Weldability of Nickel/Aluminum Dissimilar Materials Laser Welding Part I: Microstructure Development, Solidification Behavior and Mechanical Properties of Polycrystalline Nickel-based Superalloy

Abstract

Weldability-related issues, including asymmetrical weld pool shape, γ phase multicomponent microstructure development, solidification behavior and mechanical properties, of γʹʹ precipitation-strengthened polycrystalline nickel-based superalloy are experimentally analyzed during keyhole laser welding repair. The crucial relationship between them provides weldability improvement opportunities and defect-free high quality weld for feasible aerospace materials laser processing. Attractive neck transition region of weld is more vulnerable to coarser dendrite than bottom part in the vicinity of fusion boundary, and this location is also liable to abundant Laves/γ eutectic phase formation because of heat and solute accumulation. The dominant chemical inhomogeneity brings about microstructure anomalies and irregular morphology to deteriorate hot cracking resistance. Because of thermometallurgical factors of selective alloying partition in the molten pool, partition-resistant Cr is chemically accumulated in dendrite core, while partition-susceptible Ti, Al, Mo and Nb are enriched in high-supersaturation interdendrite region, which consequently result in serious segregation and extensive Nb-rich intermetallic phase formation across solidification interface. Dendrite refinement and aggressive Laves/γ eutectic phase formation suppression are not simultaneously satisfied by low heat input under nonequilibrium solidification conditions. Therefore, mechanical properties are only partially improved by diffusion-limited microstructure refinement as result of phase instability. In other words, reductions of tensile strength and ductility are attributed to brittle Laves/γ eutectic phase formation during terminal stage of solidification. The severity of eutectic reaction thermodynamically depends on nail-shaped keyhole profile, solidification conditions and solute redistribution ahead of solid/liquid interface. Additionally, viable control of weld metallurgical and mechanical properties progressively encourages optimal combination of laser power and welding speed to interdendritically minimize disadvantages of eutectic phase formation by a variety of welding conditions optimization.