Effect of Cu and Zr additions on the superplastic behavior of 6061 aluminum alloy

Experiments were conducted to evaluate the influence of zirconium and copper additions on superplastic behavior of a 6061 aluminum alloy. Fine grains were produced in a commercial grade of 6061 Al and a 0.15%Zr + 0.7%Cu-modified 6061 alloy by two-step thermomechanical processing. The superplastic properties and microstructure evolution of both alloys were examined in tension at temperatures ranging from 475 to 620degreesC and strain rates ranging from 7 x 10(-6) to 2.8 x 10(-2) s(-1). It was shown using differential thermal analysis that both alloys exhibit the highest superplastic characteristics in a partially melted state. The 0.15%Zr + 0.7%Cu-modified 6061 aluminum alloy exhibits a maximum elongation-to-failure of 1300% at 590degreesC and an initial strain rate of 2.8 x 10(-4) s(-1). In contrast, the highest total elongation of 350% (m similar to 0.6) was achieved in the commercial grade 6061 alloy at 600degreesC and a strain rate of 1.4 x 10(-4) s(-1). The results suggest the addition of Zr provides high stability of the fine-grained structure under superplastic deformation at high temperatures by precipitation of Al3Zr dispersoids. Apparently, the increased amount of liquid phase caused by the copper addition can enhance the superplastic properties of the 6061 alloy.