Deconvoluting cracking mechanisms in fusion processing of steel-copper multi-materials

This study investigates various cracking mechanisms and their prevalence in fusion processing of steel-copper multi-materials using operando X-ray diffraction and imaging during laser powder-bed fusion (LPBF) of 316L-CuCrZr multi-material. Operando X-ray imaging helped identify three main types of cracking: (i) solidification cracking, (ii) metal-induced embrittlement (MIE), and (iii) liquation cracking. All cracking types are closely related to the phase formation during processing, leading to two underlying mechanisms. First, liquid-liquid phase separation (LLPS) and the monotectic reaction in the 316L-CuCrZr system lead to the formation of Cu-rich and Fe-rich liquids with vastly different solidification ranges, causing solidification cracking at the melt pool centers. Second, LLPS and the monotectic reaction distribute the Cu-rich liquid uniformly between the Fe-rich dendrites, leading to MIE and/or liquation cracking. X-ray computed tomography indicated that smaller but more frequent cracks form due to MIE/liquation cracking compared to solidification cracking. Further experiments showed that by avoiding phase separation via process adjustments, cracking can be drastically reduced. However, the complete elimination of cracking necessitates chemical alterations of the material feedstock, as observed in the crack-free examples in the literature. These findings serve as a guideline for understanding the underlying reasons of cracking in steel-copper multi-materials and optimizing processing to effectively mitigate cracking, while also quantifying the extent to which these adjustments can achieve this outcome.