Effect of friction stir welding parameters on the mechanical performance of recycled HDPE plates

Friction stir welding (FSW) was investigated as a solid-state processing technique for joining recycled high-density polyethylene (HDPE) plates manufactured from post-consumer bottle caps and previously used as construction formworks. The effects of rotational and traverse speeds on weld densification and mechanical performance were systematically evaluated to establish processing–structure–property relationships in recycled thermoplastic systems. Mechanical characterization included ultimate tensile strength (UTS), density, and Shore D hardness, and load–elongation response, while surface and cross-sectional morphological analyses were used to assess void formation, material consolidation, and weld integrity. Optimal conditions were identified at 1600 rpm with 12.5 mm/min and 1000 rpm with 25.0 mm/min, resulting in UTS values up to 4.7% higher than those of the unwelded material. Morphological analysis revealed that excessive heat input at elevated rotational speeds promoted void formation and reduced joint integrity, whereas moderate parameters enhanced material consolidation and produced defect-free welds. A strong correlation among UTS, density, and hardness confirmed their suitability as indicators of weld quality. These findings demonstrate that controlled FSW parameters can significantly improve the structural performance of recycled HDPE, supporting their use in sustainable construction-related and non-critical engineering applications within circular economy strategies.