# Metal Additive Manufacturing for Satellites and Rockets
## 2 Metal Additive Manufacturing Techniques  
While polymer 3D printing usually works by placing a molten filament on the desired positions or polymerizing a liquid resin at defined places, metal additive manufacturing is usually based on metal powders or even metal wires. Typical materials used for metal AM include aluminum, stainless steel, titanium, or cobalt chrome [8]. According to ASTM standard F2792 [9], metal AM processes can be subdivided into powder bed fusion and directed energy deposition. Powder bed fusion processes generally include a powder bed in which energy is deposited at defined positions to either melt or sinter the metal powder by a laser or an electron beam. As an example, Figure 1 depicts the process of selective electron beam melting (SEBM) [10]. The four-step process starts with the application of a powder layer, which is heated, molten, until the layer is fully produced, and the platform can be lowered to enable application of the next powder layer. It must be mentioned that the process works in \"controlled vacuum\", i.e., the chamber is pumped to approximately \(10^{-4}\) to \(10^{-5}\) mbar before helium is inserted to reach a low pressure of about \(10^{-3}\) mbar, to prevent electrostatic charging [10]. Electrons are emitted from a tungsten filament or a LaB\({}_{6}\) cathode and, as in scanning electron microscopy, are accelerated and focused using electromagnetic lenses [11]. The object is built in layers of typical thicknesses between 0.05 and 0.15 mm. Generally, a new powder layer is preheated to temperatures between 300 and 1100 \({}^{\circ}\)C for different metals to start sintering, e.g., to around 400 \({}^{\circ}\)C for pure copper and 900-1100 \({}^{\circ}\)C for Ti-48 Al-2 Cr-2 Nb powder [12; 13], in this way increasing the electrical conductivity and thus avoiding repulsion of charged powder particles [14], then melted where desired, lowering the platform, applying a new powder layer of defined thickness on the platform, and starting with the next preheating  
Figure 1: Selective electron beam melting process. **Left**: Process chamber. **Right**: 4-step process for building one layer. From [10], originally published under a CC-BY license. step. Literature reports indicate that selective electron beam melting results in relatively coarse surfaces and internal microstructures [15], but on the other hand in low residual stress [16]. Besides selective electron beam melting, there are several laser-based processes that work with a powder bed. One is the aforementioned selective laser melting (SLM), using a laser to fully melt the powder, in this way creating dense layers so that no post-treatment (as in SLS) is necessary [17]. Instead of the electron beam described previously, here a laser beam scans the surface, focused with normal glass lenses. SLM usually works in a nitrogen or argon atmosphere to avoid oxidation during the fusion process, with nitrogen having a larger thermal conductivity, which is advantageous for cooling the specimens that are produced [18; 19]. The SLS process is in principle identical, with a different nature of powder fusion [20]. The process and the melt pool formation are depicted in Figure 2[20]. Similar to the aforementioned selective electron beam melting, here a laser beam is used to melt the powder in the upper layer, before the build plate is lowered, and a new powder layer is placed in the build area by the recoater (Figure 2a). The geometry of the melt pool, produced by laser heating, is depicted in detail in Figure 2b. In addition to these typical powder bed fusion processes, there are also directed energy deposition (DED) processes used for metal additive manufacturing. Figure 3 depicts two typical DED systems [21], working with a laser or electron beam and powder or wire feed, respectively.