A temperature-adaptive enhanced thermomechanical model for friction stir welding: Interfacial slip quantification, tool loads coupling, and asymmetric residual stress mechanism

The inherent solid-state temperature stability of friction stir welding (FSW) is crucial for producing high-quality joints, however, the description of its underlying mechanisms by conventional thermomechanical models still remains to be improved. A temperature-adaptive coupled thermomechanical model was developed by integrating interfacial contact, material plastic flow, tool mechanical loads, and process parameters, with solid-state temperature stability rationally characterized via thermomechanical dynamic-balance feedback. The model was validated by thermocouple measurements and residual stress tests during FSW of 2219-T8 aluminum alloy, with a peak temperature prediction error ≤6% and a peak residual stress error ≤10%. Additionally, the simulation results faithfully reproduced the solid-state temperature stability of FSW and successfully captured the asymmetric distribution characteristics of residual stress, with an asymmetry degree of approximately 17%. Further analysis revealed that the tool mechanical loads play a dominant role in the residual stress field: tool torque is the key factor leading to stress asymmetry, while neglecting tool forces can result in an overestimation of peak residual stress by approximately 25%. The above results provide an important reference for understanding the solid-state temperature stability and asymmetric residual stress mechanism during FSW of aluminum alloys.