This has more to do with the physics of welding. Welding under ~160A the welding arc is characterised by small chuncks of welding wirebreaking off and travelling towards the weld pool (this is known as globular transfer). The 'sizzle' you hear when welding is this process. The shielding gas has two functions, to protect the weld from oxygen contamination and guide the weld chunks to the puddle by preventing an ionized sphere of gas from draging the weld away from the centre of the welding arc.
100 % argon results in a very narrow welding transfer channel, it tends to build up upon transfer and results in very thin welds. Conversely 100% CO2 produces quite a wide weld channel and a wide, flat weld pool, because of this, less material is deposited in the center of the weld and the arc can be difficult to control at low volts/amps. A mix of the two (i.e. argosheild lite) produces a weld of medium width and an arc which is easy to control, especially at low amps/volts (hence why it is so popular).
100 % argon is required for welding alloy because this is done at >160A and at these amps the welding wire arcs off continuously to produce 'spray transfer', the narrow weld channel produced by argon is required to control the welding arc and produce a suitable weld.
100% CO2 can be used in welding sheet metal it just requires some practise, generally running at higher gas flow will aid in cooling the weld and good gun/wire feed control is required. Because it also produces a wide weld it also heats over a wider area, care must be taken in determining heat transfer and so you don't warp the panel.
Check this out for more.
Welding gasWelding gas 2NZ being the greatest country in the world
we can get owner bottles here and just pay for the gas (within 2 years my bottle would have paid for iself by not having to pay rental). Can also get disposeable bottles for the various mixes for small jobs
TM (will try these out one day).
Edited by nzstato, 09 December 2010 - 07:56 AM.