Influence mechanism of tool pin profile on heat transfer and material flow behavior in friction stir welding of aluminum alloys

The design of the tool pin is a critical factor influencing weld quality and material flow behavior in friction stir welding (FSW). This study employs a Coupled Eulerian-Lagrangian (CEL) thermo-mechanical model to systematically investigate the effects of tool pin taper angle on thermal cycles, plastic strain, and material flow during FSW of Al alloys. The model integrates interfacial friction heating, viscoplastic constitutive behavior, and thermomechanical contact conditions. Experimental validation was conducted through thermocouple-based temperature measurements and macrostructural analysis of weld cross-sections. Results indicate that larger taper angles enhance longitudinal thermal gradients and promote lateral heat dissipation, leading to a wider thermomechanically affected zone (TMAZ). A 0° taper angle induces significant adhesive friction and intense material flow, while a 60° taper reduces flow velocity near the pin tip. With increasing taper angle, plastic strain transitions from a uniform distribution to a concentrated region on the advancing side (AS) of the stir zone (SZ), where peak plasticity occurs. Conversely, deformation near the pin root is suppressed. These insights offer a theoretical basis for optimizing tool geometry to control heat distribution and material flow, improving weld quality in engineering applications.