Assessment of Pinless Tools Efficiency in FSSW of Thick High-Strength Low-Alloy Steels for Structural Lap Joints

Abstract

In this study, the maximum achievable weldable thickness in spot welding with pinless tools was investigated by welding multiple stacks of 1 mm thick high-strength steel HC420LA. This approach allowed for the evaluation of heat dissipation during the welding of progressively thicker sections, as well as for the comparison of the joints strength for increasing welded thicknesses. The methodology used also enabled the identification of the key technical challenges in achieving high-quality welds using very short welding times. Notably, even for a 5 s weld, a four-sheet configuration (4 mm thickness) was successfully joined, achieving a maximum shear load of approximately 10 kN and exhibiting a refined acicular ferritic bainitic microstructure with metallic continuity across the steel interfaces. The highest mechanical performance was obtained in the welds produced within 30 s, which reached a maximum shear load of approximately 16 kN. Importantly, the mechanical strength of these welds was benchmarked against relevant standards for the automotive and construction sectors, demonstrating that the spot welding with pinless tools enable achieving performance levels suitable for structural applications.