Elevated temperature properties of electron beam welds of an α+β titanium alloy

Abstract

The effect of base metal microstructure and post-weld heat treatment (PWHT) on the stress rupture creep and high temperature tensile properties of electron beam welds of an α+β titanium alloy, Ti-6.8Al-3.42Mo-1.9Zr-0.21Si has been evaluated. In the as-welded condition, the stress rupture properties of the welds were poor. Stress relieving improved the properties but they were still inferior to the base metal properties. Creep strains in the as-welded condition were also large. The poor stress rupture and creep properties in the as-welded condition may be due to a metastable microstructure and possible hydrogen accumulation in the heat affected zone (HAZ), the location of failure. Welds of the base metal in the β heat treated condition (β base welds), with a coarse prior β grain size at the location of failure exhibited superior stress rupture properties and predominant intergranular fracture, while welds of the base metal in the α+β heat treated condition (α+β base welds) failed in a transgranular mode. PWHT of α+β base welds with predominant acicular/lenticular α microstructure just below the β transus temperature exhibited marginally superior stress rupture strength than the base metal with an equiaxed α+lenticular/acicular α microstructure. Supertransus PWHT led to poor stress rupture properties due to thick grain boundary α, and a thick and aligned α plate colony structure. High temperature tensile properties of welds were equivalent to that of the base metal. α+β base welds with a finer prior β grain size at the location of failure exhibited superior strength and ductility. Subtransus PWHT resulted in an acicular α microstructure and low ductility. PWHT in the supertransus region resulted in poor strength and ductility due to coarse and aligned transgranular α+grain boundary α.