Magnesium-containing, aluminum-based alloy for thin-wall castings

Abstract

An aluminum-magnesium alloy is disclosed which provides superior properties for casting in steel dies and good ductility for forming castings of complex shapes, including thin-wall portions. The aluminum-based alloy contains, in weight percent, about 2-15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2% titanium, and a minimal content of iron. In its molten state this aluminum-magnesium-chromium alloy can be pushed into the molding cavities of iron-based dies in a high pressure die casting procedure and conform to complexly-shaped die surfaces with thin cavity portions without dissolving appreciable amounts of iron or experiencing die soldering on the die surfaces. The resulting castings display good strength and ductility and can be further enhanced by an artificial aging process after solution heat treatment.

1 . An aluminum-based die-casting alloy consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2 percent titanium, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum. 
     
     
         2 . An aluminum-based die-casting alloy consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2 percent titanium, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum; a cast specimen of the alloy, without heat treatment of the cast specimen, having a tensile strength of at least 250 MPa, an ultimate tensile strength of 280 MPa, and a total elongation of at least 15 percent, each determined on a test specimen at 25° C. 
     
     
         3 . An aluminum-based die-casting alloy consisting essentially, by weight, of 5 to 9 percent magnesium, 0.25 to 0.35 percent chromium, 0.15 to 0.35% manganese, 1.0 to 3.0 percent silicon, 0.05 to 0.1 percent titanium, less than 0.15 percent iron, less than 0.01 percent copper, less than 0.01 percent zinc, less than 0.003 percent phosphorus, less than 0.03% strontium, and the balance aluminum. 
     
     
         4 . A method of die-casting an article using an aluminum-based alloy, the article having at least one thin-wall section with a thickness of three millimeters or less, the article being cast in a mold cavity of a die formed of an iron-based alloy; the method comprising:
 injecting a mold-filling volume of a molten aluminum-based alloy into the mold cavity of the iron-based alloy die, the cavity having surfaces formed by separable, facing mold members and a portion of the cavity defining each thin-wall section and maintaining a predetermined pressure on the molten aluminum-based alloy to force the molten alloy into full conformance with the surfaces of the cavity, the molten aluminum-base alloy having a composition consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5% manganese, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum; and   cooling the molten aluminum-based alloy to form the solid shape of the article and removing the solid die-cast article shape.   
     
     
         5 . A method of die-casting an article as stated in  claim 4  in which the cast molten aluminum-based alloy consists essentially, by weight of 5 to 9 percent magnesium, less than 0.15 percent iron, 0.25 to 0.35 percent chromium, 0.15 to 0.35% manganese, less than 0.01 percent copper, 2.0 to 3.0 percent silicon, less than 0.01 percent zinc, 0.05 to 0.1 percent titanium, less than 0.003 percent phosphorus, less than 0.01 percent strontium, and the balance aluminum. 
     
     
         6 . A method of die-casting an article as stated in  claim 4  in which the temperature of the volume of molten aluminum-based alloy injected into the mold cavity is in the range of 670° C. to 730° C. 
     
     
         7 . A method of die casting an article as stated in  claim 4  in which the facing mold members are maintained at a temperature in the range of 180° C. to 230° C. during the formation of the cast article. 
     
     
         8 . A method of die-casting an article as stated in  claim 4  in which the facing mold members are maintained at a temperature in the range of 180° C. to 230° C. during the formation of the cast article and the cast article is removed from the mold members at a cast article temperature in the range of 180° C. to 230° C. 
     
     
         9 . A method of die-casting an article as stated in  claim 4  in which the die-cast article shape is removed from the mold and, before further cooling, re-heated in a precipitation hardening process. 
     
     
         10 . A method of die-casting an article as stated in  claim 4  in which the die-cast article shape is (i) removed from the mold and re-heated to a temperature in the range of 480° C. to 540° C. for a period of two to six hours to promote solution of separate phases in the microstructure of the cast article, (ii) rapidly cooled to a temperature below 100° C., and (iii) then reheated to a temperature in the range of 180° C. to 250° C. for a period to produce precipitation hardening in the cast article.