Achieving superior low temperature and high strain rate superplasticity in submerged friction stir welded Ti-6AI-4V alloy

Abstract

The superplastic forming of Ti alloy welds has great application prospects in producing integrated components. However, the nugget zone (NZ) of the Ti alloy welds, produced by fusion welding or conventional friction stir welding (FSW), consists of lamellar microstructure, which exhibits either low superplasticity or high superplastic temperautre and low strain rate. As a result, the NZ plays a leading role in hindering the superplastic forming of the whole welds. In this study, submerged friction stir welding (SFSW) was conducted in Ti-6Al-4V alloy for the first time, and a defectfree weld with the NZ consisting of a strip microstructure was obtained. The NZ exhibited a low-temperature superplasticity at 600°C, which was the lowest superplastic temperature ever reported in the Ti alloy welds. Besides, at 800°C, the NZ showed high strain rate (3×10−2 s−1) superplasticity and a largest elongation of 615% at 1×10−3 s−1. Compared to conventional FSW joints, the NZ of SFSW joint exhibited a much lower flow stress and a decrease in optimal superplastic temperature by 100°C. This is mainly attributed to the easy globularization of the strip microstructure, enhancing the ability of grain/phase boundary sliding.