Note: Descriptions are shown in the official language in which they were submitted.
WO 94/06586 ~ 3 ~ PCI`/AU93/00454
PARTICULATE FEEDSTOCK FOR METAL INJECTION MOLDING
The present invention relates to a particulate
material comprising an alloy or composite. The
particulate material is especially suitable for use as a
feed material in the injection moulding or casting of
thixotropic alloys. As used herein, the terms "composite"
or "alloy composite" include an alloy matrix having ceramic
reinforcement, and includes metal matrix composites.
The semi-solid processing of alloys and composites is
an area of t~ch~ology in which much interest is presently
being shown. Such processing generally requires the
formation of a thixotropic alloy which is subsequently
processed. Thixotropic alloys are produced when solid
particles of a metal or alloy are homogeneously suspended
in a liquid phase of molten metal. The semi-solid mass
thus produced has thixotropic rheology.
Thixotropic alloys may be processed to produce metal
articles by injection moulding.
A number of processes to produce thixotropic alloys
have been proposed. United States Patent Nos. 4,694,881
and 4,694,882 both assigned to the Dow Chemical Corp., the
entire contents of which are herein incorporated by
reference, describe processes for producing thixotropic
alloys which comprise feeding solid particles of a metal
alloy from a hopper into an extruder, such as a screw
extruder. In U.S. 4,694,881, the solid particles are
heated in the extruder to a temperature above the liquidus
temperature of the alloy. The molten mass thus obt~ineA is
subsequently cooled to a temperature between the solidus
and liquidus temperatures and subjected to shearing to
break the dendritic structure that would otherwise form.
The resulting liquid-solid composition of a thixotropic
alloy is injected into a mould to form a moulded product.
United States 4,694,882 describes a similar process,
except that the feed alloy particles are heated to a
temperature between the solidus and liquidus temperatures,
without complete melting of the feed metal particles taking
place.
W094/06586 PCT/AU93/00454
3 il
Both of the above processes utilise feed particles or
chips of a convenient size for h~n~l;ng. The patents
especially describe the use of chips having an irregular
shape. The size of the particles used is described as not
S being critical to the invention, although relatively small
particle sizes are preferred because of heat transfer and
handling requirements.
Experiments carried out by the present applicant have
shown that the particles used in the process described in
U.S. Patent Nos. 4,694,881 and 4,694,882 are prone to block
the hopper and seize the screw extruder. Further, the
particles do not exhibit good packing characteristics which
can cause difficulty in achieving sufficient heat transfer
rates to cause the partial melting of the metal particles
lS and also render control over the temperature more
difficult.
The present inventors have now developed particles of
metal alloys and composites that are particularly suitable
for use in producing thixotropic alloys and in the
injection moulding of such alloys.
According to the first aspect, the present invention
provides particulate material comprising particles of metal
alloy or composite, wherein a substantial proportion of the
particles are shaped such that the ratio of the length of
the largest ~im?n~ion of a particle to the effective
diameter of the particle is in the range of l.0 to 4.0 and
that the substantial proportion of particles have a
particle size wherein the largest dimension of the
particles lies within the range of 0.5 to 5 mm. Preferably,
the particles are shaped such that the ratio of the length
of the largest ~;m?n~ion of a particle to the effective
diameter of the particle is in the range of 1.2 to 3.0,
more preferably 1.2 to 2Ø As used hereinafter, the
ratio of the length of the largest ~;men~ion of a particle
to the effective diameter of the particle will be denoted
by the term "aspect ratio".
The effective diameter of a particle may be determined
by determining the smallest circle that the particle will
W094/06586 214 ~ 4 3 4 PCT/AU93/00454
--3--
be able to pass through. The diameter of this circle is
the effective diameter of the particle.
Preferably, the particles have a largest ~ir?ncion in
the range of 1 to 3 mm.
The particles are shaped such that the tap density of
the mass of particles is preferably at least 50% of the
theoretical density of the alloy or composite.
The particles preferably have a substantial smooth
surface texture.
In a preferred embodiment the substantial proportion
of particles comprise at least 40% by weight of the mass of
particles, preferably at least 60% by weight more
preferably at least 80~ by weight, most preferably at least
95% by weight of the mass of particles.
In one embodiment, the particles preferably have an
approximately ovoid shape. Such particles may also be
described as having a shape similar to a rugby football or
as being the shape formed by the solid of revolution of an
ellipse or generally elliptical shape about a longit~ n~
axis.
In another embodiment, the particles may have a
generally tear drop shaped profile or have a profile that
may be described as a flattened tear drop. In this
embodiment, in a longitll~; ~A 1 cross-section of a particle,
a first end of the particle will have a generally
hemispherical or hemi-ovoidal shaped portion. The
generally hemispherical or hemi-ovoidal shaped portion may
be flattened, usually at a l~ g edge thereof. This
portion will taper to a second end of the particle, where
the particle will terminate at a point or at a portion
having a small radius of curvature. The overall shape of
the particle may be considered to be formed generally as
the solid of revolution of the planar shape of the cross-
section profile. Although the particle should have a
substantially smooth surface texture, it will be
appreciated that the particles will have a small degree of
surface roughness (as will the football shaped particles).
In a second aspect, the present invention provides a
W094/06~86 PCT/AU93/00454
21~4~34
,;
method for producing a thixotropic alloy in which feed
particles of a metal alloy or composite are heated and
sub;ected to shear to produce a substantially homogenous
mixture of solid particles and liquid wherein a substantial
proportion of the feed particles each have a shape such
that the ratio of the length of the largest dimension of a
particle to the effective diameter of the particle is in
the range of 1.0 - 4.0 and the substantial proportion of
the particles have a particle size wherein the largest
~i ?ncion of the particles lies in the range of from 0.5 to
5 mm.
In a preferred emboA~?~t of the second aspect of the
invention, the particles are shaped such that the ratio of
the length of the largest dimension of a particle to the
effective diameter of the particle is in the range of 1.2
to 3.0, more preferably 1.2 to 2Ø
The substantial proportion of feed particles
preferably have a particle size wherein the m~x; ~l~m
~;m~n~ion of a substantial proportion of the particles is
preferably in the range of from 1 to 3 mm. The particles
preferably have a substantially smooth surface texture. In
a preferred embodiment the substantial proportion of
particles comprise at least 40~ by weight of the mass of
particles, preferably at least 60% by weight more
preferably at least 80% by weight, most preferably at least
95% by weight of the mass of particles.
The thixotropic condition may be produced by any
suitable process that involves heating and shearing the
particles. However, it is particularly preferred that the
thixotropic condition is produced by use of a screw
extruder apparatus. In this case, the feed particles may
be supplied to a screw extruder whereupon they enter a
first heating zone and are heated to a temperature above
the melting point of the alloy or composite. The molten
material may then pass to a second zone where the molten
metal is cooled to a temperature below the liquidus
temperature and above the solidus temperature.
Solidification of some of the material will occur to form
W094/06586 214 ~ 13 ~ PCT/AU93/00454
a mixture of solid particles and liquid. The screw of the
extruder is caused to rotate such that the mixture is
sheared to prevent the formation of large crystal
structures and a thixotropic material is formed.
s Alternatively, the feed particles may be heated in a
first zone of the screw extruder to a temperature above the
solidus temperature of the material but below the liquidus
temperature of the material. Shear is applied to the
resulting mixture of liquid and solid particles by rotation
of the screw of the extruder to produce the thixotropic
material.
It will be appreciated that the method of the present
invention is not restricted to use of a screw extruder, but
that any means that is capable of heating the feed
particles to the required temperature and supplying a
shearing force to the mixture of liquid metal and solid
particles may be used. For example, the mixture may be
subjected to the action of a rotating plate or it may be
forced to travel through a tortuous path extruder in order
to impart sufficient shearing force to the mixture to
produce the thixotropic material. As a further
alternative, electromagnetic stirring may be used to obtain
the thixotropic material.
The feed particles may be supplied from a hopper by
gravity feed or ~u~l~yor feed.
The thixotropic material formed by the method of the
second aspect of the invention is especially suitable for
use in the production of metal components by injection
moulding. Accordingly, the present invention also provides
a method for producing an article which comprises heating
and shearing feed particles comprising a metal alloy or
composite to produce a substantially homogenous mixture of
solid particles and liquid, injecting said mixture into a
mould, allowing the mixture to at least partially solidify
in the mould and removing the article from the mould,
wherein a substantial proportion of the feed particles are
shaped such that the ratio of the length of the longest
~im~n~ion of a particle to the effective diameter of the
W094/06586 ~ PCT/AU93/00454
particle is in the range of 1.0 to 4.0 and the substantial
proportion of particles have a particle size wherein the
largest ~ n~ion of the particles lies within the range of
0.5 mm to 5 mm.
Preferably, the particles are Rh~pe~ such that the
ratio of the length of the longest dimension of a particle
to the effective diameter of the particle is in the range
of 1.2 to 3.0, more preferably 1.2 to 2Ø
The particles of the present invention may be of any
required metal alloy or composite thereof. Some suitable
materials include metal and intermetallic alloys based on
lead, aluminium, zinc, magnesium, copper and iron. The
preferred particles are alloys of aluminium.
The invention will now be further described with
reference to the Figures in which:
Figure 1 shows a schematic profile view of "football"
shaped particles according to the invention;
Figure 2 shows a sc~nning electron micrograph of the
actual particles shown schematically in
Figure 1;
Figure 3 shows a schematic cross-section view of
another particle according to the invention;
Figure 4 shows a similar view to Figure 3 showing the
calculation of aspect ratio for such
particles;
Figures 5 and 6 show sc~nn; ng electron micrographs of
further particles according to the present
invention;
Figure 7 shows a percentage frequency distribution of
aspect ratio for granule type 1;
Figure 8 shows a percentage frequency distribution of
W094/06~86 ~ 3~ PCT/AU93/00454
the dimension "length" for granule type l;
Figure 9 shows a percentage frequency distribution of
the dimension "width" for granule type 1;
Figure 10 shows a percentage frequency distribution of
S aspect ratio for granule type 2;
Figure 11 shows a percentage frequency distribution of
the ~;~?n~ion "length" for granule type 2;
- Figure 12 shows a percentage frequency distribution of
the ~;~~n~ion "width" for granule type 2;
Figure 13 shows a sr.~nn; ng electron micrograph of
particles according to the invention which
have a more needle-like structure;
Figure 14 shows photomicrographs of a slurry produced
in crucible tests at 575C using granule
lS type l;
Figure 15 shows photomicrographs of a slurry produced
in crucible tests at 590C using granule
type 1;
Figure 16 shows photomicrographs of a slurry produced
in a crucible test at 575 using granule
type 2; and
Figure 17 shows photomicrographs of a slurry produced
in a crucible test at 590C using granule
type 2.
In a preferred embodiment, a substantial proportion of
the particles of the particulate material of the present
W094/06~86 PCT/AU93/00454
~t4~
--8--
invention have an approximately ovoid particle shape with
a ratio of the largest dimension to the effective diameter
of between 1.2 and 3.0, more preferably 1.2 to 2Ø This
ratio may be designated the aspect ratio of the particles.
These particles can be further characterised as being in
the shape of an elongated sphere or shaped like a rugby
ball. A preferred shape of the particles is shown
schematically in Figure 1. The aspect ratio for the
particles is determined from the ratio of length to
effective diameter for the particles. Thus, referring to
Figure 1, the invention requires that:
L/D - 1.0 to 4.0, preferably 1.2 - 3.0, more
- preferably 1.2 - 2.0
The dimension L preferably lies within the range of
0.5 to 5mm.
Figure 2 shows a sc~nn; ng election micrograph of
actual particles that are generally ovoid shape. The
particles may also be described as of generally cylindrical
shape and having rounded ends.
In -a further embodiment, the particles have a
generally tear drop shape that may be flattened at one end.
With reference to Figure 3, which shows a cross-sectional
view of a particle, particle 20 of generally flattened tear
drop shape has a first end 21 that is in the form of a
generally hemispherical or hemi-ovoidal shape. First end
21 may be flattened at l~ing edge 22. Particle 20 is
shaped such that first end 21 tapers towards second end 23.
Second end 23 terminates at a point or at a portion 24
having a small curvature of radius.
Figure 3 shows a cross-sectional view of particle 20.
The overall shape of the particle may be considered to be
in the form of a solid of revolution of the cross-section
about longitudinal axis 25.
Referring to Figure 4, the aspect ratio of particle 20
3~ falls within the range of 1.0 to 4.0, preferably 1.2 to
3.0, more preferably 1.2 to 2Ø As with the football
shaped particles, the aspect ratio of particle 20 is given
by the ratio L/D. Here, ~;m~n~ion L may be considered to
W094/06586 ~ 4 ~ 4 PCT/AU93/00454
be the ~ximll~ height of the particle. Dimension D is the
diameter of the smallest circle that the particle is able
to pass through.
Sc~nni ng electron micrographs of further particles
that fall within the scope of the present invention are
shown in Figures 5 and 6.
The particulate matter of the present invention should
include a substantial proportion of particles shaped
according to the embodiments described above. In producing
the particulate matter of the invention, it has been found
that a substantial proportion of irregularly shaped
particles are also formed and become included in the
particulate matter. The presence of such irregularly
shaped particles does not unduly affect the properties of
the particulate matter unless the irregularly shaped
particles are present in an lln~cceptably large amount.
When used in the methods of the present invention for
producing a thixotropic material or a metallic article by
the injection moulding of a metal alloy or composite, the
substantial proportion of the mass of feed particles are
preferably sized such that the overall length of the
particles is in the range of 0.5 to 5 mm, more preferably
l to 3 mm. This allows convenient h~n~l;ng of the
particles whilst also avoiding bi n~ i ng or clogging of the
screw, in the case where a screw extruder is used.
The particulate material of the present invention has
a combination of properties that is not found in any
metallic particulates currently known to the applicants and
these combination of properties make the particulates
especially suitable for use as feedstock in th; xo~clding
processes. The particulate material of the invention has
a tap density that is at least 50~ of the theoretical
density. This ensures good particle to particle contact
and allows adequate heat transfer rates to be achieved in
the heating zone. This allows for relatively short heating
times to be used to cause the initial melting or partial
melting of the particles and it also allows for close
control over temperature to be maintained to enable the
~4~43~
W094/06586 PCT/AU93/00454
-- 10 --
thixotropic state to be maintAine~. The particulate
material is relatively free flowing and will be unlikely to
block a feed hopper. The mixing torque required to turn
the screw when the particulate material fills a screw
extruder is not l-n~cceptably high and the particles are
sufficiently large to ensure that particles cannot slip
between the walls of the extruder and the screw to cause
b;n~;~g of the screw.
The properties of a group of particulate materials
were dete~ in order to compare them with the
properties of the mass of particles of the present
invention. The particles used for comparison purposes were
ma-de of aluminium and consisted of powder (lOO~m), needles,
granules and irregular sh~p~ ~ch;n~ng chips. Although
some of these particles showed properties in one category
that were superior to the properties of the particles of
the invention in that category, none of the comparative
particles had a combination of properties that were as
desirable or useful as the properties of the particulate
matter of the invention.
The particulate material of the present invention may
be mixed with particles of other shapes and sizes.
However, this is generally not preferred due to possible
problems associated with segregation and settling of the
resultant mixture.
In order to quantify the performance of particulate
matter of the invention, a series of comparative tests were
run to compare the properties of the "football" particles
with a series of co~?rcially available particles. The
particles used for comparison purposes were aluminium
granules, aluminium needles, aluminium spherical powder
(lO0 ~m average particle size) and aluminium m~ch; ~ery
chips. These particles were tested for particle size,
particle shape, apparent density, tap density, flow rate
through a st~n~rd funnel, ~;x~ng torque and angle of
repose. The data obtA;ne~ is shown in Table l.
Using three characterisation tests of flow time, tap
density and mixing torque, the particles were ranked
W094/06586 PCT/AU93/00454
4 3 4
according to performance (a ranking of "1" signifies the
best performance). The rankings are shown in Table 2.
WO 94/06~86 2 14 4 4 3 ~: PCI`/AU93/00454
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W094/06586 2 1~ 4 PCT/AU93/00454
At first glance, it appears that the spherical powder
provides the best performance in two of the three
categories. However, the powder seized between the screw
and the wall of the torque measuring device and it is
S likely that this will also occur in thixomolding apparatus.
Accordingly, the spherical powder is unsuitable as a
feedstock for th;xo~lding.
Once the spherical powder has been elim;n~ted as a
potential feedstock, it is apparent that the particulate
matter of the present invention is the most suitable for
use as a feedstock for th;x~- clding processes.
In order to ~emon~trate the advantages of the present
invention, a number of particles were prepared and compared
with particles that are not ~ncompassed by the present
invention.
The particles that fall within the scope of the
present invention have been denoted as "granule type #1"
and "granule type #2". The summary of the granule
dimensions is given in Table 3.
WO 94/06586 ~14 4 ~ 3 1 PCI`/AU93/00454
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W094/06586 ~ 3 ~ PCT/AU93/00454
Particle size analysis of granule type #1 and granule
type #2 was carried out and the results of this particle
size analysis, given as percentage frequency distribution
of aspect ratio, percentage frequency distribution of the
~;~?n~ion "length" and percentage frequency distribution of
the ~;m~n~ion "width" (diameter), for granule type #1 and
granule type #2, are shown in Figures 5 to 10. The
granules were produced from an A1 7% Si alloy.
Granule types #1 and #2 were found to be free flowing
as no ~;x;ng torque could be measured. In addition, the
granules transported easily along the barrel of the torque
measuring device. The granules were found to have an
apparent density of from 56-58~ of the theoretical apparent
density and a tap density of 69% of the theoretical tap
density.
For comparative purposes, samples of particles
comprising mainly needles were obtained. All of the
needles caused seizing of the screw during moulding screw
simulation. The apparent density of the needles ranged
from 39 to 45~ of the theoretical value and the top density
ranged from 50 to 59% of the theoretical value. The
needles were of a similar aluminium alloy as the granule
types #l and #2.
Several experiments with an A1 7% Si alloy were also
carried out in which the granule types #1 and #2 and the
needles were used to make a slurry of solid metal with
liquid metal. These trials simulated the formation of a
thixotropic alloy. The slurry was produced in a stirred
silicon carbide crucible. The stirrer had two flights of
blades. The procedure involved preheating a sufficient
amount of particles to 400C. The furnace temperature was
set at 590C, which is between the solidus and liquidus
temperatures for the aluminium alloy used in the particles.
The pre-heated particles were charged into the crucible
such that the second flight of the stirrer made contact
with the particles during stirring, although the particles
did not cover the second flight of blades at this stage.
The stirring speed was set at 100 rpm.
W094/06586 PCT/AU93/00454
Aluminium alloys are expected to be a difficult
feedstock for t~;xs~olding processes because at about
400C, aluminium-cont~ n; ng particles stick to each other.
This particle adhesion would tend to produce blockages in
the feed screw of a th; xo~olding apparatus.
The crucible tests to simulate the formation of a
thixotropic alloy showed that granule types #l and #2 both
produced a slurry without any difficulties. Observations
of the method were as follows:
. on initial and subsequent furnace charges, no evidence
of granule adhesion (i.e., b;~;ng together was not
apparent
after stirring for approximately 30-40 minutes the
onset of granule melting was obvious with the
formation of large, solid lumps of material
a decrease in the stirring efficiency was noticed as
material continuously built-up around the crucible
wall.
to increase stirring efficiency, stirring was
periodically stopped to allow material removal from
the crucible wall. In addition, if material build-up
was rapidly re-establ;ch~, a granule addition was
then carried out to facilitate build-up removal and
good m; x; ng
granule additions were also necessary due to a
reduction of material volume during melting.
With regard to the needles, some problems were
encountered in producing a slurry using needles. These
include:
evidence of needles binding together due to the 400C
W094/06~86 ~ PCT/AU93/00454
preheating stage. This observation was made during
- the initial and subsequent charges associated with the
trial
the bi n~; ng together of the needles was accentuated
S when the needles came in contact with the hot walls of
the crucible. On ~;x;ng, large lumps formed
~ ately causing the motor to labour. (Note:
stirring was stopped for ~ 15 minutes and the furnace
temperature increased to allow material "soft~n~ng".
. once the lumps had broken down, there were no problems
with ~;x;~g the material, except for material build-up
around the crucible wall.
In addition to the above difficulties, it is also
noted that the needles would tend to seize the screw of the
1~ th;xs~olding apparatus during feeding.
A mass of more needle-like particles, a sc~nn;ng
electron micrograph of which is shown in Figure 13, were
also subjected to a crucible test. These particles, which
had an average length of 2.8 mm and an average width of 0.8
mm (aspect ratio of 3.4) fall within the scope of the
present invention. Although the difficulties mentioned
above in respect of needles were present to some degree,
the particles of Figure 13 were able to ~orm useful
slurries and hence would be an acceptable feedstock for
tixs~olding. Seizing of the screw is likely to be less of
a problem with the particles of Figure 13 than with long,
thin needles having aspect ratios above 4.
The slurries obt~;nP~ using granule types #1 and #2
were allowed to solidify and photomicrographs were
subsequently taken. Figures 11 and 12 show
photomicrographs of the slurries obt~ine~ usiny granule
types 1 at 575C and 590C respectively. Figures 13 and 14
show similar photomicrographs for granule types 2. The
slurries were obt~ by heating the granules up from room
temperature to a temperature between the solidus and
W094/06~86 ~ 14 4 l 3 ~ PCT/AU93/004~
-- 19 --
liquidus of the alloy. The photomicrographs clearly show
solid particles surrounded by regions of solidified liquid.
A fair amount of porosity is also present, which is due to
the stirring arrangement used in the crucible experiments.
S The porosity is not expected to be present when a
thixomolding apparatus is used.
