TWI Frequently asked questions
Process gases play two important roles in high power laser welding:
Shielding: to protect the weld keyhole, solidifying molten metal, and the solidified weld bead in the case of reactive metals such as titanium, from the atmosphere thus preventing oxidation and improving weld quality. This role can be achieved by most of the commercially available, inert welding gases, delivered to the workpiece by, for example, co-axial nozzle or larger bore side jets.
Plasma control/suppression or plume control/suppression: to minimise beam attenuation and scattering by ionised vapour (=plasma) or vapour gases (=plume) above the keyhole. This function is much more difficult to achieve than shielding, and is strongly influenced by a number of characteristics of the gas species being used, depending on whether it is plasma or plume control that is required to be achieved:
For effective plasma control, the following factors are important:
Heat removal: the efficiency in cooling down the plasma created in CO2 laser welding and therefore controlling its formation is higher for high thermal conductivity gases. This is related to atomic weight and is further affected by the gas velocity. Gas atoms with high kinetic speeds result in effective heat removal properties and a reduction in temperature around the focused beam leading to recombination of ions and electrons, and hence a decrease in the plasma density.
Ionisation potential: It is important to use gases with a high ionisation potential, higher than that of iron for the CO2 laser welding of steel, such as He. A gas with low ionisation potential, such as Ar, will be easily turned into plasma itself and contribute to the build-up of plasma above the keyhole.
Dissociation properties: The dissociation of a molecule by absorption of energy can remove additional heat reducing further the size of the plasma. The dissociation potential of iron is 7.8eV, which means that only plasma control gases with a figure lower than this will be able to dissociate.
For effective plume control the momentum of the gas being used is important, as its role is more one of dispersal of the vapour gases emerging from the keyhole. Consequently, properties such as the atomic or molecular weight and the velocity of the gas are important. In that respect, the use of Ar, with its higher atomic weight, is preferable to He.
