1 4 A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 Fig. 1 — Overview of fusion-based metal additive manufacturing processes. Metal additive manufacturing (MAM) processes have matured from their early use as rapid prototyping tools to producing today’s critical end-use components[1-4]. Since the early 2010s, an increasing number of MAM processes have emerged that were initially referred to by various acronyms[5]. ASTM Committee F42 on Additive Manufacturing Technologies undertook the task of standardizing the terminology by issuing ASTM Standard F2792-12a in 2012[6]. In the most general terms, fusion-based MAM processes are characterized in terms of feedstock and the energy source used to fuse or melt the feedstock into the desired component geometry. Figure 1 provides an overview of the fusion-based processes in which either a powder or wire feedstock is combined with an energy source that melts the feedstock to either create a new freeform part or add material to an existing part. The two main categories include powder bed fusion (PBF) and directed energy deposition (DED). In PBF, a focused beam is used to trace out the part according to a defined toolpath from a CAD model in a layer-by- layer method using either a laser (L-PBF) or an electronbeam(EB-PBF)[7,8]. DED can use either a powder feedstock integrated with a laser (LP-DED) or a wire feedstock with either a laser beam (LW-DED), an electric arc (AW-DED), or an electron beam (EBW-DED)[9-12]. Other solid state MAM processes exist, but are not the focus of this article. IN THE BOX VS. OUT OF THE BOX The two primary categories of MAM can be thought of as “in the box” for PBF versus “out of the box” for DED. Although the most highly cited “in the box” metal AM process is L-PBF, the size of the build chamber restricts the final size of the component. To eliminate this size constraint, “out of the box” DED processing has emerged, although some systems may use a large purge chamber to prevent oxidation and issues with reactive alloys. The ability to fabricate outside the box removes size constraints, but tradeoffs between feature and geometric resolution, build and post-processing time, component size, microstructure and resulting properties, and process availability must always be considered. Figure 2a highlights the increased build dimensions made possible by using DED compared to PBF. In contrast, Fig. 2b shows that as build size increases, the deposition rate also increases with a resulting reduction in feature size. Figure 3 showcases several largescale structures fabricated using MAM DED. A NASA HR-1 alloy channel wall nozzle with integral internal passages for a liquid rocket engine is shown in Fig. 3a, built using LP-DED. An aluminum tank structure built using AW-DED is shown in Fig. 3b. MULTIPLE MATERIAL CHOICES DED can build using a variety of alloys including those based on nickel, iron, copper, cobalt, titanium, and Fig. 2 — (a) Selection of AM processes based on overall build dimensions[12]; and (b) relationship between feature size and deposition rate[5]. (b) (a)
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