Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROMAGNETIC PUMP
This invention relates to an electromagnetic
pump and more particularly to an electromagnetic pump
useful in low pressure permanent mold casting of molten
metals. The electromagnetic pump is advantageously used
for casting molten metals such as magnesium, magnesium
alloys, and magnesium composites.
Electromagnetic (EM) pumps are known to be
used, for exa~ple, in aluminum and magnesium processes
and in the pumping of reactor coolants in the nuclear
industries. None of the EM pumps of the prior art have
been successfully used in a low pressure permanent mold
(LPPM) process. The electromagnetic pumps of the prior
art also have the disadvantage of requiring external
cooling.
An EM pump is described in U.S. Patent No.
4,828,459, entitled "Annular Linear Induction Pump With
An E~ternally Supported Duct", filed by H.C. Behrens,
December 16, 1987. The electromagnetic pump of the
present invention is an improved pump over the pump
described in the U.S. Patent No. 4,828,459.
It is desired to provide an electromagnetic
pump which (l) has the ability to operate at molten
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magnesium temperatures without auxiliary cooling; (2)
has the ability to feed low pressure permanent mold die
casting machines; (3) offers flexibility in design size
for a wide range of casting machines; (4) provides a
wide control range for maximum versatility in utilizing
different mold configurations; and (5) incorporates into
the pump an electrically heated stand-pipe which will
keep the metal heated in the idle position just below
the point of introduction into a mold.
The present invention is directed at an
electromagnetic pump comprising a housing containing a
first set of coils for mold filling and a second set of
coils for mold filling, each of said first and second
set of coils being separately and independently
connected to a power supply and a control system, said
second set of coils being adapted for holding molten
metal at a predetermined level.
Figure 1 is a schematic view showing one
embodiment of an apparatus of the present invention for
casting billets or ingots.
Figure 2 is a front view of a billet apparatus
of the present invention for use in the process of the
present invention.
Figure 3 shows a partial cross-sectional view
of the billet apparatus of Figure 2.
Figure 4 is a top view of the billet apparatus
of Figure 2.
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Figure 5 is a side view showing one embodiment
of an electromagnetic pump for use in the process of the
present invention.
Figure 6 is a partial front view of the
electromagnetic pump of Figure 5.
Figure 7 is a partlal cross-sectional view of
the electromagnetic pump of ~igure 5.
Figure 8 is a top view of Figure 5.
Figure 9 is a cross-sectional view taken along
line 10-lO of Figure 5.
Figure 10 is a cross-sectional view taken along
line lO-lO of Figure 5.
The present invention resides in an apparatus
and process for producing LPPM castings from molten
materials including molten metal, alloys and/or
composites. An "LPPM casting" herein means low pressure
permanent mold casting. The pressures used in the
present invention are from 2 to 30 (13.8 to 207 kPa).
A single stage machine, i.e. a machine which is
capable of producing one piece or part at one time, is
used to produce a finished cast part. A finished cast
product, for example a billet, produced by the process
and apparatus of the present invention can be shipped
3 "as-is" when it is removed from the machine. The
product is useful, for example, in a remelt or extrusion
process. The process and apparatus of the present
invention provides/ for example, a sound, clean billet
with fine equiaxed grains.
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As an illustration of the billets produced by
the present invention, billets greater than 95 percent
dense have been produced. Billets having a porosity of
less than 4 percent are preferred. The billets should
contain a minimum, if any, amount of large nonmetallic
inclusions (NMI). sillets containing zero NMI per
square inch (6.45 cm2) of a size of greater than 0.020
inches (0.51 mm) in diameter have been produced. The
NMI count in a billet is measured by standard methods
such as optical microscopy of fractured surfaces. It is
preferable that the billet contain fine (about 0.10
inches in diameter or less) equiaxed grains throughout
the structure of the billet. The above billets are
characterized as being good quality billets.
A variety of well known mold shapes can be used
in the present invention such as billets, wheels,
ingots, T-bars and the like. Sand molds may also be
used in the present invention.
The yield of the operation herein depends on
the size of the casting desired. Clearly, it is desired
to produce a casting as efficiently and quickly as
possible. Generally, castings may be produced at a rate
of from ] per minute to 1 per 20 minutes and preferably
at least one casting per 10 minutes.
An advantage of the process and apparatus of
the present invention is that it provides a means for
casting molten metals such as magnesium, magnesium
alloys, and magnesium composites.
Any magnesium or magnesium base alloy may be
used in the present invention. For example, those
containing various amounts of Al, Zn, Mn, rare earth
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metals, Zr, Ag, Y, Th, and the lilce, can be used.
Alloys of magnesium such as AZ91, AZ31, EZ33, ZK60, AM
60 and other alloys listed in American Society for
Testing and Materials (ASTM) B80-1987, page 34 are
useful for processing in accordance with the present
invention. All commercial and experimental alloys are
useful in the present invention. Some examples of
magnesium composites include AZ91 reinforced with 20
volume percent 600 grit SiC particulate; a magnesium-6
0 wt percent zinc alloy reinforced with 20 volume percent
1000 grit SiC particulate and all commercial and
experimental magnesium alloys reinforced with 1 to 30
volume percent SiC, Al203 or B4C of 1 to 50 microns in
particle size.
Cooling means used in the present invention
include any means which will provide "directional
solidification" such as air, H2O, glycol and the like.
The molten material is cooled down to a temperature
substantially below the molten material's solidification
temperature before removing the product from the
machine. Of course the solidification temperature and
the cooling temperature used depends on the molten
material used. The solidfication temperature of
magnesium is 650~C. For example, when casting a
magnesium part, the casting part is cooled to a
temperature of from 200 to 400C to remove the part
from the mold.
3o
The solidification time in the present
invention is substantially influenced by part size, mold
design and type of metal used in casting. Generally,
the time of solidification of a cast part is from 2 to 4
minutes.
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The materials of construction of the apparatus
of the present invention are those used for machines
such as mild steel, cast steel, stainless steel, or a
high carbon steel such as C4140. Generally, materials
should not be reactive to the molten material and stable
at process conditions. For example, the mold parts
should be made of hard steel because the parts may be
subject to thermal shock.
"Sprue" means the entry part to the mold
cavity. In low pressure permanent mold casting, metal
may freeze within the sprue that feeds the mold. To
minimize freezing in the sprue, the sprue may be heated
by any conventional means such as by electrical heating.
Eliminating freezing within the sprue aids in increasing
the production of billets. For example, once freezing
within the sprue is eliminated, billets may be produced
at a rate of about one billet every 3 to lO minutes.
~ith particular reference to Figure l, there is
shown a casting apparatus, generally indicated by
reference numeral 10, for producing billets and/or
ingots from molten metal. Tlle casting apparatus lO
includes a billet mold machine, generally indicated by
reference numeral ll, an electromagnetic pump, generally
indicated by reference numeral 12 and a crucible 13 with
molten material 14 therein.
The billet mold machine 11, more clearly shown
in Figures 2-4, comprises two mold halves 21 and 22
diametrically opposed for forming a mold cavity 23 when
the halves are contacting each other in mirror-like
fashion. Thermocouples 24 are placed in the top, middle
and bottom of each of the halves to monitor the
temperature in the mold halves. The top thermocouple is
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preferably positioned so that it extends through the
mold half 22 to be in direct contact with the molten
metal to indicate whether the mold is full with molten
metal. The molds are slidably mounted on slide members.
The slide mem~ers are, in turn, mounted on a support
base plate 25 for slidably moving the halves to enable
one to oper. or close the mold halves and position the
halves on plate 25. The molds are connected by support
structure 26 to an actuating means 27 such as hydraulic
jacks for opening or closing the mold halves.
Cooling medium ports 28 in the top of mold are
used to introduce a cooling medium through conduits 28b
into the top of mold and cooling medium ports 29 in the
bottom of the mold are used to exit the cooling medium
such as air from the molds. The cooling medium
circulates through the inside of the mold halves 21 and
22 through cooling medium channels 28a.
The cooling medium for cooling the molten metal
in cavity 23 ~rom top to the bottom may include, for
example, air, a cooling liquid such as glycol, and the
like. Sprue 30 may also contain cooling medium inlets
(not shown) for introducing a cooling medium such as air
to the sprue to cool the metal at the inlet 33 at the
bottom of the billet machine.
The electromagnetic (EM) pump 12, is more
clearly shown in Figures 5-lO, and comprises an encased,
insulated and heated pipe which will deliver and hold
molten metal. The pump is used for feeding molten metal
to the mold ll and for maintaining the mold filled with
molten metal during the casting operation.
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More specifically, the electromagnetic pump 12
comprises an electrical box 40 with phased plug
receptical. Electrical conduits 41 and 42 connect the
electric box 40 to pump windings. A housing 43 contains
a 12 coil 240 VAC core assembly, indicated generally as
section A of the pump 12, and a 6 coil 110 VAC assembly,
indicated generally as section B of the pump 12, and a
heated stand pipe 44 for holding molten metal just below
a point of mold entry. Lifting ear lugs 45 are used to
lift and position the pump on a support platform to be
held by flange 46. Hanger brackets 47 can also be used
to support the pump 12 in position above a crucible 13.
A conduit inlet 48 is used for connecting to a heater
element 49 for heating the stand pipe 44 as shown in
Figures 7 and 9. An insulation layer 50 of any
conventional insulating material is placed around the
pipe 44. An inert gas padding may be introduced through
inlet 51 in box 40 for protecting the electrical and
core systems of the pump from oxidation and/or
deterioration at elevated operating temperatures.
In Figure 10, there is shown the housing 43
enclosing coils 52 and cores 53 around tube 55. A core
rod 55 is centered in tube 54 to form an annulus 56 for
pumping molten material therethrough to stand pipe 44.
In carrying out one embodiment of the present
process, a low pressure casting of a round billet
(cylindrically shaped body) is carried out using a
single mold billet machine substantially shown in Figure
1 as follows:
The mold and all tools are preheated before use
to above about 100C. A melt furnace pot is used and
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can be protected by a flux suitable for use with molten
magnesium or SF6 gas.
A 12 coil EM pump as substantially shown in
Figures 5-lO, operating off 240v powe. supply is
preferably used for feeding molten magnesium into the
mold. The EM pump is attached to the bottom of the
billet machine by a hanger assembly which allows the
molten metal to fill the mold. A pan with a hole in the
center is placed on the pump. The pump is turned on and
a visual inspection of the metal flow and volume is
noted for the filling speed and complete mold fill. All
working parts are cycled to insure correct response.
The pot is set at a temperature of about 690C.
A pan test is run to make sure all electrical components
are working as needed. The billet mold is preheated and
can be purged with a gas such as SF6, argon, CO2 and the
like prior to placement on pump. The mold is coated
with any compatible mold coating such as a spinel for
isolating the mold walls from the molten metal and for
preventing the molten metal from wetting the mold walls.
A mold release coating such as graphite spray is also
preferably applied to the mold walls at about 400C.
When the pump and the mold are ready for
operaticn a pumping rate, predetermined from the pan
test, will be applied to the mold. If a thermocouple in
the top of the mold does not read a full mold then more
power will be applied to the EM pump until a reading
shows that the mold is full.
For the first few mold fills, only enough
pressure will be applied to fill the mold. After
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looking at the finished billet a decision may be made to
apply additional pressure.
When it has been determined that a full mold
exists or that only a certain amount of metal can be
introduced into the mold, the pump remains energized for
a predetermined amount of time, for example, for about
three minutes. The pump is then deenergized and the
mold is left to cool for a period which will be
determined at that time for example from previously
prepared temperature charts.
The air flow rates and/or water flow rates for
cooling the molten metal used in the present invention
should be sufficient to provide the necessary cooling of
the cast part. The rates are measured and can be
controlled at a desired range depending on cooling rate
desired. Generally, for example, the water flow rates
range from 0.5 to 3 gal/min (1.89 to 11.4 lit/min) to
provide the necessary cooling.
Care must be given when opening the mold to
make sure no molten metal still exists in the mold. The
billet is inspected for quality. The next injection
will contain the previous billets data for run
parameters.
The machine as substantially shown in Figure 1
is a single mold billet machine and is very similar to a
3 low pressure die casting machine in that molten metal
fills the mold through a fill hole located in the bottom
center of the vertical mold. Magnesium metal fills the
mold at about 700C and is cooled, for example, using
air injected through cooling ports in the top of the
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mold and through the cooling area at the introduction
point of the mold.
Example 1
An apparatus substantially as shown in Figure 1
was used to prepare billets of a magnesium composite
alloy AZ31s.
Ingots of AZ31B were melted in a 1500 pound
(675 kg) steel crucible. A steel billet mold
substantially as shown in Figures 2-4, with a mold
cavity measuring 7.25 in (18.4 cm) in diameter by 25 in
(63.5 cm) long, was placed directly over the crucible.
Next, molten composite was pumped into the steel billet
mold with an electromagnetic pump. Six billets weighing
70 pounds (31.5 kg) each, were produced at a rated of
about one every nine minutes.
Billets of high quality were produced, i.e.,
the billets had reduced levels of oxide inclusions and
voids and a smooth surface finish. Billets were made to
a set shape and size without the necessary of risers.
The six low pressure permanent mold cast
composite billets were x-rayed and the results showed
that the billets had a minimal amount of internal
porosity. Some of the billets were extruded into a
2-1/4 inch (57 mm) round rod with excellent surface
quality, no porosity and very fine (i.e., 8-1~ microns)
grain size.
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