Microstructural study of keyhole TIG welded nickel-based superalloy G27

The weld fusion zone (FZ) microstructure obtained after keyhole tungsten inert gas welding and post-weld solution heat treatments (PWSHTs) of a new nickel (Ni)-based superalloy called G27 is studied, and the grain growth behavior in the base material (BM) during PWSHTs is characterized. Microsegregation-induced interdendritic microconstituents in the FZ of as-welded G27 are identified by analytical (scanning) transmission electron microscopy ((S)TEM) as niobium (Nb)-rich MC carbides, Nb-rich Laves eutectic constituents, & gamma;' and & eta; phases. The Laves eutectics are generally considered brittle and can adversely affect the mechanical properties of the weldment; thus, an hour PWSHTs were performed at 954 degrees C-1060 degrees C to eliminate the & gamma;/Laves eutectics. PWSHT up to 1010 degrees C results in only partial removal of Laves eutectics with an excessive formation of & eta; phase surrounding the Laves phase. Complete dissolution of Laves eutectics with no & eta; phase formation is achieved after a PWSHT is performed at 1060 degrees C. Relative to INCONEL & REG; alloy 718, the complete elimination of the & gamma;/Laves eutectic constituents in the FZ of G27 through a PWSHT was proven to be achieved without causing excessive grain growth in the BM, which could be due to the pinning effect of the fine molybdenum (Mo)-rich precipitates, that are formed during solution heat treatment and identified as hexagonal close-packed phase particles through extensive (S)TEM analyses.