Influence of strain rate and temperature on the compressive ductility of AI, Mg and Ti alloys

Abstract

The flow behaviour and ductility of the Al-alloy AA7075, Mg-alloy AZ80 and Ti-alloy Ti-6Al-4V were investigated in quasi-static and dynamic compression tests at strain rates in the range 0.001 s -1 ≤e ≤5000 s -1 and temperatures between 20°C and 450°C. For strain rates e≤100 s -1 , the compression tests were carried out using a numerical controlled hydraulic testing machine. High strain rate experiments with e ≥ 1000s -1 were performed on a Split Hopkinson Pressure Bar. Using of experimentally determined flow curves, the effect of strain rate and temperature on the compressive deformation at fracture was determined, showing that the forces required for forming, as well as the limits of possible deformation are controlled by strain rate und temperature. Under dynamic loading, both of AA7075 and AZ80 show an increase of the deformation at fracture with increasing strain rate whereas the Ti-6Al-4V shows a decrease of it. The investigated mechanical material behaviour (strain hardening, strain rate sensitivity and thermal softening) and metallographic investigations of the deformed specimens in dynamic compression tests allow an explanation for character, formation and evolution of damage in the deformed material. Constitutive material law, whose parameters are determined from the experimental data, can be applied to describe the influence of strain rate and temperature on the mechanical material behaviour under compression tests. This material law is to be implemented into the FE simulation, in order to determine the local stale of stress and strain at time of the fracture.