MICROSTRUCTURE EVOLUTION OF PARTIALLY MELTED ZONE OF TIG WELDING JOINT OF Ni-BASED INCONEL-718 SUPERALLOY

Abstract

Element segregation,such as Nb in Ni-based Inconel-718 superalloy,causes the precipitation of low melting point phase during solidification.The actual base metal can melt in a lower temperature and the structural continuity is damaged during welding thermo cycle curve.The liquid film easily generates between austenite grains and leads to stress concentration before solidifying into the low melting point phase.Microstructure evolution of Inconel-718 welding joint increases the hot crack sensitivity and changes mechanical property of the joint.The partially melted zone(PMZ) is close to the molten metal in the fusion zone,which is the most liquation crack sensitive region of welding joint heat affected zone(HAZ).Different microstructures exists among wrought,ascast and homogenization Inconel-718 superalloy inducing weldability differences of these material.Especially,the solidus-liquidus curve differences of low melting point phase in PMZ notably affect the high temperature mechanical property of welding joint.The wrought,as-cast and homogenization Inconel-718 superalloy sheets were respectively welded by tungsten inert gas arc welding(TIG) with different heat inputs.The microstructure of PMZ was observed by OM and SEM.Alloy element content of intradendritic austenite,interdendritic segregated region and Laves phase was investigated by EDS.The theoretical solidus-liquidus temperature of these phases was calculated by Thermo-Calc software.The melting and solidification temperature of austenite and Laves in PMZ of different base metal was investigated to analyze the temperature range for the formulation of liquid film.The results show that the microstructure heredity phenomenon obviously exists in the PMZ of Inconel- 718 welding joint.The equiaxed grains remain in the PMZ of wrought joint,and the dendritic structure is still kept in the PMZ of as-cast and homogenization joint.The slender Laves and particle MC phase precipitate along the boundary of the austenite in PMZ of welding joint.But the segregated region originally existed in base metal disperses.The calculating results show that the maximum solidus-liquidus temperature range is in as-cast base metal,secondary in homogenization,minimum in wrought.The width of PMZ is increased with the increasing heat input and PMZ of as-cast is larger than PMZ of wrought and homogenization Inconel-718 superalloy.