Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

The superplastic properties and microstructure evolution of a 0-15'%Zr and 0-7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600 C and strain rates ranging from 7 x 10(-6) to 2(.)8 x 10(-2) s(-1). The refined microstructure with an average grain size of about 11 mum was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570degreesC and epsilon over dot>=2(.)8 x 10(-4) s(-1). Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient in of about 0.65) were found at the same strain rate and a temperature of 590 C, which is higher than the incipient melting point of the 6061 alloy ( similar to 575 degreesC). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.