Numerical analysis of the thermomechanical behavior induced by uniform polygonal pin geometries in friction stir welding

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Optimizing tool pin design is absolutely critical for achieving stable thermal conditions and defect-free joints in Friction Stir Welding (FSW). This study investigates the specific influence of regular polygonal pin geometries ranging from triangular to octagonal, on the resulting thermal and strain fields of AA1050 aluminum alloy using a three-dimensional finite element model. Results reveal a fundamental thermomechanical trade-off: the triangular pin (3 faces) generates the highest peak temperature (707°C) and the widest HAZ (27.67 mm), yet the lowest peak strain (464.13). Conversely, the octagonal pin (8 faces) yields the lowest peak temperature (639°C) and highest peak strain (≈783.94). A linear spatial decay in the Large Strain Area (LSA) occurs as pins approach a cylindrical morphology, with the LSA contracting from 56.55 mm² (triangular) to 29.44 mm² (octagonal). The hexagonal pin is identified as the optimal configuration, effectively stabilizing peak temperatures (646.92 °C) within the material’s solidus–liquidus window while balancing high strain intensity (≈ 636.9) and a moderate LSA (37.68 mm²). To facilitate scalability across diverse welding conditions, a dimensionless tool template is proposed by normalizing pin features against plate thickness. This work provides a robust framework for advanced FSW tool design, transitioning from traditional empirical tool selection toward a rational, model-driven optimization paradigm.

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