Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

Abstract

In α-β titanium alloys, the high-temperature β phase can decompose in several ways, depending on alloy composition and cooling rate. In the case of welded joints, cooling rates can vary widely as a function of heat input. In the current work, a dilute α-β Ti-Al–Mn alloy was welded over a range of heat inputs using electron beam and gas tungsten-arc welding processes. A major part of the rapidly cooled electron beam weld could be identified as lath-type or “massive” martensite. In the slower cooled gas tungsten-arc welds, the transformation product was a mixture of lamellar α and β phases formed entirely by diffusion. Post-weld heat treatment resulted, in all cases, in an α-β structure that coarsened with annealing temperature and time. Tensile elongation in the as-welded condition was poor on account of a large prior-β grain size and an acicular microstructure. The ductility improved as the structure coarsened on heat treatment. Tensile fracture was always microscopically ductile, but the presence of a grain boundary α layer tended to induce intergranular rupture, especially when a hard, intragranular matrix confined slip to occur in the grain boundary regions.