Date registered: Sep 2004
Vehicle: 2014 E250 Bluetec 4-Matic, 1983 240D 4-Speed
Mentioned: 3 Post(s)
Quoted: 256 Post(s)
TIG welding is low heat, manual feed where the eletrode is not consumed in the arc if you have any skill. MIG is a process that is higher heat, and aimed at medium deposition rates. The MIG electrode is the filler wire.
If you get poor penetration, or excessive roll over of the bead, or other visual defects you need to practice more. MIG machines usually have a set of operating parameters you set to establish the rate of wire feed (tied to Amps). TNT I think already noted you get a puddle of liquid metal that you have to manipulate (stretch side to side and back and forth) to avoid overflowing the puddle or starving it. A good weld has a ridged, but smooth surface and the edges do not join the base material at an angle that exceeds 90 degrees. You add enough filler material that the puddle does not pull away base material at its edge - this leaves a sharp edged divot along both sides of the weld that goes beneath the finished height of the base material and is known as undercut - it is a stress riser you don't need in a component with high stresses. You try to control the shape and travel of the puddle to get the result you want.
Much of the same applies to TIG, but in this case the energy into the puddle is controlled by your foot and the material being fed into the puddle is also controlled by you, this time your "free" hand, the one not holding the welding torch. TIG is a much better process for sheet metal or rust or other low heat/low energy type welds. Even anything less that an eighth of an inch thick if you are a klutz. Otherwise with MIG you will find the puddle melts through the base material and you make a hole. Don't use TIG if you are going to be adding 10 lbs of filler a day - you will never get 10 lbs of TIG filler down in a day.
Holes in thin metal are more difficult to fix with MIG, in my opinion. Many exhaust pipe system welders will use MIG and turn the heat and feed rate way down. This leaves a layer of not welded shit that must be remelted with another pass or it fails under stress. Remelting is another difficult thing to do with MIG. Check the guys on Orange County Chopper. I have never seen them do a finish weld on anything - it is all MIG tacks. Which are not much more than staples. Someone else comes in, or they ship it out, to have the job done by someone with real skills. Their welds are visible and there is nothing worse than requiring your welds be ground smooth because they won't pass a visual inspection. Lots of extra work and you usually end up picking up the undercut and other junk that gets dug out of a visually shitty weld.
Your brittle or cracked welds are a typical result of lack of heat - travel rate is too fast (that is you again) even if you are depositing a bead of significant thickness. You have to melt the base metal and the filler metal in the puddle or else it is like you dribbled liquid metal on cold metal. It doesn't weld, it mechanically bonds by conforming to the surface roughness - hit it with a slag pick or chisel and it won't stick. This is actually something you learn to see in the puddle.
You also need to select compatible filler material and base materials. Trying to weld aluminum to copper using regular steel filler won't do the job. You end up ruining the base material in the area of the weld and the stuff in the puddle segregates leaving you with little more than slag again. Even regular steels turn out not to be regular from the perspective of selecting filler material. Autos use a lot of high strength stuff these days, and my experience is with submarine hulls and nuclear power plant piping - none of it regular steels. So pay attention to what the materials are and get the right filler materials.
They sell crayons you can use to help you learn what the concept of heat input is - you draw a line a few inches from the joint with one or more of these crayons and you will want one to melt away and the other to stay meaning you got it hot enough and not too hot for a good joint.
Autos have mostly chickenshit material thicknesses. I would opt for a TIG machine unless I was good with the MIG welder on the thicknesses in question. Joining a thin material to a thick material is another reason - better control of the puddle since you own the energy input rate, the feed rate for filler material and the puddle shape/travel rate all by yourself.
TIG sucks when there is a lot of shit in the way though - tight places make both hands and your head with a spot to operate the pedal with your foot a bit of challenge compared to how one handed MIG can be.
So what are you doing, how much filler do you plan to deposit a day, and what kind of skills do you have?