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Nitrogen is used to prevent porosity in the welding member by preventing oxygen and air from entering the molten metal during the welding process.

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One of the main problems with the vast majority of metals is they oxidise ...ie react with oxygen. In Iron this is rusting...in Aluminium, this can form a relatively stable, protective coating...magnesium can be very bright and includes high levels of ultraviolet light being emitted.

It is fortunate that for the more dramatic magnesium oxidisation, the enthalpy of reaction (ie the amount of energy required to start the oxidation process) is quite high...but therein lies the problem....arc, MIG, TIG and LASER welding all seem to apply reasonably high amounts of energy to the metals being welded.

The idea of adding Nitrogen (Argon, CO2 or any other INERT gas) is to eradicate the oxygen from the immediate environment, preventing the oxidation from taking place and allowing a homogenous weld. It does not act as a Flux, which technically promotes melting, but as a facilitator allowing the metal to melt and flow without reacting to the gases present in the air. Nitrogen is being used more and more due to its availability, and the use of CO2 can cause problems as it does react with some metals in the arc of a weld...argon is quite expensive.

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