AbstractThe aluminum lithium alloy AA2198-T851 has been bobbin tool friction stir welded using a tool concept with one stationary and one rotating shoulder. Defect free welds in 3 mm thick sheet have been produced featuring a high quality surface finish on the stationary side. The macrostructure forms an asymmetrical shape with microstructural characteristics known from standard friction stir welding. Because of only one rotating side a material flow direction towards the stationary shoulder has been observed. A parameter survey shows that a weld pitch of one mm per rotation combined with high pressure between the shoulders lead to good results. Mechanical performance of 82% of base metal ultimate tensile strength and 77% of base metal hardness have been achieved. The fracture analysis indicates two competing fracture modes, one being in the heat affected zone and the other at the borderline of the stirred zone on the advancing side. The first mode forms due to thermal cycle influence, whereas the second location suffers from weak bonding as a result of the thermal cycle and experienced deformation.