TWI Frequently asked questions
Laser welding is commonly carried out via a keyhole mechanism. Once the keyhole is formed, the absorption rate of laser energy is increased and the production of metal vapour is enhanced. In the case of CO2 laser welding, the metal vapour can become ionised to form a plasma.
The presence of a plasma within the keyhole can be considered beneficial as it assists in the coupling or absorption of the power in the laser beam in to the workpiece. However, during laser welding, the plasma can tend to jet out of the top of the keyhole and interact with the laser beam, particularly when welding using high powers at slow welding speeds to penetrate thick section material. The effect of this escaping plasma is to absorb and re-radiate a proportion of the laser power, and hence prevent the beam from reaching the material surface in the focused form required for deep penetration welding.
When CO2 laser welding, appropriate control of this plasma can result in an improvement in welding speed, penetration depth and/or weld quality. 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 angled jet, either alone or incorporated in a full or open-fronted shielding shoe. For thick section welding, an angled jet with both a horizontal and vertical velocity component is more effective. The horizontal component is necessary to blow away ionised plasma from the top of the keyhole, and the vertical component is necessary to suppress plasma escape 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 which is approximately 1mm ahead of the material/laser beam interaction point in the direction of welding.
Welding with shorter wavelength near-infrared lasers (e.g. Nd:YAG or Yb fibre lasers) has the reputation of being 'easier' than welding with a CO2 laser, because of the apparent absence of a plasma that absorbs these shorter wavelengths. However, what appears to happen is that a 'plume' (of 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.
