Dynamic softening behaviour of AZ80 magnesium alloy during upsetting at different temperatures and strain rates

Abstract

In order to evaluate the dynamic softening characteristics of casting AZ80 magnesium alloy, isothermal upsetting experiments with a height reduction of 60 per cent were performed at temperatures 523, 573, 623, and 673 K, at strain rates of 0.01, 0.1, 1, and 10 s⁻¹ on thermal physics simulator Gleeble 1500. The flow behaviour of the applied stress as a function of strain, strain rate, and temperature exhibited a more pronounced effect of temperature than strain rate, and a typical softening characteristic. A Fields–Backofen constitutive equation was introduced to describe this behaviour, while the flow stress calculated could not provide a fit to the measured stress–strain relationship at the dynamic softening stage. Based on the relationships established between yield stress and recrystallization grain refinement under different strain rates and temperatures, the softening behaviour was illustrated. An amended mathematical model containing an item s was introduced to characterize the dynamic recrystallization softening behaviour. Compared with the Fields–Backofen equation, the resulting flow curves calculated by the amended model offered a better representation during the softening stage.