Abstract:
A die casting method for use with semi-solid metal billets allows the removal of air from portions of the billet as it is compressed and deformed to flow into a cavity of a die. The die is formed in a manner that captures other potential defects and impurities in portions of the finished die cast product that can easily be removed subsequent to the completion of the die casting operation. The air is removed through passages formed in a generally cylindrical rod that extends through a second surface of the die to allow the air to flow from pockets formed by a concavity of the billet and through the air conduit to the atmosphere.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to semi-solid die casting and, more particularly, to a process for reducing the likelihood that impurities and air entrapment will adversely affect the quality of a die cast component. 
     2. Description of the Related Art 
     Those skilled in the art of die casting and, more particularly, die casting of semi-solid material are familiar with various processes and procedures associated with die casting methods. 
     U.S. Pat. No. 3,650,312, which issued to Allen on Mar. 21, 1972, describes a hybrid casting-hot working process for shaping magnesium, aluminum, zinc and other die casting metals. Molten metal is thickened by addition of high surface area aerogel powder to produce a thixotropic mass which is workable at low working forces and retains its worked shape after removal of the applied working forces. Cooling to below melting temperatures solidifies the mass into a product having the metallurgical structural characteristics of a forged metal notwithstanding the use of casting methods in fabrication. 
     U.S. Pat. No. 5,879,478, which issued to Loue et al. on Mar. 9, 1999, describes a process for semi-solid forming of thixotropic aluminum-silicon-copper alloys. It relates to an aluminum alloy for thixoforming with a particular composition which, when reheated to the semi-solid state to the point at which a liquid fraction ratio between 35 and 55% is obtained, has an absence of non-remelted polyhedral silicon crystals. 
     U.S. Pat. No. 5,219,018, which issued to Meyer on Jun. 15, 1993, describes a method of producing thixotropic metallic products by continuous casting with polyphase current electromagnetic agitation. The invention relates to a continuous casting method for producing thixotropic metallic alloys containing degenerated dendrites. It consists of casting the liquid metal in a movable occluded mould consisting of a hot upstream zone produced from insulating material and a cooled downstream zone in which the metal solidifies, while carrying out by means of a sliding magnetic field, obtained by a series of polyphase inductors, an electromagnetic agitation which causes the dendrites formed in the cold zone to pass into the hot zones where they change to nodules by superficial refusion. 
     U.S. Pat. No. 5,865,238, which issued to Carden et al. on Feb. 2, 1999, describes a process for die casting of metal matrix composite materials from a self-supporting billet. The billet is formed of an aluminum alloy matrix and ceramic particles. It is heated in an oxygen containing atmosphere forming an aluminum oxide surface and softening the matrix alloy. The semi-solid billet then is compressed in a die casting sleeve and the softened matrix material is displaced into a die to form the shape. 
     U.S. Pat. No. 6,098,700, which issued to Carden et al. on Aug. 8, 2000, describes an apparatus for die casting of metal matrix composite materials from a self-supporting billet. The billet is composed essentially of a metal alloy matrix and dispersed ceramic particles. It comprises heating means to soften the metal alloy, a horizontal plunger to drive and to compress the billet, a die through which the softened metal matrix and ceramic particles are formed into a shape defined by the interior surface of the die, and cooling means to maintain the temperature of the interior surface of the die at a predetermined temperature. 
     U.S. Pat. No. 6,399,017, which issued to Norville et al. on Jun. 4, 2002, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. A container system includes a vessel for holding a thixotropic semi-solid aluminum alloy slurry during its processing as a billet and an ejection system for cleanly discharging the processed thixotropic semi-solid aluminum billet. The crucible is preferably formed from a chemically and thermally stable material. The crucible defines a mixing volume. The crucible ejection mechanism may include a movable bottom portion mounted on a piston or may include a solenoid coil for inducing an electromotive force in the electrically conducting billet for urging it from the crucible. 
     U.S. Pat. No. 6,402,367, which issued to Lu et al. on Jun. 11, 2002, discloses a method and apparatus for magnetically stirring a thixotropic metal slurry. The aluminum alloy comprises a first solid particulate phase suspended in a second liquid phase so as to maintain its thixotropic character by degenerating forming dendritic particles into spheroidal particles while simultaneously equilibrating the melt temperature by quickly transferring heat between the melt and its surroundings. The melt is stirred by a magnetomotive force field generated by a stacked stator assembly. 
     U.S. Pat. No. 6,432,160, which issued to Norville et al. on Aug. 13, 2002, discloses a method and apparatus for making a thixotropic metal slurry. It comprises simultaneously controlledly cooling and stirring the melt to form solid particles of a first phase suspended in a residual liquid second phase. Vigorous stirring of the metallic melt results in the formation of degenerate dendritic particles having substantially spheroidal shapes. 
     U.S. Pat. No. 6,637,927, which issued to Lu et al. on Oct. 28, 2003, discloses a method and apparatus for magnetically stirring a thixotropic metal slurry. A melt is stirred by a magnetomotive force field generated by a stacked stator assembly. The stacked stator assembly includes a stator ring adapted to generate a linear/longitudinal magnetic field positioned between two stator rings adapted to generate a rotational magnetic field. The stacked stator rings generate a substantially spiral magnetomotive mixing force and define a substantially cylindrical mixing region therein. 
     U.S. Pat. No. 6,932,938, which issued to Norville et al. on Aug. 23, 2005, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. A crucible is preferably formed from a chemically and thermally stable material such as graphite or a ceramic. The crucible defines a mixing volume. The crucible ejection mechanism may include a movable bottom portion mounted on a piston or may include a solenoid coil for inducing an electromotive force in the electrically conducting billet for urging it from the crucible. 
     U.S. Pat. No. 6,962,189, which issued to Buckley on Nov. 8, 2005, describes a method of making precision castings using thixotropic materials. Precision castings requiring a fine finish and having complex internal geometries can be produced by casting a semi-solid thixotropic metal alloy within or about a meltaway material component in the form of a core and/or a die insert that has a lower melting point than the solid to semi-solid transition temperature of the thixotropic alloy. 
     U.S. Pat. No. 7,132,077, which issued to Norville et al. on Nov. 7, 2006, discloses a method and apparatus for containing and ejecting a thixotropic metal slurry. During processing, a molten aluminum alloy precursor is transferred into the crucible and vigorously stirred and controllably cooled to form a thixotropic semi-solid billet. Once the billet is formed, the ejection mechanism is activated to discharge the billet from the crucible. The billet is discharged into a shot sleeve and immediately placed in a mold and molded into a desired form. 
     U.S. Pat. No. 7,296,611, which issued to Hirai et al. on Nov. 20, 2007, describes a method and apparatus for manufacturing metallic parts by die casting. An injection molding apparatus includes a melt furnace and a metal supply system located in the melt furnace. The metal supply system includes a pump. The injection molding apparatus also includes a first metal inlet from the melt furnace to the metal supply and a vertical injection mechanism adapted to inject liquid metal into a die system. The injection molding apparatus also includes a second metal inlet from the metal supply system to the vertical injection mechanism. 
     The patents described above are hereby expressly incorporated by reference in the description of the present invention. 
     SUMMARY OF THE INVENTION 
     A die casting method, in accordance with a preferred embodiment of the present invention, comprises the steps of providing a die having a cavity which generally defines the shape of a component to be cast, wherein the die has an opening formed in a first surface of the cavity and a conduit formed in a second surface of the cavity, pushing a billet through the opening and toward the second surface, causing the billet to deform and flow into the cavity to form the component, and directing air to flow through the conduit and away from the cavity as the billet is deformed. 
     In a particularly preferred embodiment of the present invention, the billet is a semi-solid metal, such as aluminum, as the pushing step is commenced and the air is directed to flow through a plurality of axial passages formed in a generally cylindrical rod. The billet has a rear surface against which a force is exerted to accomplish the pushing step and a generally concave surface which is the first portion of the billet to contact the second surface. In a particularly preferred embodiment of the present invention, it further comprises the steps of moving the concave surface into contact with the second surface and exerting a continuing force to force the billet against the second surface and cause air to flow from within a depression of the concave surface and into the conduit. It can further comprise the step of selecting a billet size which results in a portion of the billet, proximate the rear surface of the billet, extending from the cavity opening after a majority of the billet has flowed into the cavity and the component is formed. It can further comprise the step of removing an outer cylindrical portion of the billet as it moves toward the second surface. The outer cylindrical portion of the billet can be removed by scraping it from the billet because the outer dimension of the billet is greater than a minimum dimension of the opening. The present invention can further comprise the step of receiving a portion of the concave surface in a part of the cavity which is associated with a part of the component that is intended to be discarded after the component is removed from the die. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully and completely understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which: 
         FIG. 1  shows a known type of die casting machine; 
         FIG. 2  shows a product that can be made by the present invention; 
         FIG. 3  shows a semi-solid billet prior to the die casting process of the present invention; 
         FIGS. 4A-4C  show three sequential stages of practicing a method of a preferred embodiment of the present invention; 
         FIG. 5  is an enlarged partial sectional view of  FIG. 4B ; 
         FIGS. 6 and 7  show an alternative embodiment of the present invention; and 
         FIG. 8  is an enlarged view of a rod having air conduits formed on its outer cylindrical surface. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the description of the preferred embodiment of the present invention, like components will be identified by like reference numerals. 
     As is generally known to those skilled in the art, certain conditions which exist during a die casting procedure can result in flaws in a finished die cast component. As an example, entrained air can be captured within portions of the finished die cast component. This can lead to unacceptable surface conditions or weakened portions of the die cast product. In addition, entrapped aluminum oxide within the die cast component can produce flaws or weakened regions. When die casting semi-solid metal, such as aluminum, the shape of the billet prior to the die casting process can adversely affect the quality of the finished product. The various embodiments of the present invention are directed to the avoidance of casting flaws that result from certain processing conditions which will be described in greater detail below. 
       FIG. 1  is a schematic illustration of a die casting machine which comprises a cylinder  10 , a plunger  12 , a trough  14  and a die  16 . The die  16  defines a cavity  18  which has an opening  20 . A billet  24  is positioned to be pushed by the plunger  12 , or piston, to the right in  FIG. 1 , through the opening  20  and into the cavity  18  to form a die cast component. The die  16  comprises two portions,  26  and  27 , which can be separated to remove the component from the cavity  18  after the metal solidifies. As described in U.S. Pat. Nos. 5,863,238 and 6,098,700, the billet  24  can be a semi-solid metal, such as aluminum. 
       FIG. 2  is an isometric representation of an anchor bracket for use in conjunction with an outboard motor. A preferred embodiment of the present invention will be described below in conjunction with the anchor bracket  30  shown in  FIG. 2 . 
       FIG. 3  illustrates a billet  40  of semi-solid metal, such as aluminum, which is intended for use in conjunction with a preferred embodiment of the present invention. The billet  40  has a rear surface  42  and a generally concave surface  44 . The concavity of the concave surface  44  results from its method of manufacture. Since the molten metal is rotationally stirred during its transition to the semi-solid state, the concave end  44  retains this shape as it becomes semi-solid. In addition, the metal contracts during this process to further deepen the depression in the concave end  44 . As will be described in greater detail below, the billet  40  will eventually become the finished component illustrated in  FIG. 2  after the die casting process is complete and certain portions of the die cast component are removed. 
       FIGS. 4A-4C  illustrate a die cast machine during sequential periods of transforming the billet  40  of  FIG. 3  into the die cast component shown in  FIG. 2 . 
     In  FIG. 4A , a die cast machine  50  is schematically illustrated. A plunger  52 , or piston, is positioned to exert a force (toward the right in  FIG. 4A ) against the rear surface  42  of the billet  40 . A die  54  comprises two separable portions,  56  and  58 . The die  54  has a cavity  60  which generally defines the shape of a component (such as the anchor bracket  30  described above in conjunction with  FIG. 2 ) to be cast. The die has an opening which is shown between points  61  and  62  in  FIG. 4A . The opening is formed in a first surface  64  of the cavity  60 . A conduit, which can be a plurality of passages  66 , is formed in a second surface  70  of the cavity  60 . The air passages  66  which make up the conduit can be a plurality of axial flat strips machined onto the outer surface of a rod  74  or any other technique which allows the passage of air from the cavity  60  to a point of atmospheric pressure, such as region  76  which is outside of the die  54 . 
     In  FIG. 4A , the billet  40  is shown prior to its passage through the opening  80 . The plunger  52 , or piston, can push the billet  40  toward the right in  FIG. 4A , through opening  80 , and toward the second surface  70 . As the plunger  52  continues to exert a force in a direction toward the right in  FIG. 4A , the billet  40  is caused to move into contact with the second surface  70  and begin to deform as it continues to be compressed by the plunger  52 . This, in turn, causes the semi-solid to metal of the billet  40  to flow into the upper portions of the cavity  60 . 
       FIG. 4B  shows the billet  40  after it has been pushed by the plunger  52  into contact with the second surface  70 . The opening  80  of the die  54  causes the diameter of the billet  40  to be reduced as a result of its outer surface being scraped by the dimension of the opening  80  compared to the original diameter of the billet  40 . This scraping tends to remove an outer oxidation surface from the billet  40  and push that cylindrical surface toward the left in  FIG. 4B  relative to the billet  40  which continues to move toward the right. In  FIG. 4B , the billet  40  is shown as it would generally appear at the instant when its concave surface moves into contact with the second surface  70  of the die  54 . 
     With reference to  FIGS. 3 and 4B , it should be understood that air is typically trapped within the concavity of the concave end  44  of the billet  40 . One of the several important functions of the present invention is to provide a way to allow the air within the concavity to be removed through the second surface  70  so that it is not entrained within the semi-solid metal as that metal moves upwardly into the component forming portion of the cavity  60 . 
       FIG. 4C  shows the procedure at a later stage than  FIGS. 4A and 4B . In  FIG. 4C , the billet has been pushed into the cavity  60  to the full operational travel of the plunger  52 . This has caused it to be deformed and the continuous force exerted by the plunger  52  also caused the semi-solid metal to flow upwardly into the cavity  60  to form the finished die cast component. 
     With continued reference to  FIG. 4C , several benefits of the present invention should be noted. First, the air is allowed to escape from the concavity of the billet through the axial passages  66  of the generally cylindrical rod  74  so that it is not trapped within the solidified metal when the component is completed. Also, the material on the surface of the concavity at the concave end  44  of the billet  40 , as described above in conjunction with  FIG. 3 , is trapped within the protrusion  90  that extends toward the right of the bottom portion of the finished part. Furthermore, the rear surface  42  of the billet is easily removed with the scrapped portion which is the lower part of the finished die cast product. That rear surface  42  remains in the region identified by reference numeral  84  in  FIG. 4C . Because the rear surface  42  of the billet  40  is typically partially solidified because of its contact with various colder plungers that are used during the die casting process, its more solid state would adversely affect the quality of the finished part if it was allowed to become a part of the final component. Instead, that rear surface  42  is contained within portion  84  after the semi-solid metal solidifies and is removed from the die  54 . Therefore, portions  84  and  90  of the finished die cast product are likely to contain impurities which can easily be removed from the final component after it is removed from the die  54 . 
     With reference to FIGS.  2  and  4 A- 4 C, it can be seen that the die cast product shown in  FIG. 4C  contains scrap portions in addition to the finished die cast product shown in  FIG. 2 . As an example, the portions of the die cast part above dashed line  100  and below dashed line  102  are removed after the completed component is removed from the die  54 . The remaining portion, between lines  100  and  102 , is the anchor bracket  30  shown in  FIG. 2 . It should be clearly understood that the section views in  FIGS. 4A-4C  are not precisely drawn with respect to a single plane cutting through the die  54  and the die cast component. Instead, the section view is selected so that it can be more easily compared to the actual component shown in  FIG. 2 . For purposes of cross reference, the extensions,  110  and  111 , of the anchor bracket  30  are identified in  FIG. 2  and extension  110  is identified in  FIG. 4C . In addition, protrusions  116  and  117  of the anchor bracket  30  are identified in  FIG. 2  and protrusion  116  is identified in  FIG. 4C . Similarly, the extensions,  114  and  115 , are identified in  FIG. 2  and extension  114  is identified in  FIG. 4C . It can be seen that the anchor bracket  30  in  FIG. 2  is the portion of the die cast product that is between dashed lines  100  and  102  in  FIG. 4C . 
       FIG. 5  is an enlarged section of  FIG. 4B  which is provided in order to more clearly show the air conduit which is provided for the purpose of reducing the amount of entrained air that is allowed to flow upwardly into the finished component along with the semi-solid metal as described above. The air conduit compresses a plurality of flat portions  66  that form air passages away from the concavity of the concave surface  44 , through the second surface  70  of the die and toward region  76  which is at atmospheric pressure. This flow of air is illustrated by the dashed line arrows in  FIG. 5 . As the billet  40  is continually compressed by the plunger, as described above, the concavity of the concave surface  44  continues to collapse and expel the air contained within it. This expulsion of air and its passage through the conduit removes the air and prevents it from being entrained in the finished die cast product. 
     With continued reference to  FIGS. 4A-4C  and  5 , a core pin plate  120  is shown with a plurality of core pins  122  attached to it. When the die cast product is properly solidified, the two sections of the die,  56  and  58 , are separated and the core pin plate  120  is pushed toward the die section  58  to eject the finished part from the cavity  60 . The generally cylindrical rod  74 , which is provided with the plurality of air passages  66 , is also urged in the same direction as the core pins  122  during this procedure. It assists in ejecting the part from portion  58  of the die. 
     During this procedure, the air that is originally within the concavity of the concave surface  44  of the billet  40  is allowed to escape through the passages  66  to region  76 . In addition, in at least one embodiment of the present invention, a small amount of semi-solid metal, such as aluminum, travels into the leftmost portions of the passages  66 . Since these passages are small, as will be described in greater detail below, the ejection of the finished component from the die results in these small extensions of metal being pulled out of the passages  66  to remain with a finished die cast product and be removed along with the remaining portions of the product below dashed line  102  in  FIG. 4C . 
       FIG. 6  is an enlarged view of the generally cylindrical rod  74 . In the alternative embodiment of the present invention shown in  FIG. 6 , the passages  66  are shown as semi-circular grooves machined in its outer surface. It should be understood that the machining of semi-circular grooves for this purpose is an alternative embodiment of the present invention. A preferred embodiment comprises flat surfaces milled in the outer cylindrical surface of the rod  74 . 
       FIG. 7  is a section view taken through the rod  74 . It shows the rod  74  disposed within a cylindrical opening of die portion  58 . It should be understood that  FIGS. 6 and 7  are intended to illustrate the air passages  66  more clearly than is possible if the actual preferred embodiment used in practice is illustrated. This can be seen in  FIG. 8  which shows an illustration of an air passage  66  formed by machining a flat surface  130  in the outer cylindrical surface  132  of the rod  74 . The actual distance, measured along a line perpendicular to surface  130  and along a diameter of the rod  74 , is approximately 0.008 to 0.010 inches in length. This very small air passage  66  is sufficient to allow the air to flow out of the concavity at the concave end of the billet, but the semicircular grooves shown in  FIGS. 6 and 7  are provided to more clearly show the relative positions of the air passages  66  and their number. In a preferred embodiment of the present invention, eight air passages  66  were formed by machining the flat surfaces  130 , described above in conjunction with  FIG. 8 , at generally equally spaced positions around the outer cylindrical surface of the rod  74 . 
     As described above in conjunction with  FIGS. 1-8 , the method of the present invention provides several important advantages when die casting a product from a semi-solid billet  44 . The manufacture of the billet  44  results in a rear surface  42  that can be partially solidified from its contact with colder plungers used to remove it from a furnace and another plunger  52  used to push it into the die  54 . This potentially solidified portion  140  of the billet  40  illustrated in  FIG. 3  could cause defects in a finished die cast component. However, that portion  140  is retained in the region identified by reference numeral  84  in  FIG. 4C . It is removed, as is portion  90 , when the part of the finished die cast product below dashed line  102  is removed. In addition to the provisions of portions  84  and  90  for these purposes, the air passages  66  and the rod  74  remove air from the concavity of the concave surface  44  to avoid entraining the air in the finished product. 
     Although the present invention has been described with particular detail and illustrated to show preferred embodiments, it should be understood that alternative embodiments are also within its scope.