Abstract
This paper is part of a Themed Issue on Additive manufacturing of metals for aerospace applications. Additive manufacturing (AM) is a technology that promises to reduce part cost by reducing material wastage and time to market.1 Furthermore, AM can also enable an increase in design freedom, which potentially results in weight saving as well as facilitating the manufacture of complex assemblies formerly made of many subcomponents.2 A basic AM system consists of a combination of a motion system, heat source and feedstock. Owing to its intrinsic characteristics, each process is naturally suitable for certain applications. For instance, selective laser melting delivers net shape components with high resolution; however, similarly to electron beam melting, deposition rates are relatively low, and part size is limited by the enclosed working envelope.3 Consequently, this class of processes is best suited to small components with high complexity. The main business drivers for their adoption are freedom of design, and possibly reduced time to market.1 The benefit associated with the reduction in material waste is limited; the mass of the components is already low to begin with. While the possibility of topologically optimizing certain components is important, there is a growing requirement for larger reductions in material waste, for the following reasons. First, with the increasing usage of carbon fiber reinforced polymers, aircraft designers are forced to shift from Al to titanium, the former being electrochemically incompatible with carbon.4 Second, with the current and forecast aircraft market expansion rate, the demand for titanium parts is increasing accordingly.5 Third, titanium is an expensive material to source and machine.6 Therefore, in the aerospace industry, there is a pressing need for the development of a process that could replace the current method of manufacturing large structures such as, stiffened panels, wing ribs, etc., which are machined from billets or large forgings, with unsustainable buy/fly (BTF) ratios. This metric is the ratio of the mass of the initial workpiece to the one of the finished products in the aerospace sector, values of 10 or even 20 are not unusual.7
