Experimental investigation of the effects of processing parameters and heat treatment on SS 316L manufactured by laser powder bed fusion

Actually, additive manufacturing (AM) is considered as a set or a class of manufacturing processes that are designed to produce complex parts. The classification of the AM processes is based on the materials that are used and the type of fed energy in the fabrication process. Then, the mechanical and physical characteristics of the produced materials mainly depend on the process parameters, their interactions and effects on the matter to be processed, and then the microscale/mesoscale transformations that are involved in a given process. This work deals with the investigation of the effects of the processing parameters and post-processing heat treatments on both hardness of the surfaces of the samples (Vickers scale) and the corresponding porosities; the process adopted in this investigation is laser bed fusion, especially the selective laser melting machine (SLM) (ISO/ASTM 52.900). The experiment plan was designed by taking into account the variation of the laser power, the scanning speed, and the heat treatment (HT). In this study, layer thickness, scanning strategy, hatch space, and HT maintaining time were fixed. The findings showed that the surface hardness evolves as a concave quadratic function of the volumetric energy density (VED) where the maximum is reached at VED around 166.67 J/m3; indeed, below this value, the hardness increased due to matter densification that takes place by increasing the specific energy VED; then the hardness decreases (softening) by the additional incrementing of the energy. The softening of the materials at higher energy levels is explained by the additional heat delay caused by the successive laser irradiations similar to an in-process annealing. Furthermore, the top surface hardness is higher than the lateral surface hardness due to the heat flux that is continuously fed to this latter by the consecutive laser shots; in contrast to the top surface constitutes a last layer, that it has not been object of additional irradiations. These observations emphasize the hardness anisotropy and, by extension, the anisotropy of the grains morphology, and the mechanical properties in general. Additionally, based on the hardness variation, it was highlighted that the annealing at 800 degrees C produced softer and more homogeneous materials than at 600 degrees C which was assigned to grain growth. Hence, for 600 degrees C treaded sample, additional heating time is required to reach the same level of materials homogeneity as for 800 degrees C annealing. Furthermore, the surface porosity was computed using image processing and revealed an eventual exponential decrease with the VED energy. Finally, the density variation was observed also according to the SLM-material and should explain the evolution of the hardness according to the process parameters.