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

72 and EBM are fundamentally welding processes. Each successive layer of metallic powder is welded to the preceding layer in a fully molten welding process – long gone are the days of sintered green pre-forms which are then infiltrated by bronze materials. Although they have their own advantages and challenges, as with plastic AM techniques it should be remembered that they are simply welding – no black magic occurs – and as a welding process, a lot of the same challenges are faced. For example ‘un-weldable’ materials cannot be readily processed by LBM/ EBM, and residual stresses or distortion are perhaps an even greater issue in AM processes than in conventional welding, as they repeat-weld several thousand times to form the end component (albeit on a smaller scale compared to conventional arc-welding processes). Typically, metallic AM processes use a powder feedstock, swept by a re-coating arm or blade onto a substrate in layers ranging from 20-100 µm depending on the process in question. In the case of EBM, the substrate and build chamber are pre-heated to about 750 C and use thicker, 70-100 µm layers, while LBM systems more typically heat the substrate to only 35 C – or up to 200 C at most – and process material in layers of 20-60 µm. EBM systems must naturally operate under a vacuum, while LBM systems use an inert atmosphere of nitrogen or argon depending on the material being processed. The differences in layer thickness and pre-heating temperatures mean that EBM and LBM have their own advantages. EBM parts experience lower residual stresses owing to smaller temperature differentials, and faster build times (and so higher productivity) due to thicker layers. Conversely, LBM systems typically provide higher resolution and so improved surface finish, although with more powerful lasers newer systems now build in thicker layers, improving productivity at the expense of resolution, while EBM systems have recently seen an improvement in surface finish. So the two technologies are perhaps beginning to converge, although they still maintain distinct differences. A particular advantage of these metallic AM processes lies in their ability to produce a complex shape such as would more usually be investment cast, but without tooling and with material properties closer to those of wrought material; however, there are important design limitations. Most parts require supporting structures where downfacing surfaces are too close to horizontal (usually below 40 0 to the build platform, depending on the material). These supports must be built from the same powder bed material, so they are usually welded to the part in question and so can be quite difficult to remove, requiring intensive manual fettling or further post- processing such as CNC machining or Electrical Discharge Machining (EDM). While the ability to manufacture complex and unconventional geometries is undoubtedly a significant advantage for metallic AM technologies, such complex geometry makes post-processing far more difficult. For example, internal cavities that allow much lighter and stiffer component designs cannot be easily finished to improve surface roughness, and so remain a liability with regard to fatigue crack propagation. The ability to manufacture complex lattice structures is also a fantastic advantage but which brings its own set of problems. New software tools allow such structures to be generated relatively easily, but characterising the mechanical properties of such structures can be difficult, making predictive analysis and design unreliable. Inspecting such a complex internal geometry is virtually impossible by any process other than CT scanning, which increases cost. Indeed, non-destructive testing of additively manufactured parts is an area of ongoing research and a substantial topic in its own right. For some time now, the greatest challenge facing metallic AM has been to convince potential end-users of the mechanical properties that can reliably be expected from additively manufactured material. Recent years have seen a general acceptance of the properties that can be achieved, but the industry must now strive to ensure a reliable and Spring 2015 | Unmanned Systems Technology LBM Ti-6Al-4V lattice samples used to determine bulk properties for a potential unmanned space application (Courtesy of EOS as part of Innovate UK’s LIGHT project) New software tools allow the generation of highly complex structures (Courtesy of Materialise)