Properties of as welded and heat treated pure titanium-7075 Al-Zn-Mg alloy friction weld joints

The effects of joining conditions and an age hardening post-weld heat treatment (PWHT) at 120° C for 24 h on the tensile strength and metallurgical properties of dissimilar friction joints between pure titanium and age strengthened 7075 Al-Zn-Mg alloy were investigated. Highest strength was achieved using intermediate friction pressure (150 MPa), short friction time (0-5 s), and high upsetting (forging) pressure (400 MPa). The joint tensile strength decreased when the joint diameter was increased from 8 to 16 mm. The joint tensile strength of as welded (AW) dissimilar joints was similar to that of PWHT joints with diameters of 8, 12, and 16 mm. Detailed TEM confirmed that there was a negligible difference in the thickness of the intermetallic layer formed at the dissimilar joint interface for AW and PWHT joints. While the intermetallic phases formed at the joint interface comprised Al₃Ti, τ (Ti₂Mg₃Al₁₈), and Al in AW joints, they consisted of Al+τ or Mg₂Al₃+τ+Al in PWHT joints. Softened regions were generated in 7075 base material immediately next to the interface in AW joints. Post-weld heat treatment increased the hardness of the softened region almost to that of as received 7075-T6 base material in 12 and 16mm diameter joints. In contrast, the hardness of the softened region in 8 mm diameter joints could not be recovered to that of the as received material. This was a result of averaging and coarse precipitates in the softened region produced during the friction welding operation.