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Optimisation of Friction Stir Welding (FSW) and Laser Beam Welding (LBW) for assembly of structural aircraft parts

Periodic Reporting for period 2 - OASIS (Optimisation of Friction Stir Welding (FSW) and Laser Beam Welding (LBW) for assembly of structural aircraft parts)

Reporting period: 2019-05-01 to 2021-03-31

With the development of precision laser beam welding (LBW) and friction stir welding (FSW), it is now possible to fabricate welded aero-structures without mechanical fasteners such as rivets and bolts. These new processes produce a lighter weight, distributed load path with the potential for enhanced strength and structural stiffness, no holes and a smoother, more aerodynamic surface. In addition to being more structurally efficient, the new processes are cheaper, should reduce inspection & maintenance requirements. Ultimately they allow manufacture of lighter-weight aircraft, with reduce fuel burn, superior operating efficiencies and reduced emissions of aeroplanes.
The achieved technical objectives of the project have advance three welding technologies (FSW, Friction Stir Spot Welding (FSSW) and LBW) for manufacturing complex lightweight aluminium structures. Inspection of the joints was be carried out in accordance with relevant aerospace standards, and an elaborate mechanical testing program has shown excellent mechanical properties for a range of alloys, joint configurations and welding processes. Upon completion of the projects, two full scale cargo door demonstrators were produced, one with friction stir welding processes and one with laser beam welding.
TWI has developed weld procedures for three different friction stir welding process variants, i.e. tee-joints, butt joints and lap spot joints in 2000 and 7000-series aluminium alloys. Weld procedures were also developed for laser beam welding of AA7000 series aluminium T-frame onto a 5000-series aluminium skin, without the use of filler material. A detailed testing programme was prepared by VZLU to accommodate all weld combinations. The testing was performed in accordance with the relevant ISO and aerospace standards to be directly applicable in the aerospace industry. Tensile, fatigue and corrosion testing was performed as well as metallurgical assessment of the joints. Romaero, with the support from ESAB and TWI, has designed and manufactured a multi-purpose fixture with integrated coolingto support both FSW and LBW, capable of maintaining the skin completely fixed at all times to prevent any movement because of the high forces involved. The fixture and weld procedures were subsequently used for fabrication of technology demonstrator 1 (friction stir welding) and technology demonstrator 2 (laser beam welding). To facilitate the selection of aluminium alloys, University West conducted Varestraint experiments, to assess which alloys can be successfully laser-welded. GEONX initiated the LBW simulation work, which was subsequently taken over by TWI to develop a simulation model for laser beam welding, to predict the distortion and residual stress in the welded structures. QUB developed a cost-model, to compare the OASIS welding processes against existing riveted solutions. This showed that the welded solutions can indeed provide significant cost and time savings. Finally, a digital twin was produced for offline programming, process flow simulation and resource optimisation.
The European and global aircraft market will benefit from new cost effective manufacturing routes for aerostructures, ensuring optimal material usage and maintain its competitiveness. The impact of the uptake of the optimised welding techniques (FSW, FSSW and LBW) for assembly of structural aircraft parts will enhance improved aerodynamics and light-weighting which subsequently will improve fuel efficiency and global competitiveness of the industry. Also in terms of productivity, joining using rivets is more time consuming than using welding approaches. In the case of rivets, holes need to be drilled and deburred prior to installing the rivets and a “redressing” step is required after the rivets are in position. Moreover rivetless assembly will enable the usage of “leaner” parts. Delivering a new cost effective manufacturing route for assembly of aircraft parts (cargo door) to the topic manager and the European aerospace industry, will make a contribution to the range of innovations being addressed within the Airframe ITD initiative in Clean Sky 2 that includes:

- The use of SSFSW (including corner welding) and RFSSW of stiffeners and frames to skins in a T-configuration, with material combination of AA2000 and AA7000 series alloys.
- The utilisation of the AA5xxx series alloy, specifically the AA5028 Al-Mg-Sc alloy, welded using LBW without a filler addition which would greatly simplify manufacturing.
- The use of SSFSW to weld curved panels in a butt-joint configuration.
- The implementation of numerical modelling to model the welding processes as their effects on structural integrity, distortion and residual stress for welded cargo doors.
- The implementation of a Digital Twin to accurately predict the associated costs for the rivetless assembly when fabricating using welding technologies.

Overall the developments have proven that by using welding techniques (FSW, FSSW and LBW), “leaner” parts can be used to fabricate aerostructures for fuselage applications, leading edge, wing panels, nacelles and doors. The light-weighting is achieved by eliminating rivets, sealant and flanges, reduced frame thickness, and the ability to have continuous joints, rather than spot joints. With the use of welding techniques to produce rivetless assemblies, joining time could be reduced, by 75%, than when using manual riveting.
The development of OASIS technologies, combined with other fuel consumption reduction steps and engine developments being undertaken in the JTI, will result in the existing fleet of civil aircraft being able to be replaced with more fuel efficient alternatives, and ultimately supporting the fabrication of a new generation of zero-emission aircraft in the next decade.
Demonstrator with different joining technologies used in OASIS
Refill Friction Stir Spot Welding joints
T-frames welded to a skin with the CornerStir process