This paper establishes the trends in structure formation of 4 mm thick 1561 aluminum alloy welded joints, depending on changes in the parameters of the MIG welding mode on a steel substrate (specifically, the speed and heat input of the process). Welding modes were selected based on the criterion of satisfactory seam formation, and the optimal welding mode was determined based on the criteria of minimizing porosity, grain refinement, and improved mechanical properties. Technological studies have shown that, based on the criterion of satisfactory seam formation with a quality level ranging from C to B according to ISO 10042, it is advisable to select a welding speed between 380 and 600 mm/min with a heat input between 217 and 240 J/mm. Metallographic studies have shown that increasing the MIG welding speed promotes weld grain refinement, decreases their shape factor, and simultaneously increases the number of pores while reducing their size. Mechanical tests demonstrated that increasing the welding speed enhances the mechanical properties of welded joints. Therefore, when using MIG welding of aluminum alloys in industry, it is recommended to increase the speed to 600 mm/min and higher.