On the effect of microstructure on the torsional response of AA7050-T7651 at elevated strain rates

A torsional split Hopkinson bar has been implemented to observe strain rate effects in the material response of aluminum alloy 7050-T7651. The material, received in the form of 32 mm thick plates, has been tested in the as received condition and also in the as friction-stir welded (FSW) condition. Microstructural observations have been performed using optical microscopy, revealing a fine, equiaxed structure of small grains in the FSW nugget material and an elongated and markedly larger grain structure in the as-received material condition, typical of rolled, un-recrystallized plate. Quasi-static tensile and high rate torsional testing reveal lower yield and flow stresses in the FSW material when compared to the base metal. Testing further demonstrates the presence of appreciable rate dependence in both material states. The present work is also indicative of an increased work hardening rate in the FSW material, while the base metal exhibits only minimal work hardening. The increased propensity for work hardening and decreased mechanical strength in the FSW material is found to be the primary difference between the two material states.