*3.3. Microhardness Distribution*

The microhardness measurement was performed on the cross section of the resistance spot welded joint. The schematic diagram of the measurement location is shown in Figure 11. The distance between two test points in the base metal and weld nugget zone was 0.5 mm. Because the HAZ width was narrow, the two test points distance was 0.25 mm in HAZ.

**Figure 11.** Schematic diagram of the microhardness measurement location of the welded joint.

Figure 12 displays the microhardness distributions of resistance spot welded joints gained with different welding currents. The microhardness of the base metal was 198 HV0.2, and the microhardness in HAZ was obviously higher than the base metal. The microhardness of the weld nugget zone was the highest, which was above 350 HV0.2. The microhardness at the edge of the weld nugget was slightly higher than that of the weld nugget center. Due to the uneven distribution of the current density at the welding joint, the current density at the edge of the weld nugget was greater than the average current density, which generated a greater resistance heat. Therefore, during the resistance spot welding process, the solidification and crystallization first occurred at the edge of the weld nugget. Meanwhile, the temperature gradient was large and the cooling rate was fast, so the martensite was large and coarse and the microhardness was higher in this zone. With the increase of the welding current, the average microhardness in the weld nugget zone decreased. While the welding current was 8.5 kA, the microhardness of the weld nugget was higher because of the faster cooling rate, and the microstructure was lath martensite, which was uniform and fine, so the hardness was higher. While the welding current was 10.5 kA, the cooling rate should decrease. The columnar crystals of the weld nugget grew up, mainly composed of lath martensite and acicular ferrite, which resulted in the decrease of hardness. When the welding current continued to increase to 12.0 kA, the microhardness of most zones of the weld nugget was higher than that of 10.5 kA due to the large heat input and coarse structure, but there were welding defects in the center of the weld nugget, so that the average value of microhardness decreased.

**Figure 12.** Microhardness distributions of welded joints with different welding currents.
