# Metal Additive Manufacturing for Satellites and Rockets
## 2 Metal Additive Manufacturing Techniques
In both cases, the material used to build an object is deposited only at the required position, either as a powder (Figure 3A) or as a wire (Figure 3B), opposite to the aforementioned powder bed processes where the powder is provided along the whole building area. These materials are again heated by laser (Figure 3A) or electron beam (Figure 3B), similar to the aforementioned techniques. Electron-DED again works in vacuum, while laser-based DED systems use inert gases as described for the powder bed fusion processes. In the powder-based DED systems, an inert gas can be introduced into the melting region together with the metal powder (Figure 3A). It is even possible to use multiple nozzles for powder introduction so that objects can be prepared from material mixtures, differing along the specimen [22]. It is also possible to use plasma as the energy source, such as gas metal arc welding [23]. Generally, laser- and plasma-produced parts usually need post-treatment due to high residual stress in the specimens, which has to be relieved and the necessity to improve the microstructure, while stress is usually lower for electron-beam produced parts which are nearly fully densely produced [24]. Figure 2: (**a**) Typical SLM or SLS process; (**b**) heat transfer paths in melt pool formation and solidification in an SLM process. From [20], originally published under a CC-BY license. Another additive manufacturing technique working with a wire is the wire-arc additive manufacturing (WAAM), in which a wire feed is molten by an electric arc source, allowing for additive manufacturing of steel and several alloys [25; 26]. Additionally, it should be mentioned that some of the alloys which are highly interesting for space applications have been processed with especially well-suited metal additive manufacturing methods. A compositionally graded alloy from CoCrMo and Inconel 718, enabling fitting the material at each position to the required working conditions, was produced by laser powder bed fusion [27]. SEBM is often used to prepare Ni-based superalloys for high-temperature applications [28]. Generally, it must be mentioned that the morphological properties of additively manufactured specimens can significantly differ from the designed structures, and the mechanical properties can show large deviations from the wrought material due to the inclusion of air voids or other microstructural differences [29; 30]. Other problems related to the different methods are the aforementioned necessity to work in vacuum or inert gas atmosphere and the often highly challenging parameter optimization for each new material, including different batches of nominally identical materials. Furthermore, each material has different challenges, such as the problem of hot cracking at grain boundaries in Ni-based superalloys, occurring during the final solidification stages [31]. As this brief overview of some common metal additive manufacturing methods shows, there are diverse techniques available with different advantages and challenges, which are steadily being developed for using metal additive manufacturing in increasingly more applications. The next sections give an overview of possible applications in aerospace research and industry.