Refill FSSW welding process

The 'Development of Refill Friction Stir Spot Welding (RFSSW) for skin-stringer-frame stiffening of Al-CuLi cryogenic tanks' project – an ESA Discovery activity led by TWI Limited and ArianeGroup – explored whether Refill Friction Stir Spot Welding could be used to manufacture internally stiffened cryogenic fuel tanks for future European launchers. 

The process works by plunging a rotating tool into the materials to be joined. The friction generates heat that softens (but doesn't melt) the metal, whilst the rotation stirs and mixes the plasticised material from both sheets together. As the tool withdraws, its independently moving components work together to refill the hole with this stirred material, creating a forged weld with a flush surface that requires no additional finishing. The project successfully demonstrated that refill FSSW can create reliable joints in advanced aluminium alloys at cryogenic temperatures.