Note: Descriptions are shown in the official language in which they were submitted.
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Title: Semi-Solid Metal Forming Process
Inventor: Gordon Woodhouse
FIELD Ol~ THE INVENTION
This invention relates generally to semi-solid metal forming and
more particularly to the formation and use of magnesium billets in semi
solid metal die casting and semi-solid forging processes.
BACKGR'.OUND
ME~tal die casting is a process in which molten metal is caused to flow
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
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
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 c=ycle time and the lower temperatures utilized result in decreased
die wear. Because of the absence of molten metal there is less pollution and
safety ha~:ards are reduced.
In semi-solid metal die casting, 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 successfully in semi-solid metal casting, the grain structure
of a billet: must exhibit the necessary degree of lubricity and viscosity to
give
good 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 optimized in order to produce successful parts.
Metal forging is another process in which metal is caused to flow into
a cavity defined by a mold. Unlike die casting, metal is not injected as a
liquid ini;o the cavity, but rather a solid billet or slug is placed between
dies
which are subsequently forced together to squeeze the billet or slug into the
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cavity as the die is closed. In semi-solid metal forging, the metal billet is
pre-
heated to a partially solid, partially liquid consistency prior to forging.
The
consisten~..~y is similar to that used for semi-solid metal die casting.
As in semi-solid metal die casting, the billet should consist of fine
grained equiaxed crystals rather than a dendritic structure to optimize the
flow of rr~etal between the dies and to optimize the physical characteristics
of
the finished parts.
An. earlier process for forming a treated billet involves the use of
magnetic 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.!i. patent no. 4,415,374 (Young et al) describes an alternate process
for forming a billet of aluminum for use in a semi-solid metal die casting
process. '.Coung et al describes a process having the following steps:
1. Melting and casting an ingot;
2. Cooling the ingot to room temperature;
3. Repeating 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. Repeating the ingot above its solidus temperature; and
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8. Forming and quenching the ingot.
The ingot produced according to the process described in Young may
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
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
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
process however involves an expensive and wasteful machining 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
for semi-solid metal die casting which avoids not only magnetic stirring, but
also eliminates many of the steps that would be required pursuant to the
Young process.
It is a further object of the present invention ro provide a semi-solid
metal die casting process which avoids the machining,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
that may be significantly greater than about one inch (approximately 25 mm)
in diameter.
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SUMMARY OF THE INVENTION
A semi-solid metal forming process using a cast billet and having the
following steps:
1. heating the cast billet to a temperature above its
recrystallization temperature and below its solidus
temperature;
2. extruding the cast billet in an extruded column;
3. cutting the extruded column into at least one billet;
4. heating the billet from step 3 to a forming temperature
corresponding to a semi-solid state; and
1.5 5. squeezing the billet from step 4 into a cavity in a metal
forming die set to form a part.
DESCRIPTION OF DRAWINGS
Preferred embodiments of the present invention are described below
with reference to the accompanying drawings in which:
Figure 1A is a schematic representation of the process of the present
invention;
Fi~~ure 1B is a schematic representation of an alternate embodiment
process according to the present invention;
Figures 2 through 30 are photomicrographs of billets cut from
extruded cast billets and are individually described in Example 1
below;
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Figure 31 illustrates sample locations in a test plate which were tested
in :Example 3;
Fig;ure 32 illustrates the locations at which photomicrographs were
taken in Example 3 below; and
Figures 33 through 36 are photomicrographs individually described in
Ex~~mple 3 below.
DESCRIPTION OF PREFERRED EMBODIMENTS
Re:Ferring to Figure 1, molten metal 10 is poured from a ladle into a
mold 12 and allowed to solidify into a cast billet 14. The cast billet 14 is
heated, for example by inductive heating coil I6 to a temperature above its
recrystallization temperature and below its solidus temperature.
The heated cast billet 14 is then extruded through an extruding die I8
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.
Alternatively, the heated billet 22 may be transferred to a forging
apparatus 40 where it is squeezed into a cavity defined between a movable
die 42 and a fixed die 44.
The present invention is further illustrated by the examples set out
below.
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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 cooled at a high chill rate utilizing copper molds and a
water spo.°ay to provide a chill rate of at least 2°C per second
at the billet
centre. T'he billets were cut into 2 ft. long sections and the diameter
machined down to 8 in. to remove imperfections to the outside edge.
Grain sizing of the 8 inch billet perpendicular to the extrusion axis
was 38 rr~icrons 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, E~4 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
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
remainder of the extrusion was cut into 7 in. sections or "slugs".
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
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
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using 2'I° nitol etchant. The micros were examined at various
magnific~~tions 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;
SOLUTION HEAT TREATMENT
Ramp 15(1C - 338C 3.0 hrs
Hold 338"C 0.1 hrs
Ramp 33fiC - 413C 1.5 hrs
Hold 413"C 0.5 hrs
Ramp 41~s°C - 426°C 0.5 hrs
Hold 426"C 12.0 hrs
Air Cool
(Furnace atmosphere 10% C02 to avoid ignition.;
The same procedure was followed in billet sectioning polishing and
etching a;s previously described with the "as extruded" billet sections.
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 9x00 x magnification of these samples.
The purpose for solution heat treating the extrusion billets and
analyzin~~ 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
equipment availability necessitated the use of convection heating rather
than induction heating. Preferably the heating cycle should not exceed 20
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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
taken are as follows:
Figure 2 is a photomicrograph of the outside edge of billet 1, as
extruded, at 200 x magnification.
Figure 3 is a photomicrograph of the outside edge of billet 1, as
extruded at 400 x magnification;
Figure 4 is a photomicrograph of the centre of billet 1, as extruded
under 10() x magnification;
2.0 Figure 5 is a photomicrograph of the centre of billet 1, as extruded
under 201) x magnification;
Figure 6 is a photomicrograph of the outside edge of billet 2, as
extruded,. at 200 x magnification;
Fi~;ure 7 is a photomicrograph of the outside edge of billet 2, as
extruded,. at 400 x magnification;
Figure 8 is a photomicrograph of the centre of billet 1, as extruded, at
400 x magnification;
Figure 9 is a photomicrograph of the centre of billet 2, as extruded, at
200 x magnification;
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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,
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 magnification;
Figure 16 is a photomicrograph of the centre of billet 1, extruded and
solution heat treated, at 200 x magnification;
Figure 17 is a photomicrograph of the outside edge of billet 2,
extruded and solution heat treated, at 50 x magnification;
Fil;ure 18 is a photomicrograph of the outside edge of billet 2,
extruded and solution heat treated, at 100 x magnification;
Fihure 19 is a photomicrograph of the outside edge of billet 2,
extruded and solution heat treated, at 200 x magnification;
Figure 20 is a photomicrograph of the centre of billet 2, extruded and
solution :heat treated, at 50 x magnification;
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Figure 21 is a photomicrograph of the centre of billet 2, extruded and
solution heat treated, at 100 x magnification;
Figure 22 is a photomicrograph of the centre of billet 2, extruded and
solution heat treated, at 200 x magnification;
Figure 23 is a photomicrograph of the centre of billet 1, as extruded,
parallel to the extrusion axis, at 100 x magnification;
Figure 24 is a photomicrograph of the centre of billet 1, as extruded,
parallel to the extrusion axis, at 200 x magnification;
Figure 25 is a photomicrograph of the centre of billet 2, as extruded,
parallel to the extrusion axis, at 100 x magnification;
Figure 26 is a photomicrograph of the centre of billet 2, as extruded,
parallel to the extrusion axis, at 200 x magnification;
Fi~;ure 27 is a photomicrograph of the centre of billet 1 parallel to the
extrusion axis, after solution heat treatment, at 100 x magnification;
Fi~;ure 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
extrusion axis, after solution heat treatment, at 100 x magnification;
Fi~;ure 30 is a photomicrograph of the centre of billet 2 parallel to the
extrusion axis, after solution heat treatment, at 200 x magnification;
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Grain Size Determination
As Extruded Billets
Billet 1 Outside Edge 10.2 microns
Billet 1 Centre 7.6 microns
Billet 2 Outside Edge 7.6 microns
Billet 2 Centre 7.6 microns
(Structure 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 Centre 20.3 microns
(W'ell defined solution heat treated grain structure)
DISCUSSION
The microstructure observed consists of magnesium primary
magnesium and aluminum solid solution crystals and eutectic consisting of
two phases, secondary magnesium solid solution crystals and Mgl~All2
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
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
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corresponding micros taken from the heat treated specimens showed a more
evenly distributed recrystallized 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
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.
SSJVI 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
software issues.
As a test piece, a welding test plate die was chosen, heated by oil to
approxirr~ately 220°C.
In general, the material was SSM-castable, but different than other
magnesium alloys. The thickwall part (l0mm thick) was perhaps not ideal
for magnesium casting.
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SSM HE~~TING
Slug heating was performed in a single coil induction heater and
optimized such that the slugs were removed from the coil just prior to the
onset of burning which corresponded to a softness which allowed dissection
with a knife. Total heating time was approximately 230 seconds. Very little
metal rur~-off was obtained during the heating 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 CA 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 flashing), the parts filled
better but lapping was still apparent. The best results were obtained using a
plunger velocity of 1.2 m/s.
The heat treated slugs appeared lighter in color after heating and had
less tendency to burn. The SSM parts produced from 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".
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T~;~o 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
will not be laminar. Too low a gate speed results in metal solidification
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.
2:0 The casting machine was a single cylinder unit having servo control
to carefuilly 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 be removed from the
virgin metal.
~5
EXAMPLE 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
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sections .and tested for mechanical properties. The plates were not heat
treated arid the results are tabulated in Table 1 below.
TABLE 1
PLATE SAMPLE SAMPLE TYPE UTS YS ELONG
NO. NO. (ksi) (ksi) %
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
35 2 .250" ROUND 33.6 14.7 12.3
35 4 .250" ROUND 31.1 13.9 10.3
35 6 .250" ROUND 33.3 13.9 13.3
Plates 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. T:he results of the tests are tabulated in Table 2 below. In Table 2
below the sample plan for the heat treated plates is the same as illustrated
in
1.5 Figure 31.
TABLE 2
PLATE SAMPLE SAMPLE TYPE UTS YS ELONG COMMENTS
NO. NO. (ksi) (ksi) %
34 1 .250" ROUND 23.4 14.1 3.0 OXIDE INCL.
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34 3 .250" ROUND SAMPLE
DAMAGED
IN
MACHINING
34 5 .250" ROUND 37.6 14.6 18.5
35 1 .250" ROUND 37.0 12.8 15.7
35 3 .250" ROUND 36.9 13.8 16.4
35 5 .250" ROUND 36.8 12.8 19.3
Photomicrographs of one of the plates were taken at locations M1 and
M2 as illustrated in Figure 32. The photomicrographs are reproduced in
Figures 3;3 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
the present invention to particular alloys, equipment and parts being cast.
For exaxrtple, although AZ61 magesium alloy was utilized in the tests no
2.0 doubt other magesium alloys could be used. The process can also be adapted
to metal systems other than magesium where the metal is capable of
forming a two-phase system comprising a solid particles in a lower melting
matrix. 7.'he process will work with aluminum and may also work with
other similar metal systems such as 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 spirit and scope of the claims set out below.
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PrE~ferably heating of the billet 22 prior to forming should be carried
out at a rate of no greater than 30°C per second and even more
preferably at
a rate of :no greater than 20°C per second if aluminum is being used.
Heating
at a rate greater than 30°C per second may result in the precipitation
of
silicon from the resulting stresses thereby deleteriously affecting
machinability of the finished part. It has been found that a three stage
induction heater is particularly well suited to maintaining a desirable
heating rate.
