Effects of heat treatment on microstructure and microhardness of linear friction welded dissimilar Ti alloys

A detailed investigation for the influence of post weld heat treatment (PWHT) on the microstructure of TC4 and TC17 dissimilar joints was analyzed. The fully transformed microstructure in the as-welded zone indicated that the peak temperature exceeded the β-transus temperature at the weld interface during linear friction welding. TC4 side was mainly composed of martensite α′ phase with random distribution and it was single β for that of TC17. In the thermomechanically affected zones of TC4 and TC17, the structure undergoes severe plastic deformation and re-orientation, yet without altering the phase fractions. After PWHT, in the weld zone of TC4 alloy, the phase transformation α′→ α+β occurred and the acicular α was coarsened, which resulted in a decrease in hardness. In the weld zone of TC17 alloy, fine α phase precipitated at the grain boundary and within β grains, which resulted in a sharp increase in hardness.