Strain hardening behavior of linear friction welded joints between TC11 and TC17 dissimilar titanium alloys

The influence of strain rate on the tensile properties and its distribution of linear friction welded (LFWed) dissimilar joints between Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) and Ti-4Mo-4Cr-5Al-2Sn-2Zr (TC17) titanium alloys were studied in this paper. Furthermore, strain hardening behavior, strain rate sensitivity and fracture characteristic of the LFWed joints have been evaluated. It was found that a great deal of ultra-fine grains appear on the TC11 side of weld zone (WZ). The microstructure on the TC17 side of WZ is composed of coarsened β phase, a lot of slip lines and dislocation networks within the boundaries of β grains generate under high temperature deformation by transmission electron microscope (TEM). A distinctive asymmetrical hardness profile is observed across the joint, with higher hardness on the TC11 side of WZ and lower hardness on the TC17 side of WZ, respectively. While the yield strength (YS), ultimate tensile strength (UTS) of the joint increase, both hardening capacity and strain hardening exponent n-value decrease with increasing strain rate, meanwhile, the strain-hardening rate θ increases at a given true stress. Strain rate sensitivity of the welding joint decreases with increasing plastic strain. Moreover, the fracture location of the LFWed joints between TC11 and TC17 titanium alloys almost locate at the regions with lower hardness. Additionally, ductile fractures and a mixed cleavage fractures appear respectively at the center and edge area of the LFWed joints.