Process parameter optimization in bobbin tool friction stir welding of AA6082-T6: A response surface methodology approach⋆

Bobbin tool friction stir welding (BT-FSW) has emerged as a promising solid-state joining technique for aluminum alloys due to its ability to generate symmetric heat input and improved through-thickness material flow. However, the complex interaction between process parameters, heat generation, and material consolidation makes parameter optimization challenging, particularly for precipitation-hardened alloys such as AA6082-T6 where thermal exposure can significantly alter mechanical properties. This study investigates the combined influence of rotational speed, feed rate, pinching gap, and tool pin profile on the mechanical performance of BT-FSW joints in AA6082-T6. A response surface optimal design was implemented to evaluate parameter interactions and develop predictive models for ultimate tensile strength (UTS) and Vickers hardness (HV). Analysis of variance confirmed statistically significant models with coefficients of determination of 0.962 and 0.983 for UTS and HV, respectively. Feed rate and pinching gap were identified as the dominant process parameters with P values for feed of 0.0031 and 0.0001 for UTS and HV models respectively and P values for Pinching gap of 0.0033 and 0.0001 for UTS and HV models respectively, while their interaction exhibited the strongest influence on both responses with P values of 0.0004 for UTS and 0.0001 for HV models respectively. Mechanistically, higher feed rates reduced thermal exposure and promoted grain refinement through dynamic recrystallization, whereas increased pinching gap enhanced forging pressure and improved material consolidation. Overlay response surface analysis revealed a practical operating window at feed rate of 500 mm/min, pinching gap of 5.8 mm, and rotational speed of 800 rpm, producing joints exceeding 200 MPa tensile strength and 78 HV hardness. Three Independent validation experiments confirmed model accuracy with prediction errors below 5%. The results provide a statistically validated and physically interpretable optimization framework for BT-FSW of AA6082-T6, offering practical guidelines for industrial fabrication of lightweight structural components.