Comparison of Microstructure Refinement in Wire-Arc Additively Manufactured Ti-6Al-2Sn-4Zr-2Mo-0.1Si and Ti-6Al-4V Built with Inter-Pass Deformation

The titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242) has been deposited for the first time by a directed energy deposition process using a wire and arc system – i.e., wire-arc additive manufacturing (WAAM) – with and without inter-pass machine hammer peening, and its microstructure investigated and compared to the more commonly used alloy Ti-6Al-4V (Ti64). The application of inter-pass machine hammer peening – where each added layer was deformed before deposition – successfully refined the strongly textured, coarse, columnar β-grain structure that is commonly seen in α + β titanium alloys, producing a finer equiaxed grain structure with a near-random α texture. The average grain diameter and texture strength decreased with the peening pitch. When Ti6242 was deposited under identical conditions to Ti64, by switching the alloy feed wire in-situ, the refined β-grain size decreased across the alloy-to-alloy transition reaching on average 25% less in Ti6242 than in Ti64. A similar 25% scale reduction was also found in the Ti6242 α-lath transformation microstructure. This comparatively greater microstructure refinement in Ti6242 was attributed to the dissimilar alloying elements present in the two materials; specifically, molybdenum, which has a lower diffusivity than vanadium and led to slower β-grain growth during reheating as well as a finer transformation microstructure.