Laser welding of a W-free precipitation strengthened Co-base superalloy

Abstract

A new class of W-free, precipitation strengthened Co-base superalloys has recently been developed with improved high-temperature strength while maintaining excellent oxidation and hot corrosion resistance. In this study, we report the impact of fusion welding on microstructure and properties of an alloy of this class having a nominal composition Co–30Ni–10Al–5Mo–2Ta–2Ti. We carry out bead-on-plate laser welding of the plates and characterize the microstructure and properties of the welded samples. Fusion zone of the welds consists of FCC γ dendrites and an interdendritic network of ordered γ′ phase. Finer γ′ precipitates are observed inside γ dendrites, which form by solid-state transformation during post-solidification cooling. Measurement of phase compositions reveals differences in solid/liquid partitioning behavior of the constituent elements and indicates significant solute trapping for Mo, Ti, and Ta. Experimentally determined partition coefficients for Co and Ni agree reasonably well with thermodynamic predictions obtained using Thermo-Calc, but the prediction for Al contradicts the observations. Although welding-induced microstructural changes lead to a drop in the hardness, it was successfully restored after a post-weld solutionizing-aging treatment.