Effects of microstructure on high strain rate deformation and flow behaviour of Al–Mg–Si alloy (AA 6061) under uniaxial compression and combined compression and shear loading

Abstract

In the present study, high strain rate deformation and flow characteristics of AA 6061 were investigated under dynamic uniaxial compression and combined compression and shear loading conditions. The uniaxial compressive behaviour was obtained at strain rates in the range of 102–104 s−1 using a split Hopkinson pressure bar (SHPB); ultrahigh strain rate plate impact compression and shear experiments were conducted to investigate the material behaviour at plastic strain rates in excess of 104 s−1. In order to understand the effect of material microstructure on the mechanical behaviour of the AA 6061 alloy, both the T6 and overaged (OA) heat treatments were investigated. The results of the SHPB experiments indicate that the two heat treatments show positive strain rate sensitivity at strain rates up to 4660 s−1. However, at strain rates in excess of 4660 s−1, Al6061-T6 exhibits a somewhat negative strain rate sensitivity. Estimates for temperature rise, assuming that all the plastic work is converted to heat, indicate that the specimen temperature may be elevated into a regime where both thermal effects on the flow stress and the resulting strain rate sensitivity change the flow stress response. This suggests that one contributing reason for the observed decrease in flow stress at the highest strain rates is due to thermal softening. Moreover, at strain rates in excess of 104 s−1, the flow stress levels for both the Al6061-T6 and Al6061-OA heat treatments are observed to approach their saturation stress levels.