Welding Tips and Tricks

I have a friend who started a car project and alot of the welding he has done is with a 115V flux core mig. I just finished tig welding school about 3 months ago and I did some structural welding for him on some other things on the car. Since he has seen the tig welds he wants me to go over what he has done with the mig. From what I learned at school i am concerned about the penetration of his mig weldds and I have brought this to his attention But I am wooried about contamination and poor weldds due to the flux core mig welds that are already there. Any input would be greatly appreciated. Can I just wire brush and clean or do I have to grind out all the welds to tig? There is no I.D. as to the filler content of the mig wire on the spool.This is all carbon steel material.

My 2c -

It is likely that the metallurgy of the flux core wire is OK for the application (mild steel parent metals), however lack of penetration, good fusion, and any flux or other contaminants included inside the weld beads are what could make the joints weak as they currently exist.

I could see you going over the old welds with a focused TIG arc to penetrate deep, fuse the metals together, and try to float any contaminants to the surface. If doing this I would hold a tight arc, probably with high frequency pulsing turned on (if available), try and keep a fresh tip, and use plenty of heat, to try to melt the metal deep through to the back side and wash to the surface any embedded flux or other contaminants that may be embedded. Filler rod should not necessarily be added everywhere, if the bead is sufficiently raised for strength, but have "it at the ready" for touching up anywhere the weld bead looks low, IE: where undercutting of the parent metals might occur otherwise. You should prep any easily accessible surfaces (front and back) by sanding/grinding away any old flux, spatter, oxides, or other surface contaminants down to bright shiney steel. You could even grind down any easily accessible old weld bead down flush, to be rebuilt with a nicely penetrated TIG bead, its a judgement call how far down to grind.

The other way would be to grind out *all* the old weld material with something like a cutoff wheel or Ive heard plasma cutter can work well to rid old weld bead, or perhaps just "vee" it out mostly, and then fill up the crack/vee with TIG torch and fresh filler rod. But to me it sounds like a whole lot more work, probably only worth it you find there are really stubborn contaminants inside the old weld beads that give you issues when attempting to just TIG over the top. Like if this vehicle has been driven around in the wet / rain for a while, etc. The water and dirt might absorb into the discontinuities and flux inclusions of the old weld beads.

My first suggestion to my friend was to grind out old welds but due to the amount of work and going backwards he wasn't to happy with my suggestion. I told him that was the only way I felt was a propoer approach to avoid a band aid situation. just thought some of the experts here might have a different or better idea than mine.

What are the thicknesses of the metal being welded and the size wire being used. What is max current/amps on the mig welder used? Is your friend a decent welder where he knows how to weld with a flux-core wire? What are the parts being welded?

Jakeru gave you great advice and Jim asked the questions you should know before proceeding. I commend you for your concern for you friends safety and stepping up to the plate to help him. You learned well in school!

Jakeru's method is very similar to what I've done. I always try to back chip a butt weld even when I'm not using flux core. I just did a trailer hitch swing away. I had to fill a 1/16" gap between 1/8" square tube and 3/16" cast steel. The solution was to MIG it with ER70-S6 to create the fill then grind it back and weave a tig torch over it adding ER70-S2 rod as needed. That gave me a nice well pentetrating weld into both pieces and a good supporting fill in the joint.

The metal I was working with was pretty nasty and oily so cleaning consited of a getting it bright as possible with a flap wheel then using a stainless steel cup brush to get into the tight spots. Following that I wiped it with Acetone then heated it a heat gun set at 600 degrees. When that cooled I wiped it again with Acetone. A similar method should work for you. You could also kick up the gas CFH and use a big cup to help push off the impurities. There's a lot of things you can try if you think it out.

Remember this is welding and you can always grind it out and start over. Well, almost always...

THANKS EVERYONE for your responses. I have made my mind up for HIS safety I am going to grind out all of the critical welds and tig them all. It's going to be a lot of work but I would hope a friend would do it for me and it will be great practice and I will really feel so much better about the situation.

Well I shot myself in the nuts on this one. Spent all day grinding and cleaning finally tonight start to weld and it sucks. I have no puddle control the material spits and on up hill the filler will not flow it just blobbed and dropped no matter how much torch angle. I am so disappointed in it. So I started to treat it like a dirty alum casting weld grind repeat to build myself a base. What a nightmare to cut out and replace is financially out of the question. its 120 wall 4x2 tubing MS. I ran the entire range as far as amps to try anything.He had some scrap left over so I welded that to see and not a problem perfect penetration and nice stack of dimes. Could it be contamination from the flux?

Sorry to hear your having difficulties Steve. Tough lesson to have to work thru for your friend and yourself. I responded on the topic days ago but something happened and it didn't go thru. I was going to say that flux core mig welds are not automatically or neccessarily "bad" or "inadequate" if done properly. Numerous NASCAR teams do their chassis and cage welds with GMAW exclusively. The biggest question no matter what the process is, are the welds sound. I had suggested as an alternative, a close inspection of all the weld joins, and perhaps have him do a couple sample welds on the same materials used, and subject them to some destructive testing of your own to gauge how sound are his welds are.

Most sanctioned racing classes now have regulations mandating certification including single pass only welds on the cages. That is in regard to generally 4130 material and race cars, but there is some wisdom to be gained from those reg's. On tubing such as yours there is a point to how much welding heat you want to subject the HAZ to. Without seeing the original welds it's hard to know exactly what is causing the problem your having now. Your best bet may be at this point to re MIG them instead of TIG. Best to use GMAW as opposed to FCAW, with 25/75 gas mix. Mig will better weld thru the contamination. Again hard to say without seeing. Another approach would be to plate re-inforce the joints. Probably the best way at this point short of sectioning out parts of the frame for complete re fabricating. Good luck!