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

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 degrees C and 3 x 10(-3) s(-1). Besides, the stir zone showed a similar superplasticity of close to 600% to the base material at 825 degrees C, 3 x 10(-4) -1 x 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 beta 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. (C) 2019 Elsevier B.V. All rights reserved.