EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Additive Manufacturing (AM) technologies can realize complex-shape structures, previously not feasible with subtractive manufacturing machines. From Computer-Aided Design (CAD) to a slicing software, which creates a G-code machine routine, it is possible to realize three-dimensional (3D) parts layer by layer. In the past, 3D printing was used only for rapid prototyping due to the required high tolerances and the low performance of the materials used. Nowadays, AM technologies are improved to the point that 3D printed parts can sometimes be used without tooling or post-processing. Directed Energy Deposition (DED) is a metal Additive Manufacturing (AM) process, where a 3 or more-axis arm moves an extruder with one or more nozzles and a laser beam that melts the powder particles. Parts realized with this technology are always near-net-shape. Due to high cooling rate of the DED process the melt pool is subjected to rapid solidification generating peculiar and thermally unstable microstructures. Therefore, the aim of this research is to characterize the microstructure of as-built DED samples and to understand the effects of a heat treatment. A set of the samples was indeed subjected to an annealing at 1150°C for 2h followed by cooling in air. Optical microscopy (OM) and scanning electron microscopy (SEM) were used together with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) to fully characterize the crystallography and the microstructural and metallurgical features of the as-build and heat-treated samples. Results showed a dendritic microstructure of the as-built 316L stainless steel which is highly influenced by the local cooling rate. Furthermore, preliminary results on the heat-treated samples highlight a peculiar evolution of the austenitic-ferritic microstructure linked to the temperature reached during the annealing phase, suggesting that tailoring the post-processing thermal treatment can be crucial to fully exploit the potential 316L processed by DED.