AbstractBobbin tool friction stir welding has high process flexibility and is particularly suitable for closed structures such as hollow extrusions or pipes. More recently, a bobbin tool concept having one rotating and one stationary shoulder has been proposed. The present investigation developed this novel semi-stationary shoulder concept to bobbin tool friction stir welding of magnesium alloy for the first time. The concept was proved to be feasible by decoupling the upper shoulder from the rotation of the bobbin tool. Holding the upper shoulder stationary during welding led to reduced handling forces as well as torque, enabling higher welding speeds up to 1500 mm/min. This speed is 50% larger than the value achieved by the standard tool concept, which makes the semi-stationary shoulder variant more attractive for industrial applications. Furthermore, holding the upper shoulder stationary modified the original symmetrical refilling of the plasticized metal into an asymmetrical pattern. In case of improper welding parameters being employed, the asymmetrical refilling led to transportation of created volumetric defects close to the upper surface from the mid-thickness and thus facilitated visual inspection of the weld. In the proper sets of welding parameters, the joints produced by the semi-stationary shoulder variant experienced lower level of strain localization during tensile testing than those produced by the standard variant. The weakened strain concentration led to an enhancement of the joint elongation from 82% to 95% of that of the base metal.