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Gas shielding, plasma and plume control in laser welding


Process gases can play up to at least three important roles in high power laser welding:

Shielding: to protect the weld pool, and the solidified weld bead also in the case of reactive metals such as titanium, from oxidation by the atmosphere, thus improving weld quality. This role can be achieved by most of the commercially available, inert welding gases. These can be delivered to the workpiece by, for example, co-axial nozzles or larger bore side jets off to one side of the laser beam.

Plasma control/suppression: to minimise laser beam attenuation and scattering by any ionised vapour (or plasma) if present in sufficient quantities above the keyhole. This function is more difficult to achieve than shielding, and is strongly influenced by a number of characteristics of the gas species being used.

For effective plasma control, the following factors are important:

Ionisation potential: It is important to use gases with an ionisation potential higher than that of iron if CO2 laser welding steel, such as helium (He). In particular, a gas with a low ionisation potential, such as argon (Ar), will be turned easily into a plasma itself, and contribute to the build-up of plasma above the keyhole, rather than helping to reduce it.

Heat removal: the efficiency in cooling down the plasma created in CO2 laser welding, and therefore controlling its formation, is higher for higher thermal conductivity gases. This is in turn affected by the velocities of the gas atoms. Gas atoms with high kinetic speeds result in effective heat removal properties and a reduction in temperature around the focused beam. A reduction in temperature results in recombination of ions and electrons, and hence a decrease in the overall plasma density.

Dissociation properties: The dissociation of a molecule by absorption of energy can act to 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.

Plume control/suppression: to minimise beam attenuation and scattering by non-ionised vapour gases (or plume) above the keyhole. 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 a high velocity jet of Ar, with its higher atomic weight, is preferable to using He.

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