OBRABOTKAMETALLOV Vol. 27 No. 3 2025 technology Results and Discussion The welds produced by resistance spot welding (RSW) demonstrated satisfactory surface quality across the entire range of tested parameters. Changes in the diameter and depth of electrode indentation were observed depending on the welding mode. Metallographic analysis revealed no internal defects such as porosity or shrinkage cavities within the cast structure of the weld nugget. Fig. 3 shows the general microstructure of the weld joint, illustrating characteristic structural zones including the fusion zone and the heat-affected zone (HAZ). In the cast structure region (Fig. 3, a, b), a fine-grained recrystallized microstructure with equiaxed grains is observed, along with insoluble FeAl₃ intermetallic inclusions (black) and a narrow zone of columnar crystals oriented along the heat dissipation direction during solidification. The heat-affected zone (HAZ), adjacent to the fusion zone (Fig. 3, c), is characterized by a dendritic structure. The base metal microstructure (Fig. 3, d) consists of grains elongated in the rolling direction. a b c d Fig. 3. Microstructure of a welded joint obtained by resistance spot welding (RSW): a – general view of the welded joint; b – microstructure of the cast zone of the weld core; c) transition zone from the weld core to the heat-affected zone (HAZ); d – microstructure of the base material To study the effect of welding period on the weld microstructure, metallographic analyses were performed on samples welded at various welding periods (from 33.4 ms to 167.0 ms) at a fixed welding current of 12 kA. It was found that increasing the welding period in this range leads to growth in grain size of equiaxed, dendritic, and columnar structures within the fusion zone. No significant changes in grain size or microstructure were detected in the HAZ compared to the base metal. However, welding periods exceeding 167.0 ms caused grain growth in the HAZ adjacent to the fusion zone relative to the base material, attributed to increased heat input during welding. In the fusion zones of welds produced at the minimum welding current, a columnar grain structure with a pronounced liquation zone at the fusion boundary was observed (Fig. 3, b). The extent of the liquation zone increased at lower welding currents. Increasing the welding current resulted in a significant enlargement of the columnar grains in the fusion zone. Additionally, equiaxed grain regions formed in the central part of the fusion zone, indicating decreased cooling rates and thermal gradients. Grain coarsening was also noted in the HAZ of welds produced at higher welding currents.
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