Shielding gas selection
Which mixture suits the material, the process and the thickness — with the EN ISO 14175 designation, so you can order it by name rather than by nickname.
Carbon steel
Argon with 8–25 % CO₂ covers nearly everything. More CO₂ means a hotter, deeper-penetrating arc and more spatter; less means a cleaner bead face and a steadier arc, and it is what spray transfer needs — above about 20 % CO₂ spray transfer stops working at all. Pure CO₂ is cheap and penetrates well, but it only gives globular transfer and it spatters.
Stainless and aluminium
Austenitic stainless takes argon with a small active addition — 2 % CO₂ or 2 % oxygen — kept small on purpose: carbon picked up from CO₂ reduces corrosion resistance. Aluminium takes pure argon, or argon with helium once the section is thick enough that argon alone will not carry the heat.
TIG
Inert gas without exception. Any active component destroys the tungsten within minutes. Argon-hydrogen mixtures are used on austenitic stainless to raise the heat and clean the surface, and on nothing else — hydrogen in ferritic or duplex steel causes exactly the cracking that preheat is meant to prevent.
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