Effect of transient change in strain rate on plastic flow behaviour of Al-Zn-Mg-Cu alloy at elevated temperatures

In industrial practice of hot forging, i. e. extrusion-type forging, abrupt changes in strain rate during the deformation of the material occur. For accurate numerical simulation of a forging process, the experimental investigation of the effect of the transient change in strain rate on plastic flow behaviour is necessary. The present paper deals with the investigation of this effect on the flow stress of an Al-Zn-Mg-Cu alloy during its deformation at 450°C. During continuous uniaxial compression loading of a cylindrical specimen, the strain rate was abruptly increased or decreased from its initial value at engineering strain of ~26 %. From the beginning of the upsetting up to the strain of ~26 % the value of the strain rate was constant and equal to either 1 s^-1 or 10 s^-1. At the strain of ~26 % the value of the strain rate was either increased to 10 s^-1 or decreased to 1 s^-1. The results of the experimental investigations were used to determine the isothermal flow stress-strain curves of the Al-Zn-Mg-Cu alloy. On the basis of these curves, the strain rate sensitivity index m as a function of true strain as well as temperature was determined.