When the world’s largest aluminium extrusions company faced a tight deadline to supply seating decks to one of the UK’s premier sporting venues, it turned to TWI for help.
TWI Member company Sapa was contracted to provide seating decks as part of the redevelopment of London’s Olympic Stadium, the centrepiece of the London Olympic Park and one of the venues for this year’s Rugby World Cup.
Sapa Extrusions, based in Harderwijk in the Netherlands, had to deliver 3500 retractable decks for the stadium, which provided an 80,000-seat venue for the 2012 Olympics and Paralympics, but will begin the 2016/17 football season as the 54,000-seat home ground of West Ham United Football Club.
Increasing production speed
The seating decks were to be made from extruded aluminium, incorporating joints made using friction stir welding (FSW). To meet its production targets, Sapa approached TWI for assistance in increasing the speed at which it was able to FSW the aluminium panels, with an aim to double the 1000mm/min rate it was limited to at the start of the project.
TWI approached the project in two phases. First, suitable process parameters and tool designs were identified through trials carried out at TWI’s Great Abington headquarters. Once these had been established, TWI took its findings to Sapa’s Harderwijk premises, to apply them in the production environment.
The phase one trials at TWI were carried out on its ESAB SuperStir gantry-type machine. This piece of equipment features a maximum traverse speed of 5000mm/min, a maximum tool rotation speed of 5000rpm, and is capable of exerting up to 100kN of z-axis force on the joint as it works.
A modular jigging system was used to ensure the components being joined were securely held in place, with adjustable side clamps along the weld length. Strong clamping was essential to ensure that the powerful lateral and perpendicular forces generated during welding did not lift the parts being welded or push them apart.
The panels were 6063-T6 aluminium alloy extrusions with 3.5mm skin thickness. TWI was able to select a FSW tool design for the task based on its past experience working with similar alloys.
Having identified a suitable FSW tool and configuration, TWI conducted extensive welding trials to identify the optimised welding conditions for this particular extrusion design. Once a number of test welds had been made, they were visually inspected, subjected to metallographic examination and tested for tensile strength. Results from these tests showed positive results from welds carried out at 3000mm/min.
From workshop to production line
TWI then travelled to Sapa’s Dutch facility to provide two days of on-site support, applying the parameters to the company’s own FSW equipment. Tests began using the optimised welding conditions achieved at TWI.
As in phase one, the resulting welds were visually inspected, subject to metallographic examination and tested for tensile strength – with surprising results. Although the welds were visually acceptable, the majority of them were found to contain voids.
The relatively poor result was attributed to a number of possible factors, such as the panels’ varying flatness along the weld’s 6000mm length; the need for the position of the tool on the z-axis to be adjusted on the fly as the weld took place, leading to inconsistent penetration; insufficient machine downforce; and fit-up issues at the joint line.
TWI subsequently reduced the traverse speed to 2000mm/min to eliminate some of these problems and enable the production of void-free welds.
Following the visit, TWI provided Sapa with suggestions on how to improve their existing clamping system, and Sapa continued to conduct welding trials of its own. Thanks to the support from TWI, Sapa technicians were able to find a ‘sweet spot’ on their machine, allowing panels to be welded at a speed of 2450mm/min, producing friction stir welded panels with specifications that met their requirements.