METHOD AND APPARATUS FOR THIXOTROPIC MOLDING OF SEMISOLID ALLOYS

METHODE ET APPAREIL POUR DES MOULAGES THIXOTROPES D'ALLIAGES SEMI-SOLIDES

English Abstract

A method and apparatus for thixotropic molding of semisolid alloys are
disclosed. The method
comprises of feeding a dendritic-free feedstock bar into an extruder barrel,
melting a terminal
portion of the feedstock bar into a semisolid slurry by heating it to a
temperature between its solidus
and liquidus temperatures, and using the solid portion of the feedstock bar as
an one-time "plunger"
to inject the semisolid slurry into a mold cavity. The apparatus based on the
present method, which
is equipped without a regular extruder screw or plunger, uses a feedstock bar
as a one-time plunger
to inject semisolid slurries formed from a terminal portion of the same
feedstock bar.

Claims

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. ~A method for thixotropic molding of semisolid alloys comprising of feeding
a dendritic-free
feedstock bar into an extruder barrel, melting a terminal portion of the said
feedstock bar in a
heating zone of the said barrel into a semisolid slurry by heating it to a
temperature between its
solidus and liquidus temperatures, and using the solid portion of the said
feedstock bar as a one-
time "plunger" to inject the said semisolid slurry into a mold cavity.
2. ~The method of claim 1 requires no regular extruder screw or plunger to
perform thixotropic
molding of semisolid alloys.
3. ~The method of claim 1 wherein the said extruder barrel contacts with the
said semisolid slurry
and is subjected to an elevated temperature and injection pressure only at its
frontal portion.
4. ~The method of claim 1 wherein the said feedstock bar serves as both a
feedstock of the said
semisolid slurry and a one-time plunger simultaneously.
5. ~The method of claim 1 wherein the said extruder barrel has a cooling zone
where leaked slurry
is frozen into a sealant, which closes the clearance between the said
feedstock bar and the inner
surface of the said barrel.
6. ~The method of claim 1 wherein the said heating zone is periodically sealed
by means of the
formation of the said sealant at one end and a solid plug formed from the
residual slurry in the
discharge nozzle at the other end upon the completion of each shot, and,
therefore, oxidation is
prevented without using a protective gas within the said barrel.
7. ~An apparatus for thixotropic molding of semisolid alloys according to the
method of claim 1,
which comprises:
a) an extruder barrel having a discharge nozzle at one end;

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b) ~a feeding means to drive a dendritic-free feedstock bar into the said
extruder barrel and
toward the said discharge nozzle;
c) ~a heating means to generate a heating zone in the said extruder barrel to
heat a terminal
portion of the said feedstock bar to a temperature between its solidus and
liquidus
temperatures;
d) ~a cooling means to generate a cooling zone adjacent the said heating zone
in the said
extruder barrel to freeze slurry leaked from the said heating zone into a
solid sealant;
e) ~a supporting means to withhold the shot pressure subjected to the heating
portion of the
said barrel.
8. ~The apparatus of claim 7 wherein the said extruder barrel is a tri-
metallic cylinder constituting a~
bimetallic and a monometallic portion joined by welding.
9. ~The apparatus of claim 8 wherein the said bimetallic portion comprises of
an outer shell made of
one material with high strength at operating temperatures and a liner made of
another material
with high corrosive resistance to the semisolid slurry and shrunk-fit onto the
said outer shell.
10. ~The apparatus of claim 7 wherein the said extruder barrel equipped with
an O-ring at its rear to
prevent air from entering the said barrel via the clearance between the said
feedstock bar and the
inner surface of the said barrel.
11. ~The apparatus of claim 7 wherein the said extruder barrel has relatively
thin walls giving
advantages in economy and precise temperature control.
12. ~The apparatus of claim 7 wherein the said feeding means comprises of a
pair of unassisted
wedges, which drives the said feedstock bar into the said barrel during
forward motion and then
slides freely back.

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13. ~The apparatus of claim 7 wherein the said heating means comprises of a
series of band
resistance heaters attaching on the outer surface of the said extruder barrel.
14. ~The apparatus of claim 7 wherein the length of the said heating zone is
adjustable by the number
of band heaters used.
15. ~The apparatus of claim 7 wherein the said cooling means is a two-part
cooling ring with internal
circulating coolant.
16. ~The apparatus of claim 7 wherein the said supporting means comprises of a
series of supporting
hoops, each of which has a groove for housing a band heater.
17. ~The apparatus of claim 7 wherein the said extruder barrel, said feeding
means, said heating
means, said cooling means and said supporting means are within a barrel
housing.
18. ~The apparatus of claim 7 wherein the said extruder barrel can be easily
detached from the said
barrel housing.
19. ~An alloy switching method, without need of barrel opening and purging,
via replacement of the
current barrel with another barrel preloaded with a feedstock bar of another
alloy.

Description

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METHOD AND APPARATUS FOR THIXOTROPIC MOLDING OF SEMISOLID ALLOYS
This invention relates to an economical procedure and apparatus for molding
semisolid metallic
materials using a dendritic-free feedstock bar as a one-time extruder plunger
to inject semisolid
slurries converted from a terminal portion of the same feedstock bar.
BACKGROUND OF THE INVENTION
Thixotropic molding is a relatively new metal forming process during which
semisolid slurry
containing round solid particles dispersed in a continuous liquid phase is
converted from a
feedstock alloy and subsequently injected into a mold cavity. In comparison
with conventional high
pressure die-casting procedures, which work with liquid metals, thixotropic
molding is capable of
producing parts that are: 1) bubble free due to non-Newtonian viscosity - an
inverse relationship
between viscosity and the rate of shear - of the semisolid slurry and 2) less
shrinkage due to the
presence of a solid phase in the slurry and lower operating temperatures. It
may also increase the
lifetime of machine components viz., mold, plunger and chamber, as semisolid
slurry is generally
less erosive to these tools than liquid metal. Generally, thixotropic molding
is more suitable than
conventional die-casting processes in producing high quality molded parts with
higher strength,
better toughness and dimensional precision. In the past few years, thixotropic
molding has bean
adopted rapidly by a variety of industries especially for molding magnesium
alloy parts. In fact,
ASM has recently listed thixotropic molding as a standard magnesium forming
method (R.D.
Carnahan, "Thixotropic Molding: Semisolid Injection Molding of Magnesium
Alloys", Magnesium
and Magnesium Alloys, ASM Specialty Handbook, Eds. M.M. Avedesiam and H.
Baker, ASM
International, pp. 90-97, 1999).
Prior to the present invention, thixotropic molding has been carried out using
machines that
resemble a thermoplastic injection-molding device as disclosed in U.S. Pat.
No. 5,040,589. In this
process, feedstoek chips are fed into a reciprocating screw injection unit
where it is externally
heated and mechanically sheared by the action of a rotating screw. As the
material is moved
forward within the barrel, it is converted into semisolid slurry containing
degenerated dendritic
particles via partial melting and mechanical shearing, and is collected at a
space between the

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injection nozzle and the screw tip. Once an appropriate amount of slurry has
been accumulated, the
screw is rapidly driven forward to inject the slurry into a mold cavity.
Although the existing procedure for thixotropic molding of semisolid alloys
has been used
successfully in the past, they suffer from a number of shortcomings and
limitations including:
1. High cost of the extruder screw and barrel. Both the screw and barrel work
under severe
conditions - erosive attack from the semisolid metal, elevated operating
temperatures, high
injection pressure, high wear, and high thermal and mechanical fatigue;
therefore, they can only
be constructed using costly materials, such as maraging-type tool steels and
cobalt-based alloys
for processing magnesium alloys as disclosed in U.S. Pat. No. 5,996,679 and
niobium-based
alloys for processing aluminum or zinc alloys as disclosed in U.S. Pat. No.
5,819,839. Such
materials have poor workability and machinability (U.S. Pat. No. 5,996,679).
All of these mean
that the extruder screw and barrel can increase manufacturing cost
considerably and can
consume a significant amount of maintenance cost as well since the screw must
be regularly
replaced due to wear and tear. It has been reported that a 600 ton capacity
barrel made of
nickel-based wrought Alloy 718 with a cobalt-based liner alone costs $150,000
(U.S. Pat. No.
6,059,012).
2. Extruder screw may not only raise the manufacturing and maintenance costs
but may also cause
operating problems, especially backward leakage of slurry. As indicated in
U.S. Pat. No.
6,474,399, under high injection pressures, semisolid slurry may, via the
clearance between the
screw and the inner wall of the extruder barrel, leak backwards into and
subsequently erode the
shaft housing, and sometimes the driving mechanism of the screw.
3. The extruder barrel has to be opened and purged for alloy switching. This
is unsafe when
magnesium alloys are processed because semisolid slurries may combust when
exposed to air.
4. A protective gas is needed in the extruder barrel to prevent oxidation;
however, the gas may
become trapped in the molded part, hence causing porosity (U.S. Pat. No.
5,501,266).

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5. Semisolid slurries are made directly from dendritic feedstock using a so-
called one-step method
as disclosed in U.S. Pat. No. 4,694,882. However, depending on the shearing
action provided
by the screw, the dendritic grains may not be fully degenerated into globular
shapes especially
when the feedstock consists ofa well-developed dendritic structure and when
the volume
fraction of solid phase in the slurry is relatively high. This may cause
porosity, especially in
thin walled parts.
In addition to the one-step semisolid slurry making method employed in the
conventional
thixotropic molding machine, which uses a shearing force to break up the
dendritic network,
semisolid slurries can also be obtained by a two-step method in which a
dendritic-free (thixotropic)
alloy is first prepared and then is reheated to a temperature between its
solidus and liquidus
temperatures (Flemings, M.C., Riek, R.G. and Young, K.P., "Rheoeasting",
Materials Science and
Engineering, vol. 25, pp. 103-117, 1976). The two-step approach has been
widely used for
thixocasting.
There are two different processes for preparation of thixotropic or dendritic-
free alloys - the first
step in the two-step semisolid slurry making method. U.5. Pat. Nos. 3,902,544,
3,948,650,
3,954,455, 4,229,210 and 4,310,352 disclosed vigorous agitation processes
where during casting an
alloy is agitated while in the semisolid state to prevent formation of a
dendritic network by either
mechanical ar electromagnetic stirring. U.5. Pat. Nos. 4,415,374, 5,133,811
and 6,120,625
disclosed strain induced and melt activated (SIMAj approaches, which use
severe deformation and
recrystallization to break-up dendritic structures.
SUMMARY OF THE INVENTION
The present invention has been devised to overcorr~e the foregoing
shortcomings and limitations
identified for the conventional thixotropic molding machine.
It is therefore a primary objective of the present invention to fulf 11 that
need by providing a
thixotropic molding method and apparatus for semisolid alloys, which is based
on an extremely
simple mechanism without use of a regular extruder screw or plunger.

Representative Drawing