The effect of laser beam parameters and welding speed

8.1.1. The effect of laser power and welding speed

Welding speed is the most important parameter in the hybrid welding which is mainly dictated by the laser beam with certain power density since it is responsible for the penetration depth.

Therefore for hybrid welding the selection of parameters primarily depends on the laser power and welding speed and can be selected from typical penetration depth versus welding speed graphs as presented in Figure 58.

With an increase in welding speed, both weld width and penetration decrease sharply due to the thermal input to the base metal. On contrary, decrease in welding speed enables deeper penetration and higher heat input. High heat input has a positive effect on microstructure since cooling rate is slower. In addition, fast solidification defects can be avoided. However decrease in welding speed means decrease in productivity. Conversely, laser power can be changed to control penetration depth and welding speed. Subsequently the higher power provides deeper penetration and more stable keyhole and laser power influences on seam geometry.

If the air gap between two workpieces is implemented, welding speed can be increased significantly. However it is difficult to determine the dependency between welding speed and air gap.

Figure 58. Typical penetration depth versus welding speed graph. All experiments were done in BOP configuration. (Vollertsen & Thomy, 2005; Ream, 2004; Liu et al., 2006)

As presented in Figure 58 it can be concluded that the optimal welding speed selection in order to achieve particular penetration depth can be very difficult task since penetration depth depends on focal point diameter and therefore also on power density, laser beam quality, and on the material being welded. Moreover, focal point position must be considered since it affects penetration depth significantly.

Many welding defects are related to improper welding speed. Typically humping, incomplete fusion and undercutting phenomena can occur during high welding speeds (more than 10 m/min) in the keyhole mode with single beam (Kannatey-Asibu Jr., 2009). At low welding speeds the porosity formation is very probable (Katayama et al., 2006).

8.1.2. The effect of focal point diameter and beam quality

Since focal point diameter determines power density, it has significant influence and controls the welding speed and the penetration depth as shown in Figure 59. Obviously, if focal point diameter gets smaller at constant laser power, the penetration depth can be increasing.

Apparently, the decreased focal point diameter (less than 200 µm) cannot facilitate significant

penetration depth. As a result, very small focal point diameters (less than 150-200 µm) will promote larger welding depths only if the beam quality (BPP) becomes higher and this statement shows that fiber lasers give almost double penetration depth compared to disk laser when both lasers are focused to 100 µm spot diameter (Weberpals et al., 2007).

Figure 59. Penetration depth (welding depth) as a function of welding speed for different focal point diameters during welding of steel. (Weberpals et al., 2007)

8.1.3. The effect of focal point position

According to studies done by (Qin et al., 2007), (Gerritsen et al., 2005), (Thomy et al., 2007), (Fellman & Salminen, 2007), (Victor et al., 2009), (Liu et al., 2008), (Yamazaki & Kitagawa, 2012), (Vänskä et al., 2013), (El Rayes et al., 2004) the position of focal point has an influence on the geometrical parameters of welds and quality since it determines the size of focal point, as a result, also the laser intensity impinging on the top surface of the workpiece.

Particularly, the focal point position greatly influence on full penetration capability which makes the statement that full penetration cannot be adjusted only by welding speed (Zhang et al., 2014)

An adequate weld could be obtained for a focal position on the surface (0 mm) or under the surface (-1 mm or -2 mm), mainly it depends on the thickness of the work piece and air gap.

The focal point position controls the penetrations of the hybrid process. The correct focal point alignment gives maximum penetration. (Antonsson & Grote, 2009), (Olsen, 2009)

Focal point position in hybrid welding differs from autogenous laser welding where the highest penetration depth is achievable at focal point position on the surface or slightly below it. In hybrid welding experiments conducted by El Rayes et al. (2004) and Gerritsen et al.

(2005) where CO₂ laser was combined with MAG source, indicate that focal point position depends on the applied arc current magnitude. With increase in arc current, the deeper focal point position should be used to achieve maximum penetration depth. Gerritsen et al. (2005) assumed that the molten weld pool surface is depressed by the arc pressure down to the plate, as a consequence, to achieve maximum beam density the focal point position should be approximate closer to the molten pool surface. However, it was assumed that the arc pressure is not so strong to depress weld pool surface significantly below the surface therefore it might be also due to increase in energy coupling effect between heat sources.

Qin et al. (2007) studied the effect of focal point position on the penetration depth and weld bead geometry by combining Nd:YAG laser with a pulsed MAG source. According to his work, the penetration depth is the highest regardless arc current when location of laser focal point is 2 mm above the surface as can be seen from Figure 60a. Figure 60b shows a steady relationship of increase in weld width depending on deeper (down from the surface) focal point position for higher currents (295 A). (Qin et al., 2007)

Figure 60. Variations of penetration depth (a) and weld width (b) during variation of the focal point position. 2 kW Nd:YAG, pulsed MAG source, trailing arc arrangement, Ar+18% CO₂,

low carbon steel, welding speed 1.5 m/min. (Qin, et al., 2007)

Victor et al. (2009) by combining 10 kW fiber laser with MAG source found out that the maximum penetration depth is achieved when focal point position located at the surface of the work piece (0 mm) as shown in Figure 61. Moreover, it can be seen that hybrid welding

produces slightly higher penetration depth that autogenous laser beam welding. In hybrid welding, the focal point position tolerance range with decent penetration depth is much wider compared to autogenous laser welding, where penetration depth starts to decrease from 8 mm below surface (from 1 mm for autogenous laser welding) and from 6 mm above surface (from 1 mm for autogenous laser welding). It can be concluded that the hybrid welding process has much wider process window according to focal point position.

Figure 61. Influence of focal point position of the fiber laser on penetration depth. (Victor, et al., 2009)

According to results obtained in HYBLAS projects, the focal point position which was varied in very wide range (from 0 mm to -12 mm) has insignificant influence on weld appearance and weld efficiency during CO₂ laser-MAG hybrid welding of 12 mm plate (see Figure 62).

Figure 62. The effect of focal point position during paraxial CO₂-MAG hybrid welding of square butt joints. (Webster et al., 2008; Petring et al., 2007)

According to Thomy et al. (2007), the focal point position below the surface in hybrid welding of X70 pipeline steel, significantly reduces the penetration depth and top bead quality when welding is performed in bead-on-plate configuration with single mode fiber laser (1 kW) combined with MAG source (118 A, 18 V).

In the experiments conducted by Fellman & Salminen (2007) where fiber laser was combined with MAG source during welding of 6 mm S355 carbon steel, focal point position 5 mm below the surface provided better penetration capability and weld quality. When focal point position was 5 mm above the surface, the geometry of welds were more similar to the arc welding produced welds accompanied by lower penetration depth. As a result, focal point position below surface provides better geometry of welds and higher penetration depth.