Seriously though, you can do this with the MIG, but it takes a bit of effort. Two things you need to know:
1. That bitch needs to be cleaner than a Catholic virgin on confirmation day.
2. The weld is never going to be as strong as welding two pieces of mild steel together.
OK, so to achieve these two things I start by cleaning the area with a knotted wheel or wire brush. Once I get the paint and old petro-chemicals out of the way, I use a wire toothbrush and plenty of brake cleaner. Get it down in the pores of the metal and let it soak. This will free up the gunk, and also evaporate the water that is caught in there. Never use a grinding wheel to clean, as it actually closes the pores and traps the gunk/water inside.
OK, now to critical issue #2. You need to cut your bracket so that it follows the contours of the casting nicely, because you want to have as much welded area as possible. A bracket that was 8 inches long per side would not be excessive for this application. Think surface area. The more the better, because you are relying on it for strength.
OK, got the bracket cut and the area cleaned. Tack the bracket in posistion with your MIG. Now, time to add a bit of heat. A propane torch works just fine for this. We are trying to achieve two things at once with this heat 1) burn out any remaining gunk/water and 2) pre-heat the area so that it doesn't undergo a bunch of thermal shock when the MIG starts buzzing. It's tough to tell you how much heat to add, it's really hard to add too much with a propane powered torch, but 45 seconds to a minute for an 8 inch surface would be about right.
Now, here's where MIG diverges from stick. With a MIG and regular old ER70S-2 wire and CO2/Argon, I lay a bead on the casting right next to the bracket. It is going to provide a more flexible thermal buffer when you lay the final bead to the bracket. Don't be shy with this prep bead. Lay it nice and wide, but keep your gun moving at a pretty good clip. I would say for a typical 175 MIG machine, you want to run this prep bead at the middle heat position, but turn the line speed up 50% or more from the recommended.
OK, now that you've got all the prep beads layed, don't dawdle about. You want to take advantage of the heat that's in the casting. Go ahead and join your part to the prep bead with normal heat and wire feed settings.
After all the burning is done, you want to find a way to help the part cool as slowly as possible. I use sand as an insulator, but a welding blanket works very well also. If you don't have access to those, it would be good to fire up the propane torch and feed it some heat every five minutes or so to help normalize the welds.
There it is. Takes a bit of time, to be sure, but I've had very good luck with this method, and never had a part break free of the cast piece.
CRASH
