Evaluation of hot formability of an AL-4.6ZN-0.8MG alloy by increasing-strain-rate torsion tests

Evaluation of hot formability of an AL-4.6ZN-0.8MG alloy by increasing-strain-rate torsion testsM. El Mehtedi, N. Ryum, S. Spigarelli, E. Evangelista and B. RonningThe hot formability of an experimental Al-4.6%Zn-0.8%Mg alloy was studied by torsion testing.Conventional constant strain-rate tests were carried out to investigate material response in selected condition of temperature, T, and strain rate, ©.In order to determine the relationship relating flow stress with strain rate and temperature, the conventional procedure, based on straining at constant strain rate, requires a relatively large number of tests.The scope for reducing the number of tests by increasing linearly , i.e. , where K is a constant, from 0 to a definite value, was explored in this work and a series of increasing strain-rate tests were carried out.Nine tests, with K ranging from 0.05 to 0.2 s-1, were performed at 500 ºC; the resulting curves were used to recalculate the isostrain rate curve; comparison between experimental and calculated curve gave encouraging results, confirming the reliability of both testing procedures. The microstructure of torsioned samples was investigated by light (LM) and scanning electron microscopy (SEM).EBSD patterns were obtained to analyse grain size distribution and the presence of substructures. The results are discussed in the light of the more recent theories of high-temperature deformation of Al alloys