U.S. patent application number 16/616648 was filed with the patent office on 2021-06-03 for electromagnetic modified metal casting process.
The applicant listed for this patent is PYROTEK, INC.. Invention is credited to Robert FRITZSCH.
Application Number | 20210162491 16/616648 |
Document ID | / |
Family ID | 1000005415758 |
Filed Date | 2021-06-03 |
United States Patent
Application |
20210162491 |
Kind Code |
A1 |
FRITZSCH; Robert |
June 3, 2021 |
ELECTROMAGNETIC MODIFIED METAL CASTING PROCESS
Abstract
A process for the electromagnetic refining of light metals being
cast is provided. The process includes applying a single phase weak
stationary field to the metal with a low frequency induction coil
during solidification.
Inventors: |
FRITZSCH; Robert;
(Trondheim, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PYROTEK, INC. |
Spokane |
WA |
US |
|
|
Family ID: |
1000005415758 |
Appl. No.: |
16/616648 |
Filed: |
May 24, 2018 |
PCT Filed: |
May 24, 2018 |
PCT NO: |
PCT/US18/34389 |
371 Date: |
November 25, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62510472 |
May 24, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 11/003 20130101;
B22D 11/115 20130101; B22D 27/02 20130101 |
International
Class: |
B22D 11/115 20060101
B22D011/115; B22D 11/00 20060101 B22D011/00; B22D 27/02 20060101
B22D027/02 |
Claims
1. A process for the electromagnetic refining of metals be cast
wherein an electromagnetic confinement field acts on molten metal
in the course of solidification, the process including applying a
single phase magnetic field to the metal, said field being applied
by a low frequency induction coil placed at only one side of the
metal.
2. The process of claim 1 wherein said low frequency comprises
0.1-120 Hz.
3. The process of claim 2 wherein said frequency is
quasi-sinusoidal,
4. The process of claim 1 wherein said low frequency comprises
pulsed DC.
5. The process of claim 1 wherein the coil is shaped and positioned
such that the associated electromagnetic field can penetrate and
induce a current in all sections of the casting.
6. The process of claim 1 wherein said field satisfies at least one
of less than 2 Tesla and 6-60 Hz.
7. The process of claim 1 wherein said coil operates at a power of
less than 800 amps.
8. A process for electromagnetic casting of light metals, wherein
an electromagnetic field acts on a molten metal in the course of
solidification, said electromagnetic field being provided by an
induction coil, wherein the induction coil provides a field of less
than about 2 Tesla during the solidification.
9. The process of claim 8, wherein an applied current is an
alternating current of single-phase.
10. (canceled)
11. The process of claim 8, wherein the frequency and/or power
and/or current s modified during the process of solidification.
12. The process of claim 8, wherein the applied frequencies
comprise 0.1-120 Hz or even 240 Hz.
13. The process of claim 8, wherein the coil is shaped and
positioned such that the associated electromagnetic field can
penetrate and induce a current in all sections of the casting
within the first two penetration depths for the lowest applied
frequency,
14. The process of claim 6, wherein the coil operates at current of
less than 800 A.
15. The process of claim 6, wherein a surface velocity of the
molten metal being solidified is between about 0 and 12 cm/s during
at least a portion of the process.
16. The process of claim 8 being a continuous casting.
17. The process of claim 8 wherein the light metal being cast
comprises an alloy and wherein the electromagnetic field is
maintained but reduced during the solidification process.
18. The process of claim 8 wherein the electromagnetic field is at
least substantially stationary.
19. (canceled)
20. (canceled)
21. The process of claim 8 wherein an inverter is used. cm 22. The
process of claim 8 wherein a power supplied to a system creating
the electromagnetic field induces less than a 10% increase in a
temperature of the metal being cast.
23. (canceled)
24. The process of claim 8 wherein current to the coil is increased
during solidification.
Description
[0001] This application claims the benefit of Provisional
Application No. 62/510,472, filed May 24, 2017, the disclosure of
which is herein incorporated by reference.
BACKGROUND
[0002] The present exemplary embodiment relates to a method of
refining the microstructure of metal castings, such as those formed
of light metals including aluminum, magnesium and titanium and
their alloys. The electromagnetic casting process described in
detail herein is primarily designed for castings containing light
metals
[0003] Casting of metal is one of the oldest manufacturing
processes, where liquid metal is poured into a mold to produce
parts. Traditional casting involves the pouring of metal into a
permanent or non-permanent mold, including runners or gating
systems and risers allowing for sufficient pressure such that
trapped gas escapes and the liquid metal completely fills the
mold.
[0004] The microstructure and the physical properties of the molded
metal can be influenced during the solidification process using
various treatments. A widespread practice is to chill the casting
mold with a cooling system, e.g. Direct Chill or DC casting,
permanent active/passive cooled molds or others to remove the
thermal energy from the mold and increase the speed of the
solidification process. The speed of the solidification affects the
microstructure by increasing the crystallization speed which
restricts the time for grains to grow, thereby generating a finer
microstructure with better physical properties.
[0005] Another way to enhance the microstructure is to add grain
refiners to the metal prior to casting. The grain refiner can act
as a nucleation grain, increasing the number of nuclei and forming
a larger number of crystals during solidification which have less
space to grow. In this manner a finer grain structure can be
achieved in the finished casting. Unfortunately, grain refiners can
be detrimental in certain applications.
[0006] Casting technology has also contemplated the use of an
electromagnetic field to contain a body of metal being cast. It is
known from French Patent No. 1 509 962, herein incorporated by
reference, that steel or aluminum ingots can be produced by
electromagnetic casting. The procedure disclosed comprises
generating an alternating electromagnetic field around a column of
metal in a molten condition, by means of an angular inductor. The
magnetic field provides a means of inducing electromagnetic
pressure within the primary casting area to prevent the molten
metal from spreading and thus impart a certain geometry to the
metal. When the metal, confined in that manner, is subjected to a
cooling effect by a suitable cooling agent, it then solidifies,
following the form imposed by the field. Unlike the conventional
casting process, solidification does not occur in contact with the
walls of a mold, but outside of any contact with a solid material.
Under those circumstances, the articles produced are generally
ingots which have a better surface condition and which, in some
cases, may be used directly in dimensional transformation
operations without the need to have recourse to particular surface
treatments, such as for example a scalping operation.
Advantageously, cold crucibles and/or contactless solidification
systems can provide superior chemical cleanliness suitable for use
with high purity metal testing and production. Of course, this
requires a very high energy process. For example, the liquid metal
is held in a confined condition by applying an electromagnetic
field which is generated by means of an annular inductor supplied
with an alternating current at a frequency which is generally
between 500 and 5000 Hertz. These process suffer from a drawback in
that they are batch processes not generally suitable to large scale
industrial applications.
[0007] In U.S. Pat. No. 3,985,179 (the disclosure of which is
herein incorporated by reference), Goodrich et al. disclose a
method using an electromagnetic casting apparatus wherein a
ring-type inductor generates an electromagnetic field having a flux
density which diminishes in intensity towards the top of the
inductor to more efficiently control the shape of the molten metal
within the inductor for use with light metals. In U.S. Pat. No.
4,004,631 (the disclosure of which is herein incorporated by
reference) a cooling jacket was added. Goodrich uses this
technology to control the shape of the solidifying metal by
electromagnetic forces to reduce the wear on the refractory and
hence increase their lifetime. When molten metal is fed to the
inner peripheral area of the inductor, the interaction of the
electromagnetic field with the eddy currents induced in the molten
metal generates the electromagnetic forces, which control the
cross-sectional shape of the solidifying metal to the same general
shape as the inductor. The radial force components generated by the
electromagnetic field prevent any significant lateral movement of
molten metal and thus allow for no contact between the molten metal
and the inductor.
[0008] Using electromagnetic fields to control the shape of a
billet or slab during the casting of aluminum alloys is shown in
U.S. Pat. No. 4,307,772 (the disclosure of which is herein
incorporated by reference) and in U.S. Re. 32,529 (the disclosure
of which is herein incorporated by reference). Using
electromagnetic levitation Hull et al. shows the possibility of
horizontal casting of thin sheets as described in U.S. Pat. No.
4,741,383 (the disclosure of which is herein incorporated by
reference). Each of these techniques use a high power alternating
electromagnetic field to maintain the shape of the casting by
induction of eddy currents and Lorentz forces.
[0009] Electromagnetic stirring during solidification has been
applied to direct chill (DC) casting of wrought alloys through the
CREM (Casting, Refining, ElectroMagnetic) process, developed by
Charles Vives in the late 1980's. In U.S. Pat. No. 4,530,404 (the
disclosure of which is herein incorporated by reference), Vives
discusses the effect of the electromagnetic field on the structure
of the metal. Within this document Vives describes the effect of
the alternating current frequency on the microstructure and the
stirring effects of the liquid metal. Vives teaches that as the
power input to the stirrer was increased, the subsequent grain size
in the ingot was reduced. In some alloys a grain size smaller than
that associated with the use of a grain-refining master alloy was
obtained.
[0010] Radjai et al. used DC magnets and AC currents to
successfully refine Mg, Al and grey iron. Mizutani et al. has
employed a similar method and for grain refinement of Al alloys and
bulk metallic glasses. Greenwich University and Valdis Bojarevics
(2015-2016) papers discuss ultrasonic refinement by EM vibration
using high frequency and immersed coils.
[0011] However, these publications all teach a high force approach,
using multiple phase approaches and frequencies of at least 50 Hz
at very high magnetic field strength. Therefore the investigations
all focus on the direction of the EM field. The present disclosure
contemplates a single phase relatively low variable force and
variable low frequency approach instead. Moreover, using
micro-movements, resonance effects, increasing the kinetic energy,
altering the critical radii of nucleation grains and the
disturbance of the growing DAS structure instead of inducing a
massive bulk flow with significant inductive heating has been found
to provide unexpected benefits.
BRIEF DESCRIPTION
[0012] According to one embodiment, a process for the
electromagnetic casting of metals is provided. The process employs
an electromagnetic confinement field on the molten metal in the
course of solidification. The process further includes applying a
single phase stationary field to the metal, wherein the field is
applied by a low frequency induction coil placed at only one or two
sides of the metal.
[0013] It is contemplated that the low frequency induction coil
will operate in about the range of 0.1-240 Hz or 0.1-120 Hz. It is
further contemplated that the process could use a coil having a
vertical axis is aligned with a vertical orientation of an
associated casting table. It may be desirable to provide a coil
that is shaped and positioned such that the associated
electromagnetic field can penetrate and induce a current in all
sections of the casting. It may be desirable to use only a single
coil.
[0014] It may also be desirable that the field satisfy at least one
of (a) less than 2 Tesla or less than 1 Tesla or less than 0.5
Tesla and (b) 6-60 Hz. In certain embodiments the coil operates at
a power of less than 500 amps or less than 250 amps or less than
0.8 kA. For example, a single plate 30 turn coil operating at less
than 500 amps with a 100 mTesla field may be suitable.
[0015] The present disclosure further contemplates the adjustment
of power, current and/or frequency during the solidification
process (optionally dependent on metal phase).
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic illustration of a first representative
electromagnetic die casting configuration;
[0017] FIG. 2 is a schematic illustration of a second
representative electromagnetic die casting configuration;
[0018] FIG. 3 is a schematic illustration of a third representative
electromagnetic die casting configuration;
[0019] FIG. 4 is a top cross section view of the configuration of
FIG. 3;
[0020] FIG. 5 is a side cross section view of the configuration of
FIG. 3;
[0021] FIG. 6 is a schematic illustration of a fourth
representative electromagnetic die casting configuration;
[0022] FIG. 7 is a perspective view of a round single coil;
[0023] FIG. 8 is a top view of a round double coil:
[0024] FIG. 9 is a schematic illustration of the pancake coil
testing set up;
[0025] FIG. 10 is a schematic illustration of the round coil
testing set up;
[0026] FIG. 11 is a schematic illustration of a representative EM
vibration model;
[0027] FIG. 12 is a schematic illustration of a representative EM
continuous casting vibration model;
[0028] FIG. 13 is a schematic illustration of a representative
strong EM vibration model;
[0029] FIG. 14 is a schematic illustration of a representative
strong EM vibration continuous casting model; and
[0030] FIG. 15 is a schematic illustration of a EM pressure
model.
DETAILED DESCRIPTION
[0031] As used herein, the term "electromagnetic solidification"
refers to the solidification of a metal or metal alloy at or below
the solidification temperature during the exposure to an
alternating or static magnetic field.
[0032] As used herein, the term "electromagnetic refining" refers
to the effect of the electromagnetic field on the solidification
process, by introducing kinetic and thermal energy to refine the
microstructure of the casting.
[0033] One goal of the present disclosure is application of an
electromagnetic field during casting and solidification of aluminum
to refine the microstructure with the direct increase of the
mechanical stability of the casting. To achieve this, the relevant
influencing variables and effects of independent input variables
have been evaluated.
[0034] In one embodiment it has been found that using a single
phase improves control of the induced Lorentz forces and eddy
currents. In this manner, the process does not induce significant
velocity in the metal (limited stirring). The single phase allows
EM flux at a portion/volume of interest without having huge coil
packs, traveling magnetic fields and strong flow fields to manage.
It also allows the design of coils, molds and solidification rates
by a variable frequency, giving high current densities at the
solidification front by the variable frequency and giving high
current densities at the solidification front without significant
mixing. Variability is advantageous because the electrical
conductivity of solid and liquid aluminum is significantly
different.
[0035] Important design parameters of the present disclosure
include power, field and geometry. As there is evidence that
velocity or vibrations influence the microstructure of the
solidifying metal, the parameters that alter this influence are
separated into 3 groups: [0036] Power: contains the current,
voltage, power factor, capacitor requirements, inductance and
resistance of the coil, cooling requirements, the coil shape and
geometry and the shielding; [0037] Field: contains the frequency,
the phase architecture, the position of the coil, the crucible
geometry and material, the used alloy, the coil shape and geometry
and the shielding; and [0038] Geometry: contains the crucible
geometry and material, wall thickness, casting shape, coatings for
controlled cooling (with higher or lower heat conductivity)
[0039] As a positive side effect the reduction of porosity,
reduction of macro segregation, homogeneity of chemical
composition, and refined microstructure can be expected.
[0040] Aluminum and its alloys are the prime choice for the
manufacture of automobile parts. Different manufacturers are using
various casting processes to produce the casts, e.g. engine blocks
and cylinder heads. The most popular processes are die casting,
precision sand casting, lost foam casting, and investment casting.
The castings can be conducted as direct chill casting (DC),
pressurized casting (counter pressure casting PCP as low and high
pressure casting) and modified casting. One commonly used aluminum
alloy in cast houses is an aluminum-silicon alloy, which provides
good fluidity, strength, ductility, good wear and corrosion
resistance.
[0041] The present disclosure finds particular usefulness with
aluminum alloys. Particularly, aluminum alloys demonstrate a
solidification process wherein a "mushy" zone occurs along the
solidification front. The low power single phase system of the
present disclosure creates just enough EM flux to cause localized
vibration and breaking of large dendrites along the solidification
front. In this regard, and recognizing that solid metal is a better
conductor of EM energy, the power to the inductor coil can be
reduced when metal at any location in the casting apparatus'
reaches its solidification temperature (e.g. between 550.degree. C.
and 660.degree. C. for aluminum alloy). It is further contemplated
that the power to the coil will be substantially continuously
reduced commensurate to the quantity of solidified metal in the
casting.
[0042] The fatigue resistance and the reliability of aluminum
alloys are directly affected by the casting process. The defects
altering the fatigue are first pores, due to shrinkage or gas, and
second exogenous inclusions and/or second phase particles, such as
intermetallic inclusions and precipitates. The distribution and the
size of these obstacles/imperfections/precipitates are introduced
by the chemical composition and the geometry of the casting,
wherein traditional casting modifies these effects by altering the
solidification rate and the casting pressure.
[0043] Fatigue cracks nucleate and grow from existing defects.
Microstructural refining of the castings, such as secondary
dendrite arm spacing is traditionally dependent on the heat
transfer rates within the metal and within the mold during the
solidification stage.
[0044] Microstructural refining can also occur by external forces.
The role of fluid flow during the solidification of aluminum is a
complex and important topic. Both, the micro- and macrostructure
and micro- and macro segregation are affected. The source of the
fluid flow can be either natural or forced convection. Natural
convection is driven by variations in density and thermal energy
occurring during the solidification process due to differences in
temperature and/or chemical composition, whilst the forced
convection can arise from mechanical or electromagnetic stirring.
The fluid flow can occur in both the bulk liquid and the liquid
portion of the semi solid metal.
[0045] Electromagnetism is one of the four fundamental interactions
that exist in nature. The focus of the electromagnetism is based on
the interaction of particles with an electric charge. When a system
is exposed to a change of electric current a magnetic flux is
induced. The magnetic flux and the electric currents are
proportional. This is defined by Amperes circuit law.
[0046] To be able to define the reaction force that is within the
magnetic flux Faraday's law can be used, which is defined as the
curl of the electric field being equal to the rate of change of the
magnetic field. The magnetic flux is dependent on the frequency and
the amount of the applied current and the kind of induction device
used. When we know the current density J and the magnetic flux B,
the reaction force F can then be found by using the simple
equation: F.sub.L=J.times.B. The force F.sub.L is called Lorentz
force and is the volume force acting on the fluid.
[0047] Since the applied current changes with time, oscillating
with the number of cycles given by the frequency, the Lorentz force
also changes its direction twice as often as the frequency of the
current.
[0048] EM stirring can be classified in linear induction machine
(LM) stirring where 2 to 3 phases of current induce a traveling
magnetic field generating a strong directed propulsion within the
liquid. A weak stirring occurs if a single-phase current is applied
to an inductor in proximity to the liquid metal, allowing the flux
to travel through the liquid metal. The magnetic flux still induces
a predominant flow field circulating the metal within the affected
area, with lower velocities primarily generating strong
electromechanical vibrations. In general, the power of the EM flux
in the affected area should be less than a level which would create
bulk flow via drag forces in the metal casting.
[0049] The effect of the electromagnetic field may be most
efficient if applied during the early stage of solidification.
Without being bound by theory, it is believed that additional
kinetic energy in the melt helps nucleation by decreasing critical
radius for nucleation, leading to higher nucleation rate and grain
refinement, but also disrupts the growing dendrites forming the
dendritic arm spacing structural (DAS) growth during
solidification. This generates additional nucleation grains within
the proximity of the solidification front. The secondary DAS are
expected to be partially suppressed by the EM vibrations and the
induced velocity gradients at the solidification front.
[0050] The concept of the Electromagnetic Modified Casting (EMC)
and Electromagnetic Modified Refining (EMR) is not only
implementing the use of EM stirring effects, but is based on a
combined effect of induced bulk flow, electromechanical vibrations,
electromagnetic pressure and the change of conductivity of aluminum
during solidification. As solidified aluminum has a higher
electrical conductivity, current will prefer to conduct along the
solid fraction of the solidification front and the already
solidified metal. The current then induces EM fields within the
solid fraction of a solidifying metal. These fields will induce
Lorentz forces and hereby vibrations at the solidification front,
which induce a EM pressure to disturb and break dendrites
(disconnected dendrites can act as a nucleation site), increasing
the energy within growing grains forming dislocations and grain
boundaries, generating nucleation sites, as a result reducing the
grain size, the dendritic arm spacing (DAS) and suppressing the
secondary dendritic arm spacing (SDAS). Additionally, detaching
dendritic structures from the solidifying grains and diluting them
within the melt might generate particles acting as nucleation
sites/grains.
[0051] By using a single phase of an AC current a steady state
condition can emerge for every frequency applied. Changing the
phase and/or designing/optimizing the inductors can alter the
vibration and flow patterns. The interaction with the solidifying
and mushy metal during the solidification stage can be adjusted by
increasing/decreasing the frequency and the current depending on
the stage of the solidification within the casting. In this regard,
a process wherein the frequency is (decreased) during metal
solidification may be beneficial. Similarly, a process where
current is (increased) during solidification may be beneficial.
[0052] The conductivity is strongly dependent on the state of a
metal. The conductivity of liquid aluminum is, depending on the
purity of the used metal and the amount of alloying elements,
approximately 30% lower than of solid aluminum. In other words, the
current will choose the path of least resistance and conduct mainly
in the solidified sections of the aluminum. Pure Al has a
25.degree. C. conductivity of 36.9 micro Si/m, the conductivity
decreases to 9.4 micro Si/m at 650.degree. C. and when liquid it
reduces drastically to 4.1 micro Si/m. This is a conductivity of
approximately 44% relative to when solid.
[0053] The existing literature focusses on the reduction of
solidification time to reduce the DAS and the SDAS. A fine
dendritic structure and a reduced DAS are leading to a shorter
periodicity of micro segregation. There is convincing evidence that
ultrasonic vibrations and electromagnetic vibrations alter the DAS
and the SDAS towards a finer structure.
[0054] The electromotive force induced by the electromagnetic flux
can be experienced as strong vibrations and a velocity gradient.
The electromotive forces generate a drag by predominating in one
direction, depending on the inductor geometry and the phase of the
electric current. The penetration depth, defined by the applied
frequency of polarization of the current, defines the depth the EM
flux interacts within the metal, inducing vibrations and velocity
within the volume.
[0055] The present disclosure contemplates an AC field design
generally providing a max B field of 1-2 Tesla. A connector for the
AC current can be selected from different conductive metal types
such as steel or copper. The design may include designated mold
sections for the AC current supply and/or a copper finger as
connectors. It is contemplated that a single or double plate
inductor coil with a single phase AC current would be employed. An
exemplary current can be in the range of about 400 A. An exemplary
DC field can be on the order of about 0.5 Tesla.
[0056] EMC can be achieved by using an induction device which uses
alternating current (AC) EM flux which penetrates into the liquid
metal. The penetration depth can be adjusted by changing the
applied frequency and the field strength of the current applied. As
the flux penetrates it interacts with the conductive medium
inducing a counteracting current (resistance force), which in
return induces a Lorentz force, generating a drag force.
Remembering the nature of EM flux being always a closed loop, for
every phase, the Lorentz force is induced twice in alternating
directions. The electromotive forces initiate a velocity and
vibrations within the metal.
[0057] In one embodiment, the coil is advantageously positioned in
order to generate Lorentz forces, which act to cause the
solidifying metal, that change with the applied frequency and
current, enhancing the refining action. Once solidified, the
microstructure of the casting is refined depending on the physical
properties applied to the coil. The casting requires less post
treatment after electromagnetic casting, generating a clear
economic advantage.
[0058] Due to the lack of space around the castings, safety issues
with water in proximity to liquid aluminum and to be able to change
the casting, while using the same coil, a coil geometry with a
magnetic flux going "top-down" or "bottom-up" are viable options. A
coil in the lid of the casting table is another viable example
where the coil geometry and shielding are restricted in space but
close to the casting.
[0059] Turning now to detailed coil design considerations, it
should be remembered that parallel wires, carrying the same current
in the same phase, such as occurring in pancake coils generate
strong attraction to each other, while generating a magnetic flux
perpendicular to the pancake shape, depending on the phase and the
geometry. However, although pancake shape is referenced in this
paragraph, the present disclosure also contemplates round coils,
plate coils, and solenoidal coils, for example.
[0060] In perhaps the simplest configuration of the disclosure, a
low frequency induction coil (0.1-120 Hz) is placed at one side of
a casting with its axis aligned in the vertical direction of the
casting crucible. The coil can be shaped and positioned in such a
way that the EM field can penetrate and induce a current in all
sections of the casting when using the lowest contemplated
frequency for the solidification process.
[0061] Alternatively, by turning a direct current (DC) applied to a
solenoid or an inductor of any geometry on and off it generates a
magnetic field during the change of current, while the inductor is
charged, similar to an AC simulation. This is also known as a pulse
width modulation (PWM). As one example, DC coils could be used with
a pulsed current from a Ac/Ac inverter or a DC/AC inverter
applicable for single phase use, but also for example by a standard
DC drive which can be pulsed at a controllable rate. This imposes a
frequency of changing current, while the polarization remains
always the same direction, hence the coil experiences a DC current,
generating magnetic flux during the electrification.
[0062] Regardless of the source of the EM field, as articulated
previously, one feature of the present disclosure is the exposure
of the casting during solidification to a relatively low power.
This has in places in this disclosure been expressed as a system
which does not create bulk flow in the molten metal being
solidified. An alternative effect of the desired low power EM grain
refinement process is only limited, if any, addition of induced
heat to the metal casting during solidification. Moreover, it may
be desirable that less than 10% or 5% or 1% or 1/2% of power
supplied to the elements of the system, depending on the applied
frequency, and used to generate inductive forces be converted to
heat when the system is being powered to refine the grain structure
in the solidifying casting.
[0063] Several conceptual ideas of the present disclosure are shown
in FIGS. 1-4 for AC and DC systems.
[0064] In FIG. 1, a schematic illustration of a DC EM die casting
apparatus is depicted using AC current applied directly through the
metal. The casting metal which can be contained in any manner known
in the art, such as a sand mold, is bound radially by a DC coil (a
pancake inductor coil on one or two sides is also viable). An AC
connection is provided at each end of the mold. In FIG. 2, the DC
coil is replaced with a pair of DC Helmholtz coils.
[0065] In FIGS. 3-5 a schematic illustration of a single AC plate
inductor coil positioned at one end of the casting mold is shown.
The 12-turn pancake coil shaped of, for example, rectangular 6.5 mm
copper tubes with 1 mm walls and a 14 cm O. A magnetic shield
surrounding the coil can be provided. In certain instances, it may
be desirable to provide a sleeve, e.g. (SiO.sub.2), around the
individual coils.
[0066] A similar concept is shown in FIG. 6 wherein a pair of AC
plate inductor coils are offset relative to one another within a
soft magnetic iron core shield. This provides an AC field with two
phases (two coils on a lamination leg). The coils are overlapping
on two legs each, with the center leg being surrounded by both
coils. This generates a traveling magnetic wave when the phases are
applied. This concept provides two different AC phases with a low
frequency inducing minor velocity stream, combining the grain
refining resulting from electromagnetic forces with a minor
stirring action (less than bulk flow) decreasing the segregating of
the applied casting alloy during solidification.
EXAMPLES
[0067] A round single coil with SiO.sub.2 sleeves as shown in FIG.
7 was used to investigate the weak linear EM fields during the
solidification of molten aluminum. FIG. 7 illustrates a round
single coil with 16 turns. In certain testing, a stronger EM field
was evaluated using a double round coil (31 turns) with SiO.sub.2
sleeves as shown in FIG. 8. The relative positioning of a pancake
coil (e.g. FIGS. 4 and 5) to the casting mold is illustrated in
FIG. 9. FIG. 10 provides a schematic illustration of the testing
setup used with the double round coil (shown) and the single round
coil.
[0068] Electrical insulation and physical integrity of the coil was
provided by glass fiber sleeves (SiO.sub.2) and high-temperature
glass fiber tape. Utilization of fiber tape, epoxy resin,
Si-polymer rubber, or other supportive materials are also
contemplated. The coils were water cooled (the disclosure also
contemplates glycol, two phase oils and fog coating or alternatives
for cooling) to remove the heat from eddy current, resistive heat
of the coil and the radiation heat from the hot metal.
[0069] Exemplary dimensions of the single, double and pancake coils
are provided in the following Table.
TABLE-US-00001 TABLE Unit Abr. Single Double Pancake Inner M
D.sub.c 0.1315 0.1315 0.2 Diameter Radius M r 0.06575 0.06575 0.14
Area m.sup.2 A 0.0135 0.0135 -- Turns N 16 31 12 Height m I.sub.c
0.1058 0.111 0.006
[0070] The outer radius of the crucible was 10 cm at the top
diameter with a capacity of approx. 0.8 l. The metal was heated and
melted in a resistance furnace using an air atmosphere. The
solidification was not forced, the crucibles were preheated. The
metal temperature was set to be 800.degree. C. in the resistance
furnace and was measured before electrifying the coils.
[0071] When the required temperature was reached the crucible was
placed on a preheated sand bed. The trials with the round coils
allowed the crucible to be placed within the coil, while the
pancake coil was placed on top of the crucible.
[0072] The coils were powered by a 50 Hz single-phase variable
power supply giving up to 400 A at 40 V. Each coil was evaluated
three times using pure aluminum.
[0073] Experiments have demonstrated that a 7-turn pancake coil was
able to generate a strong magnetic field in the center of a casting
with an EM flux generating vibrations and inducing a minor flow
field sufficient to stir the metal below the coil. The peak
velocity in the sample was calculated to be 3.4 cm/s at the wall
region. There was a curl in the upper section accelerating the
liquid aluminum with approx. 2.5 cm/s towards the wall. The curl in
the center was moving from the bottom upwards. The used frequency
of 50 Hz allowed a penetration depth of approx. 30 mm such that
Lorentz forces extended the full depth of the crucible. The
magnetic flux density to drive the velocity was modelled to have a
peak value of .about.11 mT at the metal surface within the
crucible.
[0074] Various shielding materials were evaluated with respect to
flux on the surface of the metal and the results are shown in the
graph below.
[0075] The graph shows the r distribution of the magnetic flux
density at the surface of the metal level of the crucible, as shown
in the small sketch at the right side of the image, following the
arrow pointing to the right. The different lines represent
different material characteristics and the effect of the magnetic
shield on the EM flux density.
[0076] The effect of the shielding material on the magnetic flux
strength at the center of the metal crucible (z axis following the
arrow pointing down) is shown in the graph below. The frequency of
50 Hz allowed for a penetration depth of approximately 30 mm,
evidencing that Lorentz forces can be created within the first
three penetration depths and at a frothing at full depth of 100 mm
of the crucible.
[0077] The graph shows the z-axis distribution of the magnetic flux
density at the center (r=o) of the metal in the crucible, as shown
in the small sketch at the right side of the image, indicated by
the arrow pointing down reflecting the center of the crucible
magnetic flux intensity using the pancake coil setup. The different
lines represent different material characteristics and the effect
of the magnetic shield on the EM flux density at the z-axis.
[0078] Shielding was found to provide a beneficial increase in
flux. Silicon steel, such as the GO (Grain Oriented) 3 wt % Si,
laminated or (FM-B) Si--Fe gave almost 250% more flux at the center
of the crucible. The soft magnetic ferritic iron shield gave an
extra field strength of 180%, with the remaining flux being
dissipated via inductive heating of the shielding metal.
[0079] The peak velocity in the experiments has been calculated to
be 5.9 cm/s at the wall region. There is a curl in the upper
section accelerating the liquid aluminum with approx. 4.5 cm/s
towards the wall. The curl in the center is moving from bottom
upwards, with a similar velocity. In steady state, the velocity
within the crucible homogenizes to two curls opposing each other,
inducing a downward flow in a center and an upward flow in the wall
regions. The magnetic flux density to drive the velocity has its
peak value of .about.22 mT at the metal surface of the
crucible.
[0080] According to a further set of experiments to evaluate the
flux impact on grain structure, a single coil (see FIG. 7)
providing a peak magnetic flux density of 15 mT and a double round
coil (see FIG. 8) providing a peak magnetic flux density of 28 mT,
when electrified with 100 A, were evaluated in the configuration of
FIG. 10. The pancake coil was electrically tested. The electrical
data is shown in Table 2.
TABLE-US-00002 TABLE 2 Electrical data taken with the variable
power supply at 100 A. Single Double Pancake round round coil Coil
coil Peak mT 9.8 15 28 magnetic flux Excitation A 100 100 100
current Voltage V 3.6 4.8 11 Power VAr 360 480 1100 Inductance
.mu.H 18 26 104
[0081] Each of the single and double round coils were tested in
association with casting pure aluminum that was heated to 800 C,
removed from the furnace and solidified in a sand bed.
[0082] The resultant castings were sectioned, polished, etched and
visually inspected. The aluminum grains of an untreated sample can
be spotted with the naked eye, as the grains are several mm in
diameter. The solidification front initiated from the outer shell
towards the center of the metal sample, following the temperature
gradients. A dendritic growth structure was visible in the gas
cavity (resulting from shrinkage and different solubility of
hydrogen and other gases in the aluminum alloy) in the bottom of
the sample. The 3D structure of the DAS was observed, revealing
symmetrical pyramids growing into the hollow space.
[0083] The visual inspections after etching revealed that the
experimental protocol changed the cast aluminum microstructure by
applying a weak field to the aluminum during solidification. The
round single coil was electrified with 100 A, with 0.48 kVar giving
a maximum alternating magnetic field of 15 mT at the metal crucible
interface, exponentially decaying towards the center of the
crucible. The grain structure changes and the normal solidification
structure of a pure aluminum alloy without EM treatment were
eliminated. The DAS were interrupted, but still grew, while smaller
grains were observed. In addition, the gas cavity became smaller
and was moved from the bottom to the center of the metal sample,
which can be correlated to a weak induced velocity field. The
surface of the cavity was smoother and the 3D-structures of the DAS
are smaller compared to a comparative sample without weak EM
treatment.
[0084] The strongest magnetic field tested was generated by the
double coil. The round double coil was electrified after the 800 C
hot metal filled crucible was placed within the coil. The
excitation current was 100 A, with 1.1 kVar, giving a maximum
alternating magnetic field of 28 mT at the metal crucible
interface. Visual inspection of the cast aluminum demonstrated a
refined grain structure, particularly at the lower section of the
casting. The gas cavity moved from the bottom to the top of the
metal casting, connecting with the surface and allowing the gas to
be removed during the solidification and shrinking.
[0085] The testing suggests that a variable magnetic field
generated by a single phase AC induction coil in the range of 15 to
28 mT for the 0.8 L size crucible is highly beneficial. Moreover,
it appears that the penetration depth is sufficient to allow the EM
vibrations to contribute to the solidification structure. Generally
speaking, the magnetic field in Tesla represents the relative
interaction possible between the metal being cast and the applied
EM field, while the frequency being applied represents the volume
of the metal being cast which is affected. Broadly speaking, for a
typical casting such as an engine block a variable magnetic field
in the range of 15 mT to 1 T, for example 200 mT, may be beneficial
and for a casting such as a wheel a variable magnetic field in the
range of 5 mT to 250 mT, for example as 60 mT may be
beneficial.
[0086] The present experiments further demonstrate that a minor
induced velocity is beneficial and allows the metal to solidify
without lowering the solidification temperature significantly. A
velocity within the casting of an 800 C. and below aluminum alloy
in the range of between about greater than 0 and 12 cm/s may be
desirable depending on the geometry and the alloy used. The single
coil demonstrated a maximal velocity of approximately 4 cm/s at the
crucible surface. The double coil provided a crucible surface
velocity in the range of 8 to 10 cm/s.
[0087] When looking at one single induction system and when
reducing the frequency, the magnetic field strength will also
reduce, as there is less change of current per time. Nonetheless,
the EM penetration depth will increase and thereby increase the
volume the interaction of the magnetic flux and the metal can take
place Hence, a lower frequency increases the volume, while it
reduces the force distribution over this volume. One interesting
aspect of the disclosure is that during solidification there is
variation of conductivity, changes in penetration depth with
current flowing the path of lowest resistance, and changes in the
generation of eddy currents. Without being bound by theory, it is
believed that by varying the power and/or frequency throughout
solidification to focus the energy in the region of interest, EM
interaction can support the refining of the microstructure without
bulk flow.
[0088] The present disclosure contemplates several EM casting
configurations. The systems disclosed are intended to provide EM
vibration alone or in combination with EM pressure velocity. The
systems may have the ability to provide varied frequency throughout
the solidification process such that the frequency and/or intensity
is modified based on the metal conductivity and alloy composition.
This allows the system to be tailored to induce EM vibrations and
EM pressure primarily at the solidification front This disclosure
further contemplates the combined use of AC and DC currents applied
on different coils to provide Lorentz forces at the region(s) of
interest within the casting during solidification, such as the
moving solidification front.
[0089] Referring now to FIG. 11, an EM vibration casting model is
depicted. In this configuration, AC/DC coils are provided at the
top and bottom regions of the casting. Current outlets are provided
in the top surface of the casting and a current inlet at the bottom
surface of the casting. This embodiment provides the advantages of
larg applied geometries as the coils can be at very low frequency
or DC, giving maximum penetration depth (B) while the AC currents
are directly applied to the casting (J), generating Lorentz forces
by the formula F.sub.L=J.times.B at the area where most current
passes. The current will prefer the solid fraction of the metal,
same as the magnetic field, due the differences in conductivity
within the metal from liquid to solid. This will induce Lorentz
forces within the growing DAS and SDAS, but also at the interface
of the solidification front. With continuing reference to FIG. 12,
a similar configuration can be used in association with a
continuous casting process such as employed with wire, rod and/or
thixo castings.
[0090] Referring now to FIG. 13, a strong EM vibration model is
depicted. In this configuration, a DC/low frequency AC coil is
placed adjacent the top and bottom surfaces of the casting and an
AC/DC coil placed intermediate the DC/low frequency AC coil(s).
This model provides similar advantages as the design of FIGS. 11
and 12. It gives the additional advantage of a contactless
induction of the AC field, further reducing the wear and/or
interaction of the current connectors. With continuing reference to
FIG. 14, an extrusion, wire, rod, of wire and/or rods at a
relatively high production rate as the grain refinement stage will
be contactless and therefore without wear. It should be noted that
although a set of three coils is illustrated, the commercial
embodiment may include several more (e.g. up to 10 coils or more)
and/or thixo casting can be formed using a similar arrangement
making the design suitable for production. In fact, it is noted
that any of the coil configurations disclosed herein are believed
suitable for both single mold and continuous casting processes.
[0091] Turning now to FIG. 15, an EM pressure model is depicted.
Particularly, a counter pressurized mold is employed in combination
with a first DC or low frequency AC coil and an AC coil. This is an
illustration of a standard CPC (Counter Pressure Casting) or PCP
(Pressurized Casting Process) used for car rims, pistons,
semi-forged high quality parts, or other high quality demanding
products. The pressure is usually created by a mechanical pump
within the metal bath and/or by a vacuum within the mold (counter
pressure casting). The induction coils could enhance the grain
structure, macro-segregations and homogeneity while the pressurized
casting is further reducing the shrinkage and the porosity.
[0092] The exemplary embodiment has been described with reference
to the preferred embodiments. Obviously, modifications and
alterations will occur to others upon reading and understanding the
preceding detailed description. It is intended that the exemplary
embodiment be construed as including all such modifications and
alterations insofar as they come within the scope of the appended
claims or the equivalents thereof.
* * * * *