Oliver Project Develops Near Net Shape Composite Assemblies

TWI recently completed a three year partnership with CAV Advanced Technologies (AT), Leonardo MW and TISICS Titanium Composites, along with the MTC, and the NITC at Queen’s University, Belfast, for the Innovate UK ‘OLIVER’ project.

The OLIVER project has, through a series of case studies supported by IPG Photonics, helped these aerospace supply chain companies to develop their understanding of how laser welding can fabricate new, lightweight, near-net-shape, titanium (Ti) alloy and titanium metal matrix composite (Ti-MMC) assemblies. In addition to developments in laser welding procedures, digital manufacturing, fixturing solutions, in-process NDT techniques and cost models have also been developed by the project partners.

The specific application targets for TWI’s welding activities for this project included:

• Firewalls and bulkheads: These structures are currently made out of overlapping sheets and stiffeners, riveted together. OLIVER has demonstrated the successful replacement of rivets by laser stake stitch welds, a net weight reduction then being possible. Coupon-level testing also indicates that the static and dynamic performance of such a structure can be maintained, or even bettered, when compared with riveting. Additionally, cost modelling suggests that manufacturing complexity can be reduced, with fewer processing steps, and thus the economics of that manufacture improved
• Wing leading edge structures: These are also made from sheets. Nevertheless, the maximum width of sheet available from manufacturers is limited which, in turn, limits the maximum length of leading edge structure that can be produced. The OLIVER project has developed laser butt welding procedures, with high quality weld profiles which, with minimal additional processing, are then suitable for air flow surfaces
• Struts: One production route for these involves machining from solid, with its inherently low – and uneconomic – buy-to-fly ratio. Near net-shape manufacturing and assembly methods are therefore very attractive. OLIVER has demonstrated that this can be achieved through the use of laser welded assemblies. Furthermore, the low heat input of laser welding has facilitated the incorporation of carbon-fibre reinforced Ti-MMCs to such assemblies. The consequent use of these high specific strength materials has been demonstrated to lead to weight savings

In summary of TWI’s involvement in this project:

• Laser stake welding procedures have been developed and demonstrated for both stitch and seam welding, through stacks of up to four overlapping Ti alloy sheets and/or stiffener flanges. Work on coupons and on sub-assemblies representative of firewalls and bulkheads indicates an overall mechanical performance better than with riveted structures, with a simpler, less labour-intensive manufacturing route
• High quality thin sheet laser butt welding has also been developed which, in the future, can be used to facilitate the production of larger sheets, for longer leading edge structures, whilst at the same time not compromising air flow over the surfaces of those structures.
• In thicker materials, laser butt welding has also been shown to be a means of assembling multi-material structures, as exemplified by the Ti/Ti-MMC struts targeted by this project. This manufacturing route can be used to improve buy-to-fly ratios, reduce material waste and its associated costs, whilst at the same time facilitating the introduction of novel materials to help bring down weight and increase performance