From the files - Electroforming lip skins & beyond (first published in Machinery, Aerospace supplement October 2018)

4 min read

​Doncasters has been using electroforming for more than 13 years, predominantly as erosion shields on rotor blades. Earlier this year, the company unveiled a breakthrough use, as Andrew Woods, business development manager for Electroform & Airmotive, explains

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.

Image: Parts are additively manufactured, like plating, over a mandrel, delivering precise, stress-free components

This year, at the Farnborough International Air Show, Doncasters unveiled its pioneering project that, it said, will “take the industry by storm” – a fully electroformed leading-edge erosion shield for an aerospace engine inlet lip skin on a passenger jet.

Not a new process, electroforming has been used for well over 100 years, but in terms of aerospace applications, it is a relative newcomer. The current main use of electroforming is for rotative fixed components, that protect the leading edge of composite structures. Electroformed parts are form-fitting and stress-free products that do not twist, warp or spring back. They are prepped, bonded and then mechanically fixed to components.

Nickel-based alloys are the most popular choice of material, due to their favourable mechanical properties. Nickel-cobalt, in particular, has 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 to protect them against premature wear and damage. For example, bird aircraft strike hazards pose a threat to composite blades. And while the number of major accidents involving civil aircraft is quite low, 65% of bird strikes still cause damage to the aircraft.

Nacelle lip skins, just as with rotor blades, need to be able to operate in harsh environments and be produced to tight tolerances. They are traditionally created by spin-forming aluminium alloys, where a blank of flat material is rotated on a spinning machine and material ‘pushed’ over a forming mandrel. During rotation, heat is applied to the material by a gas torch.

Such spun-formed parts require significant post-processing steps, including heat treating for stress relief, burnishing and machining. These parts are produced in a full circumference, single piece, but 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 during installation.

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 part production and subsequent installation.

While this has proved successful for many years, the Doncasters R&D team has identified three major benefits of switching to the electroforming process: cost saving: metallurgical improvements; and process benefits.

On the first, 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’.

Second, 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.

On 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, so avoiding quality issues or scrapping a part.

The first stage of the process sees metallic pellets introduced into a precisely controlled bath where they dissolve in the electroforming solution, with current passed 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 part is achieved, the mandrel is removed from the solution and the part detached 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 also 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 the 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 then be machined into segments without incurring deformation or spring back, or they can be electroformed in sections, if needed.

Depending on the size and part feature complexity, the growth time for a simple leading edge would take approximately eight-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.

Beyond aerospace and lip skins, Doncasters 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. It has also identified a plethora of sectors that electroforming has the opportunity to be involved with: automotive, for lightweight casings for transmission and electric motors; renewables, for metallic leading-edge solutions on wind turbine blades; niche vehicles, for sports exhaust parts or metallic trims; and aerospace industry, for leading edges on turboprop blades, fairings, cowls, HIP canisters, etc.