Contributions of an innovative post-weld heat treatment to the micro-tensile behavior of two mono-material linear friction welded joints using: The β-metastable Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) and the near-α Ti-6Al-2Sn-4Zr-2Mo (Ti6242) Ti-alloys

The mechanical behavior of two mono-material Ti-alloy joints obtained by Linear Friction Welding (LFW) were investigated through micro-tensile testing combined with full-field measurements; the resulting tensile behaviors were examined in dependence of the local microstructures and crystallographic features. The near-a Ti-6Al-2Sn-4Zr-2Mo (Ti6242) and the beta-metastable Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) welds were tested up to failure in both as-welded (AW) and heat treated (PWHT) configurations. In the AW Ti17 joint, the dissolution of the hardening a precipitates in favor of a beta(metastable) phase resulted in a remarkable loss in stiffness and strength compared to the as-received Widmanstatten alpha + beta base material (BM). The gradients of microstructure led to significant heterogeneities in the local mechanical behaviors and subsequent strain incompatibilities between the refined weld center line (WL) and the neighboring coarse-grained thermo-mechanically affected zone (TMAZ). These features caused an early and intense strain localization in the joint ultimately causing failure by quasi-cleavage at the WL/TMAZ border. Transforming the AW microstructures through beta annealing followed by alpha + beta ageing permitted to form back a Widmanstatten alpha + beta microstructure in the whole assembly resulting in homogenized elastic and plastic behaviors. In the joint core of the AW Ti6242, the precipitation of alpha'/alpha '' martensitic laths or alpha(secondary) fragments upon cooling the high temperature beta(transformed) microstructure caused a noteworthy refinement and a consistent weld overmatch; a slight loss of stiffness was also noted. The microstructures neighboring the refined joint did not exhibit singular microstructural changes but the dissolution of alpha(secondary) platelets initially hardening the beta matrix and a probable resorption of hardening particles. These phenomena appeared to have weakened these heat-affected zones (HAZ) leading to an intense slip occurrence in the latter and a quasi-cleavage fracture. The heat treatment consisting of an alpha + beta annealing followed by ageing permitted to homogenize the stiffness values through the reprecipitation of equilibrium alpha + beta microstructures across the joint. The formerly softened HAZ was hardened back and exhibited a mechanical behavior similar to the neighboring BM. A gradual microstructure refinement was preserved across the joint ensuring a remaining slight weld overmatch.