Superplastic deformation behavior of lamellar microstructure in a hydrogenated friction stir welded Ti-6Al-4V joint

Abstract It is rather difficult for Ti alloy welds to be superplastic formed due to their low superplasticity and high flow stress. In this study, the Ti-6Al-4V alloy after hydrogenation with 0.2 wt% hydrogen was subjected to friction stir welding (FSW). The superplastic behavior of the FSW joint was investigated. The stir zone with a fine lamellar microstructure exhibited a largest elongation of 660% at 825 °C and 3 × 10 −3 s −1 . Besides, the stir zone showed a similar superplasticity of close to 600% to the base material at 825 °C, 3 × 10 −4 −1 × 10 −3 s −1 . It is the first report on the superplasticity of FSW Ti alloy joints with hydrogenation. Compared to the FSW Ti-6Al-4V alloy joint without hydrogenation, the hydrogenated FSW joint showed superior superplastic properties, including a high elongation, less than a half flow stress and lower superplastic temperature. These superior superplastic properties were mainly attributed to the globularization of the lamellar microstructure and the high fraction of β phase induced by the hydrogen element. This study provides an effective way to reduce the difficulty of practical superplastic forming for Ti alloy welds, although a dehydrogenation process is still needed to avoid hydrogen embrittlement.