Effect of initial base metal temper on microstructure and mechanical properties of friction stir processed Al-7B04 alloy
2 mm thick Al-7B04 alloy sheets under T6, T4 and O tempers were subjected to friction stir processing (FSP). The microstructure, Vickers hardness and tensile properties of the stir zone were characterized. The results show that the initial base metal temper has a significant impact on the microstructure and the mechanical properties of the stir zone. FSP led to the formation of full recrystallized microstructure, the average grain size in the stir zone ranged from 2.1 to 2.2. μm when the base metal tempers were T6 and T4. On account of the relatively strong pin force of the precipitates, the grains in the stir zone became finer when O temper base metal was applied. Besides, the stir zones produced using the base metal under T6 and T4 tempers were softened, while a significant strengthening was observed when the base metal temper was O. The softening in the stir zone for T6 and T4 temper is attributed to the dissolution of fine strengthening precipitates during FSP, and the strengthening for O temper arises from the grain refinement caused by FSP.