TIG welding filler wire and Mig welding wire from a spool are essentially the same composition except that mig welding wire often contains more silicon and that can actually be a good thing for TIG welding steel. Don’t hesitate to use steel or stainless steel mig wire if you run out of TIG welding filler metal. If its too small, double it up and twist it up in a cordless drill
Standard Tig wire for welding mild steel is E70S2 It seems like the standard mig welding wire off the shelf these days is almost always E70S6. The 2 and the 6 indicate the addition of silicon and deoxidizers in the wire.
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