Surface aluminizing on Ti–6Al–4V alloy via a novel multi-pass friction-stir lap welding method: Preparation process, oxidation behavior and interlayer evolution

Surface aluminizing on Ti–6Al–4V alloy was successfully performed via a novel solid-state method of multi-pass friction-stir lap welding (FSLW). The process principles are elucidated. The aluminized coating was tailored as ∼500 μm in thickness after preparation and milling treatment to remove the redundant pure Al. No annealing treatment was performed after the FSLW preparation. The as-processed Ti/Al interlayer was Ti-rich in chemical composition, more than 60 μm in thickness, and had an interval banding structure, due to the FSLW thermal–mechanical effects. Oxidation tests for bare TC4 and aluminized specimens were conducted at 700 °C. Surface morphologies, phases and interlayer evolutions of the oxidized specimens were investigated. The diffusion and/or possible reaction of the Ti/Al interlayer occurred simultaneity corresponding to the oxidation behaviors in the near-surface layer. The oxidation resistant roles of the aluminide interlayer and the upper abundant Al coating produced via the method were discussed. Abundant Al content in the coating of significant thickness benefited the anti-oxidation performance and forming of Ti/Al interlayer. The aluminide interlayer of considerable thickness, with composition gradients and good density at Ti/Al interface location, played a main protection role against oxidation to Ti-substrate.