Friction stir welding is commonly used to join metals because it is environmentally friendly and provides higher fatigue strength than electric resistance spot welding, riveted connections, and screw connections. Alternative methods that do not reduce the fatigue strength, such as friction stir spot welding (FSSW), are being investigated. However, several studies have proven that FSSW joints are not sufficiently qualified for industrial usage. Defects such as keyholes, cavities, and cracks that form during classic FSSW with a screw stirring tool have not yet been eliminated. Some studies have demonstrated that the geometry of the stirring tool and the shoulder profile affect the quality of the joint. In this study, five pinless stirring tools with different shoulder profiles were designed to avoid the formation of cavities and cracks. The effect of the stirring tool on the mechanical properties and macro- and microstructural material properties during the joining process were examined.