AbstractThe Refill Friction Stir Spot Welding Process is a solid state joining process manufacturing excellent mechanical properties of the joint. The aim of this study is to analyse the influence of the tool wear on selected quality criteria such as the lap shear strength and the surface quality of the joint as well as the required electric current that is an indicator for the process forces and torques. Based on the result of 22 duration tests with 3,500 welding points the sleeve can be identified as the most stressed tool part with a maximum wear of 0.3mm. The tool wear depends on the gap size between the tool parts that are loaded with plasticized work piece material. The volume of the material influences the required torque of the main spindle and the tool temperature. At reaching 70% to 80% of the maximum tool wear the process temperature rapidly increases. Another result is the independence of lap shear strength of the joints on tool wear. But within the duration tests the mechanical characteristics were increased (about 7,000N before and 8,300N after one duration test in average) by achieving larger temperatures. The surface quality is slightly influenced by the tool wear. The height of the surrounding burr that occurs at the boundary of clamping ring and sleeve is 0.25mm for a worn out tool. Furthermore key indicators that were developed in past studies are validated to be useful for a condition monitoring approach by showing significant changes for increasing tool wear.