## 3 Experimental Results
### Macro- and Micro-Morphology
rose first and then decreased, reaching its maximum after 14 cycles with the tensile shear forces of 14.28 kN and 15.75 kN. However, the tensile shear force of HC340 LA steel without coating was higher than that of HC340 LAD + ZM steel with Zn-Al-Mg coating under the same welding parameters. When the welding time of HC340 LA without coating was one cycle, the welding core had already appeared, and the tensile shear force at this time was 5.34 kN. HC340 LAD + ZM with coating did not nucleate until the welding time was eight cycles, and the corresponding tensile shear force was 4.44 kN. Combined with the diameter and the thickness of the welding point, the existence of the Zn-Al-Mg coating reduced the weldability. Although the diameter of the welding point was large enough after six cycles, an effective welding core was still not formed at this time. Because of the existence of Zn-Al-Mg coating, the contact resistance between the two plates was smaller, and there was less heat input from the welding center area, thus failing to effectively form the welding core. Therefore, with the increase in the welding time, the impact of the coating on the welding time became smaller, and HC340 LAD + ZM with coating did not nucleate until the welding time was eight cycles. Meanwhile, the zinc with a low melting point evaporated and gasified the heat during welding, which reduced the effective heat input generated during welding, resulting in more difficult nucleation. Figures 5 and 6 show the cross-section morphologies of the HC340 LA without coating and the HC340 LAD + ZM with coating at different welding times. Figure 5 shows the cross-section morphology of HC340 LA steel without coating when the welding time was 1-18 cycles. As shown in the figure, the uncoated HC340 LA steel was very sensitive to the change in welding time, and the width of the molten core gradually increased with the increase in welding time, and the thickness of the welding point also decreased. When the welding time was one cycle, there was no complete nucleation, but a union appeared between the plates. When the welding time was two cycles, the nucleation began to obviously appear, forming a small welding core area. After four cycles, nucleation completed, and the columnar crystal appeared obviously perpendicular to the plastic ring. At this time, a shrinkage hole defect appeared. Because there was so little molten metal, it shrank as it cooled, and holes formed. As the welding time increased, the height of the melting core decreased, and the width of the melting core also gradually increased, which was basically unchanged after reaching 14 cycles. Figure 6 shows the cross-section morphology of the HC340 LAD + ZM steel with coating when the welding time was 1-18 cycles. Due to the existence of the coating, the welding of the coated HC340 LAD + ZM steel requires a greater welding heat input. On the one hand, the existence of the coating made the contact resistance between the plates smaller and made the heat input generated during welding lower, resulting in difficult nucleation. On the other hand, because of the low melting point of Zn, the main heat generated was first absorbed by the coating, resulting in less heat input obtained by the parent material, Fe, which could not effectively nucleate. Thence, nucleation appeared initially when the welding time was eight cycles; when the welding time reached 10 cycles, the welded joint nucleated completely.