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 beta-transus temperature at the weld interface during linear friction welding. TC4 side was mainly composed of martensite alpha' phase with random distribution and it was single beta 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 alpha'-> alpha+beta occurred and the acicular alpha was coarsened, which resulted in a decrease in hardness. In the weld zone of TC17 alloy, fine alpha phase precipitated at the grain boundary and within beta grains, which resulted in a sharp increase in hardness.