Thermal-mechanical response to plunge depth variation in friction stir lap welding of AA6061 and SS304-computational fluid dynamics approach

The influence of plunge depth on key thermo-mechanical performance indicators during dissimilar friction stir lap welding of AA6061 and SS304 using a finite volume method-based computational fluid-dynamics model is investigated. Simulations were conducted for plunge depth values ranging from 0 to 0.45 mm while maintaining constant rotational speed (1200 rpm), traverse speed (100 mm/min), and tool tilt angle (1.5°). The results show that the maximum workpiece temperature remains nearly constant at ~ 978 K across all plunge depth values, indicating minimal influence of plunge depth on overall heat generation. However, weld interface characteristics vary significantly with plunge depth. The maximum weld interface temperature increases from 743 to 791 K (~ 6.5 %), demonstrating enhanced interfacial heat transfer. Similarly, the maximum weld interface velocity increased from 0.07865 to 0.08121 m/s (~ 3.25 %), indicating improved material flow. In contrast, the minimum weld interface viscosity decreases from 8.94 × 10⁵ to 6.82 × 10⁵ kg/(ms) (~ 23.7 %), reflecting increased material softening. Tool-workpiece interface torque reduces from 14.96 to 13.91 Nm (~ 7 %), suggesting lower resistance to tool motion. These findings indicate that increasing plunge depth enhances bonding and material mixing, with an optimal plunge depth range of 0.15–0.30 mm for balanced weld quality and stability.