Microstructure evolution and mechanical properties of a nickel-based superalloy repaired using wire and arc additive manufacturing

In this paper, the microstructural evolution and mechanical behaviour of the repaired ATI 718Plus sample were investigated in detail. Results show that the deposited zone (DZ) mainly contained columnar grains inclined along the z axis following the steepest temperature gradient during WAAM. And local Nb segregation leads to that a large amount of Laves, η, γ’ precipitates gathered in the interdendritic region. In contrast, the substrate zone (SZ) consisted of equiaxed grains and uniform distribution of γ’ precipitates. The γ’ precipitates in DZ show an obvious height dependence with a trend of increasing with increasing distance from the top to the bottom. Due to the exposure to elevated temperature, grain growth and the dissolution of η, γ’ phase were observed in HAZ. The hardness shows a gradual increase along the negative Z axis and an abrupt decrease in HAZ. These significant changes make the HAZ a possible vulnerable region. After direct age (DA) treatment, further growth of η precipitates and sufficient precipitation of γ’ phase were observed. The repaired samples always fractured at DZ. The tensile test indicated that UTS in DA condition shows an obvious increase, but the elongation is lower due to the brittle Laves phase acting as stress concentrators in the precipitation-hardened matrix and the fact that only half of the tensile samples experienced obvious deformation.