why is gas sheilding used?

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i have been welding for 1 year on a mig machine and know how to work with the gas. i also know that it does make things cleaner. what i am wondering is why it works? i am hoping for a very scientific answer. thanks -tori
Reply:Inert gases are used to 'shield' the weld from other gases such as oxygen which will combine with the weld.  Inert gases do not combine chemically with the weld and cause contamination.Is that what you're looking for?
Reply:thats kinda what i am looking for,     why/how does the normal air contaminate the weld tho?    does it make the weld weaker or something?     thanks for your help -tori
Reply:The two biggest problem gasses for welds found in the atmosphere are oxygen and nitrogen.  Both of these gasses can combine with molten metals to form oxides and ntrides, which, when included in the weld, cause porosity, embrittlement, and trapped slag, which greatly reduce the mechanicla properties.Inert gasses aren't the only gasses used as shielding for GMAW.  CO2 (a reactive gas) is very popular for short-arcing mild steel because it is cheap.  The propereties of CO2 are deeper penetration, and higher heat conduction.  Spray metal transfer is usually accomplished with a shielding gas composed of at least 80% argon and varying percentages of oxygen.All gasses perform some function in the process other than just shielding the weld, too.  Among them are penetrationl level and bead shape, minimizing undercits, allowing faster travel speeds, providing cleaning action ahead of the puddle, and controling the mechanical properties of the weld.A good example is how the weld looks if you try it without turning on your gas!Hope that helps you.HankLast edited by hankj; 10-17-2004 at 08:34 PM.
Reply:The way I understand it, is to think about how the cutting torch process works.  When you 'cut' steel with a oxy/acetylene cutting torch you are really speeding up the oxidation process of the metal (i.e. really fast rusting) and 'cutting' through it.  It's the same thing in the welding process.  As the metal becomes molten and combines with oxygen it oxidzes ( I think of it as rusting really bad).  When you use a shielding gas it will displace the air around the weld until the weld pool cools enough to not allow the surrounding air to contaminate it.I'm sure someone else has a much better technical answer but that's how I think about the whole process.---Looks like I cross-posted with hankj.
Reply:thanks guys that was exactly what i was asking for. u guys r better than all the welding books i have tried in the past. happy welding thanks           -tori