Site specific strengthening in additively manufactured stainless steel for protection of geometrical inhomogeneities

Additive manufacturing allows the creation of components with different properties in different geometrical regions. This allows more efficient engineering structures to be fabricated as varying properties can be built into one component depending on the functional requirements. In this study, the relationship between geometrical inhomogeneity and site-specific properties was investigated. Interlayer hammer peening (HP) was used to selectively refine the grain structure and reinforce regions around notches. Notched tensile testing revealed that this led to increase in ultimate tensile strength (UTS) of upto 13% compared to a control. Bulk HP also led to an increase in UTS but came with a significant reduction in ductility, which was mitigated in the selective HP case. Digital image correlation and finite element analysis revealed that this was due to the localization of strain into the softer ligaments, thereby allowing a greater plastic deformation capacity. This effect however only occurred in the presence of geometrical inhomogeneity. Similar tests without any geometrical inhomogeneity revealed that selective HP led to a more than threefold reduction in elongation to failure. The present study provides insight into the relationship between geometrical design and site-specific material property distribution and how the design space can be extended.