
International
Class: |
B22D 001/00; B22D
025/00 |
Claims
What is claimed is:
1. A semi-solid metal (SSM) casting process, comprising: providing
an Al--Si hypereutectic alloy and an Al--Si hypoeutectic alloy;
heating at least one of the Al--Si hypereutectic alloy or the
Al--Si hypoeutectic alloy; mixing the Al--Si hypereutectic alloy
with the Al--Si hypoeutectic alloy; cooling the hypereutectic
alloy--hypoeutectic alloy mixture for a length of time to form a
semi-solid metal; and, casting the semi-solid metal.
2. An SSM casting process according to claim 1, further comprising
heating both the Al--Si hypereutectic alloy and the Al--Si
hypoeutectic alloy.
3. An SSM casting process according to claim 2, further comprising:
controlling the length of time to achieve a cooling rate by heating
the Al--Si hypereutectic alloy to a predetermined temperature,
heating the Al--Si hypoeutectic alloy to a predetermined
temperature, and mixing the Al--Si hypereutectic alloy with the
Al--Si hypoeutectic alloy.
4. An SSM casting process according to claim 3, wherein the Al--Si
hypereutectic alloy predetermined temperature is different from the
Al--Si hypoeutectic alloy predetermined temperature.
5. An SSM casting process according to claim 3, wherein the
difference in temperature of the Al--Si hypoeutectic and
hypereutectic alloys is chosen to achieve a faster rate of cooling
of the hotter alloy as compared to heating the hotter Al--Si
hypereutectic alloy and allowing the hotter alloy to cool
independently at room temperature.
6. An SSM casting process according to claim 4, wherein the
difference in temperature is chosen to achieve a cast product
having Si particles with an average diameter ranging from about 60
microns to about 100 microns.
7. An SSM casting process according to claim 6, wherein the
difference in temperature is chosen to achieve a cast product
having Si particles with an average diameter of 70 microns or
less.
8. An SSM casting process according to claim 6, wherein the
difference in temperature is chosen to achieve a cast product with
Si particles that are more uniformly dispersed than a cast product
made by a conventional SSM rheocasting process.
9. An SSM casting process according to claim 1, wherein said
hypereutectic alloy is greater than 12.6 percent Si.
10. An SSM casting process according to claim 9, wherein said
hypereutectic alloy is about 23 percent to about 25 percent Si.
11. An SSM casting process according to claim 1, wherein said
hypoeutectic alloy is less than about 12.6 percent Si.
12. An SSM casting process according to claim 11, wherein said
hypoeutectic alloy is about 7 percent to about 8 percent Si.
13. An SSM casting process according to claim 12, wherein said
hypoeutectic alloy is about 7 percent Si.
14. An SSM casting process according to claim 2, wherein the
temperature of said hypereutectic alloy ranges from about
800.degree. C. and about 900.degree. C.
15. An SSM casting process according to claim 14, wherein the
temperature of said hypereutectic alloy is 800.degree. C.
16. An SSM casting process according to claim 2, wherein the
temperature of said hypoeutectic alloy ranges from about
350.degree. C. and about 850.degree. C.
17. An SSM casting process according to claim 16, wherein the
temperature of said hypoeutectic alloy is about 500.degree. C.
18. An SSM cast product that is manufactured by an SSM casting
process, comprising Si particles having less than an average
diameter of about 100 microns.
19. A cast product according to claim 18, wherein the rate of
cooling of the Al--Si alloy yields Si particles in the cast product
that have less than an average diameter ranging from about 60
microns to about 100 microns.
20. A cast product according to claim 19, wherein the Si particles
have less than an average diameter of about 70 microns or less.
21. A cast product according to claim 18, wherein the rate of
cooling of the Al--Si alloy yields Si particles in the cast product
that are more uniformly dispersed than a cast product made by a
conventional SSM rheocasting process.
Description
PRIORITY
[0001] This application claims priority to the provisional U.S.
patent application entitled, Semi-solid Metal Casting Process and
Product Thereof, filed Sep. 20, 2002, having a Ser. No. 60/411,872,
the disclosure of which is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the process of
casting metal alloys. More particularly, the present invention
relates to a method of casting aluminum-silicon alloys for
semi-solid metal rheocasting.
BACKGROUND OF THE INVENTION
[0003] Semi-solid metal (SSM) aluminum alloy castings outperform,
in both cost and performance, other casting techniques, such as
conventional die casting, gravity permanent mold casting, and
squeeze casting. SSM casting methods, when utilized for the
manufacturing of hypereutectic aluminum (Al) alloy
products/castings have advantages over other casting techniques
because SSM castings tend to exhibit higher mechanical properties
in the areas of strength and wear resistance, ductility, and
reduced porosity than castings produced by the above-listed other
methods.
[0004] Of the several methods to achieve cast components with SSM
alloys, thixocasting is the most common approach. Thixocasting
involves the heating of a metal alloy to the liquid state and then
the electromagnetic stirring of the melt during
solidification/freezing. These billets are subsequently cut into
slugs, and re-heated to a semi-solid state before being injected
for casting. Alternatively, rheocasting, which is also known as
"slurry" or "slurry-on-demand" casting, eliminates several steps
required by thixocasting techniques. This process involves
singularly heating a metal to a liquid state and then cooling the
molten metal to the required SSM phase, before injecting the
semi-solid metal into the mold/die cavity.
[0005] The mechanical and metallurgical properties of hypereutectic
SSM castings are predicated, in part, by the microstructures of
primary Si in the final part. The size and morphology of these
particles can be controlled by the cooling rate of the
hypereutectic alloy to the required temperature and the isothermal
hold time at the SSM temperature. Because solid primary phase
particles are a part of the semi-solid metal being injected into a
mold/die cavity, the microstructure of the primary phase of an
aluminum alloy prior to injection into a mold/die is indicative of
the microstructure of the primary phase of the resulting aluminum
alloy casting. Thus, the mechanical properties of a casting can be
predicted before a casting is even produced. Accordingly, many
attempts have been made to improve methods to achieve the requisite
microstructure. Known strategies including electromagnetic stirring
and addition of grain refiners.
[0006] One concern in the casting of hypereutectic aluminum-silicon
(Al--Si) alloys is to achieve a homogeneous distribution of primary
silicon (Si) both in the melt and in the final part. Uneven
distribution of large globular primary Si aggregates can seriously
compromise the mechanical integrity of a casting. The morphology of
the primary Si depends on the imposed temperature gradient,
presence of impurities, and ease of nucleation. Most of the
research in this regard has been, however, related to conventional
casting of Al--Si alloys and little has been learned regarding SSM
casting of Al--Si alloys. Therefore, the conventional casting of
Al--Si alloys has been successful in industry, while significant
challenges remain in rheocasting of these alloys, particularly in
controlling the microstructure of Al--Si alloys during rheocasting
of SSM materials.
[0007] Accordingly, it is desirable to provide a method of
utilizing the rheocasting method of SSM hypereutectic Al--Si alloys
that can impart desirable mechanical properties. In particular,
there is a need for a process to control for the nucleation of
primary Si particles in hypereutectic Al--Si alloys with high Si
content to limit the primary Si size. Further still, it is
desirable to provide a method of producing products with Al--Si
alloy castings by rheocasting techniques wherein the temperature
and the final composition of the product can be control led.
SUMMARY OF THE INVENTION
[0008] It is therefore a feature and advantage of the present
invention to provide an SSM casting process to generate a desirable
hypereutectic Al--Si composition by mixing together a hypoeutectic
liquid with one that is hypereutectic.
[0009] It is another feature and advantage of the present invention
to provide an SSM casting process to control the temperature and
cooling rate of a hypereutectic Al--Si alloy melt from mixing a
cooler alloy with a hotter one.
[0010] It is another feature of the instant invention to
manufacture Al--Si alloy products of by mixing a hypoeutectic
Al--Si liquid with one that is hypereutectic
[0011] There has thus been outlined, rather broadly, the more
important features of the invention in order that the detailed
description thereof that follows may be better understood, and in
order that the present contribution to the art may be better
appreciated. There are, of course, additional features of the
invention that will be described below and which will form the
subject matter of the claims appended hereto.
[0012] In this respect, before explaining at least one embodiment
of the invention in detail, it is to be understood that the
invention is not limited in its application to the details of
construction and to the arrangements of the components set forth in
the following description or illustrated in the drawings. The
invention is capable of other embodiments and of being practiced
and carried out in various ways. Also, it is to be understood that
the phraseology and terminology employed herein, as well as the
abstract, are for the purpose of description and should not be
regarded as limiting.
[0013] As such, those skilled in the art will appreciate that the
conception upon which this disclosure is based may readily be
utilized as a basis for the designing of other structures, methods
and systems for carrying out the several purposes of the present
invention. It is important, therefore, that the claims be regarded
as including such equivalent constructions insofar as they do not
depart from the spirit and scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a phase diagram of the composition versus
temperature of the alloys used in the mixing experiments.
[0015] FIG. 2 shows the time versus temperature plot for various
experiments.
[0016] FIG. 3 shows representative microstructures from the
castings produced from the experiments outlined in FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0017] The present invention provides a method for controlling the
composition, temperature and microstructure of Al--Si alloys prior
to SSM casting to control the mechanical properties of the final
cast product. Generally, this is accomplished by mixing a
hypereutectic Al--Si alloy with a hypoeutectic Al--Si alloy. By
definition, aluminum alloys with less than about 12.6 percent Si
are considered hypoeutectic whereas those with greater than about
12.6 percent Si are considered hypereutectic (FIG. 1).
[0018] The metallic composition of alloys used in current methods
for SSM casting are limited to the availability and composition of
the starting materials. In contrast, according to the present
invention, a broad range of metallic compositions are achievable
from the same starting materials. This is because the combination
of a hypereutectic solution into a hypoeutectic allows for the
manipulation of the final concentration of Si in the Al--Si alloy
by controlling the composition and mass of the two liquids or
semi-solid slurries. The final concentration of Si present in the
alloy determines many of its mechanical properties. For example,
increasing amounts of Si provides greater wear-resistance and
strength with lower expansion rates.
[0019] In one embodiment, the final, mixed alloy composition is
about 17 percent to about 18 percent Si in aluminum, formed by
combining a hypereutectic aluminum alloy comprising about 23
percent to about 25 percent Si and a hypoeutectic aluminum alloy
comprising about 7 percent to about 8 percent Si. A hypereutectic
alloy can contain about 12.6 percent to over 25 percent Si in
aluminum. Conversely, a hypoeutectic alloy can contain about 12.6
percent or less Si in aluminum. One example of a hypoeutectic alloy
with about 7% Si is developed by Elkem (under the trademark of
SIBLOY.RTM.), and is preferable for SSM processing of hypoeutectic
Al--Si alloys because the alpha aluminum formed in the melt is
independent of the hold time.
[0020] In addition to imparting unique physical properties to the
end product, the concentration of Si in aluminum has consequences
in the phase profile of any given alloy at any given temperature.
FIG. 1 is a phase diagram showing the composition of alloys as
varied by temperature. According to FIG. 1, about 12.6 percent Si
in aluminum defines the eutectic point, which is defined as the
lowest melting point possible between two substances in an alloy or
solution.
[0021] The phase diagram also indicates the temperature to which
the alloys need to be raised in order to be entirely in the liquid
state; this consists of the area designated above the liquidus line
1. The shaded areas 2, 3 indicate the temperature and composition
where the alloy is in a semi-solid phase, containing both liquid
and solid matter. For any given Al--Si alloy that is in the SSM
range, the semi-solid phase is where deposits of one of the metals
in the alloy begin to form. For example, hypereutectic Al--Si
alloys begin to develop large Si particles as they begin to cool
below the liquidus and into the SSM range. The instant invention
teaches a method of mixing two Al--Si alloys at different
temperatures together so that the amount of time the mixture spends
in the transitional semi-solid phase is minimized.
[0022] Temperature control of the alloys can also be achieved by
mixing a hypereutectic alloy with a hypoeutectic alloy as in the
present invention. Generally, one alloy is heated to a liquid state
and then mixed with an alloy of cooler temperature to bring the
combined melt within the SSM range. The hypoeutectic alloy is
generally maintained at a lower temperature than the hypereutectic
alloy. Preferably, the hypereutectic alloy is generally poured into
the hypoeutectic alloy, however, it is also possible to pour the
hypoeutectic alloy into the hypereutectic alloy.
[0023] In one embodiment, the hypereutectic alloys are heated to a
range of about 800.degree. C. to about 900.degree. C. and combined
with hypoeutectic alloys which are heated in the range of
350.degree. C. to about 580.degree. C. Preferably, the
hypereutectic alloy is raised to about 800.degree. C. and the
hypoeutectic alloy to about 500.degree. C. This large temperature
gradient allows for a quicker extraction of heat from the parent
hypereutectic alloy and decreases the time necessary for the liquid
alloy to drop in temperature to a semi-solid/slurry processing
temperature.
[0024] As mentioned above, the growth of Si particles in the
semi-solid phase is directly correlated to the time in addition to
the temperature of the alloy. Longer time periods in the semi-solid
phase is conducive for undesirable growth of large Si particles.
Alternatively, shortening that period minimizes the growth of large
Si particles by maximizing the number of nucleating events,
producing more Si particles of smaller size. During the casting
process, Al--Si alloys can spend a defined length of time in the
casting machinery/device in addition to the imposed cooling times.
Therefore, in addition to temperature control, it is preferable to
define the time parameters (i.e. cooling rates) within which the
desirable properties of the alloy are realized.
[0025] Specific processing parameters including the composition,
temperature, and holding times of the alloys were varied and the
microstructure of the cast alloys were therefore analyzed (FIGS. 2
and 3). Each alloy was heated to the temperature indicated and the
hypereutectic melt was then poured into the hypoeutectic alloy. The
temperature was then recorded and plotted as a function of time for
seven experiments shown (FIG. 2). The hypereutectic alloy used had
a composition of about 25 percent Al--Si and the hypoeutectic alloy
carried about 7 percent Si. Holding times ranged from about 200
seconds to about 400 seconds to simulate real-world holding times
and to study primary Si particle formations after allowing
significant time for their growth. Seven experiments are presented
(FIG. 2).
[0026] FIG. 3 shows the microstructure of the alloys from the
experiments described after they had been quenched. Microanalysis
of the casting from experiment 7 (FIG. 3A) shows that the primary
Si particles range in size from about 60 microns to about 100
microns in diameter. The primary Si are also relatively evenly
distributed with minimal aggregate formation as compared with
controls. These results are comparable to the final parts obtained
in thixocasting which are prepared to contain Si particles of
desirable size and distribution.
[0027] FIG. 3B shows the morphology of primary Si from experiment 6
to be radiating from a given point (star-shaped). This is generally
observed when the cooling rates are slow and were controlled by
elevating the temperature of the hypoeutectic solution to about
570.degree. C. The star shaped primary Si structures were reduced
by decreasing the temperature of the hypoeutectic alloy from
570.degree. C. to 500.degree. C. as shown in FIG. 3A from
experiment LM #7. Results from experiment 5 show that the amount of
dissolved aluminum can be controlled by regulating the temperature
of the hypoeutectic solution. FIG. 3C shows the structures obtained
when the hypoeutectic alloy is heated to 350.degree. C. and then
mixed into the hypereutectic alloy. In this instance, the greater
heat required to melt the primary aluminum in the hypoeutectic
alloy led to a final microstructure that displayed regions of
undissolved primary aluminum appearing as white spots. FIG. 3D
similarly shows results from experiment 4 where undissolved primary
aluminum of the hypoeutectic alloy remain in the final casting. In
this case, the final temperature was 615.degree. C. Small primary
Si can be seen on the primary aluminum, indicating that the heat
extracted by the primary aluminum provided local undercooling and
assisted in the nucleation of the primary Si. Lastly, FIG. 3E is a
representative example of the microstructure from experiments LM
#1-3 and shows the dissolution of primary aluminum as the melts
were held at a higher temperature (ranging from about 625.degree.
C. to about 636.degree. C.). In addition, these conditions led to
an uneven distribution of primary Si size and shape. Therefore,
preferable characteristics of SSM cast hypereutectic alloys can be
attained by controlling the temperatures of the hypo- and
hypereutectic solutions and the hold times at the SSM temperature
during casting.
[0028] A more rapid drop in temperature results in greater
nucleating events than if the temperature is dropped gradually.
This has the desirable effect of generating multiple Si particles
that are smaller in size (width and length), but also generally
uniformly distributed through out the alloy. The even distribution
of the Si particles allows for better prediction of mechanical
properties with less likelihood of mechanical failure which in
effect limit the average growth of the Si particles and diminished
the likelihood of globular aggregates.
[0029] The many features and advantages of the invention are
apparent from the detailed specification, and thus, it is intended
by the appended claims to cover all such features and advantages of
the invention which fall within the true spirits and scope of the
invention. Further, since numerous modifications and variations
will readily occur to those skilled in the art, it is not desired
to limit the invention to the exact construction and operation
illustrated and described, and accordingly, all suitable
modifications and equivalents may be resorted to, falling within
the scope of the invention.
* * * * *