*3.4. Tensile Shear Strength*

The tensile shear force is often used to characterize the welded joint strength. The larger the tensile shear force is, the better the strength is. Figure 13 indicates the effect of the welding current on tensile shear force. Compared with Figure 8b, it can be found that the variation trend of the W and tensile shear force with the welding current is basically the same; that is, the larger the weld nugget width, the greater the tensile shear force. With the first increase of welding current, the weld nugget width and tensile shear force all obviously increased. While it was 10.5 kA, the weld nugget width and tensile shear force all reached the maximum value of 8.75 mm and 24.20 kN, respectively. Subsequently, the weld nugget width dropped rapidly and an inflection point appeared while the welding current was 11.5 kA, but the tensile force decreased continuously. With the large welding current, the melting amount of the base metal increased gradually, which caused the increase of the weld nugget width. Thus, the bonding strength of the resistance spot welded joint increased and the tensile shear force increased. However, when the welding current was 12.0 kA, the welding heat input was too large, resulting in a large number of spatters, shrinkage cracks, and other defects in the welded joint. Although the weld nugget width increased, the effective bonding width of the welded joint decreased, so the tensile shear force decreased.

**Figure 13.** Effect of the welding current on the tensile shear force of welded joint.

During the tensile shear experiment, there were two failure modes: Interface failure and pullout failure, as shown in Figure 14. The SEM images of fracture surfaces were indicated in Figure 15. The welded joints, which were gained with the lower welding current (≤9.5 kA) and higher welding current (≥11 kA), always ruptured along the overlap surface, as shown in Figure 14a. While the welding current was low (≤9.5 kA), the W was narrow, as well as the strength of the base metal was high. Therefore, under the tensile shear force, the crack produced on the weld nugget edge at the overlap surface at first and then extended along the overlap surface until the welded joint failed with the interface failure mode. While the welding current was high (≥11 kA), the bonding strength of welded joints was small due to the welding defects, such as spatter, shrinkage, and cracks, in the weld nugget, which resulted in the smaller tensile shear force for the welded joint, and its tensile shear specimen also ruptured along the overlap surface. As shown in Figure 15a, the river pattern was obvious on the fracture surface, which illustrated the brittle fracture characteristics. While the welding current was 10 kA, the fracture pattern of the welded joint was the pullout failure mode, as shown in Figure 14b. It can be found that the fracture surfaces are mainly dimples and a small amount of cleavage steps from Figure 15b. Under the tensile shear force, the tensile specimen first produced necking in the HAZ of the welded joint. With the increase of tensile shear force, the dimples grew up and converged, and then the tensile specimen broke down in the base metal. In the present study, the weld nugget width and tensile shear force were all maximum with a 10.5 kA welding current. During the tensile shear test for welded joints produced with a 10.5 kA welding current, because of the large weld nugget and few welding defects, the welded joint was not easy to rupture from the overlap surface. The tensile stress on the edge of the weld nugget increased gradually, owing to the angle between the overlap surface and tensile force. The HAZ was the weakest zone due to the inhomogeneous microstructure, coarse grains, and low plasticity and toughness. With the increase of tensile force, the necking occurred first in the HAZ, and a number of micro-holes began to form in the center of the necking. The micro-holes grew and formed the dimples, which converged to form the crack. Finally, the crack was torn along the HAZ to form the pullout tear failure, as show in Figure 14c. Figure 15c shows the pullout tear fracture surface, which was mainly composed of small and uniform dimples.

**Figure 14.** Three failure modes of the resistance spot welded joints: (**a**) Interface failure; (**b**) pullout failure (base metal tear fracture); (**c**) pullout failure (pullout tear fracture).

**Figure 15.** SEM images of the fracture surface: (**a**) Interface failure; (**b**) pullout failure (base metal tear fracture); (**c**) pullout failure (pullout tear fracture).
