Semi-solid molding method and apparatus

Abstract

A metal alloy is heated to a molten state, and a grain refiner may be added. The molten alloy is poured into a shallow chamber of a shot sleeve of a vertical die cast press and on top of a shot piston. The shot sleeve is transferred to an injection station while the molten alloy cools to a semi-solid slurry with a globular, generally non-dendritic micro structure. An inner portion of the shallow slurry is injected upwardly by the piston through a gate opening into a die cavity while an outer more solid portion of the slurry is entrapped in an annular recess. After the slurry solidifies, the shot piston retracts, and the shot sleeve is transferred to a position where the residual biscuit is removed. The shot piston may have internal grooves to provide a large heat transfer area for cooling water to absorb heat from the molten alloy.

1 . A method of semi-solid molding a high strength metal part within a die cavity defined by a die set mounted on a vertical die cast press, the press including a shot sleeve having a generally vertical axis and enclosing a shot piston movable axially within the sleeve with the sleeve and piston defining a shot chamber above the piston, the method comprising the steps of: 
 melting a solid metal to form a molten metal,    confining the molten metal within the shot chamber,    cooling the molten metal within the shot chamber to within a predetermined temperature range while the molten metal within the shot chamber has a horizontal width substantially greater than its vertical depth and without stirring the molten metal to form a substantially quiescent and shallow semi-solid slurry having a globular micro structure,    moving the shot piston upwardly within the shot chamber to inject an inner portion of the semi-solid slurry from the shot chamber into the die cavity through at least one gate opening within an inner portion of the shot chamber, and    allowing the semi-solid slurry to solidify within the die cavity to form the metal part.    
   
   
       2 . A method as defined in  claim 1  wherein the molten metal is cooled within the shot chamber into the semi-solid slurry while the molten metal has a horizontal width at least twice the vertical depth of the molten metal.  
   
   
       3 . A method as defined in  claim 1  including the steps of: 
 forming a downwardly facing annular entrapment recess above the shot chamber and generally in axial alignment with an inner surface of the shot sleeve, and    trapping a more solidified outer portion of the semi-solid slurry adjacent the shot sleeve within the entrapment recess in response to upward movement of the shot piston.    
   
   
       4 . A method as defined in  claim 1  wherein the molten metal is cooled within the shot chamber to a temperature range which produces a range of 40% to 60% solid to form the semi-solid slurry.  
   
   
       5 . A method as defined in  claim 1  wherein the molten metal is A356 aluminum alloy and is cooled within the shot chamber to a temperature within the range of 570° C. to 590° C. to form the semi-solid slurry.  
   
   
       6 . A method as defined in  claim 1  and including the steps of: 
 confining the molten metal within a second shot chamber receiving a second shot piston,    interchanging the second shot chamber and piston with the first shot chamber and piston after the inner portion of the semi-solid slurry is injected from the first shot chamber into the die cavity, and    cooling the molten metal within the second shot chamber to within the temperature range while the molten metal within the second shot chamber has a horizontal width substantially greater than its vertical depth and without stirring the molten metal to form a second charge of the semi-solid slurry.    
   
   
       7 . Apparatus for molding a metal part within a die cavity defined by a die set mounted on a vertical die cast press, said press comprising a shot sleeve having a generally vertical axis, a shot piston movable axially within said sleeve with said sleeve and said piston defining a shot chamber above said piston, said shot chamber having a horizontal width substantially greater than its vertical depth, said die set defining a downwardly facing annular entrapment recess above said shot chamber and generally in axial alignment with an inner surface of said shot sleeve, a power operated member connected to move said shot piston upwardly within said shot chamber to inject molten metal within said shot chamber into the die cavity through a gate opening within said die set, and said entrapment recess being effective to trap a more solidified outer portion of the molten metal adjacent said shot sleeve in response to the upward movement of said shot piston.  
   
   
       8 . Apparatus as defined in  claim 7  wherein said shot chamber has a horizontal width at least twice the vertical depth of said chamber.  
   
   
       9 . Apparatus as defined in  claim 7  wherein said piston defines passages for circulating cooling fluid within said piston, and said passages include spaced interconnected grooves within an upper portion of said piston.  
   
   
       10 . Apparatus as defined in  claim 9  wherein said interconnected grooves comprise concentrically spaced grooves interconnected by ports providing a maze path for circulating cooling fluid.  
   
   
       11 . Apparatus as defined in  claim 9  wherein said interconnected grooves are defined within a bottom surface of a removable said shot piston.  
   
   
       12 . Apparatus for molding a metal part within a die cavity defined by a die set mounted on a vertical die cast press, said press comprising a shot sleeve having a generally vertical axis, a shot piston movable axially within said sleeve, said sleeve and said piston defining a shot chamber above said piston, a power operated member connected to move said shot piston upwardly within said shot chamber to inject molten metal within said shot chamber into the die cavity through a gate opening within said die set, said piston defining passages for circulating cooling fluid within said piston, and said passages include interconnected horizontally spaced grooves within said piston to provide a large heat transfer area for the cooling fluid to absorb heat from the molten metal.  
   
   
       13 . Apparatus as defined in  claim 12  wherein said grooves comprise concentrically spaced grooves interconnected by ports providing a maze path for circulating the cooling fluid.  
   
   
       14 . Apparatus as defined in  claim 12  wherein said interconnected grooves are defined within a bottom surface of a removable said shot piston.  
   
   
       15 . Apparatus as defined in  claim 12  wherein said shot chamber has a horizontal width substantially greater than its vertical depth.