*3.2. Microstructures of a Spot Welded Joint*

Figure 5 shows the low-magnification microstructure of a whole spot welding joint, which was obtained under mid-frequency AC with cooling intervals of 6 ms between any two neighboring current pulses (named AC-6), with a peak current of 12 kA, frequency of 110 Hz, and welding time of 35 cycles. In Figure 5, a whole nugget with a diameter of about 6 mm can be clearly seen, as shown by the yellow double-headed arrow. It is known that 4√*<sup>t</sup>* (*<sup>t</sup>* denotes sheet thickness) is the minimum requirement for the nugget diameter. Thickness of the sheet samples used in this work was 1.5 mm, and thus the 4√*<sup>t</sup>* should be 4.9 mm. The actually obtained diameter was much larger than the standard requirement for a spot welding nugget.

**Figure 5.** Low-magnification optical microstructure of a whole spot welding joint obtained at AC-6, with a peak current of 12 kA, frequency of 110 Hz, and welding time of 35 cycles.

The spot welded joint includes three regions: nugget, heat-affected zone (HAZ), and base metal. There were two micro areas, just like two white arc lines, on the left and right sides of the nugget in each steel sheet, just outside the upper and under indentations, marked by "e" in Figure 5. These areas exhibited as white due to the specific microstructure of acicular ferrite. Figure 6 presents the microstructures of different regions in Figure 5. The nugget was formed by solidification of melted base metal after spot welding, and it consisted of a columnar grain perpendicular to the surface of steel sheets. Due to the strong cooling effect resulting from the flowing water inside Cu electrodes, the nugget after spot welding was cooled at a rate much higher than the critical quenching rate, and its microstructure was completely composed of martensite, as shown in Figure 6a. Although the martensitic transformation in the nugget occurred within the large columnar austenite grains, martensite laths were relatively fine and dense. The widths of the columnar austenite grains were generally small and less than 50 μm, and most martensite laths form by crossing the width of the columnar grains, therefore, the lengths of the martensite laths were limited within the widths of the columnar austenite grains and were exhibited as fine and dense.

At the interface between the nugget and HAZ, i.e., the fusion zone, extremely coarse prior austenite grains can be distinguished, as shown in the lower right part of Figure 6b. The large-sized austenite grains were related to the higher austenitizing temperature, which was close to the melting point of the steel. This region would be the weakest part of the whole spot welding joint upon loading.

Just outside the fusion zone, the microstructure consisted of relatively fine martensite, as shown on the upper left part of Figure 6b. With the distance away from the nugget center increasing, the maximum heating temperature in welding was decreased, and the size of martensite laths was decreased, as shown in Figure 6c,d. From the fusion zone to the white arc line (marked by "e" in Figure 5), the microstructure was just composed of martensite.

Figure 6e and 6e' show the OM and SEM images of the microstructure in the white arc line in Figure 5. The microstructure in this area consisted of fine acicular ferrite. The width of the white arc line region was about 100 μm and it was a part of the HAZ of a spot welding joint. This region is suggested to be completely austenitized during the welding process, but the cooling rate was too slow to obtain the martensite, since it was relatively far away from the electrodes and cannot be effectively cooled by the flowing water inside the electrodes. It should be noted that the acicular ferrite in Figure 6e is much finer than that in Figure 4b. It was reported that the pre-strain imposed on austenite could affect the phase transformation behavior in subsequent cooling process [11]. Pre-strain can produce a considerable amount of crystal defects (dislocations) within austenite grains. These defects can serve as nuclei for ferrite, therefore ferrite can not only nucleate on grain boundaries but also within the austenite grains. Abundant nucleation sites within austenite grains result in the formation of fine acicular ferrite. When the pre-strain is absent, ferrite would nucleate only on grain boundaries and grow into the austenite grain, resulting in the formation of coarse acicular ferrite, as shown in Figure 4b. From Figure 5, the white arc lines were located just outside the electrode indentation, where the pre-strain accumulated significantly in prior austenite grains. Coupled with the lowest cooling rate, fine acicular ferrite was produced in this region. This area also exhibited the lowest hardness in the whole spot welding joint.

Regions outside the white arc line areas experienced a faster cooling rate due to better heat conduction into surrounding material, and the microstructures were composed of granular bainite, as shown in Figure 6f,g. In addition, with the distance away from the nugget increasing, the density of particles in granular bainite increased and the polygonal bainite matrix evolved to a lath-shaped one. Figure 6h shows the microstructure far away from the nugget, where the peak heating temperature was lower than Ac1 and the martensite in base metal was tempered to some extent.

**Figure 6.** Microstructures of different micro regions in a spot welding joint, corresponding to (**a**)~(**e**) marked in Figure 5: (**a**) nugget, composed of fine martensite within columnar grains; (**b**) fusion zone, mix of coarse martensite and fine martensite; (**c**) martensite; (**d**) minute martensite; (**e**) acicular ferrite under an optical microscope; (**e'**) acicular ferrite under SEM; (**f**) granular bainite with polygonal matrix; (**g**) granular bainite with a lath-shaped matrix; (**h**) tempered martensite.

#### *3.3. Micro-Hardness Distribution Across a Spot Welded Joint*

Figure 7 shows the distribution of hardness along the white dashed line in Figure 5. The microhardness of the nugget is somewhat higher than that of the base metal, by about 30–50 HV. This resulted from the extra-high cooling effect of the electrodes on the phase transformation. The region just outside the nugget, which consisted of martensite, had a hardness slightly lower than that of the nugget. The lowest hardness throughout the welding joint in Figure 7 (about 310 HV) corresponds to the white arc line regions in Figure 5, which consist of acicular ferrite. Outward from the white arc line regions, the cooling rate increased rapidly and granular bainite was obtained, therefore, microhardness rose sharply. Microhardness tended to stabilize at about 450 HV as soon as the base metal appeared.

**Figure 7.** Microhardness measurement results on the typical spot welded joint in Figure 5; (**a**)–(**h**) correspond to the micro areas in Figure 5.

#### *3.4. Tensile Shear Fracture*

Spot welding joints exhibit two types of fracture modes: button-shaped fractures and interfacial fractures. A spot welded joint is generally considered to be qualified if a button-shaped fracture is produced when the applied load exceeds its capacity in tensile shearing tests. That is, the whole joint is integrally pulled away from one of the two overlapped sheets, leaving a hole in this sheet and a button on the other one. Figure 8 is an overall view of a failed specimen as a button fracture, obtained at AC-6, with a peak current of 12.7 kA, frequency of 110 Hz, and welding time of 35 cycles. In tensile shear tests, in order to discover the origin of cracks, a high-speed camera was used to take continuous photographs of the weld surface during loading. It was learned from the obtained continuous pictures that for any one of the two overlapped sheets, stresses concentrate significantly on the chuck-nearing side of the joint at an early loading stage, as shown by the two rectangles in Figure 8a. For the upper sheet in Figure 8a, stress concentrates at the right side of the joint and for the lower sheets, stress concentrates at the left side. When stress exceeds the strength of the joint, cracks originate at these stress concentration sites and subsequently fracture occurs. Once cracks appear on either side, the stress concentration on the other side would be relieved greatly, and afterwards the cracks further develop until fracture takes place at that side. From the low-magnification optical micrograph shown in Figure 8b, it is known that fracture originated on the boundary of the nugget, i.e., the fusion zone, which consisted of coarse martensite, as shown in Figure 6b. This indicates that the fusion zone is the weakest region throughout the weld joint, although the white arc line region is the softest region, consisting of acicular ferrite. Figure 8c shows a magnified SEM image of the fracture edge, clearly showing that the original fracture occurred on the boundaries of coarse grains. Later, tearing occurred along half of the nugget edge on the fractured sheet and the nugget was left on the other sheet, leaving a button. Under large compressive stress, the opposite side of the fracture

originating point in the joint was deformed significantly, and the formed fiber-like structure is marked by white arrows in Figure 8d.

**Figure 8.** Button-shape fracture images, obtained at AC-6, with a peak current of 12.7 kA, frequency of 110 Hz, and welding time of 35 cycles: (**a**) overall view of a fractured joint, (**b**) optical image showing nugget, (**c**) SEM image of fracture edge at "c" point, (**d**) SEM image of fracture edge at "d" point.

A spot welding joint with the interfacial fracture mode is generally considered unqualified. In this case, the nugget was divided into two parts along the bonding surface of two overlapped sheets. The flat fracture surface is left on each sheet. In this case, the spot welding joint fails mainly under the action of shearing force. Figure 9 shows the typical morphology of an interfacial fracture surface in a failed specimen in tensile shear tests. It can be noted that many small cracks exist in the nugget, as shown in Figure 9a. Such small cracks tend to form in the spot welding process when splash occurred, in which shrinkage stresses were more likely to be produced. High-magnification observation on regions around the cracks shows no dimple produced after fracture, as marked by white circles in Figure 9b, indicating that brittle fracture occurred around cracks. Yet regions far away from the cracks exhibit plastic fracture characteristics, with a lot of dimples left. Some areas exhibit typical shearing fracture characteristics, with small dimples stretched along a uniform direction, as shown in Figure 9c,d, indicating that shearing stress was responsible for the fracture. In addition, inclusions and shrinkage defects in the nugget were also factors leading to interfacial fracture, because their appearance would decrease the effective loading area of the joint.

**Figure 9.** SEM images of an interfacial fractured specimen: (**a**) showing small cracks at low magnification, (**b**) showing brittle fracture around cracks and plastic fracture far away from cracks, (**c**) shearing surface, (**d**) stretched dimples on shearing surface.
