## 3 Experimental Results
### Fracture Form and Hardness Distribution  
The failure modes of the welding point fracture of HC340 LAD + ZM with Zn-Al-Mg coating and HC340 LA without coating are mainly divided into two types: interface fracture (IF = interfacial failure) and pull-out fracture (PF = plug failure). Figure 7a,b reveal the morphologies of IF macroscopic tensile shear fracture and IF fracture, respectively. The macroscopic fracture shows that the fracture occurred at the two interfaces of the welding point, while the morphology of the microscopic fracture is an obvious cleavage step. Figure 7c,d show the morphologies of PF macroscopic tensile shear fracture and PF fracture, respectively. The macroscopic fracture illustrates that pull-out tear occurred in the parent material zone around the welding joint; the welding point was well fused, and no fracture occurred, while the morphology of the microscopic fracture had countless axial dimples. Figure 5: Morphology of HC340 LA joint at different welding times (10 kA, 2.6 kN). Figure 6: Morphology of HC340 LAD + ZM joint at different welding times (10 kA, 2.6 kN). According to Figure 4d, due to the existence of coating, the HC340 LAD + ZM steel did not nucleate at a low welding time. Nucleation completed after eight cycles, and the tensile shear force was 4.44 kN. In addition, the quality of the welding joint was poor, and the welding core was not firmly bonded. In this context, an IF-type interface fracture of the welding joint occurred. After 14 cycles, the quality of the welding joint nucleation was better, and the tensile shear force was 14.28 kN. At this time, a PF-type pull-out fracture occurred. The HC340 LA steel without coating had good malleability due to the existence of no coating. After one cycle, although the plates did not melt in the process of chemical metallurgy at this time, the two plates combined under the action of electrode loading. At this time, the tensile shear force was 5.34 kN, and an IF-type interface fracture occurred. When the welding time was four cycles, a welding point with good quality formed, and a PF-type pull-out fracture occurred with a pull shear force of 12.36 kN. This was because, when the welding time was short, the two plates in the welding core area were not closely combined, and the shear force between the welding points was less than the tensile strength of the parent material. At this time, the fracture position occurred in the welding core. When the welding time was long enough, the welding heat was enough; the nucleation was complete, and the shear force of the welding core area was greater than the tensile strength of the parent material, thus, the pull-out fracture occurs. Under the same welding parameters, the tensile shear force of the welding joint with a Zn-Al-Mg coating was smaller than that of the welding joint without coating, which may owe to the existence of  
Figure 7: Macroscopic and microscopic morphology of typical failure modes: **(a)** IF, **(b)** SEM of IF, **(c)** PF and **(d)** SEM of PF. the coating, which affected the temperature and thermal cycle of the welding joint, resulting in the low tensile shear force. Figures 8 and 9 show the hardness value distribution of the welding point under different welding times for HC340 LA without coating and HC340 LAD + ZM with coating. As shown in the figure, the hardness value distribution of the welding points of both steel results in a \"\(\pi\)\", high on both sides and low in the middle. As can be seen in Figure 8, nucleation occurred after one cycle, and even the obvious welding core zone, heat-affected zone and welding core zone appeared. The hardness value of the parent material was 150 HV; the average hardness of the welding core zone was 375 HV, and the hardness value of the heat-affected zone was between the two. In the horizontal direction, it is obvious that the welding core zone widened as the welding time increased. In the vertical direction, it was distributed high on both sides and low in the middle, and the hardness value near the electrode head was 280-300 HV.