The use of electroforming for aerospace lip skins

Louise Smyth

Electroforming is a proven, production-based additive manufacturing process used to physically deposit metal in a controlled process in microscopic layers to highly precise geometries. This manufacturing process has been adopted by Doncasters, a leading manufacturer of high-precision alloy components, for more than 13 years - predominantly for erosion shields on rotor blades. During that time, the team at its Bramah facility in Sheffield has been working to identify further applications to help engineers drive down cost, improve quality and extend component lifespan.

At the 2018 Farnborough Air Show, Doncasters unveiled a pioneering project: a fully electroformed leading-edge erosion shield for an aerospace engine inlet lip skin on a passenger jet.
 
Electroforming isn’t actually a new technology. It has been used for well over 100 years for a variety of applications and is commonplace in the jewellery industry where you will find electroformed pendants and necklaces. Interestingly, the women’s Wimbledon final trophy was manufactured in the 1800s by electroforming. However, in terms of aerospace applications, it is relatively new.

Early adoption in aerospace

The current main use of electroforming is for rotative fixed components that protect the leading edge of composite structures. The electroform parts are form-fitting and stress-free products that do not twist, warp or spring back, and are prepped and bonded and mechanically fixed components. Nickel-based alloys are the most popular choice of material for electroformed parts due to nickel’s favourable mechanical properties that make it well-suited to many applications. Nickel/cobalt products, in particular, have internal stress low enough for the electroforming process while enabling the creation of an alloy-enhanced finished part. The main application of nickel cobalt parts is for erosion protection. Since 2005, Doncasters has been developing helicopter blades through the process of electroforming. The high propeller speeds they operate at, coupled with the harsh environments which they are subjected to, can be catastrophically damaging to aerospace components, particularly in desert and storm conditions, where sand, water and other debris can cause severe and premature wear and damage to the blades. In addition to weather, bird aircraft strike hazards (BASH) also pose a threat to composite blades becoming damaged from a collision. While the number of major accidents involving civil aircraft is quite low, 65% of bird strikes still cause damage to the aircraft.

Aerospace lip skins

Traditionally created by spin forming using aluminium alloys, lip skins, like rotor blades, need to be able to operate in harsh environments and developed to tight tolerances. Metal-spinning is a forming process in which a blank of material is rotated on a spinning machine similar to a lathe. The blank of material is clamped onto a spin-forming mandrel and rotated by servo-controlled motors and drives. During rotation, heat is applied to the material by a gas torch affixed to a robotic armature and a roller on the spinning machine makes contact with the part blank, forcing the part blank to flow over the spin-forming mandrel surface. The spun formed part requires considerable post processing steps to finish the part, including heat treating for stress relief, burnishing and machining. The parts are also produced in a full-circumference, single piece. Operators may require the lip skin to be in multiple sections to allow for removal, and replacement of damaged sections during the life cycle. This can add further machining and stress relief steps to assure that the segments do not deform or spring back for installation onto the nacelle.

Doncasters already produces a one-piece inlet lip skin for a production turboprop engine. The electroformed process was adopted to replace a multi-piece sheet metal design that was mechanically fastened to the engine nacelle. The electroformed lip skin precisely fits the contour of the nacelle and proved to be more cost-effective for the part production and subsequent installation.

Although this has proved successful for many years, the Doncasters R&D team has identified major benefits of switching to the electroforming process.

The first benefit is cost savings. Generally speaking, the process of electroforming is more cost-effective than aluminium spinning. This is predominantly down to the reduced process steps, part count, reduction and elimination in post machining and processing of the surface, automation (and low labour costs), and defined time ‘in tank’.

Metallurgical improvement is another benefit. Nickel cobalt has much better erosion properties than alternative metals. For example, its tensile strength is three times higher than stainless steel, while its hardness is more than 13% higher than titanium 6AI-4V. It has a much better strength to weight ratio and is corrosion resistant.

THE PROCESS

Like all additive manufacturing processes, electroforming builds 3D objects by adding layer-upon-layer of a material. In this instance, it produces metal parts by electro-deposition of metal over a mandrel. It’s crucial that the area is prepared, to eliminate the risk of contamination, which could adversely affect the part quality and ultimately lead to scrapping the part. The first stage of the process sees metallic pellets introduced into a precisely controlled bath that dissolve in the electroforming solution by passing current through anodes, resulting in the formation of nickel ions. The ions then deposit onto the mandrel, which acts as a cathode, as nickel metal. When the desired thickness of the part is achieved, the mandrel is removed from the solution and the part detached. The part is subsequently removed from the mandrel as a completed structural unit. Once removed, the product is immediately stress free, negating the need for stress relief treatment, such as heat-treating, of the component. Because the finished parts match the contour of the mandrel, the features are very precise. Therefore, the outer mould line of the lip skin can provide tight tolerances, providing an aerodynamic surface that reduces drag, creating a laminar flow surface that reduces drag and improves aerodynamic performance on the aircraft nacelle. It also eliminates the need for post-fabrication machining of the contour. The parts can then be trimmed to length and other features added, including drilling of holes for attachment to the outer barrel of the engine nacelle. Another feature of of the process allows for the part to be produced with either a matte or mirror finish, if desired, eliminating the need for burnishing and polishing.

Because the parts are produced stress-free, the option exists to produce the parts in a full circumference, single piece that can be machined into segments without incurring deformation or spring back, or electroformed in sections if needed.

Beyond aerospace?

The future of electroforming is looking bright. Beyond lip skins, the Doncasters R&D team is already working on electroforming for helicopter lens surrounds and hard coating on glass-forming tools, engine spinners and leading edges on outer and inner guide vanes for turbofan engines. The team has also identified a variety of sectors that electroforming has the opportunity to be involved with. These include:

•    Automotive: lightweight casings for transmission and electric motors
•    Renewables: metallic leading-edge solutions on wind turbine blades
•    Niche vehicles: sports exhaust parts or metallic trims
•    Aerospace industry: leading edges on turboprop blades, fairings, cowls, HIP canisters, etc

Ongoing R&D activities are in place to improve efficiencies within the process, offering the customer a more cost-effective solution. The process can inherently deliver rapid prototypes, allowing for concept development to produce trial parts for Doncasters’ customers.

Although not a brand new concept, the team believes that electroforming still has much more opportunity in the aerospace industry and that the only limitation is people’s imagination!

Depending on the size, and part feature complexity of the part, the growth time for a simple leading edge would take approximately eight to 10 hours, while
a large lip skin might be over 100 hours growth time. Doncasters has an ongoing programme to reduce tank times, which will promote further cost benefits and become more cost-competitive with other processes.

Andrew Woods is with Doncasters

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