*3.2. Microstructure*

Due to the uneven heat input and different cooling conditions, the microstructure of the resistance welded joints was very inhomogeneous. As shown in Figure 9a, the whole resistance spot welded joint comprised of the base metal (a zone), uneven structure zone (b zone), fine grain zone (c zone), superheated zone (d zone), and weld nugget zone (e zone). The microstructure of the base metal consists of ferrite and martensite as shown in Figure 9b. The grain and microstructure in the uneven structure zone were obviously heterogeneous, as shown in Figure 9c. Because the temperature in this zone was between Ac1 and Ac3 during the resistance spot welding process, the phase transformation and recrystallization occurred for part of the base metal, and the fine ferrite and martensite formed; meanwhile, the ferrite, which was not austenitized, became the coarse ferrite. Therefore, there were also similarities to the structure of the base metal in this zone. During the welding process, the base metal in the fine grain zone was heated to above Ac3, and all ferrite and martensite were recrystallized to austenite. The fine and homogeneous ferrite and martensite were obtained after cooling, which were similar to the normalized microstructure of heat treatment, as shown in Figure 9d. The overheated zone consisted of coarse lath martensite and a little of the ferrite shown in Figure 9e. The temperature in this zone was between 1100◦ and the solidus temperature, and the austenite was overheated. The grain grew up seriously and the chemical compositions in the grain were uniform, hence, the coarse martensite was obtained after rapid cooling. The nugget zone comprised of coarse lath martensite and a little of ferrite with the columnar crystal morphology, as shown in Figure 9f. Because the maximum temperature gradient in the nugget zone was along the axis of the electrodes, the molten liquid metal nucleated and crystallized at the fusion line first, and then formed the columnar austenite along the direction of the higher temperature gradient. Finally, the solid austenite transformed into martensite because of the rapid cooling rate of the welding process and low carbon component in the base metal.

**Figure 9.** *Cont.*

**Figure 9.** Microstructure of (**a**) different zones of the welded joint; (**b**) base metal; (**c**) uneven structure zone; (**d**) fine grain zone; (**e**) superheated zone; (**f**) weld nugget zone.

The weld nugget is the important zone of resistance spot welded joints, and its microstructure characteristics directly affected the mechanical properties of welded joints. The heat generation and heat transfer in the nugget zone were different under different welding currents, so the microstructure was very different. Figure 10 shows the influence of a typical current on the microstructure of the weld nugget. It can be seen that while the welding current was low, the welding heat input was low, and the weld nugget was mainly lath martensite with a fine and uniform structure. At the same time, the plastic deformation of the weld nugget zone was large, and there was no welding defect in the weld nugget zone. While the welding current was 10.5 kA, the welding heat input increased and the microstructure became coarser. The decrease of the cooling rate resulted in the decrease of martensite and the increase of ferrite. The microstructure of the weld nugget was relatively uniform, and there was no obvious welding defect. If the welding current was the maximum value of 12.0 kA, a large number of spatters caused some heat loss and a rapid cooling rate, so that the weld nugget microstructure consisted of lath martensite. The grains in the weld nugget center grew up and coarsened greatly, as shown in Figure 10c. In short, when the welding current varied from 8.5 kA to 12 kA, the weld nugget columnar structure gradually coarsened, mainly because of the larger heat input and the reduction of the cooling rate.

**Figure 10.** Effect of the typical welding current on the microstructure of the weld nugget: (**a**) 8.5 kA; (**b**) 10.5 kA; (**c**) 12 kA.
