Method for producing high-strength aluminum-zinc-magnesium-copper alloys

Abstract

Methods of manufacturing extruded aluminum alloy products are disclosed. Aluminum alloy billets including Zn, Mg and Cu are subjected to a multiple-step homogenization process, followed by hot working into an extruded product, solution heat treating, quenching, stretching and artificial aging with multiple aging steps. The resultant extruded products possess desirable mechanical properties in combination with desirable electrical conductivity and corrosion resistance properties.

What is claimed is: 
     
         1 . A method of manufacturing an extruded aluminum alloy product comprising:
 homogenizing a cast unwrought billet having a composition comprising from 8.40 to 9.10 weight percent Zn, from 1.80 to 2.10 weight percent Mg, from 1.90 to 2.33 weight percent Cu, from 0.01 to 0.05 weight percent Ti, and from 0.10 to 0.14 weight percent Zr, with the balance comprising aluminum and incidental impurities in multiple steps comprising a first homogenization step at a temperature of from 820° F. to 850° F. for from 6 to 30 hours followed by another homogenization step at a temperature of from 870° F. to 890° F. for 30 to 58 hours, wherein a total equivalent time of the multiple steps at 880° F. is from 28 to 77 hours;   hot working the billet into an extruded product;   subjecting the product to a solution heat treatment at a temperature of from 870 to 890° F.;   quenching the solution heat treated product;   stretching the product with a permanent set of from 1 to 3 percent; and   artificially aging the product in multiple aging steps comprising a first aging step at a temperature of from 212° F. to 275° F. for 4 to 36 hours followed by a second aging step at a temperature of from 292° F. to 335° F. for 4 to 18 hours, wherein the artificially aged product has a density of from 0.1036 to 0.1041 lbs/in 3  and:
 (a) a combined tensile yield strength and elongation (TYS+e) greater than or equal to −3.25*X+221.75 where X is an electrical conductivity of from 35% IACS to 39% IACS, and the product has a density of from 0.1036 to 0.1041 lbs/in 3 ; or 
 (b) a combined tensile yield strength and residual strength (TYS+RS) greater than or equal to 5.57148*X′−64.343 where X′ is an electrical conductivity of from 36% IACS to 37.5% IACS. 
   
     
     
         2 . The method of  claim 1 , wherein the composition comprises from 8.60 to 9.08 weight percent Zn, from 1.90 to 2.10 weight percent Mg, from 2.10 to 2.33 weight percent Cu, from 0.01 to 0.04 weight percent Ti, and from 0.10 to 0.13 weight percent Zr. 
     
     
         3 . The method of  claim 1 , wherein the composition comprises from 8.65 to 9.05 weight percent Zn, from 1.95 to 2.10 weight percent Mg, from 2.13 to 2.33 weight percent Cu, from 0.01 to 0.03 weight percent Ti, and from 0.10 to 0.12 weight percent Zr. 
     
     
         4 . The method of  claim 1 , wherein the composition comprises 8.81 weight percent Zn, 2.01 weight percent Mg, and 2.16 weight percent Cu. 
     
     
         5 . The method of  claim 1 , wherein the composition comprises from 8.50 to 9.00 weight percent Zn, from 1.90 to 2.10 weight percent Mg, from 1.95 to 2.20 weight percent Cu, from 0.01 to 0.04 weight percent Ti, and from 0.10 to 0.13 weight percent Zr. 
     
     
         6 . The method of  claim 1 , wherein the composition comprises from 8.55 to 8.95 weight percent Zn, from 1.95 to 2.05 weight percent Mg, from 2.00 to 2.15 weight percent Cu, from 0.01 to 0.04 weight percent Ti, and from 0.10 to 0.12 weight percent Zr. 
     
     
         7 . The method of  claim 1 , wherein the composition comprises 8.76 weight percent Zn, 2.00 weight percent Mg, and 2.07 weight percent Cu. 
     
     
         8 . The method of  claim 1 , wherein the artificially aged product exhibits an REX grain fraction of less than 15 volume percent. 
     
     
         9 . The method of  claim 1 , wherein the artificially aged product exhibits an REX grain fraction of less than 12 volume percent. 
     
     
         10 . The method of  claim 1 , wherein the artificially aged product exhibits an elongation of greater than 12 percent. 
     
     
         11 . The method of  claim 1 , wherein the artificially aged product exhibits a TYS of greater than 90 ksi. 
     
     
         12 . The method of  claim 1 , wherein the artificially aged product exhibits a T90 UTS value of at least 93 ksi, a T90 TYS value of at least 90 ksi and an electrical conductivity greater than 36% IACS extruded into a section having a thickness of 0.50 inch or less. 
     
     
         13 . The method of  claim 1 , wherein the artificially aged product is a Type II seamed extruded hollow section exhibiting a UTS of greater than 92 ksi and a TYS of greater than 91 ksi. 
     
     
         14 . The method of  claim 1 , wherein the artificially aged product exhibits a residual strength greater than or equal to 9.16678*X″−287.75 where X″ is the electrical conductivity measured sub-surface. 
     
     
         15 . The method of  claim 1 , wherein the artificially aged product exhibits a TYS greater than or equal to 90 ksi, an elongation greater than or equal to 9, and a residual strength greater than or equal to 46 ksi. 
     
     
         16 . The method of  claim 1 , wherein the artificially aged product exhibits a residual strength greater than 51 ksi. 
     
     
         17 . The method of  claim 1 , wherein the artificially aged product exhibits a residual strength greater than 53 ksi. 
     
     
         18 . The method of  claim 1 , wherein the artificially aged product exhibits a residual strength greater than 55 ksi. 
     
     
         19 . The method of  claim 1 , wherein the artificially aged product exhibits a TYS greater than 72 ksi and a conductivity greater than 40.4% IACS. 
     
     
         20 . The method of  claim 1 , wherein the artificially aged product exhibits TYS greater than or equal to −3.75*X+226.75. 
     
     
         21 . The method of  claim 1 , wherein the artificially aged product is capable of passing ST SCC at 45 ksi for 20 days and exhibiting an EA EXCO rating when produced in sections greater than 4.5 inches thick. 
     
     
         22 . The method of  claim 1 , wherein the composition comprises less than 0.06 weight percent Si and less than 0.09 weight percent Fe. 
     
     
         23 . The method of  claim 1 , further comprising cold working the extruded product after the hot working and prior to the solution heat treatment. 
     
     
         24 . An extruded aluminum alloy product manufactured by the method of  claim 1 . 
     
     
         25 . An extruded aluminum alloy product comprising from 8.40 to 9.10 weight percent Zn, from 1.80 to 2.10 weight percent Mg, from 1.90 to 2.33 weight percent Cu, from 0.01 to 0.05 weight percent Ti, and from 0.10 to 0.14 weight percent Zr, with the balance comprising aluminum and incidental impurities, wherein the product has a density of from 0.1036 to 0.1041 lbs/in 3  and has:
 (a) a combined tensile yield strength and elongation (TYS+e) greater than or equal to −3.25*X+221.75 where X is an electrical conductivity of from 35% IACS to 39% IACS; or   (b) a combined tensile yield strength and residual strength (TYS+RS) greater than or equal to 5.57148*X′−64.343 where X′ is an electrical conductivity of from 36% IACS to 37.5% IACS.