Note: Descriptions are shown in the official language in which they were submitted.
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Title: Semi-Solid Metal Casting Process
Inventor: Gordon Woodhouse
FIELD OF THE INVENTION
This invention relates generally to semi-solid metal casting and more
particularly to the formation and use of magnesium billets in semi-solid
10 metal casting processes.
BACKGROUND
Metal die casting is a process in which molten metal is caused to flow
15 into a cavity defined by a mold. In conventional metal die casting, molten
metal is injected into the cavity. In semi-solid metal die casting processes, a
metal billet is pre-heated to a point of softening, to a temperature above the
solidus and below the liquidus to produce a partially solid, partially liquid
consistency prior to placing the billet or "slug" in a shot sleeve in the casting
20 machine.
Semi-solid metal die casting enables control of the microstructure
of the finished part to a degree which produces a stronger part than is
possible with conventional molten metal die-casting processes. As
25 compared with conventional metal die-casting processes, semi-solid metal
casting produces parts of improved casting quality in that they exhibit lower
porosity, parts shrink less upon cooling enabling closer tolerances and
physical properties are better. In addition, semi-solid metal casting has a
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reduced cycle time and the lower temperatures utilized result in decreased
die wear. Because of the absence of molten metal there is less pollution and
safety hazards are reduced.
In semi-solid metal die c~ting~ a billet is first formed which is treated
to form fine grained equiaxed crystals as opposed to a dentritic structure.
Subsequent heating, forming and solidification of a formed part using a
treated billet avoids the formation of a dentritic structure in the finished
part.
To work sl-ccessfully in semi-solid metal casting, the grain structure
of a billet must exhibit the necessary degree of lubricity and viscosity to givegood laminar flow in the die cavity. For example an untreated DC cast billet
will shear along its dentritic axis rather than flow hence the need for fine
grained equiaxed crystals.
Flowability is further affected by grain size and solid/liquid ratio. In
addition forming parameters such as die temperatures and gate velocity will
affect the casting process. Accordingly, all of the foregoing parameters have
to be optimi7eC~ in order to produce successful parts.
The billets were cooled at a high chill rate lltili7ing copper molds and
a water spray to provide a chill rate of at least 2~C per second at the billet
centre.
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An earlier process for forming a treated billet involves the use of
m~gnetic stirring during the cooling of a cast billet to break up and avoid the
formation of a dentritic structure. Magnetic stirring is however a relatively
slow and expensive process.
U.S. patent no. 4,415,374 (Young et al) des~ibes an alternate process
for forming a billet of aluminum for use in a semi-solid metal die casting
process. Young et al describes a process having the following steps:
1. Melting and casting an ingot;
2. Cooling the ingot to room temperature;
3. Reheating the ingot above its recrystallization
temperature but below its solidus temperature;
4. Extruding the ingot;
5. Cooling the ingot to room temperature;
6. Cold working the ingot;
7. Reheating the ingot above its solidus temperature; and
8. Forming and quenching the ingot.
The ingot produced according to the process described in Young may
20 then be subsequently heated to semi-solid casting temperature and formed
into a part in a die casting process.
Even though Young avoids the requirement for magnetic stirring, it
is nevertheless a cumbersome process including a large number of process
25 steps.
More recently a process has been proposed in which a cast ingot is
machined down to a billet of approximately one inch in diameter and
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deformed by subjection to a compressive force. The deformed billet is then
heated to a temperature above its recrystallization temperature and below its
solidus temperature. The billet is then cooled to room temperature for
subsequent re-heating and use in a semi-solid metal casting process. This
5 process however involves an expensive and wasteful ma~hining operation
and only appears to work with relatively small billet diameters of less than
about one inch (approximately 25 mm) diameter.
It is therefore an object of the present invention to provide a process
10 for semi-solid metal die casting which avoids not only magnetic stirring, butalso eliminates many of the steps that would be required pursuant to the
Young process.
It is a further object of the present invention to provide a semi-solid
15 metal die casting process which avoids the maching, cold working heating,
cooling and re-heating steps associated with other processes.
It is yet a further object of the present invention to provide a process
capable of forming billets for use in semi-solid metal die casting processes
20 that may be significantly greater than about one inch (approximately 25 mm)
in diameter.
SUMMARY OF THE INVENTION
A semi-solid metal die casting process using a cast ingot and having
the following steps:
1. heating the ingot to a temperature above its
recrystallization temperature and below its solidus
temperature;
2. extruding the ingot in an extruded column;
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3. cutting the extruded column into at least one billet prior
to its cooling;
4. heating the billet from step 3 to a forming temperature
corresponding to a semi-solid state; and
5. squeezing the billet from step 4 into a cavity in a die
casting mold to form a part.
DESCRIPTION OF DRAWINGS
Plefelled embodin~ents of the present invention are described below
with rerelel.ce to the accompanying drawings in which:
Figure 1 is a schematic representation of the process of the present
invention;
Figures 2 through 30 are photomicrographs of billets cut from
extruded ingots and are individually described in Example 1 below;
Figure 31 illustrates sample locations in a test plate which were tested
in Example 3;
Figure 32 illustrates the locations at which photomicrographs were
taken in Example 3 below; and
Figures 33 through 36 are photomicrographs individually described in
Example 3 below.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Figure 1, molten metal 10 is poured from a ladle into a
mold 12 and allowed to solidify into an ingot 14. The ingot 14 is heated, for
example by inductive heating coil 16 to a temperature above its
recrystallization temperature and below its solidus temperature.
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The heated ingot 14 is then extruded through an extruding die 18 to
form an extruded column 20. The extruded column 20 is cut to a suitable
length billet 22 for use in a semi-solid metal die casting process.
The billet 22 is heated to a forming temperature corresponding to a
semi-solid state, for example by induction coils 24, and transferred to a die
casting apparatus 26. The heated billet 22 is squeezed by the die casting
apparatus into a cavity 28 between mold parts 30 and 32 to form a part 34
conforming in shape to that of the cavity 28.
The present invention is further illustrated by the examples set out
below.
EXAMPLE 1
The microstructure of two AZ61 alloy, 3 in. diameter by 7 in. length
extruded billets in the as extruded and solution heat treated condition were
examined.
The billets were produced initially as an 8 1/2 in. direct chill cast billet.
The billets were cut into 2 ft. long sections and the diameter machined down
to 8 in. to remove imperfections to the outside edge.
Graining sizing of the 8 inch billet perpendicular to the extrusion axis
was 38 microns at the outside, 48 microns at the half radius and 48 microns
at the center. As expected, the grain size in the longitudinal or extrusion
direction was somewhat larger being approximately 51 microns at the
outside, 64 microns at the half radius and 74 microns at the center.
The billets were then heated in 4-6 minute intervals in three
induction furnaces. The furnaces heated the billets to 100~C, 200~C, 300~C
(total heating time approximately 15 minutes.) The billet was then placed in
the extrusion chamber, which was at 380~C and the billet was extruded at
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between 330~C and 350~C, in one stage down to a 3 in. diameter extrusion
billet. The first 14 ft. of extrusion and the last few feet were discarded. The
r~m~inc~er of the extrusion was cut into 7 in. sections or "slugs".
5 PROCEDURE
Two of the sections of the extrusion billet referred to as billet 1 and
billet 2, in AZ61 alloy were examined in the "as extruded" condition by
sectioning a 0.5 in. section off the end of each billet, (billets were randomly
10 selected.) A micro was taken perpendicular to the axis of the billet from the centre and from the outside edge. The micros were polished and etched
using 2% nitol etchant. The micros were examined at various
magnifications to observe grain structure. A photomicrograph was taken at
each magnification and the grain size estimated.
The two extrusion billet sections were then given the following
solution heat treatment to recrystallize the grain structure;
20 SOLUTION HEAT TREATMENT
Ramp 150~C - 338~C 3.0 hrs
Hold 338~C 0.1 hrs
Ramp 338~C - 413~C 1.5 hrs
Hold 413~C 0.5 hrs
Ramp 413~C - 426~C 0.5 hrs
Hold 426~C 12.0 hrs
Air Cool
(Furnace atmosphere 10% CO2 to avoid ignition.;
The same procedure was followed in billet sectioning polishing and
etching as previously described with the "as extruded" billet sections.
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From the same samples micros were made at the centre of each billet
parallel to the extrusion axis. These micros were taken from the as extruded
and the solution heat treated billets. Photo micrographs were made at from
100 x to 400 x magnifi~ion of these samples.
The purpose for solution heat treating the extrusion billets and
analyzing the samples was to determine the effect on grain size and shape
resulting from heating and extruding the DC cast billet. The solution heat
treating was not carried out under the optimum circumstances as
10 equipment availability necessitated the use of convection heating rather
than induction heating. Preferably the heating cycle should not exceed 20
minutes and accordingly multi-state induction heating would be preferable
over convection heating. Nevertheless the results were quite favourable as
set out below.
RESULTS
The photomicrographs which are set out in Figures 2 through 30 below were
20 taken are as follows:
Figure 2 is a photomicrograph of the outside edge of billet 1, as
extruded, at 200 x magnification.
25Figure 3 is a photomicrograph of the outside edge of billet 1, as
extruded at 400 x m~gnific~tion;
Figure 4 is a photomicrograph of the centre of billet 1, as extruded
under 100 x magnification;
Figure 5 is a photomicrograph of the centre of billet 1, as extruded
under 200 x magnification;
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Figure 6 is a photomicrograph of the outside edge of billet 2, as
extruded, at 200 x magnification;
Figure 7 is a photomicrograph of the outside edge of billet 2, as
5 extruded, at 400 x magnification;
Figure 8 is a photomicrograph of the centre of billet 1, as extruded, at
400 x m~gnifir~tion;
Figure 9 is a photomicrograph of the centre of billet 2, as extruded, at
200 x m~gnification;
Figure 10 is a photomicrograph of the centre of billet 2, as extruded, at
400 x magnification;
Figure 11 is a photomicrograph of the outside edge of billet 1,
extruded and solution heat treated, at 50 x magnification;
Figure 12 is a photomicrograph of the outside edge of billet 1,
20 extruded and solution heat treated, at 100 x magnification;
Figure 13 is a photomicrograph of the outside edge of billet 1,
extruded and solution heat treated, at 200 x magnification;
Figure 14 is a photomicrograph of the centre of billet 1, extruded and
solution heat treated at 50 x magnification;
Figure 15 is a photomicrograph of the centre of billet 1, extruded and
solution heat treated at 100 x m~gnifir~tion;
Figure 16 is a photomicrograph of the centre of billet 1, extruded and
solution heat treated, at 200 x m~nification;
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Figure 17 is a photomicrograph of the outside edge of billet 2,
exkuded and solution heat treated, at 50 x magnification;
Figure 18 is a photomicrograph of the outside edge of billet 2,
5 exkuded and solution heat keated, at 100 x magnification;
Figure 19 is a photomicrograph of the outside edge of billet 2,
exkuded and solution heat keated~ at 200 x magnification;
10Figure 20 is a photomicrograph of the cenke of billet 2, exkuded and
solution heat keated, at 50 x m~gnific~tion;
Figure 21 is a photomicrograph of the cenke of billet 2, exkuded and
solution heat keated, at 100 x m~gnification;
Figure 22 is a photomicrograph of the centre of billet 2, exkuded and
solution heat keated, at 200 x m~gnification;
Figure 23 is a photomicrograph of the cenke of billet 1, as exkuded,
20 parallel to the extrusion axis, at 100 x m~gnification;
Figure 24 is a photomicrograph of the cenke of billet 1, as extruded,
parallel to the exkusion axis, at 200 x m~gnification;
25Figure 25 is a photomicrograph of the centre of billet 2, as exkuded~
parallel to the extrusion axis, at 100 x m~gnification;
Figure 26 is a photomicrograph of the centre of billet 2, as extruded,
parallel to the exkusion axis, at 200 x magnification;
Figure 27 is a photomicrograph of the centre of billet 1 parallel to the
extrusion axis, after solution heat tre~tment, at 100 x magnification;
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Figure 28 is a photomicrograph of the centre of billet 1 parallel to the
extrusion axis, after solution heat treatment, at 200 x magnification;
Figure 29 is a photomicrograph of the centre of billet 2 parallel to the
5 exkusion axis, after solution heat treatment, at 100 x magniffcation;
Figure 30 is a photomicrograph of the cenke of billet 2 parallel to the
exkusion axis, after solution heat treatment, at 200 x magnification;
Grain Size Determin~t;on
As Exkruded Billets
Billet 1 Outside Edge 10.2 microns
Billet 1 Cenke 7.6 microns
Billet 2 Outside Edge 7.6 microns
Billet 2 Cenke 7.6 microns
(Skucture is quite broken up with very large and very small grains.)
Solution Heat Treated Billets
Billet 1 Outside Edge 25.3 microns
Billet 1 Centre 22.5 microns
Billet 2 Outside Edge 22.5 microns
Billet 2 Cenke 20.3 microns
(Well ~l~fine~1 solution heat keated grain skucture)
DISCUSSION
The microstructure observed consists of magnesium primary
magnesium and alllminllnl solid solution crystals and eutectic consisting of
two phases, secondary magnesium solid solution crystals and Mgl7Al12
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intermetallic compound. The structure was quite broken up in the "as cast"
specimens and grain size measurement is only approximate.
Recrystallized grain structure in the solution heat treated specimens
5 was more accurate and well defined in the microstructure.
The micros taken in the direction of the extrusion axis of the "as
extruded" specimens showed long stringers in the microstructure. The
corresponding micros taken from the heat treated specimens showed a more
10 evenly distributed recryst~lli7.e-1 structure.
The amount of breakdown that the grain structure of the as-cast billet
will undergo is likely a function of the amount of reduction. In the present
case 7 to 1 reduction was used. Some sources suggest that the optimum
15 degree of reduction should be on the order of from 10:1 to 17:1. In practice
however the degree of reduction required may be less if the starting alloy is
relatively fine grained.
EXAMPLE 2
OVERVIEW
3 in. diameter x 180 mm long slugs of magnesium alloy AZ61 were
tested.
10 of the slugs had been solution heat treated.
SSM casting tests were made using a Buhler SCN66 machine. It was
not possible at the time of the trials to store the injection curves due to
30 software issues.
As a test piece, a welding test plate die was chosen, he~te-l by oil to
approximately 220~C.
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In general, the material was SSM-castable, but different than other
m~gnesium alloys. The thickwall part (lOmm thick) was perhaps not ideal
for magnesium casting.
SSM HEATING
Slug heating was performed in a single coil induction heater and
10 optimized such that the slugs were removed from the coil just prior to the
onset of burning which co~les~onded to a softness which allowed dissection
with a knife. Total heating time was approximately 230 seconds. Very little
metal run-off was obtained during the he~ting process.
A single stage induction heater was utilized for the test as multi-stage
induction heating was not available at the test facility. It is expected that
better heating would have been obtained with multi-stage induction
heating. Ideally at the end of the heating cycle the billet should have a
uniform temperature throughout with a well controlled solid to liquid
ratio.
SSM C~STING
The first parts were cast using a plunger velocity of 0.3 to 0.8 meters
per second. These conditions barely filled the die and visual laps were
apparent at the end of the part.
With a velocity increase to 1.8m/s (onset of fl~hing), the parts filled
better but lapping was still apparent. The best results were obtained using a
plunger velocity of 1.2 m/s.
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The heat treated slugs appeared lighter in color after heating and had
less tendency to burn. The SSM parts produced *om these slugs also
appeared lighter in color.
Even at plunge velocities as low as 0.05 m/s and up to above 0.5 m/s,
it was not possible to achieve a smooth metal front. In all cases the alloy
flowed as individual "glaciers".
Two plates (numbers 34 and 35) which were formed at a plunger
velocity of 1.8 m/s were subjected to metallurgical evaluation (see Example
3).
As can be seen, the only parameter varied in making the test plates
was the gate or plunger velocity. Accordingly none of the resulting plates
could be considered high quality castings. It is expected that much better
results would have been obtained if the die temperature had been increased
to approximately 300~C and the slugs were heated in the multi-stage
induction heater.
As illustrated by the tests, if the gate speed is too high, the metal flow
is atomized rather than l~minar. Too low a gate speed results in metal
solillification before the mold cavity fills.
Despite the less than optimal casting conditions, as illustrated by
example 3 below, the cast plates show good physical properties.
The casting machine was a single cylinder unit having servo control
to carfully control the force driving the slug into the closed die. Optimally
the casting process will cause the outer skin of the slug which contains
surface oxides resulting from the heating process to "skin" from the virgin
metal.
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EXAMPT.~ 3
Plates 34 and 35 were sectioned into six sections as illustrated in
Figure 30. One quarter inch (1/4 in.) round samples were removed from the
sections and tested for mechanical properties. The plates were not heat
treated and the results are tabulated in Table 1 below.
TABLE 1
PLATE SAMPLE SAMPLE TYPE UTS YS ELONG
NO. NO. ~ksi) ~si) %
34 2 .250" ROUND 31.5 13.9 10.9
34 4 .250" ROUND 33.2 14.2 14.1
34 6 .250" ROUND 32.9 14.5 13.6
2 .250" ROUND 33.6 14.7 12.3
4 .250" ROUND 31.1 13.9 10.3
6 .250" ROUND 33.3 13.9 13.3
Mates 34 and 35 were subsequently solution heat treated for 12 hours
at 426~C and still air cooled. One quarter inch (1/4 in.) round samples were
cut from the plates and the mechanical properties of those samples were
tested. The results of the tests are tabulated in Table 2 below. In Table 2
20 below the sample plan for the heat treated plates is the same as illustrated in
Figure 31.
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TABLE 2
PLATE SAMPLE SAMPLE TYPE UTS YS ELONG COMMENTS
NO. NO. (ksi) (ksi) %
34 1 .250" ROUND 23.4 14.1 3.0 O~aDEINCL.
34 3 .250" ROUND SAMPLE DAMAGED IN MACHINING
34 5 .250" ROUND 37.6 14.6 18.5
1 .250" ROUND 37.0 12.8 15.7
3 .250" ROUND 36.9 13.8 16.4
.250" ROUND 36.8 12.8 19.3
Photomicrographs of one of the plates were taken at locations M1 and
5 M2 as illustrated in Figure 32. The photomicrographs are reproduced in
Figures 33 through 36 as follows.:
Figure 33 is a photomicrograph of sample M1 at 50x magnification;
Figure 34 is a photomicrograph of sample M1 at 100x magnification;
Figure 35 is a photomicrograph of sample M2 at 50x magnification;
Figure 36 is a photomicrograph of sample M2 at 100x magnification.
The above description is intended in an illustrative rather than a
restrictive sense. One skilled in the art would recognize that the specific
process perameters used in the examples would have to be varied to adapt
20 the present invention to particular alloys, equipment and parts being cast.
For example, although AZ61 magesium alloy was ulilized in the tests no
doubt other magesium alloys could be used. It is quite likely that the process
could be adaped to metal systems other than magesium so long as the metal
is capable of forming a two-phase system comprising a solid particles in a
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lower melting matrix. Examples of such metal systems include aluminum
and copper. It is intended that any such variations be deemed as within the
scope of the present patent as long as such are within the spirt and scope of
the claims set out below.
