## 2 AM Categories and Common Metal AM Processes
### Powder Bed Fusion (PBF)  
The majority of metal AM systems apply powder bed fusion (PBF) processes, and 54% of the metal AM market belongs to these processes as of 2020 (Figure 2). Common metal PBF processes are Direct Metal Laser Sintering (DMLS), Selective Laser Melting (SLM), Electron Beam Melting (EBM), and Direct Metal Laser Melting (DMLM). In general, there is not a great deal of difference in the versions of these laser technologies made by different companies. Moreover, there is little difference in the materials used between the different machines. This section describes the slight differences that exist between the different PBF technologies. Among these technologies, SLM and DMLS processes are exclusively used to produce metallic components [22]. In general, these two technologies use the same basic tenet. SLM, which is also called Laser Metal Fusion (LMF), is mainly used for single component metals such as Aluminium and DMLS is commonly used for metal alloys such as Aluminium and Titanium Alloys [23]. The DMLM process is very similar to the DMLS process, except that in the DMLM process, laser beams create ultra-thin metal layers and provides a homogenous melt pool [24]. Improved surface quality and lower levels of porosity are key advantages of this process over DMLS. In EBM technology, a high energy electron beam is used to fuse metal powder together, instead of a laser in SLM printers. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**AM Category** & **Metal AM Process** & \begin{tabular}{c} **Metal Printer** \\ **Manufacturer** \\ \end{tabular}  & **Process Description** & **Pros/Cons** & \begin{tabular}{c} **Metallic** \\ **Material and** \\ **Manufacture** \\ \end{tabular}  \\ \hline Digital Light Processing & \begin{tabular}{c} Asiga, DWS, Carbon, \\ Lithoz, Rapid Shape, \\ Kudo3D, FlashForge \\ MoonBay, Hunter, \\ CoiLo, Moncure \\ 3D, Teflon 3D, Profways \\ \end{tabular}  & \begin{tabular}{c} As shown in Figure 3a, VAT \\ Photopolymerisation \\ processes use a tank/ vat of \\ liquid material, and a light \\ is used to cure the liquid \\ material and turn it into \\ solidified layers, the build \\ planform then moves away \\ from the liquid material \\ after each layer is solidified. \\ SLA is the oldest and most \\ common technology and \\ uses a UV laser to cure the \\ liquid material/ resin [18]. \\ DLP technology cues the resin by using a digital light \\ projector, and illuminates \\ images for each layer onto \\ the bottom of the resin \\ tank/vat [19]. \\ This group of AM processes \\ covers just 2\% of the metal \\ AM market in 2019 \\ (Figure 2). Two common \\ technologies in this category \\ that are used for printing \\ partially metal components \\ are SLA and DLP [25]. It \\ should be highlighted that \\ parts produced using these \\ methods are not metallic \\ parts, but rather parts with \\ metal content. These two \\ technologies enable \\ building metal containing \\ parts through the use of \\ composite resins that \\ include metallic particles \\ (see Metallic Material \\ column of this table). The \\ metallic particles are added \\ to photopolymer resins as \\ nano-fillers to improve the \\ \end{tabular}  & \begin{tabular}{c} **Pros**: High level \\ of surface finish \\ and accuracy \\ such as tungsten \\ carbide (WC), \\ iron (Fe) or cobalt \\ and post \\ processing \\ Aluminium \\ operations due to \\ the small surface \\ of the laser beam, \\ \end{tabular}  & \begin{tabular}{c} Composite resins \\ that include \\ that includes \\ such as tungsten \\ carbide (WC), \\ iron (Fe) or cobalt \\ Aluminium \\ (Al)-based \\ particles [27; 28]. \\ \end{tabular}  \\ StereoLithogArphy (SLA) & \ \begin{tabular}{c} Moai Formlabs, DWS \\ Systems, UNIZ3D, \\ XYZprinting, \\ Photocentric, Pegasus \\ Touch, Tethon 3D \\ \end{tabular}  &
\begin{tabular}{c} **Pros**: High level \\ of surface finish \\ and accuracy \\ (microns) \\