Effect of conical versus cylindrical tool pin on material flow, microstructural evolution, and particle distribution during friction stir processing

The tool pin geometry significantly influences material flow and particle distribution during friction stir processing (FSP). However, a systematic understanding of this relationship remains limited. This study systematically compared conical (with a 6 mm base diameter tapering to a 4 mm root diameter) and cylindrical (5 mm diameter) pin geometries using integrated computational fluid dynamics (CFD) simulation and experimental validation. The conical pin generated more complex material flow patterns and higher strain rate gradients due to a 55.6 % cross-sectional area reduction. Despite producing higher strain rates, the conical pin consistently yielded larger average grain sizes in AA6061 aluminum alloy, attributed to the exponential influence of temperature on the Zener-Hollomon parameter. The conical pin promoted stronger deformation textures, with the Cu {112} <111> content reaching 33.9 % (single-pass) and 55.3 % (four-pass) compared to 6.1 % and 30.5 % for the cylindrical pin. For 70 wt% B 4 C + 30 wt% h-BN reinforced composites, the conical pin produced larger particle-depleted regions and persistent “onion ring” structures, while the cylindrical pin achieved a more uniform particle distribution. Multi-pass processing with alternating directions effectively reduced particle agglomeration for both geometries. Cylindrical pins are recommended for uniform particle distribution applications, while conical pins offer advantages in enhanced shear deformation and processing efficiency through reduced pass requirements. This work establishes fundamental correlations between pin geometry and processing outcomes, enabling rational tool selection and reduced processing costs.