Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Favorable Dendrite Tip Undercooling

Abstract

Two important thermometallurgical factors, i.e. aluminum redistribution and dendrite tip undercooling, are numerically analyzed to better understand growth kinetics of microstructure development with favorable and unfavorable crystallographic orientations in three dimensions during advancement of Ni-Cr-Al ternary single-crystal melt-pool solidification interface to optimize microstructure stability within gamma γ phase. Inappropriate growth crystallography dominates profiles of aluminum redistribution and undercooling of dendrite tip in the crack-susceptible region, simultaneously. For beneficial (001)/[100] growth crystallography, the profiles of solid aluminum concentration and undercooling ahead of dendrite tip are clearly symmetrical throughout solidification interface. For detrimental (001)/[110] growth crystallography, the profiles of solid aluminum concentration and undercooling ahead of dendrite tip are quite asymmetrical within two sides of weld, and this asymmetrical phenomenon is of particular interest. The same energy of heat input is accessible to each half of molten pool, however, the difference of dendrite growth driving forces between right side and left side of weld pool kinetically exacerbates microstructure development. The complexity of (001)/[110] welding configuration is consistently attributable to larger overall solid aluminum concentration and undercooling of dendrite tip in [100] dendrite region than that of [010] dendrite region. In the two welding configurations, the size and shape of [001] dendrite growth region modifies growth kinetics of dendrite tip to extend columnar morphology of epitaxial growth for crack-resistant microstructure development with smaller solid aluminum concentration and narrower dendrite tip undercooling by either decrease of laser power or further increase of welding speed, while [100] dendrite growth region preferentially aids columnar/equiaxed transition (CET) with severe aluminum redistribution and wider dendrite tip undercoolig. The smaller heat input is imposed, the smaller solid aluminum concentration and narrower dendrite tip undercooling are crystallographically induced that is capable of elimination of metallurgical contributing factors for microstructure anomalies and vice versa. Optimum (001)/[100] growth crystallography and low heat input with decrease of laser power and increase of welding speed minimize columnar/equiaxed transition and stray grain formation, improve resistance to solidification cracking and ameliorate weld integrity, while (001)/[110] growth crystallography and high heat input with increase of laser power and decrease of welding speed significantly contribute to weldability exacerbation and microstructure degradation. The crystallography-dependent mechanism of efficient microstructure anomalies reduction, which is attributed to decreasing aluminum enrichment and undercooling ahead of dendrite tip, is therefore proposed. Consequently, comparison between calculation results and experiment results is plausible and acceptable. The usefulness application of this numerical analysis improves predictive capability and enables attractive microstructure control of single-crystal superalloys with similar materials properties.