U.S. patent application number 11/765753 was filed with the patent office on 2008-01-03 for method of unidirectional solidification of castings and associated apparatus.
Invention is credited to Men G. Chu, Alvaro Giron, Ken Kallaher, Ho Yu.
Application Number | 20080000608 11/765753 |
Document ID | / |
Family ID | 37660607 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080000608 |
Kind Code |
A1 |
Chu; Men G. ; et
al. |
January 3, 2008 |
METHOD OF UNIDIRECTIONAL SOLIDIFICATION OF CASTINGS AND ASSOCIATED
APPARATUS
Abstract
Molten metal is injected uniformly into a horizontal mold from a
feed chamber in a horizontal direction at a controlled rate,
directly on top of the metal already within the mold. A cooling
medium is applied to the bottom surface of the mold, with the type
and flow rate of the cooling medium being varied to produce a
controlled cooling rate throughout the casting process. The rate of
introduction of molten metal and the flow rate of the flow rate of
the cooling medium are both controlled to produce a relatively
uniform solidification rate within the mold, thereby producing a
uniform microstructure throughout the casing, and low stresses
throughout the casting.
Inventors: |
Chu; Men G.; (Export,
PA) ; Yu; Ho; (Murrysville, PA) ; Giron;
Alvaro; (Murrysville, PA) ; Kallaher; Ken;
(Trafford, PA) |
Correspondence
Address: |
INTELLECTUAL PROPERTY
ALCOA TECHNICAL CENTER, BUILDING C
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
37660607 |
Appl. No.: |
11/765753 |
Filed: |
June 20, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11179835 |
Jul 12, 2005 |
7264038 |
|
|
11765753 |
Jun 20, 2007 |
|
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Current U.S.
Class: |
164/122.1 ;
249/105 |
Current CPC
Class: |
B22D 7/00 20130101; B22D
27/045 20130101 |
Class at
Publication: |
164/122.1 ;
249/105 |
International
Class: |
B22D 30/00 20060101
B22D030/00; B22C 9/08 20060101 B22C009/08; B22D 45/00 20060101
B22D045/00 |
Claims
1. A method of casting metal, comprising: providing a mold having a
bottom surface and four sides defining a mold cavity therein, with
a first molten metal inlet structured to introduce a first molten
metal horizontally and directly above metal already within the mold
cavity; introducing molten metal into the mold cavity through the
inlet; and simultaneously directing a cooling medium against the
lower surface of the mold; whereby the molten metal is cooled
unidirectionally through its thickness.
2. The method according to claim 1, wherein a rate of introduction
of molten metal into the mold cavity is coordinated with the rate
of cooling.
3. The method according to claim 2, wherein the cooling rate is
about 0.5.degree. F./sec. to about 3.degree. F./sec.
4. The method according to claim 2, wherein the rate of
introduction of molten metal into the mold cavity slows as the
casting progresses.
5. The method according to claim 4, wherein the cooling rate slows
from about 3.degree. F./sec. to about 0.5.degree. F./sec. as
casting progresses.
6. The method according to claim 2, wherein the rate of
introduction of molten metal into the mold cavity is about 0.5
in./min. to about 4 in./min.
7. The method according to claim 6, wherein the rate of
introduction of molten metal into the mold cavity is slowed as
casting progresses.
8. The method according to claim 7, wherein the rate of
introduction of molten metal into the mold cavity slows from about
4 in./min. to about 0.5 in./min. as casting progresses.
9. The method according to claim 1, wherein a rate of application
of cooling medium is increased as casting progresses.
10. The method according to claim 9, wherein the coolant is applied
by spraying against the mold or against solidified metal.
11. The method according to claim 9, wherein at least one material
within the coolant is selected from the group consisting of air,
water, and an air-water mixture.
12. The method according to claim 11, wherein casting begins with
air being used as coolant, with the coolant changing first to an
air-water mixture and then to water as casting progresses.
13. The method according to claim 1: wherein the bottom surface of
the mold includes a removable portion; and further comprising:
placing the removable portion underneath the sides of the mold at
the beginning of casting; and removing the removable portion after
solidification of metal within a bottom portion of the mold
cavity.
14. The method according to claim 1: wherein the bottom surface of
the mold is formed by a conveyor having a solid section and a mesh
section; and further comprising: placing the solid section
underneath the sides of the mold at the beginning of casting; and
moving the conveyor so that the mesh section is underneath the
sides of the mold after solidification of metal within a bottom
portion of the mold cavity.
15. The method according to claim 1, further comprising: providing
a second molten metal inlet structured to introduce a second molten
metal into the mold cavity; introducing the second molten metal
into a bottom portion of the mold cavity; introducing the firs
molten above the second molten metal; and introducing the second
molten metal above the first molten metal.
16. A mold for casting molten metal, the mold comprising: a
plurality of sides defining a mold cavity therein; a bottom; at
least one metal feed chamber disposed adjacent to one of the sides;
at least one gate between the feed chamber and the mold cavity, the
gate being structured to selectively permit and resist the
introduction of molten metal into the mold cavity.
17. The mold according to claim 16, wherein the gate further
comprises: a rotatably mounted cylindrical member defining an outer
circumference and helical groove defined around the outer
circumference, a wall disposed on either side of and abutting the
cylindrical member and in contact with the cylindrical member; and
the cylindrical member and walls being structured to permit flow of
molten metal through a portion of the helical channel adjacent to
one of the two walls, and to resist passage of molten metal through
any other portion of the gate.
18. The mold according to claim 16, wherein the gate is a slot
defined within one wall of the mold.
19. The mold according to claim 16, wherein: the molten metal feed
chamber includes a plurality of walls, one of the walls defining a
substantially vertical slot; one of the walls of the mold cavity
defines a substantially vertical slot corresponding to the slot
defined within the wall of the molten metal feed chamber; the gate
comprises a substantially H-shaped member having a pair of
substantially vertical slot-closing flanges connected by a
substantially horizontal member defining a channel therethrough,
the gate structured to resist the flow of molten metal through the
slot in the feed chamber wall and the slot in the mold cavity wall
except through the channel, the gate being slidable from a lower
position wherein the channel is located adjacent to a bottom of the
slot in the mold cavity wall, and an upper position wherein the
channel is located adjacent to a top of the slot in the mold cavity
wall.
20. The mold according to claim 16, wherein the bottom is formed by
a conveyor having a solid section and a mesh section.
21. The mold according to claim 16, wherein the bottom is formed by
a cloth having a substrate disposed below the floor, the substrate
being movable between a first position wherein it is directly
underneath the cloth, and a second position wherein it is
sufficient distance away from the cloth to permit a spray box to be
placed between the cloth and substrate.
22. The mold according to claim 16, wherein the bottom includes a
fixed portion and a removable portion.
23. The mold according to claim 22, wherein the fixed portion
defines a slot structured to receive the removable portion.
24. The mold cavity according to claim 16, wherein the bottom
includes a substrate having a plurality of holes defined therein,
the holes being sufficiently large to allow cooling mediums to flow
therethrough, and sufficiently small to resist a flow of molten
metal therethrough.
25. The mold cavity according to claim 24, wherein the holes have a
diameter between about 1/64 inch and about one inch.
26. The mold according to claim 16, further comprising a coolant
manifold disposed under the bottom.
27. The mold according to claim 16, wherein the coolant manifold is
structured to selectively spray air, water, or a mixture thereof
against the bottom.
28. The mold according to claim 16, further comprising at least a
pair of molten metal feed chambers, each feed chamber having gates
associated therewith, and the gates associated with each feed
chamber being controlled independently of the gates associated with
the other feed chambers.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to casting methods. More
specifically, the present invention provides an apparatus and
method of unidirectionally solidifying castings to provide a
uniform solidification rate, thereby providing a casting having a
uniform microstructure and lower internal stresses.
[0003] 2. Description of the Related Art
[0004] Various methods of directional solidification of castings
within the mold have been attempted in an effort to improve the
properties of castings. An example of a presently available
directional solidification method includes U.S. Pat. No. 4,210,193,
issued to M. Riihle on Jul. 1, 1980, disclosing a method of
producing an aluminum silicone casting. The molten material is
poured into a mold having a bottom formed by a tin plate. A stream
of water is applied to the bottom of the tin plate, and a
thermocouple inserted through the tin plate into the casting is
used to monitor the temperature of the casting, and thereby
properly control the cooling stream. Cooling is stopped when the
temperature in the bottom portion of the mold falls from
575.degree. F. to 475.degree. F., until heat from the surrounding
melt increases this region to 540.degree. F. When the aluminum
silicone alloy is removed from the mold, the tin plate has become a
part of the casting. The result is a fine grain structure in the
lower portion of the casting. This method fails to produce a
uniform structure with low stresses, and would likely result in
waste due to the necessity of cutting away the tin plate if it is
not to form a part of the final casting.
[0005] U.S. Pat. No. 4,585,047, issued to H. Kawai et al. on Apr.
29, 1986, discloses an apparatus for cooling molten metal within a
mold. The apparatus includes a pipe within the mold through which a
cooling liquid is passed. The pipe is located in a lower portion of
the mold, resulting in directional solidification of the metal from
the bottom of the mold to the top. Once the casting is solidified,
the excess portion of the casting is cut away from the casting, and
then melted away from the pipe so that the pipe can be reused. The
necessity of cutting away the portion of the casting surrounding
the pipe results in added manufacturing steps and waste. The
apparatus further fails to provide for a uniform structure within
the casting or the low stresses within the casting that would
result from a directional solidification.
[0006] U.S. Pat. No. 4,969,502, issued to Eric L. Mawer on Nov. 13,
1990, disclosed an apparatus for casting of metals. The apparatus
includes an elongated pouring device structured to pour molten
metal against a vertical plate, thereby dissipating the energy of
the flowing molten metal. alternatively, a pair of elongated
pouring devices are used to pour molten metal towards each other,
so that the interaction of the two strains of metal flowing towards
each other dissipates the energy of the metal. The result is a
reduced wave action within the mold, so that the cooled casting has
a more uniform thickness. The apparatus fails to provide for a
uniform structure within the casting. It also fails to provide low
stresses within the casting.
[0007] U.S. Pat. No. 5,020,583, issued to M. K. Aghajanian et al.
on Jun. 4, 1991, describes the directional solidification of metal
matrix composites. The method includes placing a metal ingot above
a mass of filler material and then melting the metal so that the
metal infiltrates the filler material. The metal may be alloyed
with infiltration enhancers such as magnesium, and the heating may
be done within a nitrogen as environment to further facilitate
infiltration. After infiltration, the resulting metal matrix is
cooled by placing it on top of a heat sink, with insulation placed
around the cooling metal matrix, thereby resulting in directional
solidification of the molten alloy. This patent fails to provide
for control of the rate of solidification, for a uniform structure
within the casting, or for low stresses within the casting.
[0008] U.S. Pat. No. 5,074,353, issued to A. Ohno on Dec. 24, 1991,
discloses an apparatus and method for horizontal continuous casting
of metal. The system includes a holding furnace connected to a hot
mold having an open section at its inlet end. Heating elements
around the sides and bottom of the hot mold heat the mold to a
temperature that is at least the solidification temperature of the
casting metal. A cooling spray is applied to the top of the hot
mold. A dummy member secured between upper an lower pinch rollers
is reciprocated into and out of the outlet end of the mold to drew
out the metal as it is solidified. The method of this patent is
likely to result in waste due to the need to separate the casting
from the dummy metal. The apparatus further fails to provide for a
uniform structure within the casting or the low stresses within the
casting that would result from a directional solidification.
[0009] Accordingly, there is a need for an improved apparatus and
method of unidirectional solidifying of casting, providing for a
relatively uniform, controlled cooling rate. Such a method would
result in greater uniformity within the crystal structure of the
casting, with lower stresses within the casting, and a reduced
tendency towards cracking.
SUMMARY OF THE INVENTION
[0010] The present invention provides a method of casting including
a method of unidirectionally solidifying the casting across the
thickness of the casting, at a controlled solidification rate. The
method is particularly useful for casting commercial size ingots of
7xxx series aluminum alloys and Al--Li alloys. For purposes of this
description, thickness is defined as the thinnest dimension of the
casting.
[0011] A mold of the present invention is preferably oriented
substantially horizontally, having four sides and a bottom that may
be structured to selectively permit or resist the effects of a
coolant sprayed thereon. One preferred bottom is a substrate having
holes of a size that allow coolants to enter but resist the exit of
molten metal. Such holes are preferably at least about 1/64 inch in
diameter, but not more than about one inch in diameter. Another
preferred bottom is a conveyor having a solid section and a mesh
section. Other preferred bottoms include bottoms structured to be
removed from the remainder of the mold upon solidification of the
molten metal on the bottom of the mold, with a mesh, cloth, or
other permeable structure remaining to support the casting. A
trough for transporting molten metal from the furnace terminates at
one side of the mold, and is structured to transport metal from the
furnace or other receptacle to a molten metal feed chamber disposed
along one side of the mold. The molten metal feed chamber and mold
are separated from each other by one or more gates. A preferred
gate is a cylindrical, rotatably mounted gate, defining a helical
slot therein, so that as the gate rotates, molten metal is released
horizontally into the mold, only at the level of the top of the
molten metal within the mold. Another preferred gate is merely
slots at different heights in the wall separating the mold and feed
chamber, so that the rate at which molten metal is added to the
feed chamber determines the rate and height at which molten metal
enters the mold. Another preferred gate is a flow passage between
the molds and the feed chamber having a vertical slider at each
end, so that the vertical slider resists the flow of molten metal
through a slot in both the mold and the feed chamber, while
permitting the flow of molten metal through the channel. The flow
of molten metal is thereby limited to a desired height within the
mold, set by the height of the channel. In some embodiments, a
second trough and molten metal feed chamber may be provided on
another side of the mold, thereby permitting a second alloy to be
introduced into the mold during casting of a first alloy, for
example. to apply a cladding to cast item. The sides of the mold
are preferably insulated. A plurality of cooling jets, for example.
air/water jets, will be located below the mold, and are structured
to spray coolant against the bottom surface of the mold.
[0012] Molten metal is introduced substantially uniformly through
the gates. At the same time, the cooling medium is applied
uniformly over the bottom area of the mold. The rate at which
molten metal flows into the mold, and the rate at which coolant is
applied to the mold, are both controlled to provide a relatively
constant rate of solidification. The coolant may begin as air, and
then gradually be changed from air to an air-water mist, and then
to water. After the molten metal at the bottom of the mold
solidifies. the bottom of the substrate may be moved so that the
solid section underneath the mold is replaced be the mesh section,
thereby permitting the coolant to directly contact the solidified
metal, and maintain a desired cooling rate.
[0013] In the case of the perforated plate substrate, the mold
bottom need not be removed.
[0014] Accordingly, it is an object of the present invention to
provide an improved method of directionally solidifying castings
during cooling.
[0015] It is another object of the invention to provide a method of
maintaining a relatively constant solidification rate during the
solidification of the casting.
[0016] It is a further object of the invention to provide a casting
method having minimized waste.
[0017] It is another object of the invention to provide a casting
method resulting in a uniform crystal structure within the
material.
[0018] It is a further object of the invention to provide a casting
method resulting in lower stresses and a reduced probability of
cracking and/or shrinkage voids within the casting.
[0019] It is another object of the invention to provide a casting
having a more uniform structure.
[0020] It is a further object of the invention to provide an
apparatus and method for producing a cladding around the casting,
with the cladding having better adhesion than prior claddings.
[0021] These and other objects of the invention will become more
apparent through the following description and drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings(s) will be provided by the Office
upon request and payment of the necessary fee.
[0023] FIG. 1 is a top isometric view of a mold according to the
present invention, showing the solid portion of the conveyor below
the mold.
[0024] FIG. 2 is partially sectional isometric top view of a mold
according to the present invention, taken along the lines 2-2 in
FIG. 1.
[0025] FIG. 3 is an isometric top view of a mold according to the
present invention. showing the mesh portion of the conveyor below
the mold.
[0026] FIG. 4 is a partially sectional isometric top view of a mold
according to the present invention, taken along the lines 4-4 in
FIG. 3.
[0027] FIG. 5 is a top view of a gate according to the present
invention.
[0028] FIG. 6 is a front view of a gate according to the present
invention.
[0029] FIG. 7 is a side view of a gate according to the present
invention.
[0030] FIG. 8 is a side isometric, partially cutaway view of
another embodiment of a mold according to the present
invention.
[0031] FIG. 9 is a cutaway side isometric view of another
alternative embodiment of a mold according to the present
invention.
[0032] FIG. 10 is a side isometric view of the mold according to
FIG. 9.
[0033] FIG. 11 is a graph showing temperature of the casting with
respect to time during an example solidification process.
[0034] FIG. 12 is a graph showing cross-sectional stress
distribution across an ingot make according to the present
invention.
[0035] FIG. 13 is a graph showing stress at various locations
within an ingot cast using prior art methods.
[0036] FIG. 14 is a cutaway isometric view of yet another
embodiment of a mold and transfer chamber according to the present
invention.
[0037] FIG. 15 is a cutaway front isometric view of a mold cavity
for a mold according to the present invention.
[0038] Like reference characters denote like elements throughout
the drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] The present invention provides an apparatus and method of
unidirectionally solidifying a casting, while also providing for a
controlled. uniform solidification rate.
[0040] Referring to FIGS. 1-4, a mold 10 includes four sides 12,
14, 16, 18, respectively, with a mold cavity 19 defined therein.
The sides 12, 14, 16, 18 are preferably insulated. A bottom 20 may
be formed by a conveyor having a solid portion 22 and a mesh
portion 24. The conveyor 20 is continuous, wrapping around the
rollers 25, 28, 30, 32, respectively, so that either of the solid
portion 22 or mesh portion 24 may selectively be placed under the
sides 12, 14, 16, 18. The conveyor may be made from any rigid
material having a high thermal conductivity, with examples
including copper, aluminum, stainless steel, and Inconal.
[0041] A molten metal feed chamber 34 defined by sides 36, 38, 40
is defined along the side 12. Likewise, a similar molten metal feed
chamber 42 is defined by the sides 44, 46, 48, along side the sides
16. Some embodiments of the present invention may only have one
molten metal feed chamber, and others may have multiple molten
metal feed chambers. A feed trough 50, 52 extends from a molten
metal furnace (not shown, and well known in the art of casting) to
a location directly above each of the molten metal feed chambers,
34, 42, respectively. A sprout 54 extends from the feed trough 50
to the molten metal feed chamber 34. Likewise, a spout 56 extends
from the feed trough 52 to the molten metal feed chamber 42.
[0042] The side 12 includes one or more gates 58, 60 structured to
control the flow of molten metal from the feed chamber 34 to the
mold cavity 19. Likewise, the side 16 includes gates 62, 64,
structured to control the flow of molten metal from the feed
chamber 42 into the mold cavity 19. The gates 58, 60, 62, 64 are
substantially identical, and are best illustrated in FIGS. 5-7. The
gate 58 includes a pair of walls 66, 68 defining s substantially
cylindrical channel 70 therebetween. The channel 70 includes open
sides 72, 74, on opposing sides of the walls 66, 68. A cylindrical
gate member 76 is disposed within the channel 70. The cylindrical
gate member 76 is substantially solid, and defines a helical slot
78 about its circumference. The channel 70, cylindrical gate member
76, and helical slot 78 are structured so that molten metal is
permitted to flow through a portion of the helical slot 78 that is
directly adjacent to one of the walls 66, 68, and molten metal is
resisted from passing through any other portion of the gate 58. A
drive mechanism 80 is operatively connected to the cylindrical gate
member 76, for controlling the rotation of the cylindrical gate
member 76. Appropriate drive mechanisms 80 are well known to those
skilled in the art, and will therefore not be described in great
detail herein. The drive mechanism 80, may, for example, include an
electrical motor connected through a gearing system to the
cylindrical gate member 76, with the electrical motor being
controlled either through manual switching by an operator observing
the casting process, or by an appropriate microprocessor.
[0043] Referring back to FIG. 1-4, a coolant manifold 82 is
disposed within the conveyor 20, and is structured to spray a
coolant against the bottom surface 22, 24, of the mold cavity 19. A
preferred coolant manifold 82 is structured to supply air, water,
or a mixture thereof, depending upon the desired rate of
cooling.
[0044] In use, the conveyor 20 will be in the position illustrated
in FIGS. 1-2, with the solid portion 22 directly under the mold
cavity 19. Molten metal will be introduced form the feed trough 50,
through the spout 54, into the feed chamber 34, The gates 58, 60
will have their cylindrical gate members 76 rotated so that the
lowest portion of the helical slot 78 is adjacent to the wall 66 or
the wall 68, thereby permitting molten metal to enter the mold
cavity 19 by flowing substantially horizontally onto the conveyor
surface 22. At the same time, air will be sprayed from the coolant
manifold 82 onto the underside of the surface 22. As the mold
cavity 19 is filled with molten metal, the cylindrical gate members
76 will be rotated so that increasingly elevated portions of the
helical slot 78 are adjacent to either of the walls 66, 68, so
that, as the level of metal within the mold cavity 19 is raised,
the portion of the helical slot 78 through which molten metal is
permitted to pass will be raised a corresponding amount so that
flow of molten metal from the chamber 34 to the mold cavity 19 is
always horizontal, and always on top of the metal that is already
within the mold cavity 19. The horizontal flow of metal into the
mold cavity 19 will permit the molten metal to properly find its
own level, thereby insuring a substantially even thickness of
molten metal within the mold cavity 19.
[0045] As additional metal is added to the mold cavity 19, the
cooling rate for the metal within the mold cavity 19 will slow. To
maintain a substantially constant cooling rate, the mixture of
coolant from the coolant manifold 82 will be changed from air to an
air-water mist containing increasing quantities of water, and
eventually to all water. Additionally, as the metal at the bottom
portion of the mold cavity 19 solidifies, the conveyor 20 will be
advanced so that the mesh 24 instead of the solid portion 22 forms
the bottom of the mold 10, thereby permitting coolant to directly
contact the solidified metal, as shown in FIGS. 3-4. Additionally,
the rate of metal addition into the mold cavity 19 may be slowed by
controlling either the rotation of the cylindrical gate members 76
of the gates 58, 60, and/or the rate of introduction of metal into
the feed chamber 34 from the feed trough 50. Typically, the cooling
rate will remain between about 0.5.degree. F./sec. to about
3.degree. F./sec., with the cooling rate typically decreasing from
3.degree. F./sec. at the beginning of casting to about 0.5.degree.
F./sec. towards the completion of casting. Likewise, the rate at
which molten metal is introduced into the mold cavity 19 will
typically be slowed from an initial rate of about 4 in./min. to a
final rate of 0.5 in./min. as casting progresses.
[0046] If desired, a second alloy may be introduced into the feed
chamber 42 from the feed trough 52, and through the spout 56. This
second alloy may be used to form a cladding around the first alloy.
For example, the cladding may be a corrosion resistant layer. One
example of a cladding may be formed by first introducing an alloy
from the feed chamber 42, through the gates 62, 64, into the mold
cavity 19 by rotating the cylindrical gate members 76 of the gates
62, 64, so that metal flows from the bottom portion of the helical
channel 78 within these gates into the mold cavity 19, and then
closing the gates 62, 64. The cylindrical gate member 76 of the
gates 58, 60 are then rotated to permit the flow of molten metal
from the feed chamber 34 into the mold cavity 19 at increasingly
elevated portions of the helical slot 78, until the mold cavity 19
is filled almost all of the way to the top, at which point the
gates 58. 60 are closed. The cylindrical gate members 76 of the
gates 62, 64 are then rotated to permit the flow of metal from the
feed chamber 42 into the mold cavity 19 at the highest portion of
the slots 78 within the cylindrical gate members 76 of the gates
62, 64, thereby permitting this molten metal to flow to the top of
the metal already in the mold. The resulting substrate formed from
the alloy within the feed chamber 34 will have a cladding on the
top and bottom made from the alloy within the feed chamber 42.
Because the different alloys are brought into contact with each
other while one is liquid, and possibly while the other is mushy,
adhesion between the two alloys will be high.
[0047] Another embodiment of a mold 84 is illustrated in FIG. 8.
The mold 84 includes four sides, with three sides 86, 88, 90
illustrated. The sides 86, 88, 90, and the fourth substantially
identical but not shown side may be insulated, with a preferred
insulating material being graphite. The bottom of the mold 84 is
formed by a cloth 92, which may be made of the same material as the
bottom conveyor 20 of the previous embodiment 10. A bottom
substrate 94 is structured to move between an upper position
illustrated in solid lines in FIG. 8, wherein it supports the cloth
92, and a lower position, illustrated in phantom in FIG. 8, wherein
the substrate is removed form the cloth 92 a sufficient distance so
that the spray boxes 96, 98 may be positioned therebetween. The
spray boxes 96, 98 are structured to be moved from a position below
the clothe 92 to a position wherein movement of the substrate 94
between its upper and lower position is permitted. The spray boxes
96, 98 will therefore supply air, water, or a mixture of both, or
possibly other coolants, to either the bottom of the substrate 94
or the bottom of the cloth 92, depending upon whether the substrate
94 is above or below the spray boxes 96, 98.
[0048] In use, the substrate 94 will be in its upper position,
supporting the cloth 92. Molten metal will be introduced into the
mold 84, with air being applied to the bottom of the substrate 94
to provide cooling. As the mold 84 is filled with molten motel, and
the molten metal on the bottom solidifies, the spray boxes 96, 98
will be briefly withdrawn from their position under the substrate
94, thereby permitting the substrate 94 to be removed from its
position under the cloth 92. The spray boxes 96, 98 will then be
placed back underneath the cloth 92, so that they may apply air, an
air/water mixture, or water to the bottom of the cloth 92, with
increasing amounts of water being applied to the bottom of the
cloth 92 as casting progresses.
[0049] FIGS. 9 and 10 illustrate yet another embodiment of a mold
100 that may be used for a method of the present invention. The
mold 100 includes side walls 102, 104, 106, and 108, which may be
insulated, with a preferred insulating material being graphite. The
bottom includes a fixed floor plate 110 defining an opening below
the walls 102, 104, 106, 108. wherein a removable floorplate 112
may be inserted. The removable floorplate 112 may be made from a
material such as copper. The fixed floorplate 110 may in some
embodiments defined a slot 114 structured to receive the edges of
the removable floorplate 112, thereby supporting the removable
floorplate 112. The walls 102, 104, 106, 108, and the removable
floorplate 112, define a mold cavity 116 therein.
[0050] A molten metal feed chamber 118 is defined by the walls 120,
122, and 124 along with the wall 108 and fixed floorplate 110. A
gate 126 is defined within the wall 108, and in the illustrated
examples formed by a pair of slots defined within the wall 108. A
feed trough 128 extends from a molten metal furnace to a location
directly above the molten metal feed chamber 118. A spout 130
extends from the feed trough 128 to the molten metal feed chamber
118.
[0051] A coolant manifold 132 is disposed below the removable
floorplate 112. The coolant manifold 132 is preferably configured
to selectively spray air, water, or a mixture of air and water
against the removable floorplate 112. The illustrated embodiment
further includes a catch basin 134 disposed below the feed chamber
118. The entire mold 100 is supported on the base 136.
[0052] In use, the removable floorplate 112 will be contained
within the slot 114. Molten metal will be introduced from the feed
trough 128 into the feed chamber 118, until the level of molten
metal within the feed chamber 118 reaches the bottom of the slots
126. The slots 126, combined with an appropriately selected feed
rate into the feed chamber 118, will ensure that the feed rate of
molten metal into the mold cavity 116 is controlled. As the level
of molten metal within the mold cavity 116 rises, the feed rate of
molten metal into the feed chamber 118 may be adjusted so that
molten metal is flowing out of the slot 126 directly on top of the
molten metal within the mold cavity 116, thereby ensuring a
substantially horizontal flow of molten metal into the mold cavity
116. Coolant will be sprayed against the removable floorplate 112
through the coolant manifold 132, beginning with air, and then
switching to an air/water mixture, and finally all water. As molten
metal within the bottom of the mold cavity 116 solidifies, the
removable floorplate 112 may be removed, thereby permitting coolant
to directly contact the underside of the ingot within the mold
cavity 116.
[0053] In one example of a casting process according to the present
invention, 7085 aluminum alloy was cast into a
9''.times.13''.times.7'' ingot using a mold 100 as shown in FIGS.
9-10. The initial metal temperature was 1,280.degree. F. The
removable floorplate 112 was made from a 0.5'' thick stainless
steel plate. Thermocouples were placed along the center line of the
ingot at 0.25 inch, 0.75 inch, 2 inches and 4 inches from the
removable floorplate 112. The mold cavity 116 was initially filled
at a rate of 2 inches every 30 seconds, with a fill rate slowing as
casting progressed. The initial water flow rate as 0.25 gallons per
minute, in the form of a combined air/water mixture. The removable
floorplate 112 was removed when a thermocouple located 0.25 inch
from the removable floorplate 112 read 1,080.degree. F. At this
point, the flow rate of water was increased to 1 gallon per
minute.
[0054] FIG. 11 shows the cooling rate at each of the four
thermocouples. As can be seen from this figure, the cooling rate
ranged from 1.5 to 2.12.degree. F./sec., a substantially uniform
cooling rate.
[0055] FIG. 12 is a graph showing residual stresses throughout a
cross-section of the ingot. This data was collected by cutting the
ingot in half in the 9'' direction, and then measuring the
resulting surface deformation as the stresses within the material
relaxed. With the exception of one tensile stress in the lower
left-hand corner of FIG. 12, and one compressive stress in the
lower center portion of FIG. 12, the magnitude of the stresses
throughout the ingot is 0.6 to 3 ksi. The larger compressive stress
at the center of the ingot's bottom is of little concern, because
compressive stress generally does not result in cracking. The high
compressive stresses at this location and high tensile stresses in
the lower left corner are probably the result of molten metal first
impinging on the substrate at these locations, resulting in the
formation of cold shots and possibly other defects. The highest
tensile stress was +6e.sup.+02 PSI.
[0056] Referring to FIG. 13, the residual stresses across the
cross-section of a 4 inch by 13 inch 7085 aluminum alloy DC cast
ingot are illustrated. As the figure shows, the residual stresses
resulting from presently performed DC casting can be as high as 10
ksi. However, the stresses in this ingot were likely even higher,
because the ingot already had a longitudinal crack when the stress
was measured, which would have relaxed these stresses. As used in
the figure, sigma refers to tensile or compressive stress, tau
refers to sheer stress, LT refers to the direction substantially
parallel to the length, and ST refers to a direction substantially
parallel to the thickness.
[0057] The application of coolant to the bottom of the mold, along
with, in some preferred embodiments, the insulation on the sides
12, 14, 16, 18, results in directional solidification of the
casting from the bottom to the top of the mold cavity 19.
Preferably, the rate of introduction of molten metal into the mold
cavity 19, combined with the cooling rate, will be controlled to
maintain about 0.1 inch (2.54 mm.) to about 1 inch (25.4 mm.) of
molten metal within the mold cavity 19 at any given time. In some
embodiments, the mushy zone between the molten metal may also be
kept at a substantially uniform thickness. As a result of this
directional solidification, uniform temperature, and thin sections
of molten metal and mushy zone, macrosegregation is substantially
reduced or eliminated.
[0058] Referring to Figure 14, another mold assembly 138 is
illustrated. The mold assembly 138 includes 140, 142, 144, and a
fourth side that is not illustrated in the cutaway drawing,
opposite the side 142. All four walls 140, 142, 144, and the
unillustrated wall may be insulated, with the preferred insulating
material being graphite. The mold 138 further includes a bottom
146, which preferably includes a plurality of apertures 148 (best
illustrated in FIG. 15) having a diameter sufficiently large to
permit the passage of typical coolants such as air or water, while
also being sufficiently small to resist the passage of molten metal
there through. A preferred diameter for the apertures 148 is about
1/64 inch to about one inch. The mold's cavity 150 is defined by
the walls 140, 142, 144, the fourth wall, and the bottom 146. Wall
144 defines a slot therein, the edge 152 of the slot visible in
FIG. 14.
[0059] The molten metal feed chamber 154 is defined by the walls
156, 158, 160, a fourth unillustrated wall, and the bottom 162. A
feed trough 164 extends from a molten metal furnace to a location
directly above the molten metal feed chamber 154. A spout 166
extends from the feed trough 164 to the molten metal feed chamber
154.
[0060] A gate 168 is an H shaped structure, having a pair of
vertical slot closure members 170, 172, connected by a horizontal
member 174 defining a channel 176 therethrough. Slot closure member
170 is structured to substantially close a slot in the wall 144 of
the mold cavity 150, while the closure member 172 is structured to
substantially close the slot defined within the wall 156 of the
molten metal feed chamber 154. The gate 168 is structured to slide
between a lower position wherein the channel 176 is located
adjacent to the bottom 146 of the mold cavity 150, and an upper
position corresponding to the tip of the mold cavity 150. The slot
closure member 170, 172 are structured to resist the flow of molten
metal through the slots defined in the walls 144, 156 at any point
except through the channel 176, regardless of the position of the
gate 168.
[0061] A coolant manifold 178 is disposed below the bottom 146. The
coolant manifold 178 preferably configured to selectively spray
air, water, or a mixture of air and water against the bottom
146.
[0062] A laser sensor 180 be disposed above the mold cavity 150,
and is preferably structured to monitor the level of molten metal
within the mold cavity 150.
[0063] In use, molten metal will be introduced through the feed
trough 164 into the feed chamber 154. Molten metal may then flow
through the channel 176 into the mold cavity 150. As the level of
molten metal within the mold cavity 150 arises, the gate 168 will
be raised so that molten metal always flows horizontally from the
feed chamber 154 directly on top of the molten metal already in the
mold chamber 150. The feed rate of molten into the mold chamber 150
may be slowed as cooling progresses to control the cooling rate.
Additionally, coolant flowing from the coolant manifold 178 will
change from air to an air/water mixture to all water as casting
progresses to control the cooling rate of the molten metal within
the feed chamber 150. Because coolant may impinge directly on the
metal within the feed chamber 150, it is unnecessary to remove the
bottom 146 during the casting process.
[0064] The present invention therefore provides an apparatus and
method for producing directionally solidified ingots, and cooling
these ingots at a controlled, relatively constant cooling rate. The
invention provides the ability to cast crack-free ingots without
the need for stress relief. The method reduces or eliminates
macrosegregation, resulting in a uniform microstructure throughout
the ingot. The method further produces ingots having a
substantially uniform thickness, and which may be thinner than
ingots cast using other methods. The large surface area in contact
with the coolant results in relatively fast cooling, resulting in
higher productivity.
[0065] While specific embodiments of the invention has been
described in detail, it will be appreciated by those skilled in the
art that various modifications and alternatives to those details
could be developed in light of the overall teachings of the
disclosure. Accordingly, the particular arrangements disclosed are
meant to be illustrative only and not limiting as to the scope of
the invention which is to be given the full breadth of the appended
claims and any and all equivalents thereof.
* * * * *