Thermal Cycling, Microstructure, and Mechanical Properties of Al-Mg-Si-Cu Alloy Bobbin Tool Friction Stir Welded Joints Based on Thermal Index

The two main process parameters of Bobbin tool friction stir welding (BT-FSW) are ω (rotational speed) and v (traverse speed). Both of these factors have a significant effect on heat input, microstructure, and mechanical properties. At present, most studies on friction stir welding adopt the control variable method to study the thermal cycling during the welding process and the mechanical properties of joints, and there are few studies on changing the two process parameters at the same time, because it can be difficult to assess the correlation between heat input and mechanical properties when changing both factors at the same time. In this study, the w/v ratio is defined as the thermal index, which is a characteristic value of heat input. The study uses ABAQUS 6.5 software to establish a BT-FSW CEL (coupled Eulerian–Lagrangian) thermal coupling model. This model explores the relationship between joint thermal cycles, microstructure, and mechanical properties for different w and v values with the same w/v ratio. The results show that increasing rotational and traverse speeds under the same w/v ratio leads to an increase in the peak temperature of the nugget zone (NZ). However, the peak temperature of the thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) remained almost constant. Joint strength was highest at a rotational speed of 750 r/min and a traverse speed of 650 mm/min, with a yield strength of 227 MPa. As rotational and traverse speeds increased, the recrystallized grain content of the NZ showed an increasing trend followed by a decreasing trend. The recrystallized grain content of the advancing side thermo-mechanically affected zone (AS-TMAZ) and retreating side thermo-mechanically affected zone (RS-TMAZ) showed a decreasing trend. Joint hardness had a “W” shaped distribution, with the highest average hardness value found in the NZ.