A comprehensive review of friction stir lap welding

Friction stir welding (FSW) is a solid-state joining technique developed to overcome the limitations of conventional fusion-based welding processes, particularly for materials with poor weldability under melting conditions. Among the various joint configurations used in FSW, the lap joint configuration has gained increasing attention due to its importance in lightweight and multi-material structural applications. However, friction stir lap welding (FSLW) presents additional challenges associated with complex material flow, asymmetric thermal distribution, and the formation of lap-specific defects. This review provides a comprehensive overview of friction stir lap welding, focusing on the influence of process parameters, microstructural evolution, defect formation, and mechanical performance. The effects of key welding parameters, including tool rotational speed, traverse speed, tool geometry, tilt angle, and plunge depth, on material flow behavior, and joint integrity are systematically discussed. The characteristics of the stir zone, thermo-mechanically affected zone, heat-affected zone, and parent material are examined in relation to their microstructural features and mechanical response. Common defects in FSLW, such as hooking, kissing bond defects, and cavity formation, are critically reviewed with emphasis on their formation mechanisms and impact on joint strength and fatigue behavior. In addition, recent advances in numerical modeling and machine learning applications are summarized, highlighting their potential for process optimization and intelligent quality control. This review identifies current research gaps and provides guidance for the reliable industrial implementation of friction stir lap welding.