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.

We claim: 
     
         1 . A cast aluminum part 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, 
 a cast aluminum part is produced from a process comprising
 producing a melt comprising the alloy having a fluidity which exceeds a length of 7 cm in a spiral mold for the casting process; 
 casting at least a portion of the melt into a mold to provide the cast aluminum part; and, 
 heat treating the cast aluminum part to an overaged condition, the process including a T6 heat treatment and aging at a temperature greater than about 340° F. for greater than about four hours; 
 
 wherein, the cast aluminum part has a time-to-failure of greater than 96 hours under ASTM G103 testing conditions for stress corrosion cracking. 
 
     
     
         2 . The cast aluminum part of  claim 1 , wherein heat treating the cast aluminum part to the overaged condition further comprises:
 heating the cast aluminum part 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.   
     
     
         3 . The cast aluminum part of  claim 1 , wherein the aluminum alloy melt comprises about 0.65 wt. % to about 0.85 wt. % Cu., and the aging of the cast aluminum part occurs at a temperature ranging from about 340° F. to about 380° F. for greater than about four hours 
     
     
         4 . The cast aluminum part of  claim 1 , wherein the time-to-failure under ASTM G103 testing conditions is greater than 96 hours. 
     
     
         5 . The cast aluminum part 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. 
     
     
         6 . The cast aluminum part of  claim 1 , wherein the Cu concentration ranges from 0.65 wt % to 0.85 wt % and the ratio of Zn to Mg ranges from about 2.7 to about 3.8. 
     
     
         7 . The cast aluminum part of  claim 1 , wherein the copper concentration ranges from 0.65 wt % to 0.85 wt % and the ratio of Zn to Mg ranges is about 2.7, about 3.3, or about 3.8. 
     
     
         8 . The cast aluminum part of  claim 1 , wherein the Cu concentration ranges from 0.65 wt % to 0.85 wt % and total Mg and Zn content ranges from 5.2 wt % to 5.7 wt %. 
     
     
         9 . A method of making a cast aluminum part comprising:
 creating an aluminum alloy 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 cast aluminum part; and,   heat treating the cast aluminum part to an overaged condition, the heat treating including a T6 heat treatment and aging at a temperature greater than about 340° F. for greater than about four hours;   wherein,   the melt has a fluidity which exceeds a length of 7 cm in a spiral mold for castability; and,   the cast aluminum part produced by the method has an elongation of 8% or greater;   and, a time-to-failure of greater than 96 hours under ASTM G103 testing conditions for stress corrosion cracking.   
     
     
         10 . The method of  claim 9 , wherein heat treating the cast aluminum part to the overaged condition further comprises:
 heating the cast aluminum part 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.   
     
     
         11 . The method of  claim 9 , wherein the aluminum alloy melt comprises about 0.65 wt. % to about 0.85 wt. % Cu., and the aging of the cast aluminum part occurs at a temperature ranging from about 340° F. to about 380° F. for greater than about four hours. 
     
     
         12 . The method of  claim 9 , wherein the ratio of Zn to Mg ranges from about 2.66 to about 3.75. 
     
     
         13 . 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 fluidity which exceeds a length of 7 cm in a spiral mold for a shaped casting process; and, 
 when used in a casting process that includes T6 heat treatment and aging at a temperature greater than about 340° F. for greater than about four hours, provides a shaped aluminum casting having (i) an elongation of 8% or greater and (ii) a time-to-failure of greater than 96 hours under ASTM G103 testing conditions for stress corrosion cracking. 
 
 
     
     
         14 . The aluminum alloy of  claim 13 , wherein the alloy comprises about 0.65 wt. % to about 0.85 wt. % Cu. 
     
     
         15 . The aluminum alloy of  claim 13  further comprising at least one grain refiner selected from a group consisting of boron, carbon and combinations thereof. 
     
     
         16 . 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. 
     
     
         17 . 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. 
     
     
         18 . The aluminum alloy of  claim 1 , wherein said magnesium is at a concentration of about 1.5 wt. % to 1.8 wt. %. 
     
     
         19 . The aluminum alloy of  claim 1 , wherein the ratio of Zn to Mg is less than about 3.0. 
     
     
         20 . The aluminum alloy of  claim 11 , wherein said copper is at a concentration of about 0.7 wt. % to 0.8 wt. %. 
     
     
         21 . The aluminum alloy of  claim 1 , wherein the ratio of Zn to Mg ranges from about 2.77 to about 3.0. 
     
     
         22 . The aluminum alloy of  claim 1 , wherein the concentration of copper is selected from the group consisting of about 0.6 wt. %, about 0.7 wt. %, and about 0.8 wt. %. 
     
     
         23 . The aluminum alloy of  claim 1 , wherein the ratio of Zn to Mg ranges from about 2.66 to about 3.75.