Laser welding of dissimilar materials
Welding or bonding of dissimilar metallic materials is receiving much attention because of the great demand for high quality and high performance industrial products. However, fusion welding of dissimilar alloys is notoriously difficult due to the ready occurrence of cracking in the intermetallic compounds formed. Nevertheless, some good results have recently been obtained in laser lap-joint or lap and butt joint one-pass welding of dissimilar materials such as aluminum alloy and steel, as shown in Fig. 6.19.63-66 Intermetallic compounds should be very thin and their thicknesses are controlled by melting aluminum alloy sheet only at confined depths in the plate. When the bonding
Laser Forward: 20°; Tilt: 3°
6.19 One pass welding of a butt and lap joint configuration for welding aluminum alloy and steel.
area is wide enough, the mechanical properties of the joints are so good that the fracture occurs in a base metal in the tensile test63. Moreover, the possibility of laser welding of SPCC or Type 304 to Mg alloy is confirmed when the laser is shot on the steel side without melting of the joint interface so that the heat of the laser weld fusion zone can melt Mg alloy plate.67 The feasibility of laser welding of copper to steel or Ti, and of other high temperature metals to normal materials has been investigated.
Laser welding of plastics has been actively investigated with the evolution of diode lasers all over the world. The plastics are readily joined using diode, YAG and fiber lasers of low power under defocused conditions, as shown in Fig. 6.20, since the absorption and transparency are easily controlled by the concentration of such substances as carbon black.68,69 The melting points of plastics are generally lower than those of metals and the processing temperature ranges are narrow because of low evaporation temperatures. LD welding of plastic parts is applied practically in mass production by the Toyota Motor Corp. and its group.69 LD welding of the intake manifold, canister, cutoff valves in the fuel tank and fog lamps are examples.69 In addition, almost transparent plastics can be lap-bonded by using slightly higher temperatures generated at the joint-bonded area and using jig plates with a larger radiation heat in CO2 laser welding.
The understanding of welding phenomena and porosity formation mechanisms is important in laser welding. This means that plume and keyhole behavior, melt flows and bubble formation and porosity formation are relevant. Their observations have been taken during spot and bead welding with pulsed
YAG laser and CW YAG and CO2 lasers under various conditions using high speed videos and X-ray transmission systems.21,22,24,70
In spot welding, the rapid collapse of a deep keyhole and subsequent rapid solidification are causes of bubble and porosity formation, as shown in Fig. 6.21.71 The porosity can be suppressed by the application of pulse shaping and the addition of correct tailing power.24-26,71
6.21 Porosity in laser spot welding and porosity formation mechanism.
For bead welding, welding phenomena and porosity formation are schematically summarized in Fig. 6.22.70 Keyhole behavior, melt flows and bubble and porosity formation depend apparently upon the kind of material as well as on welding conditions such as laser power and welding speed. Such keyhole behavior and melt flows are best understood by considering the factors determining evaporation. Such factors are the different locations of laser-material interaction and keyhole collapse, the content of volatile elements and such physical properties as the vaporization temperature and the surface tension. At low welding speeds, a deep keyhole is liable to collapse, a laser beam is shot on the liquid wall of the collapsed keyhole, and consequently the downward melt flow along the keyhole wall is induced by the recoil pressure of evaporation. Moreover, intense evaporation takes place at the front wall of the keyhole and thereby many bubbles are generated from the keyhole tip. As the welding speed increases, vapors generated from the keyhole wall are more strongly ejected upwards through the keyhole inlet. As a result, upward melt flow is induced near the keyhole inlet and downward flow near the keyhole tip is reduced. A bubble moves a short distance and the formation location of porosity is limited to the area near the bottom. As the welding speed increases considerably, the keyhole becomes narrower and shallower, bubbles become smaller and consequently the number of pores
6.22 Schematic representation of welding phenomena: plume, keyhole, melt flows and bubble and porosity formation under various conditions.
decreases and the bubbles do not reach the surface of the molten pool. This means that the formation ratio is high and volume of porosity large at a certain welding speed. The formation of bubbles and of porosity are almost similar in YAG and CO2 laser welding.19-22,72,73
Certain laser welding conditions reduce porosity or large pores. Such conditions include proper pulse modulation, moderate power density, a forwardly declined (tilted) laser beam, a very low speed or a high speed, the weaving method, a twin-spots laser, full penetration welding, vacuum welding, the use of a tornado nozzle and hybrid welding with a TIG or MIG arc at high currents.19-22,70-73