*3.3. E*ff*ect of Laser Speed*

It is found that the laser power at A553-1 and A36 welds is related only to the penetration depth and does not affect the melt width and height.

However, as can be seen from Figures 13–15, as the welding speed increases, the bead height, width and penetration depth are reduced. In particular, the change in welding speed shows only the characteristics of A553-1.

Figure 13 clearly shows the difference in bead height between A36 and A553-1. As the welding speed increases, A553-1 continues to decrease in bead height, resulting in under-fill. In the case of A553-1, surface beads without under-fill are formed at a speed of 1.2 m/min or higher. On the other hand, at the high speed of 1.5 to 3.0 m/min, it is confirmed that the under-fill appeared as an average number of times. However, in the case of A36, under-fill does not occur, but the bead of uniform height is formed even when the welding speed is changed. A36 also under-fill under certain conditions, but on average, it proves the above. The occurrence of under-fill is described later. In this way, A553-1 can confirm the formation of bead height, depending on the welding speed.

**Figure 13.** Height fluctuation of surface beads, according to welding speed.

In the case of the width of the weld bead shown in Figure 14, A553-1 showed a tendency to decrease as the height of the bead changes, with the change of the welding speed, but it is confirmed that the A36 is kept constant despite the change of the welding speed. This is the laser-welding characteristic ofA553-1, and it is a result of proving that the welding speed is one of the important parameters for bead width control of A553-1.

**Figure 14.** Width fluctuation of surface beads, according to welding speed.

For the depth of penetration shown in Figure 15, both A553-1and A36 have the same results. As the welding speed increases, it is similar that the penetration depth decreases in both materials, but on average, it is confirmed that the penetration depth of A36 is formed deeper under the same welding conditions. These results show that the welding speed is controlled to control the penetration depth in general A36 welding. However, in the case of A553-1, it is possible to control both the welding depth and the width and height of the surface bead. This is evidence that welding speed has a large impact on heat input in A553-1 laser welding. This is because the size of the melted portion decreases proportionally as the amount of heat input decreases.

**Figure 15.** Penetration fluctuation of surface beads, according to welding speed.

The under-fill is reported to be caused by the synergy of the physical phenomenon of the molten part [24]. This is due to the complexities of the melt, such as volumetric shrinkage, surface tension, gravity, vapor pressure and phase transformation [25]. In this study, the cause of under-fill is explained by the phenomenon of the flow path of the exposure plasma vapor generated in the molten part. Although the steam generated by vaporization or organic plasma must be discharged to the outside

through the occurrence of the keyhole of the welded portion, the discharge channel is clogged by the fast welding speed. Consequently, the molten pool explosion (spatter) is generated, and the surface depression cannot be filled again. To prevent weld under-fill, welding must ultimately be performed at a speed equal to or less than the constant speed. Figure 16 illustrates the under-fill phenomenon occurring in the high-speed section, when the welding power is constant at 3.5 kW and only the speed changes. Figure 17 is a graph that identifies the boundary of the under-fill zone, depending on the welding speed. It can be observed from this graph that the under-fill phenomenon occurs at a constant speed in all laser-power sections.

**Figure 16.** Under-fill phenomenon of A553-1 weld in high-speed section.

**Figure 17.** Under-fill section, according to welding speed.
