We have a head start.
Friction Stir Welding (FSW) is one of the most sustainable joining processes. It is based on the simple principle of using frictional heat and mixing, to join two solid bodies into one. The low operating temperature, below the melting point of the solid bodies, and lack of external heat sources makes it a process with a low energy input. But there is more. The process only relies on electricity as the energy source to drive the motors in the welding equipment, making it a fully carbon-neutral process when powered by renewable sources. FSW does not need filler materials, thus does not require the manufacturing and transport of heavy rolls of filler wire. It does not require increasing scarce shielding gases like helium. It does not produce ultraviolet radiation or intensive light. There is no spatter and there are no fumes, which have been associated with increased health risks to welders including cardiovascular diseases. The whole process is fully mechanised, meaning that there are greatly reduced health and safety risks when compared to manual arc welding. All this, not only makes for a carbon-neutral process, but also a sustainable and safe process with minimal use of rare earth elements.
So, what’s the catch?
Heavy machinery is required to produce a friction stir weld; this could be an industrial robot, a machining centre, a gantry machine etc. For a new gantry machine of 20 tonnes producing 100km of welds per year, our calculations indicate that the carbon emissions per metre in typical materials are:
The maths on carbon emissions associated with FSW
Carbon footprint analysis is challenging, using limited publicly available data, and making lots of assumptions. We therefore encourage you to challenge us on these figures, allowing us to improve our sustainability analysis. The carbon footprint associated with the manufacturing, transport and installation of the equipment is often forgotten but not to be neglected. While it is difficult to give an exact figure, we base our estimations on the average for long haulage trucks, i.e. 58gCO2e/ton-km. A 20 tons machine transported a 1000km by truck/ship through Europe into the UK would mean a footprint of 1.2 tons CO2e. This would be for the lifetime of the machine. The footprint associated with machine manufacturing is harder to identify. However, the major contributor would be the fabrication of the steel used to build the machine. Based on various sources, the average CO2 emissions per ton of steel are 1.5 to 3 tons, depending on how the steel is produced. Therefore, we assumed that in the worst-case scenario, the 20 tons machines would require 30 to 60 tons of CO2e. A review of existing FSW equipment shows that they are easily capable of 10 years of continuous manufacturing without significant hardware replacements. At a realistic production rate of 100km weld per year*, this would imply 30 to 60g CO2 per meter weld. The tool material, assumed to be an H13 tool steel, accounts for a smaller but not insignificant portion of the carbon footprint. A typical FSW tool of 500g, lasting for 500m weld length would thus result in 2 to 4g CO2 per meter for the material, using the assumptions above. In addition, there is a small contribution for the machining itself, estimated to be less than 1g.
* 50 working weeks, 40hrs a week, production speed 1m/min, idle time 20%