7018/TIG cross-contamination?

I am welding together a roll cage, its made out of 2" OD 1026 DOM. We used 7018 to tack the tubes together while it was in the truck, now I
am using the TIG to finish welding it.
I think using the 7018 to tack was a mistake as whenever I get near the 7018 tacks the puddle goes sour and starts bubbling. Repeated grinding out and rewelding is not helping much, although I did figure out if I go to more heat and a larger filler rod and I focus on the rod in the beginning then let the droplet flow into the ground out groove I can mitigate the contamination a bit.
Anyone care to comment on this? am I correct in thinking the 7018 tacks were a bad idea? Any other tips for eliminating contamination without totally butchering the weld area with the grinder?
Would adding in a root-pass in the ground out areas with Silicon-bronze be a totally bad idea? I know in the "Old Days" they used to braze cages together, and I have had good luck with silicon bronze on dirty old sheet metal, but I dont know how it would do in a structural application...
Thanks for any advice...
-Zach
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I'm not an Ernie by any means - but with that in mind, It seems that the filler rod material doesn't want to mix with 7018. Maybe a change there might do the job. Likely it (filler rod) was chosen for the job not to mix with the stick rod. Bet you are right on ill use of 7018. Check alloys and see how they match. Assume it was clean.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
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I don't know the mechanics of it but similar things happen when you tack with 7018 and then weld over with hard wire. It can reach a problem level if you tack weld with gas shielded flux core then run submerged arc over the tacks. Run sub arc over hard wire tacks and there is not problem. I suspect it is the amount of free deoxidizers in the weld alloy. Trace amounts of silicon and aluminum are used to consume any oxides in the metal. 7018 is using a "basic" flux system which is different. My guess is that if you tacked with E 6010 there would have been much less of a problem. Try it on a scrap piece and see if there is a difference. Randy
I am welding together a roll cage, its made out of 2" OD 1026 DOM. We used 7018 to tack the tubes together while it was in the truck, now I am using the TIG to finish welding it.
I think using the 7018 to tack was a mistake as whenever I get near the 7018 tacks the puddle goes sour and starts bubbling. Repeated grinding out and rewelding is not helping much, although I did figure out if I go to more heat and a larger filler rod and I focus on the rod in the beginning then let the droplet flow into the ground out groove I can mitigate the contamination a bit.
Anyone care to comment on this? am I correct in thinking the 7018 tacks were a bad idea? Any other tips for eliminating contamination without totally butchering the weld area with the grinder?
Would adding in a root-pass in the ground out areas with Silicon-bronze be a totally bad idea? I know in the "Old Days" they used to braze cages together, and I have had good luck with silicon bronze on dirty old sheet metal, but I dont know how it would do in a structural application...
Thanks for any advice...
-Zach
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If I had to guess - and I suspect is isn't far off - The Tig metal absorbs material from the base (7018) metal leaving the latter weak and spongy. Something like amalgamation is going on.
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
R. Zimmerman wrote:

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We've had similar problems with using MIG to tack DOM roll cages. I'd suggest grinding the tacks down as far as possible and switch to a filler rod with more deoxidziers. Try some ER70S-6, it is supposed to handle contamination better. I'm no expert on these, a call to Lincoln or one of the other consumables tech support would probably give you a solid answer.
I don't have access to the ASME or AWS specs on 7018 as deposited. I did find this http://content.lincolnelectric.com/pdfs/products/msds/usm282.pdf that shows all sorts of things in the original rod that could cause some contamination issues with a pure steel base and plain steel filler rod. Not sure how much of that is left in the weld, and how much shows up in the slag or fumes.
I would NOT put in a root pass with silicon-bronze. No need to add another material that will boil out with a full steel weld (lower melting point of the silicon-bronze)
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There's nothing in the 7018 that would inherently interfere with the GTA welding that you want to use to weld over it. But as a tack weld, you might not have given the shielded metal arc weld enough time to reach the steady state where the heat of welding is sufficient to allow the flux to rise to the surface of the puddle as slag. Cold weld starts often need to be ground away to avoid porosity in finished welds. A quick tack is bound to be loaded with it. If you put in a hot tack weld, you might still have to feather the start back so as to have a tapered end onto which you'd run up your GTA weld bead. One poster suggested a 6010 or a 6011 weld for tacking. That's probably a pretty good idea since those electrodes don't have such a heavy slag potential.
Contrary to Roy's opinion for the 70S-6, I'd probably go with ER70S-2 filler metal which is optimized for GTA. The other is optimized for GMA welding. But it shouldn't matter all that much..
For optimum quality welds, either tack weld the assembly with GTA or plan on grinding away all the 7018 and re-exposing the root condition after sufficient GTA welding has secured the assembly. Use a thin grinding wheel and take your time. By all means you want to weld hot with that GTA to make sure you get good fusion. Use a tight arc so you can control your puddle and make sure that you have a flat to convex weld bead when you're done.
j

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