Interfacial characteristics and cohesion mechanisms of linear friction welded dissimilar titanium alloys: Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti17) and Ti–6Al–2Sn–4Zr–2Mo (Ti6242)

A detailed microstructural examination endeavoring to understand the interfacial phenomena yielding to cohesion in solid-state assembling processes was performed. This study focuses on the transition zone of a dissimilar titanium alloy joint obtained by Linear Friction Welding (LFW) the β-metastable Ti17 to the near-α Ti6242. The transition zone delimitating both alloys is characterized by a sharp microstructure change from acicular HCP (Hexagonal Close-Packed) α′ martensitic laths in the Ti6242 to equiaxed BCC β (Body-Centered Cubic) subgrains in the Ti17; these α′ plates were shown to precipitate within prior-β subgrains remarkably more rotated than the ones formed in the Ti17. Both α′ and β microstructures were found to be intermingled within transitional subgrains demarcating a limited gradient from one chemical composition to the other. These peculiar interfacial grains revealed that the cohesive mechanisms between the rubbing surfaces occurred in the single-phase β domain under severe strain and high-temperature conditions. During the hot deformation process, the mutual migration of the crystalline interfaces from one material to another assisted by a continuous dynamic recrystallization process was identified as the main adhesive mechanism at the junction zone. The latter led to successful cohesion between the rubbing surfaces. Once the reciprocating motion stopped, fast cooling caused both materials to experience either a βlean→α′ or βlean→βmetastable transformation in the interfacial zone depending on their local chemical composition. The limited process time and the subsequent hindered chemical homogenization at the transition zone led to retaining the so-called intermingled α’/βm subgrains constituting the border between both Ti-alloys.