Laser welding is commonly carried out via a keyhole mechanism. Once the keyhole is formed, the absorption rate of laser energy is increased significantly, and the production of metal vapour is enhanced. In the case of CO2 laser welding, this metal vapour can become ionised to form a plasma.
For one part, the presence of this plasma within the keyhole can be considered beneficial, as it assists in the coupling, or absorption of the power in the laser beam by the workpiece. However, during laser welding, particularly when welding using higher powers at slower welding speeds, typically to penetrate thicker section material, the plasma can tend to jet out of the top of the keyhole and interact with the laser beam. The effect of this escaping plasma is to absorb and re-radiate a proportion of the laser power, and hence prevent the full power density in the incident beam from reaching the material surface, in the focused form required for deep penetration welding.
When CO2 laser welding, failure to control this plasma build can result in a loss of penetration and a deterioration in the weld quality. Conversely, appropriate control of this plasma can result in an increase in weld quality and penetration depth, allowing higher welding speeds to be used.
Plasma control can be achieved by using a so-called 'assist gas'. Assist gas delivery systems can vary, from a simple coaxial nozzle to a high momentum jet angled off to one side of the laser beam, either set up alone or incorporated in to a full or open-fronted shielding shoe. For thicker section welding, an angled jet with both a horizontal and vertical velocity component is most commonly used. The horizontal component is necessary to disperse or blow away any ionised plasma from the top of the keyhole, and the vertical component helps to suppress the escape of the plasma from the keyhole and to hold the keyhole open, allowing the beam to penetrate into the workpiece. Typically, plasma control is best achieved with the nozzle aligned in the plane of the joint and set to give a plasma control jet impingement point approximately 1-2mm ahead of the material/laser beam interaction point in the direction of welding. To avoid the assist gas itself from ionising and becoming a plasma, a high ionisation potential, high thermal conductivity gas, such as helium, is used.
Welding with shorter wavelength near-infrared lasers (e.g. Nd:YAG, Yb fibre or Yb:YAG disk lasers) has the reputation of being 'easier' than welding with a CO2 laser, because of the apparent absence of a significant plasma that absorbs these shorter wavelengths. However, what appears to happen is that a 'plume' (of very fine scale non- or only weakly ionised particles) forms above the keyhole. This plume appears to scatter near-infrared radiation, giving rise to the need for 'plume control', in a manner not unlike plasma control. In this situation, high momentum (high atomic weight) assist gases can be more useful, such as argon.
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