Diffusion-limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials during Laser Welding Part I: Precipitation-strengthened Polycrystalline Nickel-based Superalloy

Abstract

Polycrystalline heterogeneities of grain growth, phase transformation and tensile properties are discussed by metallography, spectroscopy and fractography when choosing laser welding with butt-joint configuration to satisfactorily minimize effect of inappropriate weld pool shape on microstructure and mechanical properties of γʹʹ precipitation hardable nickel-based superalloy in aerospace industry. There is parabolic relationship between weld penetration and diffusion-limited nucleation, growth and coarseness of secondary dendrite arm spacing alongside fusion boundary through nonequilibrium solidification process, which is dendritically susceptible to grain growth variation and metallurgical discontinuities. The amount, size, morphology and distribution of nonequilibrium intermetallic phase near the dendrite boundaries are kinetically and thermodynamically rely on weld pool shape whose crystal structure is incoherent with γ solid solution austenite phase and increases preference for crack initiation and propagation of brittle intergranular and ductile dimple fracture failures to enormously contribute to losses of strength and ductility. Unsymmetrical keyhole weld is thermometallurgically inconvenient for amelioration of microstructure and mechanical properties on either side, and adversely mitigates resistance to hypereutectic-aided dendrite embrittlement. Beneficial grain refinement and suppression of detrimental nonequilibrium intermetallic phase at the same time are challenging. This problem is an integral part of inhomogeneous microstructure and mechanical properties. The finer grain size occurs, the larger grain boundary area is available for Laves/γ eutectic-type reaction and vice versa. Contributions of coarse grain kinetics and metallurgical reaction thermodynamics to weld disintegration and fracture failure mechanism of tensile properties are explained by microstructure characterization of multicomponent and multiphase weld. Finally, it is imperative to dendritically balance these important factors to minimize inevitable interface solidification products and anomalous substructure growth, and reasonably advance superior mechanical properties of reliable weld.