*3.2. E*ff*ect of Laser Power*

In laser welding, laser power is one of the important control variables. It has been reported that the shape of the fused portion is controlled according to the laser power. In this study, its influence on three bead shape was analyzed. The difference in the shape of the beads, according to the laser power, is shown from Figures 9–11.

In the case of bead height, shown in Figure 9, the increase in bead height of A553-1 was not uniform as the laser power increased. Generally, as the welding power increases, the surface beads are under-filled, due to an increase in the amount of spatter generated. However, this result does not necessarily indicate under-fill as the laser power increases. Moreover, when compared with A36, it is confirmed that the height of bead is similar to that of laser power, despite the difference of physical properties of the two metals. There is variation in some sections (3.5 kW), but it can be concluded that the overall trend is almost consistent. It can be predicted that the laser power of both metals has no influence on the height of the weld bead.

**Figure 9.** Height fluctuation of surface beads, according to laser power.

For A553-1 (the width of the surface bead shown in Figure 10), the width of the bead tends to widen as the laser power increases in the range of 3.5 to 4.5 kW. However, it does not continuously increase in proportion to the laser. In comparison with A36, on average, the bead width of A36 is narrower than the bead width of A553-1. This can be inferred from the fact that the direction of diffusion

of the welding heat source is rapidly diffused in the thickness direction of the base material, so that the formation of the welded keyhole is very fast. The difference in the thermal conductivity coefficient of the two materials can be evidence of the above reasoning. The difference in thermal conductivity coefficient between the two materials is more than twice that of A36. Thermal conductivity coefficient of A553-1 and A36 are 17.43 and 52.7 (W/(m2K)), respectively. When a heat source is supplied to material of A36 with a relatively high thermal conductivity characteristic, heat is transferred in all directions of the base material. It accelerates the continuous penetration of the heat source in the thickness direction of the base material. However, 9% nickel steel (A553) has low thermal conductivity, and as the energy source is concentrated on the surface, the width of the surface beads increases. In conclusion, it is evident that laser power is not an important parameter for the width of the bead.

**Figure 10.** Width fluctuation of surface beads, according to laser power.

The most important penetration depth shown in Figure 11 is directly related to welding efficiency. If it is possible to obtain deep penetration at low power, it is very advantageous in terms of productivity. It is also important to determine what conditions need to be adjusted to ensure sufficient penetration depth. As shown in Figure 11, the relationship between penetration depth and laser power shows that the penetration depth linearly increases with increasing laser power. By comparing the penetration-depth changes of the two materials, it can be confirmed that two materials penetrations are formed at almost the same level. The penetration depth of the two materials is highly dependent on the laser power, despite the difference in properties of the two materials.

**Figure 11.** Penetration fluctuation of surface beads, according to laser power.

Figure 12 shows that penetration depth increases of A553-1 with increasing output at the same welding speed (1.8 m/min) and same defocus (0, surface). This result shows that laser-power control is required to control the penetration depth efficiently. Further, since the influence of the laser power on the width and height of the surface beads is small, it can be a means for independently controlling the penetration depth.

**Figure 12.** Variation of penetration depth, according to laser-power change by macro cross-sectional shape analysis.
