OBRABOTKAMETALLOV technology Vol. 27 No. 3 2025 Raising the welding current to 17 kA led to the formation of finer equiaxed grains in the fusion zone. The increase in welding current from 12 kA to 17 kA caused deeper electrode indentation into the sheets, reducing the distance between the cooled electrode and the center of the fusion zone. Consequently, the thermal gradient (G) in the fusion zone increased. The diameter of the fusion zone was largely determined by welding current and welding period. A higher thermal gradient in the weld metal promotes the formation of a fine-grained microstructure during solidification, which is consistent with findings reported in references [1–7]. The dependence of the fusion zone diameter on welding current and welding period is presented in Figs. 4 and 5. A monotonic increase in the fusion zone diameter is observed with increasing welding current up to the maximum value achievable by the equipment used (Fig. 4). This trend is attributed to the increased heat input with rising welding current and is consistent with findings reported for carbon steels [1]. The results indicate that the applied electrode force was insufficient to cause the expulsion effect and the subsequent reduction in fusion zone diameter, since despite a small amount of expulsion occurring at 28.7 kA, no decrease in fusion zone diameter was observed (Fig. 4). Several researchers have noted the possibility of increasing welding current without metal expulsion by increasing electrode force [2–5]. Metallographic analysis of samples welded at high current values revealed incomplete metal expulsion. Furthermore, welding current affects the depth of electrode indentation on the metal surface. All welds exhibited electrode indentations, the depth of which was largely determined by the welding current magnitude. Electrode force has a minor effect on the fusion zone diameter for values up to 4,000 N. However, when the force exceeds this value, a slight decrease in fusion zone diameter is observed (Fig. 6). This effect can be explained by improved contact between the welded surfaces, resulting in reduced electrical resistance and heat input at higher electrode forces. The influence of the contact interface between the sheets on changes in fusion zone size depending on electrode compression force is also reported in studies [19–22]. The effect of electrode force on indentation depth was negligible within the investigated range. For detailed analysis of the electrode force effect on thickness reduction of the welded sheets, measurements were conducted at an intermediate welding current of 26.4 kA and welding period of cycle over a load range from 2,354 to 4,709 N. It was found that electrode force does not significantly affect indentation depth, while all welds showed an approximate 10% reduction in sheet thickness. To evaluate the strength and load-bearing capacity of the welds, tensile tests were performed (Table 1). Fig. 4. Dependence of the nugget diameter on the welding current (RSW) Fig. 5. Effect of welding period on the nugget size Fig. 6. Effect of the electrode force on the nugget
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