This study presents a comprehensive analysis of dissimilar lap joints made of S235JR (4 mm) and S355MC (3 mm) structural steels using G3Si1 (1 mm) filler wire. Welding was performed using the MAG (Metal Active Gas) process in CMT (Cold Metal Transfer) mode. Four specimens were welded with different heat inputs (Q) from 0.4 to 0.6 kJ/mm. The welding parameters had a significant effect on the weld geometry, particularly on the weld height, which increased with higher Q, and on undercuts, which decreased with higher Q. Specimens with lower Q contained undercuts that served as crack initiation sites during tensile tests. The specimen with the highest Q had minimal undercuts, and the fracture occurred within the base material (BM). The fatigue behaviour of these joints was evaluated using FAT curve analysis, achieving values in the category 614 (FAT36 to FAT60). An analysis of the welding process was also performed using the finite element method (FEM), which generally confirmed the experimental results. The results demonstrated good correlation between the experimental results and the numerical simulations.
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