MIG weld - leak proofing technique?

[PoorOwner]

Can you guys help me with this. Say you are welding together 2 tubing to make an exhaust. Let's say 16 gauge because it's not too thin or thick.

What is the motion you use on the MIG to joint it? Note that if too hot is blow through.

Also I am having alot of trouble with leaks. I can butt join 2 pipes and it does not leak on a good day. If I flare one pipe and do a slip joint I use very high heat and definitely no leak.

There is one big problem with leak when I weld on a flange. I go around the outside, it takes about 7-8 passes with breaks in between or the flange will warp.
There is little penetration because I see the bead is thick. After I am done I check with some water from inside, and leaks like a sieve. I run another pass from the inside and the leak is fixed. This makes me think there is cold casting, or my motion which is small, circular actually create leaks. Please advise.

When I check a exhaust shop's work on their pipes and flage. It is a big thick splat all around about 1/2" bead. I don't know how they don't warp the flange. Their welder has more amps than mine for sure.

[MXtras]

The trick for MIG welding thinner stuff is to use short bursts. You will likely want to use a higher heat setting - much like you would for tack welding. You basically stitch or tack the entire weld.

Scott

[PAToyota]

Although MIG for tubing is possible, I usually just turn to the TIG for stuff like that. It takes quite a bit of skill to do that sort of stuff with MIG, have it look good, and have it work well (not leak).

[PoorOwner]

Local exhaust guy just kneel on the floor and do this all day long with MIG, Piecing mandrel bends as many as needed to make full flow exhaust for cars.
I don't even think his helmet is autodarkening.. lol He is amazing.

[shumakerscott]

I have used the Phenolic Resin gas tank sealer from Caswell Plating many times with great sucess. You do NOT have to weld the holes. Put duct tape along the bad seam and swush the sealer around. let cure and you are done. When mixed acording to the instructions this stuff is tough. It is totally different than the White Kreem stuff. I have no luck at all with it. To clean the inside of the tank use nuts and washers, not rocks, that way you can use a magnet to get them out.

[Fry]

Although MIG for tubing is possible, I usually just turn to the TIG for stuff like that. It takes quite a bit of skill to do that sort of stuff with MIG, have it look good, and have it work well (not leak).

IF a guy can't figure out how to weld exhaust with a mig, then a tig is out of the question.
Just do stacked tacks with you mig, joining each one together. Or turn your heat down and do small stitches. Or take a welding class.

[PAToyota]

I just find I have a lot more control with the TIG.

[punkabilly136]

totally agree with the tig idea, with tig you can control every angle of the welding process, from how much heat your puttin in to how fast you wanna go, how much filler, to even just fusing the two together..BUT tig is expensive, so just do short stitches with your mig, and worry more about making a sound waterproof weld instead of a pretty "dime on dime" look, you can always grind the welds down and make it smooth

[Vicegrip]

Back up and start each MiG stitch a little over the top of the end of the last stick. This will cover any cold start problems. Not as purdy but work one stitch then move to another place on the seam and bounce around. I have been known to fully cool tubing with a damp rag after 2 or 3 stitches. This stops the problems with the heat soaking expanding the metal. Work on scrap until you get it down. I have to agree anyone that can weld up thin wall tubing with a TiG can likely Mig it too.

[stimpy]

when I weld pipes together I ussually take a 3 inch grinder and put a small bevel on the edge of the pipe which allows me better heat penetration at the joint when I did this to my exaust system on my truck it went better than with the blunt ends which either would burn thru or give me a cold bead and I also used the whip stich technique instead of a circular motion it allows the puddle to cool a little without blowthru and gives a nice looking weld. and practice ,practice, practice before you start a finish weld project .

[RichardNorman]

I use mig to do exhaust work often

Recently I used mig to weld up intercooler pipes. I pieced it all together using JC Whitney sourced mandrel bent exhaust tubing. That involved lots of butt welds as well as welding on two flanges.

I also pieced together the down pipe coming off the turbo which involved a very large and odd shaped flange for the turbo outlet as well as a standard flange at the other end.

Exhaust pipes SHOULD be sealed well....intercooler pipes HAVE TO BE sealed well.

Most of my welding experience comes from very thin sheet welding (body work) so welding exhaust pipe seems easy to me maybe. I didn't much trouble getting reasonably air tight welds.

From your description, I'd guess you're running too fast a wire speed and not enough heat. You don't want clops of "bead" on top of the joint. I think you guessed right that you have a cold weld.

For me, with flanges I start with the tip of the wire on the flange with a fairly hot setting. I strike the arch and push the puddle to the pipe diagonally..then push it back down to the flange diagonally and so on....like an M sort of. Then I do another 'M' directly on the other side of the pipe and so on.

You want to get in and out quick rather than laying a lot of heat and fill material while you "caulk" the joint rather than weld it. Getting in and out fast keeps it from getting super hot and warping.

Obviously you need to make sure the pieces fit well so you're not welding air too.

Also, make sure you're motions are controlled, and steady. Getting the correct wire speed and the correct speed and motion with your hands may take some experimenting but once you figure it out write it down.

I also like to push my puddle rather than pull it. It's easier to control and to see the pieces coming together. Make sure you are in their really looking. Make sure you have good lighting etc.

I know this all sounds like basic welding 101 stuff but sometimes reading it again makes a difference.

[PoorOwner]

Thanks guys I am going to practice by putting stubs of pipes on a steel plate and make sure they can hold water.

[Franz©]

Don't want to break your heart, but MIG is inherently a poor process for leakproof welds. Unless you can make one continuous pass, you will have problems where the bead begins and ends. The MIG process will require a LOT of learning to make nonporous welds.