Microstructure and mechanical properties of a friction stir processed Ti-6Al-4V alloy

Friction stir processing (FSP) trials were performed on 2 mm-thick Ti-6Al-4V sheets using various processing parameters including tool rotational speed (800-1000 RPM) and tool traverse speed (1-4 IPM). A processing window was established with the W-1% La2O3 tool to produce defect-free friction stir processed (FSP'd) materials. The stir zone (SZ) material of the FSP'd samples showed a fully β transformed microstructure characterized by typical basket-weave lamellar α/β structure. The prior β grains in the SZs contained multiple α variants, and the average sizes ranged from 12 μm to 38 μm influenced by the processing parameters. Based on metallurgical evaluation, the microstructural evolution in different regions from base material (BM) through heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ) to SZ was established. The mechanical properties of the SZs were evaluated by tensile tests. Compared to the base material, the processed samples exhibit higher tensile strength and comparable ductility. The tensile strength was affected by the microstructure of the prior β grain size and α colony size, which are controlled by processing parameters. The lower tool rotational rate and/or higher traverse speed produced a lower peak temperature and a shorter dwell time above the β-transus temperature in the SZ, which resulted in finer prior β grains and smaller α colonies, leading to higher tensile strength.