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Hi Loren, The below list was made for SS but in theroy can be appiled to all common metals: Pure Argon with Mig works well on light gauge SS SM as it gives lite penetration and reduces weld splatter. Argon adds arc stability with shallow penetration, lowers weld spatter and removes oxides at the weld surface. Helium adds high thermal conductivity with-in the arc reducing it's density. Read this as a cheap(free) amperage increase. Increases penetration and flatens the weld bead contour. As we move to heavier SS metals we can add 1% to 5%, o2 with the Argon that helps with undercutting and adding penetration for Mig welding. Adding more than 5% o2 would cause porosity with-in the weldment. Qxygen in amounts upto 5% can be added for broading the penetration in the center of the weld bead, controls undercutting and helps improve arc stability. Co2 can be added but has a spatter problem on most thin gauge metals and defeats my useage. O2 on the other hand enhances the tri-mix. When Helium is added with Argon, 75%Ar/25%He the weld bead is broader and penetration is improved on SS. Then we can add Co2 with Argon for a mix of 75%Ar/25%Co2. The Co2 is primarly added to reduce the cost of pure Argon. We can also use pure Co2 for some SS types but weld splatter is a big problem. Now we get the best of all worlds and end up with a tri-mix of Argon/Helium/Co2. With this mix we get the flatest weld bead profile, good penetration and very little weld splatter on SS. We also can use a heavy consentrate of Helium and add Ar, Co2 or o2 for Mig welding SS. What determines the cover gas, is the composistion of the filler wire and what the filler wire mfg spec's. These cover gasses can also be used for welding other metals. T_Bone
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