High strain rate properties of selected aluminium alloys

The strain rate sensitivity of two commercial aluminium alloys AA6082 and AA7108 in peak temper T6 and overaged T79 condition is analysed. These alloys are precipitation hardenable and as such, are expected to have a relatively low strain rate sensitivity. Their response to rapid loading has been tested over a wide range of strain rates, from 0.1 to 3000 s−1 at room temperature, 375 and 515°C for AA6082 and room temperature, 280 and 340°C for AA7108. Specimens were cut from planar extruded sections and tested in uniaxial compression with the deformation axis parallel to the extrusion direction. Intermediate strain rate testing was carried out using a computer controlled servo-hydraulic testing machine and high strain rate testing above 500 s−1 on a Split Hopkinson Pressure Bar (SHPB). Owing to inaccuracies of the conventional Kolsky solution of the SHPB, a numerical simulation was performed. The flow stress at a plastic strain of 5% was plotted as a function of strain rate and temperature. Up to 2000 s−1, there is very little change of the activation volume with increasing temperature for both alloys, indicating that barriers to thermal activation are large and approximately the same. However, at higher strain rates, the rate controlling mechanism appears to change and at lower temperatures, a trend towards negative strain rate sensitivity was observed. Metallographic examination of AA7108 revealed strain localisation, seen as parallel deformation bands. The strain localisation could be the cause of the negative strain rate sensitivity.