The use of lasers has become common in the fields of cutting, welding and surface treatment applications. Various different lasers are used for laser welding. CO2 lasers provide high laser powers of up to 10 kW and more, but require a gantry system to position the laser beam precisely. Solid state lasers such as Nd:YAG, Yt:YAG, diode or fibre lasers are more limited in terms of laser power, but are easier to manipulate via flexible fibre optics that may be guided by a robot, for example.
In laser welding, gases are crucial for obtaining the desired weld quality. Similar to ordinary MIG/MAG/TIG welding, air must be displaced because oxygen and nitrogen react with the melt, form oxides and nitrides and become dissolved and trapped in pores in the course of solidification. Surface oxidation and weld metal embrittlement are just two of the possible defects.
‘Heavy’ inert gases displace air in a gravity position and protect the weld metal. These gases are usually argon gases. Unfortunately, pure argon does not work with high-power CO2 lasers, because it forms a compact plasma cloud that readily absorbs the laser energy above the weld. Consequently, an inert gas with a high ionisation potential is required. Helium meets this criterion, but it is a ‘light’ gas and can not protect the weld pool adequately. Argon and helium are admixed, and the helium content is defined by the laser power level needed to obtain proper blanketing and plasma control.
Diode lasers offer laser radiation in the multi-kW range, but the beam quality is low, and intensity in the spot is generally not sufficient for deep penetration welding. This can be altered by using a gas that affects the surface tensions and by achieving deep penetration by means of argon CO2 mixtures.
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