Damage tolerant design by microstructural gradation - Influence of processing parameters and build orientation on crack growth within additively processed 316L

Powder bed based additive manufacturing techniques, such as selective laser melting (SLM) and selective electron beam melting (EBM), attract a lot of interest due to the possibility of producing geometrically complex and individualized components. Additionally, the dependency of the resulting microstructure on the processing parameters applied enables for fabrication of graded structures having locally different mechanical properties. In this study, the effect of different types of gradation on the crack growth behavior of 316L stainless steel processed by SLM was investigated. Using an approach combining mechanical testing, local strain analysis by means of digital image correlation (DIC) and electron backscatter diffraction (EBSD), insights into the relationships between the local microstructure, the crack growth rate (CGR) and the crack deflection on macroscopic and microscopic scale were obtained. The results show clear dependencies of the CGR on the local microstructure, which can be rationalized based on effects stemming from intercrystalline and transcrystalline crack growth. In graded structures, arrest of priorly growing cracks was observed upon reaching a coarse grained area, when the growth direction was perpendicular to the building direction during processing. This highlights the capability of achieving damage tolerant structures without substantially sacrificing strength.