## 3 Results and Discussion
### Continuous Welding Process and Electrode Sticking  
The nugget sizes and tensile shear strengths of the welds produced using the as-received and abraded electrodes were compared to investigate the influence of the electrode surface roughness on the weldability during the continuous RSW process, which was performed up to 50 welds. Nugget size and tensile shear strength were measured at intervals of five welds. Figure 7 shows the nugget size measurement results corresponding to the different electrodes, during the continuous RSW process at 400 kgf. All welds produced with the as-received electrode comprised nuggets with sizes between 7 mm and 7.5 mm. However, when the abraded electrode was used, at weld numbers 7, 22, and 27, nuggets of about 6 mm were formed, and at all other weld numbers, nuggets of about 7 mm were formed, similar to the nuggets produced with the as-received electrode. Figure 8 shows the size of the nuggets formed at 600 kgf. For the welds produced with the as-received electrode, the nuggets were larger than 7.5 mm, except at weld number 37. On the other hand, the nugget size of welds obtained using the abraded electrode was 7.0 mm or more, except at weld number 12. Overall, the nuggets produced with the as-received electrode were slightly larger than those produced with the abraded electrode. Figure 6: Microstructures of HAZ and fusion zone obtained with the abraded electrode at electrode forces of 400 kgf and 600 kgf. Figures 9 and 10 show the effects of the electrode force and surface type on the tensile shear strength. At 400 kgf, the tensile shear strength of welds produced with the two electrodes was similar, i.e., between 4.5 kN and 5.0 kN, except at weld number 1 (Figure 9). However, at 600 kgf (Figure 10), the tensile shear strength of the welds produced with the as-received electrode was larger than 5.0 kN (up to weld number 40) and was slightly larger than that of the welds produced with the abraded electrode. As shown, there was no significant difference in the nugget size and tensile shear strength with variations in the electrode force and electrode surface type. Nevertheless, the tensile shear strength and nugget size of the welds produced with the as-received electrode were slightly larger than those of the welds produced with the abraded electrode. The Al\({}_{2}\)O\({}_{3}\) oxide layer on the aluminum base material could be broken in the case of the abraded electrode because it had a rougher surface than the as-received electrode. Notably, it was more difficult to break the oxide film when using the as-received electrode than when using abraded electrode. Consequently, the contact resistance was higher for the as-received electrode. This, in turn, led to an increase in the resistance heat and nugget size, which resulted in tensile shear strength enhancement. Figure 8: Measured nugget size during continuous RSW under the electrode force of 600 kgf. Figure 7: Measured nugget size during continuous resistance spot welding (RSW) under the electrode force of 400 kgf. The effect of electrode surface roughness on electrode sticking was investigated using a carbon imprint test. Figures 11-14 show the results of the carbon imprint test performed on the (\(-\)) and (\(+\)) faces of the as-received and abraded electrodes at 400 kgf and 600 kgf. In the case of the as-received electrode, there was no significant change up to five welds at the electrode force of 400 kgf; however, sticking occurred on the (\(+\)) electrode face at weld number 10. Subsequently, sticking occurred on both the (\(-\)) and (\(+\)) electrode faces at weld number 15, and on all surfaces of the electrode at weld number 50. However, at 600 kgf, sticking first occurred at weld number 20, which was later than that at 400 kgf; moreover, sticking preferentially occurred at the (\(+\)) electrode face, similar to what observed at 400 kgf. On the other hand, in the case of the abraded electrode, sticking first occurred at the (\(+\)) electrode face after about 35 welds at both 400 kgf and 600 kgf.