By 2012 TWI had extended earlier research into surfacing with friction welding to assess the potential for manufacturing blisks and guide vanes from a range of high performance alloys using friction welding-based additive manufacturing.
The relatively low temperatures associated with friction welding created interest in the process for joining dissimilar materials. The lower heats reduce the potential formation of intermetallic compounds, residual stresses, and shrinkage occurrence. TWI’s experts drew upon this knowledge for a 2013 CRP project to assess friction welding for joining titanium to stainless steel.
We returned to the topic of using linear friction welding for the solid-phase additive manufacture of semi-finished, close to net shape products in 2015. This project work, which was aimed at the oil and gas industry, sought to reduce the volume of raw material used and the machining required for part production, leading to cost savings, time savings and the associated environmental benefits. Also using linear friction welding, but this time for the aerospace industry, a 2020 CRP project investigated the use of the procedure with aluminium-copper-lithium alloy to demonstrate the process capability to build structural parts and/ or add materials / functionality, locally, to primary product forms, where desired (Figure 1). Also working on behalf of the aerospace industry was a 2020 project for the linear friction welding of carbon fibre reinforced thermoplastic (Figures 2-3).
Joint Industry Programme (JIP) Projects
As well as our core research, TWI also delivers joint industry programme (JIP) projects. These projects tend to address a specific industry challenge, allowing interested parties to invest into the project in return for exclusive access to the results and the opportunity to direct the aims of the project.
Friction welding-related JIP projects undertaken in recent years at TWI include the friction stir spot welding of high strength steels for transport industries in 2006, and stationary shoulder friction stir welding (SSFSW) for the joining of titanium alloys in 2007.
The following year, in 2008, we completed joint industry research into bobbin friction stir welding for the joining of aluminium alloys, including process industrialisation, tool design, tool materials, joint quality, fixturing, component tolerance, process repeatability and the production of prototype components. This project work was revisited in a 2012 project to further develop the FSW floating bobbin technique.
Also working with friction welding and aluminium alloys was a 2010 JIP project to assess the feasibility of applying FSW corner welding to a range of commercially available aluminium alloys and section thicknesses chosen by the sponsors. This work was expanded upon in 2016 with another JIP project, this time investigating the use of FSW to join thick section aluminium alloys, delivering benefits including increased production rates, lower tooling costs, improved weld properties and reduced distortion.
Public Funded Projects
In addition, TWI has built up decades of collaborative work assisting with a range of public-funded projects.
The FrictionHarmonics project developed a prototype, non-linear, high frequency ultrasonic non-destructive testing (NDT) system for the in-process inspection of friction stir welds for kissing bonds in aluminium, titanium and nickel alloys as well as for steel structural components. SignaStir also focused on quality assurance in FSW, although this time for the manufacture of aluminium rolling stock and marine vessels.
The European Commission-funded Lostir project aimed to make FSW more accessible to manufacturers by adapting and retrofitting milling machines with compatible tools and process monitoring equipment to ensure high quality welded joints. Also funded by the European Commission and focused on FSW was the MicroStir project, which worked to develop small scale FSW for use in electronic connections and encapsulations seeing harsh service environments.
Combining FSW with NDT was the StirScan project, which aimed to detect kissing bonds in friction stir welds in aero structures, while the Mobi-Weld project developed a prototype mobile FSW system for on-site marine fabrication. In this project we addressed one of the major limitations of FSW at the time, which prevented its use on a mobile machine, in that the process requires relatively high levels of force to be applied during welding. The challenge was therefore to develop a mobile FSW system incorporating a crawler system that would be low-cost, transportable and have no limit on panel and weld length. The system had to be capable of resisting the welding forces and provide a precise, smooth FSW tool path movement. At the time it was believed to be the first machine in the world of its type, representing a major breakthrough in friction welding (Figures 4-5).
One of the advantages of friction welding is the ability to aid lightweighting in industries such as aerospace, rail and automotive. The LightBlank project used FSW alongside hot form quenching to create innovative lightweight aluminium alloy pressed parts for the transport industry, enabling both lighter and cheaper manufacture (Figures 6-7). The TiFab project introduced innovative linear friction welding technology for the near net shape manufacture of advanced titanium (Ti) aerospace components, boosting productivity and reducing costs (Figure 8), and the FlexiFab project aimed to create an automated robotic system that uses FSW to weld aluminium parts for the transport industries. The Ultra Lightweight Automotive project investigated new methods of FSW to join automotive body structures made from novel lightweight alloys in order to help meet CO2 emissions targets.
The benefits of FSW were applied to shipbuilding through the collaborative HILDA (High-Integrity Low-Distortion Assembly) project, which investigated the viability of friction stir welding of steel for shipbuilding applications (Figure 9). This work was built upon further with the RESURGAM project, with TWI’s expertise underpinning the steel FSW technology required for the project, which demonstrated applicability to not just shipbuilding, but also the energy, oil and gas, civil engineering and defence industries (Figure 10).
Elsewhere, TWI’s friction welding expertise was used to develop the ‘AdStir’ technique, which uses filler wires to add material into gaps between components to overcome issues related to alignment and condition of supply. This work led to the development of a unique technique called Friction Stir Gap Bridging (Figure 11), opening up the use of FSW to join components without all of the costs of pre-machining for large structures.
We also joined seven other organisations for the LinFric project, which aimed to drastically reduce the cost of linear friction welding equipment, making the technology more accessible to potential users especially from the power generation, automotive and aerospace industrial sectors (Figure 12).
Dedicated Industrial Member Support Projects
Of course, much of our work is conducted on behalf of specific Industrial Members to improve processes and solve challenges related to their industry and operations. As an impartial and independent organisation, TWI typically conducts these in strict confidence with exclusive access to the results being provided to the Member company. However, there are some instances where we are able to provide examples of the type of work undertaken directly on behalf of Industrial Members in different industry sectors…
- Friction Stir Welding (FSW) Moving into the Medical Sector
TWI supported Abingdon-based LTi Metaltech to bring FSW to their in-house production capabilities as they sought to move away from autogenous DC arc welding for their operations. This involved the conducting of two FSW procedure development studies to determine appropriate tool designs, welding parameters and weld quality in accordance with ISO 25239:2011 Part 1 to 5: FSW of Aluminium.
- Rotary Friction Welding for Medical Application
Also working for the medical industry, we undertook a project to join a difficult alloy (Nitinol – an alloy of nickel and titanium) by rotary friction welding using an interlayer (Figure 13). Rotary friction welding was chosen as the potential solution as the lack of melting associated with the process would minimise the reduction of desirable properties and any joining difficulties previously experienced using a fusion process. From the project findings TWI was able to recommend that further work would be worthwhile to assess the effects of torque together with interlayer thickness, on corrosion resistance and fatigue performance.
- Friction Stir Welding of Combat Vehicles
Our work has also seen us offer advice for the military, including the use of FSW for the manufacture and repair of assets such as lightweight tanks (made from aluminium alloys or aluminium based metal matrix composites (MMCs)), military bridges and amphibious personnel carriers, steel tanks (friction stir welded from extra high strength armour plate using two passes from two sides after further tool and parameter development), and titanium lightweight field howitzers. In addition, the development of Whorl™ friction stir welding tools offer a number of benefits for defence applications, including high welding speeds, encouraging tensile, fatigue, deformability and ballistic impact properties, no need for joint preparation, low distortion and high reproducibility (Figure 14). We have also found applications for rotary and linear friction welding, such as track rollers for tracked vehicles, bimetallic tipping of projectiles and armour piercing shells, machine gun barrel liners, and fuse liners. Other friction-based applications suited to military uses include friction surfacing, radial friction welding and friction stud welding.
- Friction Stir Welding Variant for Transport Applications
TWI was contacted by Nippon Light Metal for assistance in the application of stationary shoulder friction stir welding (SSFW) for the fabrication of vacuum chambers as an alternative to fusion welding.
- High Integrity Welding of Thick Section Magnesium
Vibration testing systems manufacturer, LDS Test and Measurement reached out to TWI for support in exploring an alternative route for manufacturing bespoke large vibration tables. Wirth a requirement to be lightweight but with a high strength, these tables are manufactured from magnesium, which also possesses vibration damping properties. We conducted welding trials using a high force FSW machine capable of welding 75mm thick plates in a single pass. The welding process proved to be a success, and the component was machined with no evident signs of the joint, demonstrating that the consolidated material along the weld line was free from porosity. Not only did the welding process prove successful for the application, but it was shown to represent a significant cost saving over the traditional fabrication process.
- Advanced Techniques used for Sealing Nuclear Waste Canisters
Having already proven the efficacy of reduced pressure electron beam welding for the manufacture and sealing of nuclear waste canisters, TWI were once-again approached by SKB – who manage the storage and disposal of Sweden's nuclear waste - to develop FSW in 50mm thick copper. The aim was to demonstrate high integrity full circumferential lid to canister welds (Figure 15). Our experts developed unique tools for the process alongside a novel weld cycle control system as well as providing ongoing support towards production.
- Delivering FSW Support for Formwork Specialist
TWI had worked closely with Malaysia-based MFE Formworks in 2011 to support their adoption of FSW. They contacted us again when they decided to more their formwork manufacturing over entirely to FSW in 2016, allowing us to create a three-stage work programme comprising a procedure development study at TWI UK, on-site training in Malaysia, and machine testing and commissioning in Taiwan. Our experts helped identify optimal welding conditions, train staff and test MFE Formwork’s FSW production line.
- TWI Doubles Welding Speed for London Olympic Stadium Seating
Aluminium extrusions company, Sapa were contracted to provide seating decks for the redevelopment of the London Olympic Stadium (now the London Stadium) following the 2012 Olympic Games. Sapa were asked to provide 3500 retractable decks from extruded aluminium and incorporating joints made using friction stir welding (FSW). In order to meet their production targets, Sapa called for assistance from TWI who identified suitable process parameters and tool designs through trials carried out at our headquarters before the findings were taken to Sapa’s Harderwijk premises, to apply them in the production environment (Figures 16-17).
- Friction Welding Processes for Floating Solar Application
Sunlit Sea AS, a technology provider to the floating solar industry contacted TWI with regard to the development of FSW and friction stir spot welding solutions for their solar power installations. The solution was based on the prefabrication of serially connected solar panels with back plates made from an intrinsically robust and heat conductive aluminium structure, facilitated for and integrated with a new and effective solution for logistics. The final float assembly (Figures 18-21) consists of aluminium pressings with dimples in a honeycomb structure. The joining method needed to weld the outer perimeter of the pressings to create a leak-tight edge, weld the flat section of the dimples to increase the stiffness of the assembly, and create leak-tight holes for attachments. Our experts recommended the use of FSW as the joining solution along with friction stir spot welding (FSSW) to weld the dimples and create leak-tight holes in the same fixture used for FSW, increasing productivity. TWI developed the process parameters to be used as well as the dedicated FSW and FSSW fixtures before fabricating a demonstrator assembly to validate the solution.
- Friction Stir Welding for Low-Cost Titanium Propellant Tanks
TWI worked with Airbus Defence and Space to meet a European Space Agency (ESA) request to investigate a cost-effective method for the manufacture of titanium propellant tanks. Our experts opted to use stationary shoulder friction stir welding (SSFSW) techniques for the cylindrical welding of titanium alloys suitable for launch vehicle propellant tanks, with the aim of reducing lead times and costs as well as raising the Technology Readiness Level (TRL) of SSFSW of titanium alloys to prototype demonstrator (TRL6). In August 2016, following extensive reviews of materials and manufacturing processes, TWI successfully performed the world’s first full circumferential SSFSW of two 420mm cast titanium cylinders. The test pieces were then subjected to full NDT evaluation and mechanical assessment to confirm joint quality and properties. In winter 2016, TWI had fabricated and delivered the first one of four SSFSW cast titanium prototype demonstrator (TRL6) propellant tanks to the European Space Agency (Figure 22), leading to the awarding of the Raiser 2017 Award for Friction Welding Innovation.
- Linear Friction Welding for Manufacturing of Space Hardware
Our work using friction welding for space applications continued with a project funded by both the European Space Agency (ESA) and the UK Space Agency for the development and qualification of a linear friction welding based manufacturing route for a satellite application. This would save raw materials costs and improve manufacturing flexibility. This included a survey of candidate applications/parts, the development of an LFW procedure to meet the acceptance criteria for the selected application, validate the LFW-based manufacturing route by fabricating of a representative technology demonstrator, qualify the LFW technology demonstrator using a testing protocol based on the relevant space standard (ECSS-Q-ST-70-39C1), and determine the potential improvements in manufacturing cost, efficiency and environmental friendliness of the LFW based manufacturing route via a life cycle and economic assessment (Figures 23-24).
- Linear Friction Welding for Spacecraft and Launch Vehicles
Also investigating linear friction welding (LFW) for space applications was a project completed in partnership with Airbus UK and ArianeGroup to lower the cost of manufacturing structures for spacecraft and launch vehicles. This would be achieved by replacing processes to machine components from solid plates or forgings with LFW. Specifically, this project developed the joining of pipeline attachment bosses on cryogenic fuel tanks for Ariane 6 as well as the connection of tabs on propellant fuel tanks for the Eurostar Neo satellite (Figures 25-28).
- Linear Friction Welding for Aerospace Engine Fittings
Linear friction welding (LFW) was also investigated as a process for manufacturing aircraft structural components, particularly from titanium alloys and nickel superalloys. Working on behalf of Parker Lord, our experts noted that LFW manufacture of near-net-shape components would save over 50% in material, production and overall part costs compared to their existing approach. Following this investigation, TWI created a full scale demonstrator of the rear engine fitting (Figure 29).
Example Process Advancements
TWI’s research work has led to a number of important breakthroughs in friction welding, including the invention of friction stir welding in 1991.
- Friction Stir Welding Adopted for Invisible Stainless Repair
Friction stir welding (FSW) had been shown to join sheet materials but we saw a gap for sheet metal repair that could be filled by FSW. The main difficulty in using FSW for sheet metal repair was the hole created by the rotating tool at the exit point. To solve this, we adapted the process to include an artificially-created ramp of material a few millimetres above the substrate where the tool can be removed from the process and the ramp machined off once the weld was complete. Advantages include being able to operate the repair process in wet environments while the process is programmable and repeatable.
- Dual-Rotation Friction Stir Welding
In traditional friction stir welding (FSW), overheating or melting along the shoulder contact side can occur, creating fusion-related defects along the shoulder contact side weld surface. To solve this problem, TWI’s experts developed a variant of the process where the probe and shoulder rotate separately. Known as dual-rotation FSW, this process uses a welding tool with a central probe and an outer shoulder that rotate independently with respect to both direction and speed. This allows for a high rotational speed to be achieved with the probe but without a corresponding increase in the shoulder speed. Being able to optimise the combination of rotational speeds reduces the likelihood of defects being introduced by rotational shoulder contact.
- Continuous FSW in Steel
TWI’s experts used a new high-temperature composite tool to create a continuous weld of over 6 metres long in 6mm thick steel. This was achieved by keeping the shoulder temperature below 750°C, by controlling the welding parameters, for a high-quality weld and low tool wear rate. The new tool, when combined with careful application of the FSW process was shown to be able to extend tool lives to over 45 metres. This opened up the use of FSW for applications that required long, uninterrupted steel welds, such as in shipbuilding, bridge decking or pipe seam welding (Figure 30).
- Bobbin Tool Friction Stir Welding Developed
TWI developed a new method of friction stir welding (FSW) using a ‘floating’ bobbin shaped tool which was shown to be able to create thick section welds of up to 25mm thick. Ideal for joining low temperature softening materials such as 6082 aluminium, our experts were also able to join several two and seven thousand high strength aluminium alloys, as well as grade 5083. The process uses a one piece tool, unlike some FSW variants which require complex multi piece tooling, which is free to move in the vertical or Z plane about a centimetre up and down. Because the bobbin creates a full penetration weld, the potential for creating kissing bonds, usually associated with lack of tool penetration, is eliminated. In addition, the process involves zero vertical force, unlike earlier adaptations of the FSW process and no backing bar is required because the bottom shoulder supports the underside of the weld (Figure 31).
- Advanced Simulation of Friction Stir Welding
In an effort to improve the friction stir welding process (FSW), TWI explored the use of a new modelling approach; the Coupled Eulerian-Lagrangian (CEL) method. FSW of high temperature materials often meant that the tool material would lose strength or suffer excessive wear during the process. Being able to simulate this allows the user to accurately predict the influence of tool geometry on the integrity of the weld. To validate this new modelling approach, TWI generated models of previous experimental weld trials where temperatures, tool forces and tool torques were measured. Our experts then performed a directly coupled thermo-mechanical analysis, comparing the results of the simulation to the experimental measurements and showing a strong agreement between the experiments and the simulations (Figures 32-33).
- CoreFlow®: A Sub-Surface Machining Process
Through the development of friction stir welding (FSW) and friction stir channelling (FSC) TWI created a new, highly disruptive, sub-surface machining technique called CoreFlow®. This new technique allows sub-surface networks of channels to be integrated into two-dimensional or three-dimensional monolithic parts in a single manufacturing step. These channels could then be used for heat exchange or other applications. This new technique allows for i) the formation of a closed channel within the workpiece and ii) the production of extruded wire.
- Efficiency Improvements for Aluminium Coil Processing
TWI researched a solution for the processing of aluminium coils. These coils are made from rolled sheets of aluminium with a hollow cylinder being used to create an internal diameter around which the sheets can be wound (Figure 38). These coils can be treated to suit the requirements of its eventual application. This is achieved by coil coating, which can improve corrosion resistance and the aesthetic appeal of the coil material. Coil coating requires the coil to be unwound to be cleaned, etched, anodised, coloured and sealed before being rewound ready for shipping. However, because the sheet materials are non-continuous the end of one coil needs to be joined to the start of another. Typically achieved with adhesive taping, this process requires the production line to be stopped while the joint passes through each treatment station. This leads to significant manufacturing downtime, material waste and introduces an element of risk when resuming production. To solve this issue, TWI investigated the use of refill friction stir spot welding (Refill FSSW) to join the coil ends.
- Joining Al Stranded Cable Using Rotary Friction Welding
TWI conducted a series of trials to use rotary friction welding (RFW) to join stranded aluminium cable to aluminium bar as a proof of concept for using RFW to join two segments of aluminium stranded cable. This work aimed to create a cost-effective, efficient, high speed, and automated solution. Our team successfully joined stranded aluminium cable to a solid aluminium bar through RFW and also created a ‘double-sided’ welded demonstrator of two cables joined together via a solid Al puck using RFW. The specimens were tested to assess the mechanical properties the inside and outside cable strands.
Of course, this list of projects is not exhaustive, although it should provide you with an overview of the types of friction welding work undertaken at TWI over the decades. For more information on our work and how we can support your to meet your organisation's goals, please email contactus@twi.co.uk.