Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-based Single-crystal Superalloy Part I: Columnar/Equiaxed Morphology Transition

Abstract

The thermo-metallurgical modeling of stray grain formation was further developed by couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model, nonequilibrium solidification model and minimum undercooling model during three-dimensional nickel-based single-crystal superalloy weld pool nonequilibrium solidification over a wide range of welding conditions (laser power, welding speed and welding configuration). Welding configuration simultaneously influences distributions of stray grain formation and columnar/equiaxed transition (CET). The stray grain formation and dendrite morphology ahead of solid/liquid interface are symmetrically distributed about the weld pool centerline in the (001)/[100] welding configuration. The stray grain formation and dendrite morphology ahead of solid/liquid interface is asymmetrically distributed in the (001)/[110] welding configuration. Vulnerable [100] dendrite growth region is suppressed in favor of epitaxial [001] dendrite growth region to predominantly facilitate single-crystal dendrite growth with further reduction of heat input. Stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. The smaller heat input is used, the less nucleation and growth of stray grain formation with decreasing constitutional undercooling ahead of solid/liquid interface is incurred with mitigation of metallurgical driving forces for solidification cracking and columnar dendrite morphology is increased and vice versa. Symmetrical crystallographic orientation of dendrite growth spontaneously ameliorates microstructure development, and improves resistance to solidification cracking. The mechanism of asymmetrical solidification cracking because of crystallography-dependent stray grain formation and morphology instability is therefore proposed. Optimum low heat input (low laser power and high welding speed) with (001)/[100] welding configuration essentially minimizes both stray grain formation and columnar/equiaxed morphology transition and is beneficial to weldability and weld integrity through morphology control, while undesirable high heat input (high laser power and slow welding speed) with (001)/[110] welding configuration leads to microstructure anomalies and worsens solidification cracking susceptibility. The stray grain formation and morphology transition in the [100] dendrite growth region on the right side of the weld pool are more severe than that in the [010] dendrite growth region on the left side, although the same heat input imposes on both sides of the weld pool in the (001)/[110] welding configuration. The theoretical predictions agree well with the experiment results. Moreover, the promising and reliable model is also applicable to other single-crystal superalloys with similar metallurgical properties for successful crack-free laser welding or laser cladding.