Effects of building direction and defect sensitivity on the fatigue behavior of additively manufactured H13 tool steel

Abstract

Selective laser melting (SLM) is a laser powder-bed fusion process, which produces near-net shaped components with layered microstructures leading to anisotropy in the mechanical properties. The intrinsic process-induced defects, particularly microporosity and lack of fusion, as well as their size and relative orientation with respect to the building direction, affect the fatigue performance of the SLM-processed components. Herein, the influence of building direction and defect sensitivity on the fatigue behavior of SLM-fabricated AISI H13 steel is investigated. Samples are tested after tempering and turning to find a compromise between tensile strength and toughness and minimize the effect of geometrical distortion. The fatigue strength is assessed by employing the Murakami model, and short-crack effect is evaluated by calculating the El Haddad–Smith–Topper length parameter. An inferior fatigue performance of the SLM parts is observed in comparison to that of the conventionally processed materials, owing to the presence of defects that act as origins of crack initiation. The fatigue strength is significantly influenced by the defect size, shape, and the distance from surface, which in turn are affected by the building direction. A slightly higher fatigue resistance is observed for the 90°-oriented samples, owing to the smaller defect size and the lower stress concentration factor. The data for the three building directions show a high scatter, attributed to the process-defect-induced stress concentration factors.