Friction stir spot welding lap-shear force analysis of automotive low-carbon steel sheets joint

This study aims to provide reference values and outcomes to support the practical implementation of friction stir spot welding in the automotive industry. This study investigated the friction stir spot welding of AISI/SAE 1006 cold-rolled low-carbon steel sheets commonly used in automotive lightweight structural components. Employing a design of experiment-composite central design approach, process parameters like tool rotational speed, plunge depth, and dwell time were examined using a series of experiments. The friction stir spot welding process was conducted using a tungsten carbide M30 tool. The study analyzed the influence of these parameters on mechanical properties such as lap-shear force, axial force, torque, and temperature. Plunge depth showed as a statistically significant factor affecting axial force. Torque demonstrated sensitivity to plunge depth and tool rotational speed, while temperature primarily depended on tool rotational speed. The tool rotational speed and plunge depth combination led to substantial temperature increments, promoting thermal structural transformations, including recrystallization. The analyses of microstructures revealed distinct zones within the welded joint, highlighting the recrystallization phenomena primarily in the mixing zone beneath the tool. The results showed satisfactory mechanical joint quality with ductile failure modes. No significant correlation was found between the lap-shear force and the dependent parameters. The selected range of variables and the chosen tool profile suggest a viable approach for producing friction stir spot welding joints in low-carbon steel and a robustness of the process in achieving an average tensile lap-shear force of 7.48 ± 0.64 kN, surpassing the minimum specification stipulated in the American National Standard AWS D8.1 M, which is 1.5 kN. This finding highlights the complex nature of the friction stir spot welding process, suggesting that other factors beyond the investigated parameters may influence the lap-shear force and indicate the potential to achieve even higher tensile stress values in lap joints by exploring alternative parameter combinations.