Unmanned Systems Technology 002 | Scion SA-400 | Commercial UAV Show report | Vision sensors | Danielson Trident I Security and safety systems | MIRA MACE | Additive manufacturing | Marine UUVs

73 Additive Manufacturing | Focus repeatable set of mechanical properties for a given component – quite a difficult task when one considers the number of variables involved in the process. For example, tight controls are required on the feedstock powder, ensuring not only a chemical composition that is within specification but is also free from detrimental elements (for example, the fatigue properties of titanium alloys are adversely affected by a high oxygen content, either from the feedstock powder or during processing). The particle size distribution (PSD) and morphology of the feedstock powder are also important, as they affect the packing density of powder in a layer and the interaction of the energy beam with the new layer of powder, and hence the resulting part density. Humidity in the feedstock powder can also affect its flowability and so re-coating behaviour, as well as the weld behaviour. All of these factors must be controlled not only for the first build but for the life of the powder batch, as it is sieved and re-cycled for multiple builds. Each build influences the PSD and composition, and so affects the final part quality, and once a powder batch falls out of specification then it may have an influence on the properties that can be expected of the final product. While the feedstock powder contributes to the part’s quality, there is also an interaction with the process parameters used – laser power, scanning speed, hatching distance, layer thickness and so on – which must be tuned to deliver acceptable properties of the part for a given specification of powder and within a given vendor’s machine. A ‘good’ powder in one machine may not be adequate in another or with a different set of laser parameters, for example. Not only are the manufacturing parameters important, so too is the configuration of parts for the process and in the build chamber. To obtain a good final part, it is important to design for the process as well as design an effective support strategy in cases where supports are required. Consideration must be given to the orientation of the part in the chamber, how it will be removed from the substrate (for example, by wire EDM or bandsaw) and how each potential orientation might impact the number of supports required, how easy they are to remove, the resulting surface quality and the overall economics of the build. Any supports are likely to increase the time and cost to manufacture the part, the material waste and the post-processing costs. Supports are not only required to allow downfacing surfaces to be built, but also to conduct heat out of the part during the build process, and to restrain the part against movement due to residual stresses, particularly in LBM processes. The relative importance of the support’s different functions can vary depending on the material. For example, maraging steel tends to need more attention to thermal management during a build, while titanium alloys need more stress management, so support design must focus on minimising stress raisers to prevent cracking during the build and on restraining the part from any movement that can lead to inaccurate parts. Despite the complex issues that must be overcome to manufacture a high- quality metal part though, the ultimate advantages of metal AM processes in reducing part weight, simplifying assemblies and reducing manufacturing time/cost in hard-to-machine materials can be highly worthwhile. While there are appreciable technical barriers to manufacturing parts for UAVs or satellites, many of the barriers to wider adoption of these processes are regulatory. As a relatively new process, the relevant understanding and knowledge base of metal AM is only now maturing, and so manufacturing standards and best practices have yet to be fully adopted or even written. Applications Metallic AM components are ideally suited to various applications in unmanned vehicles, for example in manufacturing stressed structural parts. The design freedoms offered allow a more efficient part design that can reduce weight and improve functionality by incorporating functions once performed by several parts into a single component. More efficient heat exchangers are an ideal application, as are any fluid flow components where internal losses may be minimised. The ability to manufacture hard-to-machine alloys such as Inconel 718 or Ti-6Al-4V as near-net shape products is also advantageous for parts such as engine exhausts or moderately sized structural parts, which can be complex to fabricate or expensive to machine using conventional methods. The manufacture of aluminium parts is less common, however, as currently only die casting alloys are readily available on AM platforms, and there is only any real benefit if a complex and hard- Unmanned Systems Technology | Spring 2015 Quality control of the raw powder is an important issue for the AM industry (Courtesy of Innovate 2 Make)