Penetration depth and resulting weld profile

As mentioned previously, if the separation between the laser and arc is reduced in the CO2-MIG hybrid process, the penetration depth is increased. One explanation for this is that when the distance is short the weld metal formed by the arc process does not have time to start solidifying before the laser beam impinges the same position and starts penetrating. The hot metal improves absorption of the beam by the metal and therefore creates a deeper weld penetration. (Abe, N. & Hayashi, M., 2002)

According to research using a 5 kW CO2 laser and MIG arc (200 A and 25 V) for hybrid welding of bead-on-plate welds with a welding speed of 0.6 m/min, the penetration depth is at its lowest when the laser beam and arc are aimed at the same point. When the process separation is too short, the molten metal produced by the arc makes it difficult for the laser to reach the deeper parts of the joint and therefore penetration is shallower. In cases when a keyhole tries to generate where there is too much deposited material, its generation and maintenance become difficult and cause a reduction in the penetration depth or the instability of the keyhole, and in turn the production of a blowhole.

(Abe, N. & Hayashi, M., 2002)

According to Abe et al. (1997b, 1998b, 2002), with distances 1-7 mm between the processes there is little variation in penetration depths and little effect of a leading or trailing arc, with no significant difference in penetration depths, Fig. 2.25. When the laser beam is irradiated into the front edge of the molten pool of the arc, near the boundary of the molten pool of the laser, the amount of molten metal is reduced and the generation and maintenance of keyhole becomes more easy, causing a greater penetration depth. When the distance becomes too great, there is no interaction and the blowholes will most likely form as a result of gas in the keyhole and the assist gas mixing with the molten metal. The reason for the blow holes might be that as the distance increases, the molten metal starts to cool and might even start solidifying before the trailing process arrives at the same point. If the laser beam is irradiated far from the molten pool of the arc, the molten pools of both laser and arc are separated and inclined to exhibit a lack of fusion.

Process separation also affects the bead shape. With a leading arc and a 1 mm process separation, bead shape has been found to improve. With short distances (e.g. 1 mm) the electric pole of the arc lies at the upper point because of the height of the molten metal. This means that the heat supplied to the upper portion of the specimen increases and the weld surface becomes wider, resulting in a wine cup shaped weld. When the distance was increased, the height of the molten metal decreased and the point of interaction between the laser and specimen shifted downward. The metal melted by the laser and arc mixed well and the bead became wedge-shaped. The more the distance was increased, the rougher the weld bead surface became. With a trailing arc, the bead shape was not disturbed when the process separation varied from 1 to 5 mm. In this case it was thought that with the leading arc, the surface bead shape was disturbed by the assist gas blowing into the molten pool made by the arc. If the arc trailed, the assist gas did not affect the molten pool made by the arc and therefore the bead shape was not disturbed within a wide range of distances between the processes.

When the distance was too great (in this case 5 mm) portions of metal melted by the laser and arc were completely separate and therefore the deposited metal was also separated. (Abe, N. et al., 1997a), (Abe, N. et al., 1997b)

Figure 2.25. Effect of process separation on weld penetration in CO2 laser-MIG hybrid welding.

(Abe, N. et al., 1997a)

The situation changes slightly when there is more space inside the groove, for example with V- or Y-grooves, or grooves with an air gap. According to a studies using a 6-7 kW CO2 laser with the MIG process when welding into a Y-groove of 16 mm thick plate (welding speed 0.7 m/min), full penetration was achieved when the process separation was either 3-6 mm or 20 mm. There was no full penetration when the distance was 10-15 mm; this was thought to be a result of the laser beam power being absorbed by the MIG arc. Alternatively, the laser induced plasma could be increased and the laser beam energy impinging the base metal reduced. When the arc and laser processes were aimed at the same location, full penetration was not achieved. This was thought to be caused by laser beam energy being absorbed in the laser induced plasma and MIG arc plasma, and also because the weld metal added from the wire was present at the laser focal point. (Makino, Y. et al., 2002), (Minami, K. et al., 2002)

Ishide et al. (2003) studied hybrid welding with 3 kW Nd:YAG laser and MIG arc. They noticed that the penetration depth decreased to some extent in case when the arc and beam irradiation position coincided. When the beam was set either 2 mm ahead or 2 mm behind the arc, the penetration depth increased. With a process separation of 4 mm, the penetration depth again decreased, see Fig. 2.26. They suggested that when the beam and arc are aimed to same point, the laser energy is used for wire melting rather than for keyhole formation.

Figure 2.26. Effect of distance between Nd:YAG and MIG on penetration depth in hybrid welding.

Laser power 3 kW, MIG current 140 A, voltage 24 V, welding speed 1 m/min. (Ishide, T. et al., 2003)

Hayashi et al. (2004) studied CO2-laser-MIG hybrid welding of thick steels plates By using bead-on-plate welding in the flat position they observed that the penetration depth tended to increase with a smaller distance between laser and arc, independent of whether the arc or laser was leading, Fig.

2.27. Maximum penetration was obtained with a leading laser beam, with the laser and filler wire tip aimed at the same location.

Figure 2.27. Effect of laser-MIG distance on penetration depth produced when welding in the flat position. In the study, the laser power used was 30 kW, arc current 500 A and voltage 47 V, with helium used as the shielding gas. (Hayashi, T. et al., 2004)

Hayashi et al. (2004) also studied the effect of process separation on different parts of the cross-sectional geometry of the weld, see Figure 2.28.

Distance between MIG electrode and laser L (mm)

Penetration depth (mm)

Welding direction

Figure 2.28. Typical geometry of laser, MIG and laser-MIG hybrid weld beads. a) laser only, b) leading arc, c) trailing arc and d) MIG only. (Hayashi, T. et al., 2004)

The penetration depth of the nail head part of the weld bead in hybrid welding with a leading arc (Figure 2.28b) was found to be much the same as that produced with trailing MIG welding when the distance between the arc and laser varied between 2.5 and 10 mm. When the processes are aimed at the same location, or when the arc was trailing and the distance between the arc and laser was 0 to 7.5 mm, the penetration of the whole weld was either equivalent to or greater than that observed during laser welding alone. Independent of the process separation, the upper bead area width was similar to the bead width found during MIG welding alone (Figure 2.29c), and the width of the lower regions of the weld was similar to laser welding alone, Figure 2.29b. It can be summarized that in arc leading hybrid welding, the lower part of the laser weld bead (nail part in 2.28a) is added to the base of the MIG weld bead and the penetration depth is slightly increased in comparison to a laser weld. (Hayashi, T. et al., 2004)

Figure 2.29. Effect of laser-MIG distance on a) nail head penetration depth, b) nail bead width, and c) nail bead width of hybrid welds produced in flat position. Backhand welding stands for trailing arc and forehand welding for leading arc. (Hayashi, T. et al., 2004)

Concerning the weld penetration in CO2 laser-MIG hybrid welding, it is evident that the driving forces generating convection inside the molten pool affect the penetration shape, i.e.

electromagnetic force, gravity, surface tension and pressure induced by plasma flow. Of these, the electromagnetic force strongly affects the penetration shape of the upper part of the weld bead.

When the arc leads and the distance between the laser and arc is small, a molten pool forms in front of the laser beam under the effect of the arc force, and the laser beam forms a keyhole with lower

energy expenditure than when irradiated on a solid surface; therefore deeper penetration is obtained than by laser welding alone. (Hayashi, T. et al., 2004)

Naito et al. (2003a&b) found out that in Nd:YAG laser-TIG hybrid welding, with leading TIG the weld penetration became slightly deeper and the bead width to some extent narrower as the distance between the laser beam axis and the TIG electrode increased. But when the TIG trailed, the penetration was deepest when the separation was 2 mm and became shallow when the separation increased, Figure 2.30. (Naito, Y. et al., 2003a), (Naito, Y. et al., 2003b)

Figure 2.30. Effect of distance between the laser beam axis and TIG electrode on penetration depth in Nd:YAG laser-TIG hybrid welding of AISI 304. Laser power was 1.7 kW, TIG current 100 A and welding speed 10 mm/s. Backhand=trailing arc, forehand=leading arc. Naito, Y. et al., 2003b)