Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Salient Aluminum Redistribution Melioration

Abstract

Correlation between welding conditions, such as laser power, welding speed and welding configuration, microstructure anomalies, including columnar/equiaxed transition (CET) and stray grain formation, and metallurgical factors, such as aluminum redistribution and supersaturation near dendrite tip, is elucidated by thorough numerical analysis to explain prevalent phenomenon of insidious microstructure development and face the challenge of microstructure amelioration as well as solidification cracking resistance improvement during course of ternary single-crystal Nickel-Chromium-Aluminum superalloy melt-pool solidification for crack-free laser surface modification. Metallurgical factors of γ gamma phase microstructure development strongly depend on welding configuration, and heat input is not as important as welding configuration. Although melted-pool center region is susceptible to microstructure anomalies, (001)/[100] welding configuration possesses auspicious growth crystallography under which the bimodal profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are symmetrically distributed to potentially decrease solute redistribution and increase resistibility to solidification cracking. Dissimilarly, (001)/[110] welding configuration possesses insidious growth crystallography under which the profiles of solid aluminum concentration and liquid aluminum supersaturation ahead of dendrite tip are asymmetrically distributed to preferably facilitate alloying plentiful enrichment and differentiate problematical microstructure instability on half side of molten pool. In the bottom of molten pool, beneficial Al-barren [001] dendrite is kinetically driven by homologous single-crystal epitaxial growth without columnar/equiaxed transition. On the right side of molten pool, detrimental Al-rich [100] dendrite is spontaneously susceptible to stray grain formation with equiaxed morphology. Symmetrical microstructure development is more enrichment-resistant than asymmetrical microstructure development, which is crystallographically ascribable to favorable thermo-metallurgical factors, i.e. aluminum redistribution mitigation and supersaturation relief, and substantially reduce metallurgical degradation and microstructure failure. The faster welding speed and the lower laser power are used, the smaller aluminum concentration and supersaturation ahead of dendrite tip are kinetically incurred to suppress columnar/equiaxed transition and stray grain formation with a number of thermometallurgical factors contribution to considerable microstructure amelioration and increasingly improve solidification cracking resistance and vice versa. Shallow molten-pool shape with symmetrical growth crystallography efficiently advances anomalies-resistant microstructure development with diminution of partition driving forces for solute redistribution and supersaturation during nonequilibrium solidification instead of deep molten-pool shape with asymmetrical growth crystallography. The latter importantly aggravates stray grain formation. Useful combination of optimum solidification conditions and favorable growth crystallography predominantly minimizes diffusion-controlled solute deviation and microstructure anomalies for excellent single-crystal superalloy laser processing without cracking, and possesses tenable relationship between welding conditions, solute redistribution and microstructure anomalies. The mechanism of crystallography-aided concentration fluctuation and copious supersaturation behind prominent phenomenon of microstructure anomalies, where nucleation and growth of stray grain formation near dendrite tip are activated, is consequently proposed. The comparison between numerical analyses and experiment results are valid and satisfactory. Microstructure anomalies are predictable for proper understanding of multicomponent microstructure development in the interior of fusion zone. The theoretical methodology is reproducible as consequence of availability of thermodynamic and kinetic properties of Nickel-based or Iron-based single-crystal superalloys.