In-situ formation of γ-TiAl based alloy coatings on a titanium alloy by tungsten inert gas cladding

The γ-TiAl based alloy coatings were synthesized on the Ti-5Al alloy substrates through in-situ surface alloying using the tungsten inert gas (TIG) cladding method. The effects of welding parameters on the microstructure, phase composition and microhardness of the coatings were investigated. The temperature gradient and cooling rate distributions during the melting-solidification process were calculated by means of a numerical simulation method. The results demonstrated that a higher welding current resulted in a narrower and deeper welding pool, while an appropriate increase in welding speed led to a shallower and wider one. A continuous cladding layer without microdefects was obtained at the welding current of 80 A and the welding speed of 200 mm/min. The layer contained about 47.62 at.% Al and was composed of γ-TiAl and α₂-Ti₃Al phases. The crystallizing morphologies in the welding pool transitioned from planar grains at the bottom to dendrites growing from the bottom to the center, and finally to dendrites aligned with the welding direction at the top. The phase transition of the dendrites proceeded as L→α→γ+α₂, and the interdendritic constituents evolved along L→α→γ, ultimately forming a microstructure of massive γ distributed between the α₂/γ lamellar colonies. The microhardness of the cladding layer was above 350 HV_0.1. The as-synthesized coating had the potential to improve surface properties of the titanium alloy.