*3.1. Weld Quality Evaluation*

In this study, we evaluated the weld quality, by examining the cross sections of the weld joints, shown in Table 2. Three pre-defined types of joints were observed: unwelded, incomplete penetration, and full penetration, according to different welding speeds and laser power. The "unwelded" type was mainly observed in case of a relatively low power range of 1000–1500 W, and the type "incomplete penetration" was observed in the range of 1500–2000 W. The type "full penetration" was observed in the range of 2500–4000 W. Because the back plate was used in this study (shown in Figure 2), it seems that burn-through was not created when a high laser power was used. That might have led to a larger range of the "full penetration" type. Figure 6 shows the top, bottom, and side views of the weldment, having a gap of 0.8 mm. Although a welding speed of 1.5 m/min and laser power of 3000 W produced a "full penetration" joint, in the case of zero gap (Table 2), a weld joint was not seen, in the case of 0.8 mm gap. This is because the molten metal was insufficient to fill the gap, which was larger in size. This results in the formation of the last type of joint, "unwelded," which is due to the unfilled gap between the workpieces in the lap joint configuration.



**Figure 6.** Workpiece appearance with a gap of 0.8 mm after welding with a welding speed 1.5 m/min and a laser power of 3000 W: top view, bottom view, side view.
