## 2 Powder Bed Fusion
### PBF Definition and Way of Working  
As shown in Table 1, Reference [2] presented many AM processes that can be used for the production of complex parts, and PBF is one of the most promising due to its ability to process a wide range of hard metals.  
Reference [3] presented AM manufacturing technologies classification based on materials according to ISO/ASTM 52900, as shown below in Figure 1:  
PBF processes are metal-based AM where the laser beam scans at a controlled speed selected locations of a powder bed and then fuses the powder to obtain solid material [13] layer by layer by either partial melting, such as in SLS, or full melting, such as in the metal-based process SLM [6]. Reference [13] presents Figure 2, which illustrates the elements of laser-based PBF process.  
The first crucial part of the SLM process is the build job preparation for any given 3D model. It consists of four main steps: geometry importation, alignment/orientation within a build envelope (critical as surfaces with an angle less than 45\({}^{\circ}\) need to be supported), support creation to allow stable processing conditions, and finally slicing based on machine-specific requirements. After preparing and loading a build job onto a machine, the manufacturing process can be initiated starting with the alignment of the recoating device or levelling of the substrate to enable the deposition of powder. Next, the laser scans a specific area based on a predefined scanning strategy and controllable parameters to selectively solidify the material. After finishing a layer scanning, the build plate is moved down, and new powder is deposited on top of the build area to produce a new layer [14]. Reference [14] presented Figure 3 that illustrates the influential factors of a typical SLM process:  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Process** & **Techniques** & **Materials** \\ \hline \multirow{4}{*}{1} & & & Direct metal laser sintering (DMLS), selective laser sintering (SLS), selective laser melting (EMM) and electron beam melting (EBM) & Polymers, metals: miraging steel, stainless steel 316L, 15-5 PH, nickel-based superalloys: Hastelloy X, Inconel 625, Inconel 718, chrome-cobalt, titanium TA6V, aluminum AISi10 mg \\ \hline \multirow{2}{*}{2} & & \multirow{2}{*}{Material extrusion} & Fused deposition modelling (FDM) & Thermoplastic filament \\ \hline \multirow{2}{*}{3} & & & \\  & & & \\ \hline \multirow{2}{*}{3} & & & \\  & & & \\  & & & \\ \hline \multirow{2}{*}{4} & & & \\  & & & \\  & & & \\  & & & \\ \hline \multirow{2}{*}{5} & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \multirow{2}{*}{6} & & & \\  & & & \\  & & & \\  & & & \\ \hline \multirow{2}{*}{7} & & & Laser engineered lens shaping (LENS), direct metal deposition (DMD), laser metal deposition (LMD), blown powder, laser cladding & Metals such as aluminum nickel-based alloys \\  & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Additive layer manufacturing processes (Farinia Group).  
Figure 1: AM technologies classification according to ISO/ASTM 52900 [3].  
Figure 3: Ishikawa diagram of influential factors in SLM process [14].  
Figure 2: Elements of powder bed fusion process [13].  
There are many applications of the SLM process in real life, such as in the medical field with dental implants (highly complex, small in dimension, and one-off patient products) or with specialized surgery equipment. Another large field is in the aerospace industry where it is hard to manufacture materials, such as nickel-based superalloy, and requires high quality, complex shapes, and low to medium production lots like hydraulic manifolds [14].