High Speed Forming of the Light-Weight Wrought Alloys

Abstract

The deformation and fracture behaviour of the Al-alloy AA7075, Mg-alloy AZ80, and Tialloy Ti-6Al-4V were investigated in quasi-static and dynamic uniaxial compression and tension tests at strain rates in the range of 0.001 s^(-1) ≤ ε̇ ≤ 5000 s^(-1) and temperatures between 20°C and 500°C. Shear tests with hat shaped specimens of AZ80 were carried out by quasi-static and dynamic loading in the shear rate range of 0.01s^(-1) ≤ γ̇ ≤ 116000s^(-1) at a temperature of 20°C. For strain rates of ε̇ ≤ 10 s^(-1), the tests were carried out using a computer numerical controlled hydraulic testing machine. High strain rate experiments with ε̇ ≥ 1000 s^(-1) were performed on a Split Hopkinson Pressure Bar. Using the 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 the possible deformation are controlled by strain rate und temperature. Under dynamic loading, both AA7075 and AZ80 show an increase of the deformation degree 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 laws, 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 in compression, tension and shear tests. These material laws are to be implemented into the FE simulation, in order to determine the local state of stress and strain at time of the fracture. Through combination of experiment and simulation, a failure criterion for ductile fracture could be determined for AA7075 under quasi-static and dynamic tensile loading.