## 4 Conclusions  
(1) The orthogonal tests showed that the optimal parameters of HC340 LAD + ZM steel resistance spot-welding are as follows: welding current, welding time and electrode loading were 10 kA, 14 cycles, and 2.6 kN, respectively. The influence of welding current, welding time and electrode loading on the tensile shear force can be successively ranked as current > pressure > time. (2) When the fixed welding current (10 kA) and the electrode loading (2.6 kN) increased, the welding point diameter of both steel plates also increased as the welding time increased. Under the same welding parameters, the mechanical properties of the spot-welding joint of the HC340 LA steel without coating were slightly better than those of the HC340 LAD + ZM with coating. The HC340 LA steel without coating began to nucleate after one cycle, and the tensile shear force was 5.34 kN. The nucleation completed after four cycles, and the tensile shear force reached 12.36 kN. The HC340 LAD + ZM with coating started to nucleate after eight cycles, and the tensile shear force was 4.44 kN. (3) In the nucleation test, the tensile shear force of the welding joint of the HC340 LA steel without coating was greater than that of the welding joint of the HC340 LAD + ZM steel with zinc-aluminum-magnesium coating. The tensile shear forces of the zinc-and-magnesium-coated steel and the uncoated steel increased first and then decreased as the welding time increased, and the tensile shear reached the maximum after 14 cycles. At this time, the tensile shear forces of the welding joint of the zinc-and-magnesium-coated steel and the uncoated steel were 14.28 kN and 15.75 kN, respectively. (4) The fracture modes of the welding joint of the two kinds of steel can be divided into two types: pull-out fracture and interface fracture. When the welding time was short, and the heat input was small, the nucleation of the welding joint was insufficient, and the tensile shear force was small. Then, an interface fracture occurred. When the welding time increased, and the heat input increased, the nucleation of the welding joint was fully complete, and the tensile shear force exceeded 11.6 kN. At this time, the welding joint was pulled and broken at the parent material near the heat-affected zone. (5) The hardness of the welding point of the uncoated steel and the coated steel was distributed in a \"\(\pi\)\" shape, which was high on both sides and low in the middle. The HC340 LA steel without coating completed nucleation after one cycle, and the corresponding hardness values of the parent material and the welding joint were 150 HV and 375 HV. However, the hardness value for the HC340 LAD + ZM steel with coating was very low after one and two cycles, and the hardness values of the welding core zone were 180 HV and 250 HV, respectively. A large number of martensite appeared in the center of the welding point after four cycles, and the hardness value of the welding core zone reached 355 HV. Compared with the hardness value of the welding joint of the HC340 LA steel without coating and the HC340 LAD + ZM steel with coating, the hardness value in the welding core zone and near the surface of the welding joint was higher than that of the same area with the coating. (6) The corrosion resistance of the welding point was lower than that of the parent material. The corrosion current of the HC340 LAD + ZM with zinc-aluminum-magnesium coating was 2.23 times that of the parent material, and the corrosion current of the welding joint of the HC340 LA without coating was 8.46 times that of the parent material. The corrosion current of the welding joint of the zinc-aluminum-magnesium-coated steel was 27 times that of the welding joint of the uncoated steel, while the corrosion current of the parent material of the zinc-aluminum-magnesium-coated plate was 7.17 times that of the uncoated plate. Conceptualization, X.C. and X.Y. ; investigation, Y.L. ; resources, Y.L. ; data curation, X.C. ; writing--original draft preparation, X.C. and X.P. ; writing--review and editing, X.C. and X.P.