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69 ultimate strength is required. In cases where packaging volume is limited, a high-strength steel can usually outperform any other material on a strength-to-volume basis, and excels compared to other metallic alloys when used in relatively thin sections. Since stainless steels are created by alloying steel with high proportions of chromium to provide corrosion resistance, they also provide a highly durable material for use in hostile environments such as sea water or those with high temperatures. Aluminium alloys are a mainstay of the aerospace industry, where low density coupled with reasonable strength provide advantageous specific strength and stiffness. Indeed, many aluminium alloys were developed specifically for use in aircraft structures and skins, and continue to serve those functions. By extension, unmanned systems – particularly UAVs – have adopted many of the same alloys for the same uses. Recently though, new alloy systems have become more readily available, and offer some performance benefits over traditional alloys. In particular, the aluminium-lithium (Al-Li) alloy system can serve as a direct replacement for conventional 2024 or 7075 aluminium alloys. The particular benefit of Al-Li alloys is that adding lithium lowers the density. Up to the solubility limit (4.2%), every 1% or so of lithium added to the alloy decreases its density by 3%, while raising its stiffness by 5% when properly heat treated. The improvement in the properties of Al-Li alloys impairs their weldability though, so joining by conventional welding is made more difficult. Until recently the greatest barrier to their adoption had been cost and availability, but the growing use of new generations of the alloys is alleviating these issues. Titanium is often used for aerospace applications where good strength-to- weight characteristics and corrosion resistance are required. As with steels, titanium alloys are able to deliver higher absolute performance than aluminium alloys when section thicknesses are constrained by packaging volume, but lower than high-performance steels. Titanium is also a good candidate for applications requiring high strength and corrosion resistance at reasonably high temperatures, delivering higher strength than stainless steels up to about 600 C. Although weaker, stainless steels will continue to operate at higher temperatures than titanium alloys. Most titanium components are made from Ti-6Al-4V (known as Ti64), the generally accepted aerospace titanium alloy, and suppliers can push the boundaries of these conventional materials a little further, for example by using certain forging techniques to increase strength or ductility. Alternatively, alloys such as Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo – referred to as Ti6242 and Ti6246 – can provide some improvements over the benchmark Ti64 alloys, particularly with regard to strength and corrosion resistance. Nickel alloys typically provide strength and corrosion resistance at much higher temperatures than steel, aluminium or titanium are capable of. Although slightly denser than steel, nickel alloys are the only real choice for very high temperature metallic structures such as turbine engine components or outside the engine for parts close to or in the exhaust stream. Manufacturing methods The vast majority of metallic alloys used in unmanned systems are supplied from forgings or extrusions in stock sizes ready for machining into the final geometry. Machining produces highly accurate components with high-quality surfaces, and modern CAM software and five-axis equipment can produce surprisingly complex shapes. The cost of machining generally increases with the quantity of material to be removed and the difficulty of machining a given alloy (which dictates the rate at which excess material can be removed and the associated tooling wear/cost). Aluminium is very easy to machine at high speeds and so is particularly cost-effective, whereas titanium or nickel alloys are reasonably hard to machine and so carry a high cost of manufacture. As an alternative to being machined from solid, components can be produced as near-net shape items using various other processes such as casting, hot-isostatic pressing or additive manufacturing (AM). In general this reduces the waste material and the cost of machining, as only key features need be machined. Advanced materials | Focus Unmanned Systems Technology | December/January 2017 High-performance metallic alloys are continuously evolving (Courtesy of Smiths High Performance)