HIGH STRENGTH, HIGH STRESS CORROSION CRACKING RESISTANT AND CASTABLE Al-Zn-Mg-Cu-Zr ALLOY FOR SHAPE CAST PRODUCTS

Abstract

The present invention provides an Al—Zn—Mg—Cu casting alloy that provides high strength for automotive and aerospace applications and optimized stress corrosion cracking resistance in highly corrosive and tensile environments. The inventive alloy composition includes about 3.5 wt. % to about 5.5 wt. % Zn; about 1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; and a balance of Al and incidental impurities.

1 . An Al—Zn—Mg—Cu aluminum alloy comprising:
 about 4.0 wt. % to about 4.5 wt. % Zn; 
 about 1.2 wt. % to about 1.8 wt. % Mg; 
 greater than about 0.5 wt. % to about 0.85 wt. % Cu; 
 less than about 1.0 wt. % Si; 
 less than about 0.30 wt. % Mn; 
 less than about 0.30 wt. % Fe; 
 a total Mg and Zn content of less than about 6%; and, 
 incidental impurities; 
 wherein, 
 the alloy has a desired fluidity for a shaped casting process and, when the process includes T6 heat treatment, provides a shaped aluminum casting having an improved stress corrosion cracking resistance over a second casting produced from the same composition modified to have less than about 0.5 wt. % Cu and from the same process. 
 
     
     
         2 . The aluminum alloy of  claim 1 , wherein the alloy comprises:
 about 4.0 wt. % to about 4.5 wt % Zn;   about 1.2 wt. % to about 1.8 wt. % Mg;   about 0.65 wt. % to about 0.85 wt. % Cu;   less than about 1.0 wt. % Si;   less than about 0.30 wt. % Mn;   less than about 0.30 wt. % Fe;   a total Mg and Zn content of less than about 6%; and,   incidental impurities; wherein,   
       the process of producing the aluminum casting comprises producing a melt comprising the alloy and casting at least a portion of the melt into a mold to provide a shaped casting; and, heat treating the shaped casting to an overaged condition; and, 
       the time-to-failure under ASTM G103 testing conditions is greater than 96 hours. 
     
     
         3 . The aluminum alloy of  claim 1  further comprising at least one grain refiner selected from a group consisting of boron, carbon and combinations thereof. 
     
     
         4 . The aluminum alloy of  claim 3 , wherein said at least one grain refiner includes a boride in a range from about 0.0025 wt. % to about 0.05 wt. %. 
     
     
         5 . The aluminum alloy of  claim 3 , wherein said at least one grain refiner includes a carbide in a range from about 0.0025 wt. % to about 0.05 wt. %. 
     
     
         6 . The aluminum alloy of  claim 1  further comprising at least one anti-grain growth agent selected from the group consisting of zirconium, scandium, manganese and combinations thereof. 
     
     
         7 . The aluminum alloy of  claim 6 , wherein said at least one anti-grain growth agent includes zirconium in a range below 0.2 wt. %. 
     
     
         8 . The aluminum alloy of  claim 6 , wherein said at least one anti-grain growth agent includes scandium in a range below 0.3 wt. %. 
     
     
         9 . The aluminum alloy of  claim 1 , wherein the magnesium concentration ranges from a concentration of about 1.5 wt. % to 1.8 wt. %, and the ratio of Zn to Mg is less than about 3.0. 
     
     
         10 . The aluminum alloy of  claim 1 , wherein said magnesium is at a concentration of about 1.5 wt. % to 1.8 wt. %. 
     
     
         11 . The aluminum alloy of  claim 1 , wherein the ratio of Zn to Mg is less than about 3.0. 
     
     
         12 . The aluminum alloy of  claim 11 , wherein said copper is at a concentration of about 0.7 wt. % to 0.8 wt. %. 
     
     
         13 . The aluminum alloy of  claim 1 , wherein the ratio of Zn to Mg ranges from about 2.77 to about 3.0. 
     
     
         14 . The aluminum alloy of  claim 1 , wherein the concentration of copper ranges from about 0.6 wt % to about 0.8 wt. %. 
     
     
         15 . A method of making a shaped casting comprising:
 providing an aluminum melt comprising
 about 4.0 wt. % to about 4.5 wt. % Zn; 
 about 1.2 wt. % to about 1.8 wt. % Mg; 
 greater than about 0.5 wt. % to about 0.85 wt. % Cu; 
 less than about 1.0 wt. % Si; 
 less than about 0.30 wt. % Mn; 
 less than about 0.30 wt. % Fe; 
 a total Mg and Zn content of less than about 6%; and 
 incidental impurities; 
   casting at least a portion of the melt into a mold to provide a shaped casting; and,   heat treating the shaped casting to an overaged condition;   wherein,   the alloy has a suitable fluidity for castability and, when cast in a process that includes T6 heat treatment, provides a shaped aluminum casting having an improved stress corrosion cracking resistance over a second casting produced from the same composition modified to have less than about 0.5 wt. % Cu.   
     
     
         16 . The method of  claim 15  wherein, heat treating the shaped casting to the over-aged condition further comprises:
 heating the shaped casting from about room temperature to a temperature in a range of about 200° F. to about 300° F. within a time period of one hour; and, 
 aging the shaped casting at a temperature greater than about 340° F. for greater than about four hours. 
 
     
     
         17 . The method of  claim 16 , wherein the temperature of the aging of the shaped casting ranges from about 340° F. to about 380° F. for greater than about four hours. 
     
     
         18 . The method of  claim 16 , wherein the aluminum melt further comprises a total Mg and Zn content of less than or equal to 6.0 wt. %. 
     
     
         19 . A shaped casting for automotive use, the casting comprising the aluminum alloy of  claim 1 , wherein
 the alloy is cast into a mold to provide the shaped casting for use in an automotive assembly; and, the shaped casting is heat treated to an overaged condition.   
     
     
         20 . The casting of  claim 19 , wherein the time-to-failure under ASTM G103 testing conditions is greater than 96 hours. 
     
     
         21 . The casting of  claim 19 , wherein the ratio of Zn to Mg is less than about 3.0. 
     
     
         22 . A shaped casting comprising an Al—Zn—Mg—Cu alloy, wherein:
 the alloy comprises
 about 4.0 wt. % to about 4.5 wt. % Zn; 
 about 1.2 wt. % to about 1.8 wt. % Mg; 
 about 0.6 wt. % to about 0.85 wt. % Cu; 
 less than about 1.0 wt. % Si; 
 less than about 0.30 wt. % Mn; 
 less than about 0.30 wt. % Fe; 
 a total Mg and Zn content of less than about 6%; and, 
 incidental impurities; 
 
 and, 
 the casting is produced from a process comprising producing a melt comprising the alloy having a desired fluidity for the casting process and casting at least a portion of the melt into a mold to provide a shaped casting; and, heat treating the shaped casting to an overaged condition, the process including a T6 heat treatment. 
 
     
     
         23 . The casting of  claim 22 , wherein the time-to-failure under ASTM G103 testing conditions is greater than 96 hours. 
     
     
         24 . The casting of  claim 22 , wherein the magnesium concentration ranges from a concentration of about 1.5 wt. % to 1.8 wt. %, and the ratio of Zn to Mg is less than about 3.0.