A review of multi-pass and multi-track challenges in thick section friction stir welding

Thick-section welding is critical in aerospace, shipbuilding, railway, and energy industries, yet it continues to face major challenges. Conventional fusion welding often leads to wide heat-affected zones, porosity, residual stresses, and microstructural degradation, reducing joint reliability under demanding service conditions. Friction stir welding (FSW), a solid-state joining process, offers advantages by avoiding melting and minimizing defect formation. However, single-pass FSW of thick plates (>10 mm) is limited by steep thermal gradients, incomplete penetration, and non-uniform microstructures. This review critically synthesizes recent experimental work on multi-pass and multi-track FSW of thick aluminum and steel alloys. Emphasis is placed on the role of process parameters, tool design, and tool materials in governing heat generation, material flow, microstructural evolution, and mechanical performance. Reported results show that multi-pass FSW improves penetration depth, enhances dynamic recrystallization, and mitigates defects such as tunnel voids and kissing bonds. Multi-track strategies, particularly with overlap ratios of 20–50%, promote uniform microstructures and achieve joint efficiencies of 80–98% of the base material. Nevertheless, challenges related to tool wear, residual stress control, and productivity remain. Overall, multi-pass and multi-track FSW extend the applicability of solid-state welding for thick sections, with future progress relying on advanced tool materials, thermal modeling, and real-time monitoring.