Building large metallic parts with wire arc additive manufacturing

complexity study - ref [8]

​In the aerospace industry, large structures such as cruciforms, stiffened panels or wing ribs are machined from billets or large forgings, with unsustainable buy/fly (BTF) ratios. Could wire-AM be a suitable alternative process?

​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 [3]. What’s more, titanium is an expensive material to source and machine [4]. 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 [5].

WAAM deposition rate [1]

​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 [1]

​Welding wire is a cheap form of feedstock. Depending upon wire diameter and alloy composition, steel is priced between £2/kg and £15/kg, aluminium between £6/kg and £100/kg, and Ti–6Al–4V between £100/kg and £250/kg. In addition, wire avoids many of the challenges associated with powders such as control of particle size or distribution, which affect process performance. Finally, at the point of deposition, the wire is entirely molten and becomes part of the final structure, and the likelihood of contamination is low.

Applications

​Given the typical 1-2mm layer thickness applied during WAAM and the resulting surface waviness (~0.5mm [6]), WAAM is better suited to form low to medium complexity and medium to large scale preforms that will require post-machining. The following components have been made at Cranfield University, UK.

titanium wing spar [1, 8]

Titanium wing spar [7] (BAE Ssystems)
 
In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles (or thereabouts depending on wing sweep) to the fuselage. The spar carries flight loads and the weight of the wings while on the ground.
A 1.2m Ti–6Al–4V wing spar made for BAE Systems [7] was deposited in a flexible enclosure using plasma arc welding [link] with a seven-axis robotic system. The part features straight and curved features, all perpendicular to the substrate, and T junctions. Two parts were built simultaneously by alternating deposition on either sides of a sacrificial substrate to mitigate residual stresses [link]. The deposition rate was 0.8 kg/h with a BTF ratio of 1.2.

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wing spar [7]

​Built-to-fly ratio (conventional machining): 11.5:1 – from a 250Kg billet to a 21Kg finished part
Built to fly ratio (WAAM): 1.14:1 – from a 24Kg near-net shape deposited part to a 21Kg finished part – 24h build (30% cooling)

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landing gear [8]

Titanium landing gear (Bombardier aerospace) [8]

​With the same set up, a 24kg Ti–6Al–4V external landing gear assembly was also built for Bombardier at 0.8 kg/h on either side of the plane which gave the largest symmetry. This part features T junctions, crossings, perpendicular and slightly tilted walls. With a BTF ratio of 1.2, WAAM enabled material savings in excess of 220 kg.

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Alu wing rib set up with two robots and rotating table [1, 8]

Aluminium wing rib [1,8]

A 2.5m aluminium wing rib is currently machined from solid with a BTF ratio of 37: 670kg are required for a finished product of 18 kg.
Using Fronius CMT Advanced11 setup coupled with a part rotator, the rib feet were added by WAAM at 1.1kg/h on either side of the plane of symmetry, which coincided with the substrate. Owing to the size of the part, two robots were depositing material simultaneously. No enclosure was required. The final part has a stiffening web in between the rib feet, which was not deposited but will be machined out of the thicker substrate, which is accommodating it. This results in a BTF ratio for WAAM of 12, sufficient to enable material savings of roughly 500kg per part.

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wing - high strength - [1]

High strength steel wing [1]
​
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.

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Large scale profiled cone [1]
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.

steel cone deposited with WAAM [1]

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flat rib

Flat rib demonstrator (Fokker aerostructure) [8]

Tall walls, complex intersections, thin walls

BTF (WAAM): 6.3:1 in 9h;

BTF (conventional machining from 53Kg billet): 37:1

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Take away 

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.

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References
[1] 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
[2] C. Vargel: ‘Corrosion of aluminium’, 1st edn, ; 2004, Oxford, Elsevier Ltd.
[3] 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.
[4] G. Lu¨ tjering and J. Williams: ‘Titanium’, 2nd edn, ; 2007, New York, Springer.
[5] 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.
[6] 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.
[7] 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.
[8] 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