I've got a Miller Passport Mig/Tig with CO2. it's metal fatique (twisting), all the failures are right where the weld terminates (with thin wall mild steel the molecular properties change enough to make it slightly harder/brittle due to full penetration) With thicker mild steel and only half +/- penetration this usually doesn't occur. I'm going to have to spread out the welds and loads forces with gussets.
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