Multi-scale and multi-technic microstructure analysis of a linear friction weld of the metastable-β titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) towards a new Post-Weld Heat Treatment

Linear Friction Welding (LFW) is a solid-state joining process producing narrow joints mainly developed for the aircraft industry. The Thermo-Mechanical Processing (TMP) loads imposed on the material cause complex microstructure transformations. This study aimed at characterizing the local microstructures and crystallographic configurations to identify the mechanisms impacting the stability and the mechanical properties of a Ti17 LFW joint with the objective of establishing an innovative homogenizing Post-Weld Heat Treatment (PWHT). The process caused a narrow zone of +/- 2 mm from the friction interface to reach the isothermal a -> beta transformation temperature under a heating rate of 500 K/s followed by quenching (-100 K/s). The TMP resulted in the formation a +/- 3 mm wide soft Process Affected Zone (PAZ) delimited by three characteristic zones: the Welding Line (WL width approximate to 400 mu m), The Thermo-Mechanically Affected Zone (TMAZ width approximate to 1 mm) and the Heat Affected Zone (HAZ). beta TMP resulted in the total dissolution of the alpha phase in the joint core (WL + TMAZ) and formed strongly textured {110}< 011 > beta recrystallized grains in the WL by continuous dynamic recrystallization of squeezed prior-beta grains in the TMAZ. alpha + beta TMP led to a gradual loss of hardness in the HAZ whose microstructure is similar to the one of the base material (BM) but is subject to gradual a laths dissolution due to the local temperatures experienced during LFW. A PWHT consisting in a beta annealing followed by an alpha + beta ageing resulted in a uniformly hardened weld with a homogenized Widmanstatten microstructure in the whole assembly.