The welds in your pics don't look terrible, Pepe`. Also, penetration appears to be okay as the edges of the weld bead are "wet in" decently and aren't mounded up and rolled-over/undercut. I'd say those welds would stick metal together and hold. For the A-arm, you can opt to drill a small hole at either end of the crack to head it off and prevent "creeping" and grind into the crack a little with the side of a hard wheel to back-fill with a weld bead. This ensures more complete penetration. Also, try to pull the weld puddle rather than push it. Pulling tends to send the weld deeper, as you're pumping wire into a pre-heated section, and pushing tends to make it flatter with slightly less penetration because you're "biting" into new/cold material with the weld arc. Don't grind down a weld if it's application is structural. The weld material is where much of the strength comes from as the wire used in MIG process is typically ER70S-x. This means the wire maintains a ~70,000 psi yield strength and usually exceeds the strength of whatever base material you're welding on. Fishplating the back of the item, over the crack on the opposite side, with a box-welded piece, is a good idea but might be overkill for just a crack/fracture. That would be along the lines of "reinforcement" rather than "repair." It's just a matter of how much effort you deem appropriate for the application/use/abuse of the subject item.Pressure is regulated at 20, while the trigger is live. That's in accordance with the Hobart instructions for setting up the Arg/CO2 mix gas.
Appreciate the input gentleman. Beyond the adjustments I mention before, part of my problem may be that all my experience had been sheet metal, not thick steel so maybe I'm expecting better penetration than I really need? On the flip side of the welds, you can see the discoloration and a little metal flake but that's about it.
When I get to repairing the A-arm, would zapping a line down both sides be better to ensure the weld is bonding the crack completely through or could that weaken the surrounding metal too much? I was planning to weld it, grind it down flat and then zap a cover plate to stiffen it up more. Possibly even add a matching plate to the other side, that appears to be ok, so far.
I don't have a flap wheel yet. Just a very old, large, electric buffer/grinder with a hard, I wanna say... 8" disk. I have an air-disk cutter as well and the only other thing I have in my tools for cutting thick metal is my sawzall. Woefully low on metal working tools.
Just my $0.02.