Analysis of solidification microstructure and cracking mechanism of a matrix high-speed steel deposited using directed-energy deposition

This study investigated the microstructure and cracking mechanism of a matrix high-speed steel fabricated by direct energy deposition. The combined effect of rapid solidification and chemical composition on microstructure and cracking mechanism during deposition were investigated. Excessive solute segregation into inter-dendritic regions due to rapid solidification caused formation of retained austenite in the inter-dendritic region and formation of α'-martensite in the dendritic region. The excess solute segregation decreased equilibrium solidification temperature and caused formation of low-melting eutectic carbides in the inter-dendritic region. These carbides increased hot-cracking susceptibility, and caused solidification cracking and liquation cracking in the inter-dendritic region. In contrast, tensile residual stress in deposited layers may have caused cold cracking in α'-martensite near the hot crack tips. Cold cracks contributed to growth of macroscopic longitudinal cracks throughout the specimen by bridging the hot cracks formed during solidification or reheating.