Effects of building direction and heat treatment on the local mechanical properties of direct metal laser sintered 15-5 PH stainless steel

The complex thermal cycle in additive manufacturing (AM) strongly depends on process parameters such as the building direction, scanning strategy, layer thickness and particle size. It is difficult to predict the mechanical properties determined by the process parameters because different parameters lead to various thermal gradients, which influences the microstructural evolution and mechanical properties of the materials. In this study, we investigated the microstructural evolution and local mechanical properties of direct melting laser sintered (DMLS) 15-5 PH stainless steel as a function of the building direction (i.e., vertical and horizontal) and the heat-treatment conditions (i.e., solid solution, aging). AM samples are composed of much smaller grains and exhibit a finer microstructure, together with a higher hardness, compared to conventional 15-5 PH samples. The AM process transforms martensite to austenite, but the solution heat treatment impedes this transformation. The austenite obtained using the AM process reverted to martensite with decrease in hardness after solid solution treatment. In case of aging, although the hardness increased, values are independent of the grain size and microstructural morphologies. 15-5 PH fabricated in the horizontal direction have higher hardness than those in the vertical direction, except for aged samples. The grain size and morphology, which were similar in both building directions, does not have any significant effect on the hardness anisotropy. The AM process, which has building directionality in heat dissipation, exhibits hardness anisotropy from differences in the inherent structure on a much smaller scale.