**2. Materials and Methods**

In this study, the base metal is galvanized dual phase steel (Trademark: CR590T/340Y) sheet, which is always used to manufacture the automobile's front longitudinal beam part. The thickness of the base metal is 2 mm, and its .hemical compositions are listed in Table 1. The XRD pattern result indicates that the base metal consisted of ferrite (86.2 vol.%) and martensite (13.8 vol.%), as shown in Figure 1. The galvanized dual phase steel sheets were conducted with double-side hot galvanizing and the weight of the Zn layer is 80 g/m3. The yield strength and tensile strength of the base metal are 356 MPa and 605 MPa, respectively.

**Table 1.** Chemical compositions of base metal (wt.%).

**Figure 1.** XRD pattern of base metal.

The resistance spot welding robot (Shougang MOTOMAN Robot Co., Ltd., Beijing, China) was employed to weld the overlap joints, and the working face diameter of Cu-Cr-Zr alloy electrodes was 6 mm. The dimensions and configuration of the joint are indicated in Figure 2. The welding time and electrode force were set at 20 cycles and 4.0 kN by preliminary process experiments, and the main experimental parameters are given in Table 2.

**Figure 2.** Schematic diagram of dimensions and configuration of resistance spot welding joint.

**Table 2.** Experimental parameters.


After welding, the appearance of the resistance spot weld was observed by digital camera (Canon Ixus1000, Cannon, Tokyo, Japan). The resistance spot weld was cut from weld nugget center, and then mounted, polished, and etched for the microstructure observed and analysis. The etching solution was 4 vol.% nitric acid. The microstructure of base metal, heat affected zone, and weld nugget was examined by Evol-18 scanning electron microscopy (SEM) (Carl Zeiss, Jena, Germany) with energy dispersive spectroscope (EDS). The phase structures were identified using the X-ray diffractometer (XRD) (D/MAX-2500PC, Rigaku, Tokyo, Japan) machine with 50kV voltage, 300 mA current, Cu Kα radiation, and 4 ◦/min scanning rate. The Vickers hardness was measured on the cross section of the welded joint using an MH-3 microhardness test machine (Shanghai Tuming, Shanghai, China), and the test load and load time were 1.961 N and 10 s, respectively. The tensile shear test was performed using CSS-44100 material testing equipment with a 100 kN maximum load (China Mechanical Testing Equipment Co., Ltd., Changchun, China), and the strain speed was 6 mm/min.
