## 3 Experiment Results and Discussions  
### Optimization of Weld Bonding Conditions  
Spot welding was performed according to each welding condition (1-4), and a salt spray test was conducted with these welded specimens. Figure 4 shows the results of the tensile shear tests on specimens as they reached 640 h, 1280 h, and 1920 h during the corrosion test. In order to compare the tensile shear characteristics of the conventional spot-welded specimen and the adhesive-added spot-welded specimen (weld bonding), the apparent maximum tensile failure load was recorded instead of the stress unit. It was strenuous to calculate the area required for stress calculation of the conventional spot-welded specimen and the weld bonding specimen, to which an adhesive was added, so the failure load under the tensile lap shear test was compared. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Process** & **Welding Condition** & **Heat Treatment (170 \({}^{\circ}\)C, 30 min)** \\ \hline
1 & DeltaSpot welding only & 1.9 kN, 9.5 kA, 240 ms * & \(\times\) \\
2 & Weld bonding & 4.5 kN, 13 kA, 180 ms & \(\Circarrow\) \\
3 & Weld bonding & 4.5 kN, 13 kA, 240 ms & \(\Circarrow\) \\
4 & Weld bonding + Roughening & 4.5 kN, 13 kA, 240 ms & \(\Circarrow\) \\ \hline \hline \end{tabular}  
* Electrode force (kN), welding current (kA) and welding time (ms). \(\times\) means the welding experiment is carried out without heat treatment. \(\Circarrow\) means the welding experiment is carried out with heat treatment.  
\end{table} 
Table 2: Spot welding process conditions for optimization. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **KS D 9502** & **Condition** \\ \hline NaCl concentration (g/L) & \(50\pm 10\) & 50 \\ Density & 1.029\(\rightarrow\)1.036 & 1.031 \\ pH & 6.5\(\rightarrow\)7.2 & 6.59 \\ Spray pressure (kPa) & 98 \(\pm\) 10 & 100 \\ Temperature (\({}^{\circ}\)C) & \(35\pm 2\) & 35.1 \\ Air saturation temperature (\({}^{\circ}\)C) & \(47\pm 2\) & 47.2 \\ Sprayed capacity (mL/h) & \(1.5\pm 0.5\) & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Salt spray test conditions. Among the results in Figure 4, the maximum tensile shear loads at 1920 h are shown in Figure 5, along with the weld nugget diameters of the as-welded specimens. Specimen 1 had the largest nugget diameter, but the shear tensile strength before the corrosion test was the lowest. This is because the other specimens (except the first one) were not only spot welded, but also bonded with an adhesive so that the bonding area and the resulting strength increased. In addition, since it was a lap joint shape rather than a butt, during the shear tension test, the line of action of the force did not coincide during tensioning, resulting in a coupling force, and thus a tearing force at the joined interface was generated. In addition, Specim 1 showed the greatest drop in shear strength as the corrosion test proceeded (Figure 4). This is because the corrosion solution not only contacted the specimen surface, but also penetrated into the faying interface, overlapping between the specimens, and corrosion proceeded to the nugget region. In the case of Specimens 2 and 3, the tensile strength also decreased as the corrosion time progressed. However, compared with Specim 1, the decrease in strength was not significant, even after 1280 h, because the penetration of the corrosion solution at the faying interface of the specimen was delayed due to the adhesive, which slowed the corrosion progress. Comparing the welding conditions of Specimens 2 and 3, the nugget diameter of Specim 2 was expected to be smaller than that of Specim 3, because the welding time of Specim 2 was shorter than that of Specim 3. However, as a result of observing the cross section, contrary to expectations, the nugget diameter of Specim 2 was observed to be larger than that of Specim 3. The reason for this result was that the adhesive was applied  
Figure 4: Tensile shear strength test results at each salt spray test time.