Landing gear systems are one of the most critical components of an aircraft, providing principal support for the aeroplane when parked, taxiing, taking off or landing.
To meet the stringent demands of bearing large loads in long-term service, 300M ultra-high strength steel (UHSS) is regarded as one of the most ideal materials for manufacturing aircraft landing gear, benefiting from its high strength, sound toughness and excellent fatigue performance. However, severe notch sensitivity and local stress concentration make 300M UHSS challenging to be processed into complex structures using conventional forging techniques. Moreover, a massive post-machining workload is required in the manufacturing process after forging, significantly increasing the Buy-To-Fly (BTF) ratio and prolonging the lead time. Currently, industry-wide, manufacturers across the world are seeking manufacturing processes, designs and methods with the purpose of reducing the cost of manufacture, lead times and supporting the race to net zero. Wire Arc Additive Manufacturing (WAAM) offers an alternative approach with great promise for 300M UHSS landing gear due to high cost-effectiveness, short lead time and design flexibility.
The Welding and Additive Manufacturing Centre at Cranfield University recently teamed up with Airbus to demonstrate the feasibility and reliability of using WAAM technology in depositing steel landing gear components. The project was funded by the Aerospace Technology Institute (ATI), the ‘Hybrid Direct Energy Deposition Sprint’ project, with partners including National Manufacturing Institute Scotland (NMIS), Cranfield University and the Northern Ireland Technology Centre (NITC) at Queen’s University Belfast, along with an industry steering group of more than ten companies.
To ensure that WAAM can produce aircraft landing gear that is free from defects and performs to the required standard, Cranfield conducted a comprehensive investigation of WAAM 300M UHSS. This involved feature deposition studies, interface studies, and post-heat treatment studies. In the feature deposition work, both gas metal arc (GMA) and plasma transferred arc (PTA)-based WAAM processes were applied to deposit various features under different shielding conditions. It was found that both GMA and PTA-based WAAM are feasible for the manufacture of defect-free 300M features. However, all the as deposited 300M features had lower strength but higher ductility compared with forged ones. To overcome this, a post-heat treatment investigation was done by austenisation using oil quenching, followed by low-temperature tempering with water cooling. These treatments refined primary austenite grains through recrystallisation, dissolved the carbide precipitates, and eliminated residual stress, thereby obtaining good comprehensive performances. The results show that this heat treatment scheme could normalise
To ensure that WAAM can produce aircraft landing gear that is free from the microstructure and mechanical performance evolution along the building direction and make the strength and fracture toughness of the WAAM 300M deposits meet the qualification. These fundamental studies provide sufficient confidence to apply WAAM to produce aircraft landing gear.
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Subsequently, a 300M UHSS Landing gear demonstrator was deposited using the PTA-based WAAM method with optimised process parameters. The demonstrator with dimensions of Ф200mm × 700mm was designed to replicate the main features of a landing gear system, with WAAM deposited features extruding away from a forged rod to connect braces and actuators. The deposition work was conducted in a global shielded inert environment with argon, 7kg of material was deposited at a deposition rate of 1.3kg/hr and took 16 hours to finish, including setup. The BTF ratio was decreased from 5.9 to 2.0 for the demonstrator when compared with conventional subtractive manufacturing, reducing the material used by 65%. This demonstrated that WAAM can not only produce geometrically accurate features without any defects on radial forged substrates but also greatly increase cost-effectiveness and reduce lead times.
Cranfield University
An environmental impact assessment study based on a “cradle-to-gate” life-cycle approach (including the cumulative effect from raw materials extraction until the shipment of the component to the customer) has shown very promising results. Dr Emanuele Pagone, researcher in the Sustainable Manufacturing Systems Centre at Cranfield University, observed: “Our study shows that producing the landing gear component with WAAM rather than traditional, subtractive approaches reduces significantly the required amount of raw materials to be extracted and refined. This also has a knock-on effect on all manufacturing processes with less material to be heat treated, machined, and transported. We have estimated that the carbon footprint of the component can be roughly halved when substituting traditional machining with WAAM. Embodied energy consumption estimates show similar results and suggest that, approximatively, other environmental impact indicators (including emissions to air, water and land) are roughly halved as well. Furthermore, WAAM creates the opportunity to repair a damaged part re-depositing only what is necessary, an option not available by traditional means. Our studies in this area have shown that repair with WAAM can reduce the environmental impact by orders of magnitude, even including the additional transportation required to repair.”
Therefore, WAAM appears as one critical technology to significantly reduce the environmental impact in manufacturing and to support the industry in the ambitious, yet absolutely important race to Net Zero.
Overall, this investigation will promote many opportunities and benefits for the whole aerospace industry as we incorporate additive manufacturing into a hybrid solution that can realise significant savings and achieve a pathway for critical component manufacturing to access more sustainable modern technologies.
For the next step, Prof. Stewart Williams, the Director of the Welding and Additive Manufacturing Centre, added: “We are currently applying our newly developed CW-GMA process to the manufacture of aircraft landing gears. This process can achieve high deposition rates of up to 15 kg/h whilst providing precise thermal control with a wide heat input range.”
Beyond that, a further exciting new development in WAAM technology is the RoboWAAM coherent hardware-software ecosystem developed by WAAM3D. It generates programmes seamlessly with any CAD model and oversees the whole deposition process, providing process monitoring and governance over the many health and safety capabilities, as well as a fully auditable process trail.
