*3.1. Measurement of Bead Penetration Shape*

In this study, laser-welding power was experimented with on five cases (3, 3.5, 4, 4.5 and 5 kW), and laser-welding speed was experimented with 11 cases (0.3, 0.5, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0, 2.2, 2.5 and 3.0 m/min) for A553-1. The total number of experiments was 55.

Figure 5 shows the results when the laser power was 3 kW and the welding speed was 0.5, 1.0, 1.5, 2.0 and 3.0 m/min, respectively.


**Figure 5.** Bead geometry on BOP welding test (laser power: 3 kW).

Figure 6 shows the results when the laser power was 4 kW and the welding speed was 0.5, 1.0, 1.5, 2.0 and 3.0 m/min, respectively.


**Figure 6.** Bead geometry on BOP welding test (laser power: 4 kW).

Figure 7 shows the results when the laser power was 4 kW and the welding speed was 0.5, 1.0, 1.5, 2.0 and 3.0 m/min, respectively.


**Figure 7.** Bead geometry on BOP welding test (laser power: 5 kW).

In the case of the A553-1, which was the focus of this study, micro-cracks were observed in the center line of the welded section with the welding conditions of 5 kW power and 0.5 m/min speed. The internal porosity was confirmed, as shown on the left side of Figure 8. It is a pore that often occurs because of excessive heat, incomplete penetration, gravity-laser angle, etc. [23]. On the contrary,

as shown on the right side of Figure 8, unsafe infusion occurred with the welding speed of 2.5 m/min, and the fine particles melted in an unstable molten state spatter.

**Figure 8.** Crack, porosity, spatter and under-fill of A553-1 after laser welding.
