New developments in advanced welding

Laser-arc hybrid welding

Hybrid welding with CO2, YAG or LD lasers and TIG, MIG, MAG (metal active gas) or another heat source has been receiving considerable attention regarding such factors as depth of penetration, higher welding speeds, wider gap tolerance and lower porosity.87100 The production of a compact head is

6.28 Schematic experimental set-up of an in-process monitoring and adaptive control system for pulsed laser spot welding.

6.29 Pulse shape used for on-site hole repair and monitoring results of heat radiation during laser spot welding of A3003 alloy under adaptive control.

6.30 Schematic mechanism for in-process repairing of a through-hole defect during spot welding.

necessary in industrial applications, as shown in Fig. 6.31,89-92 and coaxial heads with TIG electrode/MIG wire and a YAG laser beam have been developed.90 Laser welding is carried out with a focused beam, and consequently an underfilled laser weld bead is easily formed in any gap joint however slight; use of a filler wire for reduction in underfilling renders welding speed slow. The effect of laser-MIG hybrid welding on gap tolerance has been frequently demonstrated and is shown in Fig. 6.32.91 The mixing of filler wire components also takes place more completely in hybrid

welding than it does in laser welding due to the effect of arc electromagnetic convection.

The effects of various welding parameters on weld penetration and porosity formation in hybrid welding with CO2, YAG or diode laser and TIG, MIG and MAG have been reported.96-100 It is understood that weld bead penetration and geometry are greatly affected by the electrode-to-laser beam target distance and the welding direction. In YAG-TIG hybrid welding, the deepest penetration can be obtained at short distances of 1 to 2 mm; however, the penetration becomes equal to or shallower than that of laser welds at distances of 5 to 9 mm. On the other hand, in TIG-YAG hybrid welding, the penetration depth is always deeper than that of laser welds and the deepest penetration is attained at the distance of about 5 mm. The effect of oxygen in air on the surface tension and arc constriction in the normal Ar flow is revealed to be the same as that of sulfur in arc and hybrid welding of stainless steel. The formation of the deepest weld bead is attributed to several superimposed downward flows along the keyhole wall. These are caused by recoil pressure against the keyhole wall or collapsing keyhole, marangoni (surface tension driven) convection from low to high temperatures, the arc constriction and electromagnetic convection due to a high content of O2, in addition to the keyhole depth. On the other hand, shallow penetration is due to the collision between the downward flow along the keyhole wall caused by the recoil pressure and the other downward flow near the central molten pool caused by marangoni convection and the electromagnetic convection due to TIG arc constriction.99

TIG-YAG hybrid welding phenomena in air at 100 and 200A are schematically shown in Fig. 6.33.99 The diameter at the upper part of the keyhole becomes larger with increasing arc current. At 100A, a keyhole was slightly larger and deeper than that produced in laser welding; then the downward flow of the melt near the keyhole wall became dominant as the

6.34 Effect of MIG arc current on penetration of YAG-MIG hybrid welding at 40 mm/s.

melt flowed from the keyhole tip to the rear along the molten pool bottom. The latter flow deepened the bottom of the molten pool, leading to the deeper weld bead. Under these conditions large bubbles are often generated to form larger-sized pores. On the other hand, at 200 A, the molten pool surface was depressed, the keyhole diameter near the top of the surface was larger, and other fast melt flows were observed around the keyhole near the surface, resulting in the formation of wider bead widths in the upper part. Moreover, no or reduced generation of bubbles was observed, leading to no or reduced porosity at high arc currents.99

In the welding of aluminum alloys, hybrid welding with YAG-MIG heat sources can produce superior weld beads without undercuts giving a good appearance to top and bottom surfaces in comparison with those produced by laser welding. MIG-YAG welds generally appear to be slightly deeper and larger than are YAG-MIG welds. The surfaces of YAG-MIG welds always appear better than welds MIG-YAG.100 Figure 6.34 shows cross­sectional YAG-MIG weld beads made at 40 mm/s as a function of MIG arc current.100 The weld beads become larger and deeper with an increase in the MIG current.

Porosity is reduced in A5052 hybrid weld beads at the laser power of 3kW and at a high MIG current of 240 A.100 Welding phenomena, molten pool geometry, melt flows inside the pool, bubble and porosity formation are illustrated schematically in Fig. 6.35.100 At 120A, many bubbles are generated and trapped by the solidifying front, resulting in porosity formation. On the other hand, at 240 A, some bubbles are generated and all bubbles disappear from the concave surface of the molten pool, resulting in no porosity.

It is interesting to know that the concave molten pool surfaces induced at high TIG and MIG currents suppress bubble formation due to the formation of a more stable keyhole in stainless steels or act as a disappearance site for bubbles in aluminum alloys.

Laser beam

Welding

direction

°

Bubble Molten pool Porosity YAG laser welding

Hybrid welding

6.35 Schematic representation of welding phenomena, molten pool geometry, melt flows inside the pool, bubble and porosity formation during laser and YAG-MIG hybrid welding.