OBRABOTKAMETALLOV Vol. 27 No. 3 2025 technology Ta b l e 1 Values of ultimate strength, nugget diameter, and average microhardness Exp. No. Tensile strength, MPa Nugget diameter, mm Average microhardness, HV Base metal 272 – 94 1 231 7.91 110 2 220 7.68 105 3 228 7.62 103 4 218 7.59 98 5 198 6.94 103 6 187 6.85 101 7 210 7.20 106 8 203 6.87 101 9 189 7.12 106 Table 1 also presents results reflecting the influence of welding parameters on joint strength, fusion zone diameter, and hardness. It was found that increasing electrode force from 2 kN to 3 kN and welding current from 7 kA to 8 kA leads to a significant increase in tensile strength. Changes in welding period from 15 ms to 25 ms had little effect on joint strength. To characterize the mechanical properties of the weld joint, Vickers microhardnesswasmeasured in the weld zone (Fig. 7). It was found that microhardness in the fusion zone increases with rising welding current and welding period. Maximum hardness values reached 110 HV and 107 HV. Average hardness values in the fusion zone exceeded that of the base metal (Fig. 7, red line), indicating a reduction in weld ductility compared to the base material. Resistance spot welding (RSW) is a thermomechanical joining process in which heat plays a central role in forming a bond between the welded components. According to the Joule-Lenz law, the amount of heat generated during RSW is determined by the welding current, welding period, and the electrical resistance of the materials involved. Therefore, welding current, welding period, and electrode force are the primary parameters governing the welding process and, consequently, the quality of the weld joint (see Fig. 2). The RSW process cycle diagram typically reflects the variation of these three parameters over time and helps identify optimal ranges to achieve the desired weld characteristics [1–7]. It is well known that insufficient welding current can result in cold welding, whereas excessive welding current may cause metal expulsion from the fusion zone, as well as the formation of internal porosity or cracks within the cast microstructure. Insufficient electrode force may lead to molten metal spreading along the fusion boundary, while excessive force can reduce heat generation efficiency due to lowered contact resistance [1]. During welding, as the metal temperature rises, its electrical resistance also increases. The total resistance in the welding circuit (including the resistance of the welding machine, electrodes, and welded parts) determines the welding current magnitude. To form a molten zone in RSW, a certain value of total electrical resistance in the circuit must be ensured, which is the sum of resistances at each current path section through the welded workpieces [1, 2]. Higher total resistance improves weldability [1]. The total resistance depends Fig. 7. Values of microhardness of the weld versus experiment number
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