OBRABOTKAMETALLOV technology Vol. 27 No. 3 2025 on the surface condition of the welded materials and electrodes, electrode surface geometry, and electrode force. Maximum heat generation occurs at the contact interface between the welded parts, where the main electrical resistance is concentrated. Meanwhile, the high thermal conductivity of copper electrodes and their intensive water cooling prevent the base metal surface from reaching melting temperature. As the temperature rises in the zone of maximum electrical resistance, metal melting and molten zone (weld nugget) formation occur. Simultaneously, the welded sheets thin, and the distance between electrodes decreases under electrode force, reducing the overall dynamic resistance. If the molten metal volume becomes too large for the surrounding solid metal to contain under the applied force, molten metal expulsion from the weld zone occurs. Increasing electrode force reduces electrical resistance by improving the contact between sheets and smoothing surface irregularities. The efficiency of energy absorption and the growth rate of the fusion zone depend on the geometrical dimensions of the welded parts [10–15]. However, classical RSW studies [1, 2] often overlooked this factor, and most RSW control systems are optimized for welding parts of identical dimensions. Many researchers [1–12] aim to optimize RSW parameters to achieve a stable process and produce welds with specified properties. The significant influence of welding current and welding period on the quality of spot welds is consistently emphasized. Authors [1–5] identify welding current, welding period, and electrode force as the main parameters of the resistance spot welding (RSW) process. To achieve an optimal fusion nugget diameter, increased values of welding current and welding period are recommended [1–3]. At the same time, other studies demonstrate a direct correlation of the fusion zone diameter with welding current and welding period, and an inverse correlation with electrode force [5–8]. The morphology of RSW joints in metal-to-metal connections is characterized by three distinct zones: the fusion zone (FZ), the heat-affected zone (HAZ), and the base metal (BM) (Fig. 3). The fusion zone represents the cast nugget formed due to melting and subsequent solidification of the welded metals. The heat-affected zone is the region that does not melt but undergoes microstructural changes due to heat transfer from the fusion zone. Microstructural analysis of samples obtained in this work also revealed these three characteristic zones (Fig. 3), with significant differences in microstructure within each zone. Both FZ and HAZ exhibit columnar dendrites oriented in a specific direction. Porosity formation in the cast structure is typically associated with surface contamination and possible hydrogen saturation of the metal. The absence of porosity in the fusion zone in this study indicates sufficient heat input to ensure quality melting of the base metal and formation of a strong joint. Comparison of the microstructure between the HAZ and FZ shows larger columnar dendrite grains forming at the fusion boundary. The formation of columnar dendrites in both zones is driven by a high solidification rate (R) and a steep thermal gradient (G) between the molten metal (approximately 600 °C) and the base metal (at room temperature). Under these conditions, the undercooling criterion required for planar solidification at the solid-liquid interface is not met [1–7], meaning the G/R ratio is insufficient to suppress dendritic growth. The smaller size of columnar dendrites in the fusion zone is related to a higher cooling rate (i.e., faster solidification), attributed to the high thermal conductivity of aluminum alloys (120– 180 W/m·K) [1, 5, 9, 12–15]. The cooling rate decreases from the fusion zone through the HAZ to the base metal, which acts as a heat sink. This is because thermal conductivity is the primary factor controlling cooling rate. Consequently, the G × R value in the HAZ is lower compared to the fusion zone, resulting in coarser grains. The size and shape of the fusion zone are key criteria for assessing RSW joint quality (Fig. 3, a) [1, 2, 5, 16–19]. In this study, the fusion zone diameter (DFZ) ranged from 1.33 to 7.61 mm. Each value represents an average of at least three measurements. A fusion zone diameter exceeding 7 mm is considered critical by several authors [1, 2] regarding its influence on the joint’s mechanical strength. The increase in fusion zone size is attributed to the high heat input under the applied welding conditions. Shear tensile strength is another important criterion used to evaluate the quality of resistance spot welded joints. In the conducted experiments, the shear tensile strength of nine welded samples ranged from 179 to 231 MPa (Table 1). The maximum shear tensile strength was achieved at a fusion zone diameter of 7.91 mm.
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