Issue 53

P. Ferro et alii, Frattura ed Integrità Strutturale, 53 (2020) 252-284; DOI: 10.3221/IGF-ESIS.53.21 253 “ASTM F42 - Additive Manufacturing”, formulated a set of standards that classify the range of Additive Manufacturing processes into seven categories (Standard Terminology for Additive Manufacturing Technologies, 2012) [1]: VAT Photopolymerisation, Material Jetting, Binder Jetting, Material Extrusion, Powder Bed Fusion, Sheet Lamination, Directed Energy Deposition. Additive manufacturing offers several advantages throughout the design workflow including little lead-time, few constraints, little-skill manufacturing. Furthermore, it is a low environmental impact process because it is characterized by less waste and energy saving. In fact, when compared with traditional manufacturing processes, additive manufacturing can significantly reduce energy usage by using less material and eliminating steps in the production process. Among the different AM technologies, Powder Bed Fusion Processes (PBFPs) are largely used with respect to metallic materials, and for this reason they were chosen as the focus of the present review. PBFPs use a high-power density source to melt or sinter a thin metallic powder layer [2,3]. The solid part is built layer by layer, as schematically shown in Fig. 1. Both selective laser melting (SLM) and electron beam melting (EBM) belong to PBFPs. The first one uses a laser source to melt the powder, the second one uses an electron beam. But compared to SLM, electron beam systems produce lower residual stresses, resulting in less distortion of the printed part and less need for anchors and support structures. Moreover, EBM uses less energy and can consolidate powder layers at a faster rate than SLM, even if the surface finish is typically of lower quality. EBM also requires the parts to be produced in a vacuum and the process can only be used with conductive materials. Figure 1: Schematic of PBFP working principle. Despite the numerous advantages offered by PBFPs, the complex interaction between the heat source and the powder layer, as well as the complex thermal phenomena that occurs during the printing process may result in different kinds of defects. Gas pores, for example, may arise from powder surface chemistry modification and/or trapped gas in particles that are released during melting and locked in during solidification. But they may also be due to key-hole effect for deep melt pools. Elongated pores are process induced defects and are due to an inefficient melting regime or spatter and fumes ejection [4]. Unfused power also is a process induced defect while balling [4], that refers to solidification of melted material into spheres, is due to lack of wettability with previous layer, driven by surface tension and directly related to melt pool characteristics [5]. Cracking may also happen because of solidification problems (liquation), residual stresses, surface roughness and other macroscopic defects [6]. Warping can occur between two layers or at the boundary between support and part layer (curling) when the build is stopped and re-started [7]. Delamination - separation between two layers - is another serious defect that cannot be repaired by post processing. It is due to inappropriate melting overlap with previous Powder  delivery  Roller Powder  fabrication  Heat source Fabricated part Melt  Powder  l Heat source  Solidified