GKN plc is a global engineering business with four divisions; GKN Aerospace, GKN Driveline, GKN Powder Metallurgy and GKN Land Systems, which operate in the aerospace, automotive and land systems markets respectively. Founded more than 250 years ago, the UK-based company has adapted, developed and grown into a business with 56,000 employees and 2015 sales in excess of £7.5 billion. It serves most of the world’s leading vehicle, machinery and aircraft manufacturers.
Through the acquisition of strategic elements of leading aerospace manufacturers, GKN Aerospace has grown to establish itself as a world-class business. Prominent steps in this growth path, with particular relevance to the division’s global AM development activities, date back to 2001 with the acquisition of the St. Louis, Missouri, USA, operation from Boeing. This acquisition created a strong partnership with Boeing in both metallic and composite technologies. This was followed in 2009 with the acquisition from Airbus of the UK’s Filton operation, significantly enhancing the business’s expertise in metallic aerostructure assembly. In 2012 GKN acquired Volvo Aero, Sweden, creating a market leader in aero-engine components and significantly expanding GKN’s aero engine components business. The most recent acquisition, of Fokker Technologies, the Netherlands, in 2015 strengthened its market leading position, expanded its technology offering and increased content on key aerospace platforms, broadening its global footprint.
As a result of these acquisitions, GKN Aerospace can now claim to be the leading global Tier 1 aerospace supplier with an unrivalled breadth of capabilities in areas including:
- Engine systems
- Rocket engine subsystems
- Special products (e.g. window transparencies for flight deck and passenger cabins, ice protection systems, lightweight missile canisters)
- Landing gear
- Wiring interconnect systems
- Global services, including MRO (Maintenance, Repair and Overhaul), conversion and completion for mature and legacy aircraft
Today, GKN Aerospace has around 17,500 employees at 62 locations in fifteen countries. 2015 sales were around £2.5 billion, or approximately 33% of total GKN group sales.
Additive Manufacturing as a cross-divisional activity at GKN
Robert Sharman was keen to emphasise that GKN, as a group, sees Additive Manufacturing as a high priority manufacturing technology, stating, “AM development is a cross-divisional activity, particularly involving cooperation across a network of Centres of Excellence that span the Aerospace and Powder Metallurgy divisions of the group. In addition there is collaboration with a number of external parties such as equipment suppliers and research institutions.”
AM related activities in the Centres of Excellence in Powder Metallurgy parts (GKN Sinter Metals, Radevormwald, Germany) and metal powder production (GKN Hoeganaes, Cinnaminson, New Jersey, USA) operations have been reviewed in separate reports [1, 2].
The focus of AM developments within these PM Centres of Excellence is largely on powder bed fusion and binder jetting technologies to produce series ferrous automotive components. In addition, GKN Hoeganaes has introduced the gas atomisation of titanium alloys to its powder making capability and this was further strengthened through the announcement of a joint venture agreement with the specialist German powder maker, TLS Technik .
Sharman stated that GKN Sinter Metals’ interest in binder jetting was supported by its ability to leverage its expertise in Metal Injection Moulding, a technology that shares the process stages of debinding and sintering after the forming (or building) of the green parts. The potential of binder jetting to reduce AM part cost is of great interest for automotive applications. The binder-based AM technology is not, however, of interest to GKN Aerospace because of the inability of the process to achieve full density in the final product.
Global AM activities within GKN Aerospace
Sharman highlighted that the Aerospace division’s focus is on metallic AM technologies and, specifically, on the processing of titanium and nickel-based alloys, although there is also polymer capability and application, as will be discussed later in this report.
GKN’s AM developments began over fifteen years ago on rocket engine nozzle reinforcements through wire deposition. The company was also involved from a very early stage in EU-funded AM research programmes. The EU’s Framework 6 VITAL project involved the Trollhättan, Sweden, operation (then Volvo Aero and now GKN Aerospace Engine Systems, AES), while the Regional Growth Fund ecoHVP project involved the Filton operation. The company continues to be involved in collaborative R&D programmes, including the Aerospace Technology Institute (ATI) funded Horizon project.
The Fokker acquisition also brought new AM capabilities and opportunities into the business, especially on the polymer side of AM, as well as opening up new applications for GKN’s AM technology in wiring, landing gear and Maintenance, Repair and Overhaul. Consequently, the division’s AM capabilities have grown significantly in scope to cover all of the major AM processing techniques and the entire value chain, from raw material to design, process and applications development.
Developments in the broad range of available AM technologies are led by the separate Centres of Excellence in GKN Aerospace and these encompass the following technologies:-
Large-scale wire-based deposition
Large-scale deposition from a feedstock in wire form is undertaken using laser beams as the energy source. This is a high throughput process and is focused on large-scale (50 cm+) parts. Applications include large aerostructure components and the initial introduction of the technology is driven by the cost benefits arising from significantly enhanced buy-to-fly ratios. The North American Centre of Excellence is leading the development of this technology variant.
Fine-scale deposition is undertaken either from wire using a laser beam or from a powder feedstock using a laser beam with local atmosphere shielding. The focus is on titanium and nickel-based alloys and applications include the building of add-ons and features on welded structures, castings or forgings. This technology enables a reduction of part numbers, a reduction of the finish machining envelope, enhanced buy-to-fly ratios, high value component repairs and modifications and the building of a broad range of medium-sized aero-engine, space and aerostructure components and fabrications. The AM Centre of Excellence in Trollhätten, Sweden is leading this technology development.
Powder Bed Fusion
Powder Bed Fusion technologies use either a laser beam (Selective Laser Melting, SLM) or an electron beam (Electron Beam Melting, EBM) in a chamber to produce a part. The focus for EBM is on Ti-6Al-4V for highly net-shape small to medium parts. Selective Laser Melting is the lowest material throughput process of all of the available metal AM options. The focus here is on titanium and nickel-based alloys and on the building of intricate, complex, high-value components. The Filton Centre of Excellence takes the lead in the development of these powder bed processes.
Polymer processing at GKN Aerospace involves either Selective Laser Sintering (SLS) in a powder bed or the deposition of extruded polymer, which solidifies layer-by-layer (Fused Deposition Modelling, FDM). The former process is restricted to a nylon feedstock, whereas the latter process is applicable to a wide range of thermoplastics. Both processes can deliver highly net-shape products and are applicable to tooling manufacture, rapid prototyping and component building. The Filton Centre of Excellence again takes the lead in the development of these processes.
Sharman stated, “All of these process options have both strengths and limitations and, therefore, they will all find a place within the AM processing scene of the future. Notwithstanding that the Centre of Excellence at Filton is specifically focused on powder bed processes, my view is that, in the future, the current dominant position of these processes will diminish to some degree and that the relative importance of the deposition technologies will increase.”
As identified earlier in this report, an in-house powder supply capability is being developed in cooperation with the GKN Hoeganaes powder division, which has world-class capabilities in powder development, supply and handling.
Applications and developments are being pursued across the GKN Aerospace product portfolio in aerostructures, aero-engines and space applications. Examples of these application areas are cited in later sections of this report.
The perceived benefits to be derived from AM
The drivers for the adoption of AM were classified in three categories; delivery, cost and performance. Delivery drivers primarily relate to AM’s ability to significantly compress application development lead times, from initial data release to first article production. An example was quoted where a lead-time of almost two years in conventional processing was compressed to less than twelve weeks with AM.
Cost drivers relate to the much higher material utilisation and energy efficiency levels offered by AM compared with conventional processing. Material wastage levels of 90% – or buy-to-fly ratios of 10:1 – are not untypical for parts fabricated from titanium plate, whereas material utilisation can be close to 100% in AM processes. This comparison becomes even more significant in times of material price volatility. While the above categories of drivers relate to situations where AM is in competition with other manufacturing technologies, performance drivers often relate to AM’s ability to offer unique solutions, making its use an imperative. The use of topology optimisation to deliver component weight savings and the application of unique geometric capabilities for functional applications are both competitive advantages for AM in this category.
The adoption of bionic concepts in the design of AM components is an important aspect of GKN’s developed capabilities, but there is a further concept that is borrowed from nature and that, as a metallurgist, holds particular attractions for Sharman. “Whereas the achievable microstructures in conventional manufacturing processes are largely constrained by the ‘bulk’ microstructures of the starting raw materials, AM has the ability to grow tailored microstructures ‘in-situ’ on a micro-scale. By judicious control over processing parameters, this offers the potential for placing the desired microstructures, and consequent properties/performance, precisely where they are needed in the built component. This implies the need for a detailed understanding of the relationships between AM process parameters, derived microstructures and consequent properties. The development of this understanding constitutes an important element of the development work in GKN’s AM Centres of Excellence,” stated Sharman.
As a further issue related to the tailoring of performance, it was noted that through the use of multiple feedstock wires, the wire-based deposition technologies in particular have great flexibility for local adjustment of chemical composition during a component build, enabling the direct building of functionally graded materials.
The requirement to control the levels of residual stresses in larger AM components often drives AM practitioners to the selection of processes such as EBM, which uses a pre-heated powder bed, or binder-jetting, which is a close-to-ambient temperature build process that therefore avoids residual stress generation. GKN Aerospace adopts a different philosophy through a modelling and prediction approach to define the required process parameter control to minimise stress generation. This builds on established expertise at the Trollhätten Centre of Excellence in the modelling and control of stress generation during welding.
The AM Centre of Excellence at Filton
A tour of the facilities at Filton was provided by Tim Hope. The complement of AM machines in the centre was in the process of being increased from eleven to twelve, with the delivery of an additional polymer AM machine on the very day of the visit.
The equipment is housed in three separate cells, two dedicated to EBM and one to laser powder bed processing. The work involved in the first of the EBM cells has a major target in developing an understanding of the relationships between build process parameters and the consequent microstructural and property/performance control. This information is then used to set process parameters in the second cell, which is dedicated to the series production of titanium components using EBM.
The centre also includes a dedicated materials laboratory for powder characterisation and quality control and an in-house metrology and materials testing facility. The centre works closely with conventional manufacturing techniques on the Filton site.
The focus at Filton is firmly on titanium and nickel-based alloys. On a case-by-case basis, the centre has been able to demonstrate significant cost savings in the replacement of conventional processing with the near-net shape AM approach. Part integration and structural optimisation have been shown to lead to higher performance and further cost reductions. Functional systems for acoustic liners and embedded anti-ice systems are examples of products that have been developed.
The Business Unit Space in Trollhättan
The Business Unit Space was one of the early adopters of AM, researching both fine deposition for its rocket nozzles and powder bed technology for its turbines. Today it works closely with the different GKN AM centres and several institutes. The first AM application in Europe that was hot fired on a rocket engine was GKN’s Vulcain 2 demonstration nozzle. This had over 50 kg of fine deposition features added to solve a number of functions (Fig. 10). Today AM is implemented on the Vulcain 2.2 nozzle for the Ariane 6 rocket.
The AM Centre of Excellence in Trollhättan
The Trollhättan centre is based at the Innovatum Production Technology Centre (PTC) in conjunction with the city’s University West. The development cells house:
- One 3 m size laser cell for AM and welding demonstrations
- One 1.5 m size laser cell for AM wire deposition
- One 1 m size laser cell for adaptive laser welding
- One 1 m size laser blown powder and welding cell
- One EBM system for nickel alloys
Laser wire AM is also used for feature deposition on large cast or forged titanium or nickel-based alloy components. This is a qualified process in GKN Aerospace Engine Systems for bosses and grow-outs and is also in use in space systems.
Powder deposition has been developed for feature deposition, component modification and component repair and has both titanium and nickel alloy capability. As previously mentioned, the centre has the capability for thermal distortion management and modelling, leveraged from its expertise in welding process control.
The US AM Centre of Excellence, St. Louis
The US Centre of Excellence in St. Louis is a collaboration with Oakridge National Laboratory. The division drives the development and application of large-scale deposition technologies that use a feedstock in wire form.
This technology is suitable for the production of large aerospace components and for the addition of features to large titanium forgings. The local deposition of flanges, details and sections for net or near-net preforms and the fabrication of entire components are possible options. GKN’s St Louis facility pairs Additive Manufacturing technology expertise with the site’s significant end-to-end manufacturing capability. As a secured facility, the development and implementation of AM for U.S. defence applications can be undertaken.
Future outlook for the exploitation of AM
Serial production of metal AM aerospace components is already underway at GKN Aerospace alongside each Centre of Excellence and, in relation to the future exploitation of the technology, Sharman anticipates that growth will be at a fast pace in-line with the current market and that even further acceleration will arise as new platforms are launched.
“The reality is that the aerospace industry recognises all of the benefits that can accrue from the use of metal AM and is embracing the wider adoption of the technology. As with all technologies, its adoption is more linked to the market opportunity for new products and so we expect to see each new aerospace platform to have an increasing AM content,” stated Sharman.
In addition to the successful use of AM for the Vulcain rocket nozzles, the extension of GKN Aerospace’s risk and revenue sharing partnership (RRSP) with Rolls-Royce on the Trent XWB-84 large aero engine will involve the use of a range of design methodologies and fabrication technologies, including AM processes, to create the lighter weight, higher performance Intermediate Compressor Casing (ICC). The supplier of the ICC will again be GKN Aerospace Engine Systems in Sweden.
In a sector where qualification issues for any new production technology are recognised to be extremely rigorous, Sharman stated that the biggest challenges in enabling the adoption of AM relate to concerns over the reliability, quality and repeatability of the processing equipment and the raw material supply. The company’s strategy in responding to these challenges is to run test samples in builds and to ensure that the most rigorous controls and specifications are in place to ensure the necessary quality.
As GKN Aerospace seeks to leverage its expertise in metal AM, a number of options are potentially open, including the building of components for incorporation in its own assemblies and sub-systems, the contract manufacture of individual components for aerospace customers or the development of components, the manufacture of which could be sub-contracted. Sharman remarked, “All of these options are possible, depending on the application and the customer. However, in the immediate term, we would expect to keep most of the manufacture in-house because of the need for development and manufacture to be closely linked.”
Currently, production is carried out alongside the company’s Centres of Excellence but, in response to the question as to whether GKN Aerospace might contemplate the building of a dedicated AM factory as production volumes ramp up, Sharman remarked, “As with all business decisions, the timing, scale and location of future facilities would depend on a range of factors.”
 “GKN Sinter Metals: Global Tier 1 automotive supplier anticipates opportunities for Additive Manufacturing”, Schlieper, G, Metal Additive Manufacturing, Vol. 2, No. 2, p. 55-61. Available to download from www.metal-am.com.
 “Hoeganaes Innovation Centre: Technical support and powder development for the next generation of PM applications”, Whittaker, P, Powder Metallurgy Review , Vol. 5, No. 3, p 47-56. Available to download from www.pm-review.com.
 Hoeganaes to form new joint venture to produce titanium powder in North America, www.pm-review.com, retrieved November 2016.
Dr David Whittaker is a consultant to the Powder Metallurgy industry. Tel: +44 (0)1902 338498
Email: [email protected]