Frictional behaviour of AA7075-O aluminium alloy in high speed tests

The influence of strain rate on the frictional behaviour of AA7075 aluminium alloy, in the O-annealed temper state, was studied. Attention was focused on the evaluation of the mechanisms affecting friction at high strain rate. To this purpose, axisymmetric and ring compression tests were carried out using the split Hopkinson tension-compression bar in the direct version. The flow curves were investigated in a wide range of strain rates using cylindrical specimens characterised by different slenderness ratios. The results of the axisymmetric compression tests showed an appreciable strain rate sensitivity of the deforming material with a marked increase in flow stress with strain rate in the dynamic loading condition. As far as the ring compression tests are concerned, different strain rates were obtained using hollow cylinders with different initial sizes. The analysis of the results showed that strain rate affects the variation of the inner ring diameter, irrespective of the height reduction and frictional condition. Axysimmetric and ring compression tests were also performed under the quasi-static loading condition and the experimental results did not show any appreciable effect of strain rate on flow curves and frictional behaviour. Finally, the different frictional behaviour between the quasi-static and dynamic loading conditions was attributed both to the reduction in the lubricant film thickness and change in the viscous actions at the specimen-tool interface.