On the effect of a thermal treatment on the tensile and fatigue properties of weak zones of similar Ti17 linear friction welded joints and parent material

The tensile and fatigue properties and the damage mechanisms of as welded (LFW-AW) and post-weld heat treated (PWHT) similar Ti17 linear friction welded (LFW) joints as well as Ti17 parent material (PM) have been studied together with their underlying microstructures. The LFW-AW joints exhibited a depletion of the alpha phases in the weld centre zone (WCZ) and in the thermo-mechanically affected zone (TMAZ), which was previously qualified as a weak zone. Stereo Digital Image Correlation (SDIC) analysis of the strain field for monotonic loading of the PM showed strain heterogeneity that was linked by EBSD and Schmid's factor analysis to the large beta grains containing alpha precipitates. Compared to the PM tensile specimens, the LFW-AW joint highlighted lower ultimate tensile strength (UTS) and a strongly reduced and scattered macroscopic ductility. Maps and spatio-temporal graphs of normalized strain rate fields obtained by SDIC clearly highlighted localization at the weak zone. For the target fatigue life of 10(5) cycles, the fatigue strength of the AW joint was about half that of the PM with fatigue crack initiation at the beta grain boundary in the weak zones. Remarkably, the PWHT made the AW joints comparable to the PM in terms of microstructure and mechanical properties. The former weak zones recrystallized into beta grains with alpha, the UTS and fatigue strength were almost recovered, macroscopic and local ductility were enhanced, failure during monotonic and cyclic loadings occurred far from the former weak zones.