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TIG on 20ga steel weld bead problem

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  • TIG on 20ga steel weld bead problem

    I have a 185A tig welder. I have gotten pretty good with it welding sheet metal patches for car restorations. Usually butt welding 20ga mild steel at 35A. But I alway have this problem. My weld bead has a channel on each side like a little ditch. The bead is higher than the surrounding metal and the ditches lower. I metal finish this out with hammer and dolly but it always take a long time. My HAZ is about 1/2" on either side. Any suggestions on how I can change my technique to eliminate or minimize this effect?

  • #2
    thats undercut... what type of metal are you using?


    • #3
      Originally posted by bodydropcris View Post
      thats undercut... what type of metal are you using?
      I bought an 4 X 8 sheet of 20 mild steel about 8 yrs ago from a sheet metal supplier. I have forgotten the exact type. It was supposed to be suitable for automotive body repair.


      • #4
        I think a picture would help here. The only time I ever have undercut with TIG is when I'm just fusing parts together without filler. Of course the bigger the gap, the more the undercut.

        Fusing is usually good enough for sheet metal - I say usually because modern unibody cars, sheet metal is used for structural points as well.

        Easily solved by adding filler.
        In the case of 20 gauge, maybe not so easy. I'd grab some thin filler wire - maybe .023 MIG wire and probably just lay it in there. Even if it looked ugly, a 120 grit flap wheel should take it down just fine.
        Con Fuse!
        Miller Dynasty 350
        Millermatic 350P
        -Spoolmatic 30A

        Hypertherm PowerMax 1000G3
        Miller Multimatic 200 - awesome portable MIG (and stick and TIG)
        Miller Maxstar 200DX - portable TIG and stick


        • #5
          I have had that problem with tigging stainless steel but usually on one side, it would show undercut, I believe it was a difference in the types of stainless some of foreign manufacture causing the weld to undercut one side.

          I would try a different piece of sheetmetal and see what happens, maybe a damaged piece of the same material etc.
          One other thing Is if there is no back gas or purge, that can cause problems as well if your getting it a bit too hot, thin material is prone to this as the weld will penetrate all the way thru and cause it to sink in because of lack of back gas or purge.
          It hard to tell without a good picture of your situation.

          It may be as simple as the steel you are using or a coating on it etc.
          I do a lot of .025 sheetmetal to thicker .0625 material and I really dont have a problem with undercut.
          mike sr


          • #6
            try cleaning the edges where you are welding.. just run over them with a flap disk.. i dont ever see 20ga with millscale on it, but that sounds like what happens when i try and weld HR plate without grinding back the millscale, or sometimes hardened steel..