Residual stress measurements on AISI 316L samples manufactured by selective laser melting

Abstract

This paper aims to understand the formation and the effect of residual stress on selective laser melting (SLM) parts. SLM is a powder bed based additive manufacturing (AM) process and can be compared to a laser welding process. Due to the high temperature gradients and the densification ratio, which are characteristic of this process, residual stresses occur. The investigation of residual stress is performed using X-ray diffraction (XRD) for samples made of austenitic stainless steel AISI 316L (EN 1.4404). This research examines residual stress at different depths and at two outer surfaces. For the measurement of stresses at different depths, the samples’ surface layers were removed by electropolishing. At sufficiently large distances from the top surface, the stresses in the area of the edge layer initially increase strongly and then decline again. The value and orientation of the resulting main stress components are dependent on the examined layer. At the top surface, the residual stresses are higher in scan direction than in perpendicular direction. In contrast, at the lateral surface the maximum main stress is perpendicular to the scan and parallel to the building direction. These two cases can be described very well by the two mechanisms in SLM, namely the temperature gradient mechanism (TGM) and the cool-down phase. It is also shown that at samples with a relative structural density of >99%, the residual stress values are independent of the applied energy density.