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 beta-grain structure that is commonly seen in alpha + beta titanium alloys, producing a finer equiaxed grain structure with a near-random alpha 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 beta-grain size decreased across the alloy-to-alloy transition reaching on average 25 pct less in Ti6242 than in Ti64. A similar 25 pct scale reduction was also found in the Ti6242 alpha-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 beta-grain growth during reheating as well as a finer transformation microstructure.

Automated summary

In this study, the microstructure of the titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242) was compared to the commonly used alloy Ti-6Al-4V (Ti64) when deposited using a wire-arc additive manufacturing (WAAM) process. The application of inter-pass machine hammer peening successfully refined the grain structure of Ti6242, producing a finer equiaxed grain structure with a near-random alpha texture. This refinement was attributed to the presence of molybdenum in Ti6242, which led to slower grain growth during reheating and a finer transformation microstructure.