Investigation of Friction Stir Welded Al-Zn-Mg-Cu Aluminum Alloys

This research evaluates the influence of scandium on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloy extrusions joined through the friction stir welding (FSW). Two alloys were investigated: a commercial 7136-T76 containing zirconium (no scandium) and SSA038 (produced by UES, Inc.) containing both zirconium and scandium. The addition of scandium resulted in the formation of minute coherent Al-3(Sc, Zr) precipitates. The friction stir welding applied to these alloys produced high quality, sound welds. The center of welds was composed of extremely small and equiaxed grains with an average diameter of several micrometers. The grain size in the weld center was smaller for the alloy containing scandium. The microstructures on the advancing and retreating sides differed: on advancing side there was a sharp boundary between the TMAZ and the SZ while on the retreating side, the microstructure changed continuously between the two regions. There was also a significant difference between hardness profiles for AA7136 and SSA038 alloys: the profiles for SSA038 exhibited singular minima resembling those profiles usually encountered in work hardened aluminum alloys. The beneficial role of Sc in the Al-Zn-Mg-Cu alloys was also manifested by the higher joint efficiency, defined as the ratio of the yield strength of welded samples to the baseline yield strength, for the alloy containing scandium.