HAZ Liquation Cracking Mechanism of IN-738LC Superalloy Prepared by Laser Solid Forming

The heat-affected zone (HAZ) liquation cracking mechanism of nickel-based superalloys with high (Al + Ti) content during the laser solid forming (LSF) process was investigated via laser remelting of an as-deposited IN-738LC superalloy. Microstructural HAZ analysis revealed that cracks consistently propagated from the HAZ to the remelting zone along the grain boundary (GB). The formation of a liquid film during GB liquation was mainly owing to localized melting of the semicontinuous gamma-gamma' eutectic distributed along the GB. The solute segregation behavior of the IN-738LC alloy during LSF was analyzed using the Giovanola-Kurz model and Scheil models, revealing that a significant enrichment of gamma-gamma' eutectic-forming elements in the residual liquid at the final stage of solidification (solid fraction similar to 0.87) in the molten pool was the main cause of semicontinuous gamma-gamma' eutectic formation along the GB. Further, a B enrichment at the GB was identified in LSF-fabricated IN-738LC, which promoted cracking by lowering the GB liquation temperature and promoting wetting of the GB by the liquid film. Unlike the phenomenon observed in the welding of cast IN-738LC, the coherence between the gamma' phases and the gamma matrix in LSF-fabricated IN-738LC can suppress the occurrence of constitutional liquation of the gamma' phase. To understand the interaction between the thermal stress and the liquid film in the LSF process, thermal stress as a cracking driving force was also estimated based on the measurement of the residual stress from the substrate to the remelting zone of the IN-738LC deposit by the Vickers micro-indentation method. (C) The Minerals, Metals & Materials Society and ASM International 2018