With the increasing usage of carbon fibre reinforced polymers, aircraft designers are forced to shift from aluminium to titanium, the former being electrochemically incompatible with carbon [1,2]. In addition, with the current and forecast aircraft market expansion rate, the demand for titanium parts is increasing accordingly . What’s more, titanium is an expensive material to source and machine . 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 which are currently 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 product; in the aerospace sector, values of 10 or even 20 are not unusual .
WAAM deposition rate 
WAAM deposition rates are sufficiently high to make the deposition of large scale parts achievable in reasonable times. With rates ranging from 1kg/h to 4kg/h for aluminium and steel respectively, most parts can be manufactured within one working day. Higher deposition rates can be achieved (e.g. 10 kg/h), but this then compromises the fidelity of the part. For instance, at 10 kg/h, the BTF ratio can be as high as 10 for the final deposited part,16 which is effectively a preform, thus requiring significant machining as well as the deposition of much more material, making the process less attractive from an economic point of view. Keeping the deposition rate at medium levels (e.g. 1kg/h for titanium and aluminium, and 3kg/h for steel) ensures that a BTF ratio of v1.5 is always achieved, maximising the cost saving.
WAAM material cost and utilisation 
High strength steel wing 
A 0.8m wing was built for wind tunnel testing in partnership with Aircraft Research Association. Specifically, Aircraft Research Association is aiming at reducing the time between the release of design surfaces to the gathering of data in the wind tunnel. The deposition process was Fronius CMT11 with a deposition rate of 3.5kg/h. The wing features a hollow structure up until its midpoint (Fig. 2f) and will be machined to an accuracy of 0.05 mm.
Large scale profiled cone 
A steel profiled cone also built by Fronius CMT11 at 2.6kg/h. The deposition parameters produced a wall thickness of 2.5mm; against a target of 2mm, the BTF ratio was 1.25. Further to this, lead time can be cut potentially from 6 months to just a few hours.
Tall walls, complex intersections, thin walls
BTF (WAAM): 6.3:1 in 9h;
BTF (conventional machining from 53Kg billet): 37:1
High deposition rates, low material and equipment costs, and good structural integrity make Wire & Arc Additive Manufacturing a suitable candidate for replacing the current method of manufacturing from solid billets or large forgings, especially with regards to low and medium complexity parts. Besides Ti–6Al–4V and aluminium; steel, invar, brass, copper and nickel have been successfully deposited.
Parts made to date, prove it is possible to build components with high integrity and good material properties [link] in the as-deposited conditions and can be further improved by the inclusion of a cold rolling step and a machining step (=ready to use additive manufacturing).
When considered against conventional machining, WAAM shows great potential to reduce material consumption reduce reliance on large and expensive forgings and improve design flexibility, and reduce lead time for new and legacy parts.
 S. W. Williams, F. Martina*, A. C. Addison, J. Ding, G. Pardal and P. Colegrove, Wire þ Arc Additive Manufacturing Materials Science and Technology DOI 10.1179/1743284715Y.0000000073
 C. Vargel: ‘Corrosion of aluminium’, 1st edn, ; 2004, Oxford, Elsevier Ltd.
 C. Cui, B. Hu, L. Zhao and S. Liu: ‘Titanium alloy production technology, market prospects and industry development’, Mater. Des., 2011, 32, (3), 1684–1691.
 G. Lu¨ tjering and J. Williams: ‘Titanium’, 2nd edn, ; 2007, New York, Springer.
 J. Allen: ‘An investigation into the comparative costs of additive manufacturing vs. machine from solid for aero engine parts’ ‘Cost effective manufacturing via net-shape processing’, Proc. Meet. RTO-MP-AVT-139, Neuilly-sur-Seine, France, May 2006, NATO.
 F. Martina, J. Mehnen, S. W. Williams, P. Colegrove and F. Wang: ‘Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti–6Al–4V’, J. Mater. Process. Technol., 2012, 212, (6), 1377–1386.
 baesystems.com: ‘Growing knowledge, growing parts: innovative 3D printing process reveals potential for aerospace industry’; 2014. http://www.baesystems.com/article/BAES_163742/growingknowledge- growing-parts.
 A. Addison, J. Ding, F. Martina, H. Lockett, S. Williams, Manufacture of complex titanium parts using WAAM, Titanium 2015, International Titanium Association May 11-1 2015
Laser Direct Deposition