Patent Application
20070012416
1. Field of the Invention
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.
2. Description of the Related Art
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. Ruhle 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° F. to 475° F., until heat from the surrounding melt increases this region to 540° 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.
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.
U.S. Pat. No. 4,969,502, issued to Eric L. Mawer on Nov. 13, 1990, discloses 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.
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 gas 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 castin, or for low stresses within the casting.
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 and lower pinch rollers is reciprocated into and out of the outlet end of the mold to draw 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.
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.
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.
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 a 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.
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 by the mesh section, thereby permitting the coolant to directly contact the solidified metal, and maintain a desired cooling rate.
In the case of the perforated plate substrate, the mold bottom need not be removed.
Accordingly, it is an object of the present invention to provide an improved method of directionally solidifying castings during cooling.
It is another object of the invention to provide a method of maintaining a relatively constant solidification rate during the solidification of the casting.
It is a further object of the invention to provide a casting method having minimized waste.
It is another object of the invention to provide a casting method resulting in a uniform crystal structure within the material.
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.
It is another object of the invention to provide a casting having a more uniform structure.
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.
These and other objects of the invention will become more apparent through the following description and drawing.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
Like reference characters denote like elements throughout the drawings.
The present invention provides an apparatus and method of unidirectionally solidifying a casting, while also providing for a controlled, uniform solidification rate.
Referring to
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 spout 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.
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
Referring back to
In use, the conveyor 20 will be in the position illustrated in
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
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.
Another embodiment of a mold 84 is illustrated in
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.
A molten metal feed chamber 118 is defined by the walls 120, 122, and 124 along with the wall 108 and fixed Doorplate 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.
A coolant manifold 132 is disposed below the removable Doorplate 112. The coolant manifold 132 is preferably configured to selectively spray air, water, or a mixture of air and water against the removable Doorplate 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.
In use, the removable Doorplate 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.
In one example of a casting process according to the present invention, 7085 aluminum alloy was cast into a 9″×13″×7″ ingot using a mold 100 as shown in
Referring to
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 and solidified 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.
Referring to
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.
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 top of the mold cavity 150. The slot closure members 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.
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.
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.
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 metal 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.
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.
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.
