Friction stir welding of polymer: A comprehensive review of techniques, materials, applications and challenges

Friction stir welding (FSW) has emerged as a highly promising solid-state joining technique for polymeric materials, offering an effective alternative to conventional fusion welding, adhesive bonding, and mechanical fastening. This review critically analyzes recent progress in the friction stir welding of polymers, with emphasis on quantitative relationships between process parameters, tool design, thermal behavior, and joint performance. Reported studies indicate that optimized tool pin geometries, particularly threaded cylindrical and frustum profiles, can achieve joint efficiencies exceeding 85%–95% of the base material strength in polymers such as HDPE, PP, and PMMA. The weld-to-velocity (w/v) ratio is identified as a key governing parameter, with optimal ranges varying across polymers (e.g. ∼25–60 rev/mm for PP and ∼100–130 rev/mm for PE), directly influencing heat input, material flow, and defect formation. Due to the inherently low thermal conductivity of polymers, heat accumulation and dissipation play a decisive role in microstructural evolution, including crystallinity changes and zone formation within the weld region. Recent advances in tool tilt control, plunge depth optimization, and sensor-based monitoring have significantly improved process reliability and repeatability. By synthesizing the experimental, numerical, and monitoring-based studies published recently, this review provides a comprehensive understanding of polymer FSW mechanisms, performance limits, and industrial applicability, particularly for lightweight and hybrid structures in automotive and aerospace sectors.