Orientation dependent fatigue performance and mechanisms of selective laser melted maraging steel X3NiCoMoTi18-9-5

Abstract

Additive manufacturing (AM) of the maraging steel X3NiCoMoTi18-9-5 (or 18Ni300) is readily used for close contour cooling of complex forming dies or injection moulding dies, and can be used at long-term operating temperatures up to 400 °C. Therefore, this study aims to establish an understanding of process inherent defects of selective laser melting (SLM) on fatigue performance at room temperature but also for the long-term operating temperature of 400 °C. Additionally, the dependence on built orientation is analyzed. By employing static and cyclic mechanical tests and a full SEM-investigation of the fracture surfaces, a highly anisotropic fatigue performance dependent of the build direction is revealed. Fatigue strength values at 107 cycles at room temperature were found to be 42% and 30% lower for horizontal and vertical build directions compared to the fatigue strength of bulk samples. Likewise, the fatigue strength found for long-term operating temperature of 400 °C was 50% and 33% lower for horizontally and vertically built samples in comparison to conventionally cast samples. This was attributed to a microstructural crack initiation, while common process inherent defects like porosity and inclusions only play a tangential role. The anisotropy in cyclic performance is attributed to orientation of the cellular microstructure, that is dependent on the orientation of the melt pool. Defects of SLM samples did initiate on Ti/Al-oxides in the sizes of 10–40 μm, in contrast to bulk material, where TiN inclusions are commonly found as initiating defects in the range of 10 μm for high cycle fatigue (HCF) fracture surfaces.