Microstructure evolution during linear friction welding of dissimilar titanium alloys TC4 and TC17

This study investigated the microstructure evolution during dissimilar linear friction welding (LFW) of bimodal TC4 and basket-weave TC17. Experimental results show that the superior high-temperature rheological property of TC4 (compared to TC17) leads to more severe plastic deformation and plastic flow of materials on the TC4 side of the TC4/TC17 titanium alloys LFW joint than on the TC17 side during the welding process. At the friction interface, TC4 undergoes severe plastic flow, whereas TC17 undergoes partial plastic flow. TC4 undergoes plastic instability and severe plastic deformation in sequence as it exits the base material to the plastic flow zone, whereas TC17 only undergoes plastic instability. After cooling, the original microstructures transformed into secondary and metastable phases in the plastic flow zone, whereas considerable metastable phases formed in the TC17 plastic instability zone. After annealing, metastable phases of TC4 decompose to form the lamellar α phases. The metastable phases of TC17 decompose to form the fine basket-weave α phases. In addition, the Zener-Hollomon parameter is used to characterize the thermal-rheological characteristics of materials in different zones of the joint. The results show that the formation process of the fine microstructures at the center of the weld is not a conventional dynamic recrystallization process. It is a fragmentation-refinement process in the original microstructures, dominated by frictional motion and pressure during the welding process.