Evolution of grain structure, γ’ precipitate and hardness in friction welding and post weld heat treatment of a new Ni-Fe based superalloy

Friction welding of a new Ni-Fe based superalloy was investigated, focusing on the evolution of grain structure, nano-scale precipitates as well as micro- and nano-hardness of joints, and the effects of post weld heat treatment (PWHT). The EBSD analysis showed a 4 mm gradient microstructure from the weld center to the base material, where fine equiaxed grains (average size of 3.7 μm) are formed in the weld center from dynamic recrystallization with a preferred orientation along <001>, which are eliminated by PWHT. The TEM study identified ultra-fine γ′ arising from natural cooling in the as-welded joint, but at a much lower volume fraction. PWHT produces γ′ of comparable size and amount to the base material. Theoretical calculation on critical resolved shear stress indicates that the γ′ phase in both the as-welded and heat-treated joint possess the same shearing mechanism of weakly coupled dislocations. However, the role played by the ultra-fine γ’ (average size of 6.7 nm) in hardness is much weaker than those typical ones formed following PWHT, as the nano-hardness measurements indicate.