## 3 Experimental Results
### Macro- and Micro-Morphology  
According to the orthogonal experiment results, the optimal welding process parameters were as follows: welding current, welding time and electrode loading were 10 kA, 14 cycles and 2 kN, respectively. Then, the two kinds of steel with coating and without coating were welded separately on the basis of the optimal welding process parameters by fixing the welding current and electrode loading and by changing the welding time to explore the dynamic nucleation process of the spot-welding process and the influence of the coating on the nucleation quality of the welding joint. The macroscopic morphology of the welds under different welding times is shown in Figure 3. With the increase in welding time, the diameter of the welds of the two plates of steel also increased. In addition, with the increase in welding time, the HC340 LA steel without coating showed a better quality welding spot surface without splash, while HC340 LAD + ZM steel with coating had rough joints on the welding spot surface with fluctuations. Furthermore, the splash phenomenon appeared after 10 cycles, and the center of the welding point became yellow. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & **Welding** & **Welding** & **Electrode** & \multirow{2}{*}{**Pull Shear**} & \multirow{2}{*}{**Splash**} & **Melting** & **Welding** & \multirow{2}{*}{**Down Rate**} \\  & **Current** & **Time** & **Loading** & & & & **Core** & **Point** & **Thickness** \\  & **(kA)** & **(Cycles)** & **(kN)** & **(kN)** & **(kN)** & **(mm)** & **(mm)** & \\ \hline
1 & 8 & 12 & 1.8 & 6.58 & CCCD & 5.73 & 2.70 & 3.57 \\
2 & 8 & 14 & 2.2 & 9.41 & CCDD & 6.16 & 2.69 & 4.11 \\
3 & 8 & 16 & 2.6 & 9.51 & CDDD & 6.17 & 2.72 & 3.04 \\
4 & 8 & 18 & 3 & 7.74 & CDDD & 6.41 & 2.55 & 8.93 \\
5 & 9 & 12 & 2.2 & 8.75 & CCCC & 6.44 & 2.61 & 6.79 \\
6 & 9 & 14 & 1.8 & 7.47 & CDCC & 6.41 & 2.63 & 5.98 \\
7 & 9 & 16 & 3 & 9.54 & BBBC & 6.44 & 2.47 & 11.88 \\
8 & 9 & 18 & 2.6 & 8.74 & BBBC & 6.67 & 2.35 & 16.25 \\
9 & 10 & 12 & 2.6 & 10.67 & BBBD & 6.44 & 2.66 & 5.18 \\
10 & 10 & 14 & 3 & 10.21 & BBBB & 6.54 & 2.59 & 7.50 \\
11 & 10 & 16 & 1.8 & 8.55 & BBBC & 6.67 & 2.45 & 12.41 \\
12 & 10 & 18 & 2.2 & 9.85 & BBBB & 6.83 & 2.38 & 15.18 \\
13 & 11 & 12 & 3 & 9.07 & BBBB & 6.72 & 2.38 & 15.18 \\
14 & 11 & 14 & 2.6 & 9.34 & BBBB & 6.44 & 2.24 & 20.09 \\
15 & 11 & 16 & 2.2 & 10.21 & BBBB & 6.67 & 2.30 & 17.86 \\
16 & 11 & 18 & 1.8 & 10.55 & BBBB & 6.86 & 2.38 & 15.00 \\ \hline \hline \end{tabular} Note: Splash “A”, “B”, “C” and “D” mean “A splash”, “B splash”, “C splash” and “no splash”, respectively.  
\end{table} 
Table 2: Welding results of HC340 LAD + ZM orthogonal test. Figure 4 shows the change in the welding joint's diameter, thickness, pressure down rate and tensile shear force with welding time. It can be clearly seen from Figure 4 that the joule heat and the welding joint diameter of HC340 LAD + ZM and HC340 LA increased with the increase in welding time. When the welding time was one cycle, the welding joint diameter of HC340 LA without coating was 0.83 mm larger than that of the HC340 LAD + ZM steel with coating. With the increase in welding time, the gap in the welding joint diameter became smaller. When the welding time was 10 cycles, the welding joint diameter was almost the same, and then the increase in the welding joint diameter tended to be flat, while the thickness of the welding points was the opposite, and the thickness of both plates of steel showed a downward trend. When the welding time exceeded 14 cycles, the welding point thickness of the two plates of steel tended to coincide. The welding joint thickness of the 16 cycles was 2.56 mm and 2.5 mm, respectively. The corresponding downpressure rate was 14.88% and 10.71%. Figure 4: The change in (**a**) welding diameter, (**b**) thickness, (**c**) down\(-\)rate and (**d**) tensile shear with welding time. Figure 3: Macroscopic morphology under different welding times (10 kA, 2.6 kN).