Solidification cracking in steel/copper alloy multi-material structures deposited by laser-based directed energy deposition

This study investigates the susceptibility to solidification cracking in steel/copper alloy multi-material structures and identifies critical influencing factors. SS316L and H13 were deposited on copper-beryllium substrates using laser-based directed energy deposition. Solidification cracks were observed at the steel end, with a downward trend as the dilution ratio increased. Crack-free single-layer SS316L specimens were successfully deposited when the dilution ratio exceeded 44%. SS316L and H13 displayed similar cracking morphology and susceptibility, indicating that the cracking susceptibility of steel/copper alloy systems can be simplified to an Fe–Cu binary issue due to the limited impact of alloying elements. Thermodynamic calculations using the CALPHAD method reveal that Cu concentration in the steel predominantly influences cracking susceptibility through two metallurgical factors: (i) the solidification temperature range and (ii) the amount of peritectic liquid. Cracking susceptibility sharply increases when Cu concentration surpasses approximately 9 wt%, driven by the combination of a wide solidification temperature range and a limited terminal liquid quantity. Beyond this threshold, the cracking tendency continuously decreases as Cu concentration rises until reaching a crack-free region where Cu concentration is around 50 wt% or higher. A large solidification temperature range results in a long, narrow liquid channel at the final stages of solidification, which promotes crack formation. Conversely, a larger quantity of terminal liquid enhances the backfilling effect, facilitating crack healing and thus reducing the tendency for cracking. Furthermore, the solidification microstructure also affects cracking susceptibility. The cellular and columnar dendritic microstructure typical of low Cu concentrations increases cracking tendency, as thin, elongated intercellular and interdendritic films are weak and prone to crack initiation. However, at higher Cu concentrations, spinodal decomposition is more likely, producing a finely distributed, grid-like arrangement of Cu-rich phases within the steel matrix, which inhibits crack formation.