Newbie MIG question: Butt welding without warping?

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Hi, I'm pretty new to MIG welding, and using a Hobart Handler 140. A goal that I've set for myself is to be able to butt weld 2 small pieces of 16 gauge or 18 gauge steel without warping them, and without hard clamping, but I'm finding it nearly impossible!I start with 2 pieces of 16 gauge mild steel, each about 2 inches wide, by 5 or 6 inches long. I clamp them onto a flat surface, and align them along their long edges, and tack the top and bottom.Then, I unclamp and start "subdividing" by tacking in the middle, then half again, then half again until my tacks are about 0.75 inches apart. At this point, the 2 pieces are still essentially flat. All is well.Then, I weld the first 0.75 inch long section (usually about 2 inches from the top edge, but I've tried it all kinds of ways). I get a good weld, but as the metal cools, the piece starts to warp. It both "folds" up along the welded edge, as well as "curls" in the long dimension.The more sections I weld, the more I get this effect, even if I never weld 2 adjacent sections one after the other. The more time I wait between welds, the less distortion I get, but the bending/warping seems inevitable. I've tried waiting up to 15 minutes between each 0.75 inch section, and it doesn't help much.My questions are:1. Is it possible to butt weld like this without clamping, and without introducing distortion/warping?2. If so, how?And, related:3. If I'm welding in a small replacement section for a body panel on my car, how do I cope with this sort of distortion, when hard clamping may not be possible?
Reply:Well, I´m not pretty good on this...but based on what I´ve seen on TV shows....it appears that all of them just use the "tacking method" pressing the trigger and running tack next to the next tack without runnning full beads or long beads. I asumme that will get distortion to the minimumMy Babies: HF Drill pressHF Pipe Bender3   4.5" Black and Decker angle grindersLincoln Electric PROMIG 175that´s it!
Reply:I can't think of any practical purpose for butt welding two pieces of 18g material together.2" wide by 6" long isn't very big.... not much room for the heat to go.Not to discourage you, but without a real-world reason to butt weld two 2" wide strips 6" long to each other, it might not be the best practice for you.  I would practice laying down beads on a thick piece of plate.Hopefully someone else has ideas for you.
Reply:Try this for fun:  Take one of your strips and tack it to another strip in a tee joint.  Have one leg of the tee only 1/4 inch.  Over the length only tack in three places and only on one side so that it will snap apart.  Make a pair of these.  Now set the short legs of the Tees to form a butt joint and tack together.  Do your multiple tacks and short welds.  Cool with a wet rag after each short weld.  After everything is cooled snap off the two stiffener plates.  These two plates will stop longitudinal shrinkage but will not stop angular shrinkage.  If you can create a flange or bent edge when you are joining plate it will stiffen the joint and reduce the shrinkage.  That is why often edges are flanged before welding together.  You will never defeat shrinkage but you can reduce it.  Putting temporary stiffeners on light gauge is not practical.  You can always resort to hammering and stretching the weld afterward to remove the effect of the shrinkage.
Reply:The most practical reason for trying to butt weld with car repair in-mind is that it is the easiest joint to correct with planishing.  Also gives the best finished appearance, that doesn't allow for trapping of moisture like a lap weld.What is done when fabricating or replacing panels, (22-18 g steel).  Do a couple of 3/4" tacks (copper plate behind the weld will help with the heat) grind, hammer and dolly the weld/seam, repeat tacking, grinding and planishing.   No way to avoid distortion it's a fact of life, but the copper will help, and you can correct the distortion, but do it before the bead is completed.
Reply:I'll be the first one to admit that I know nothing about bodywork and very little about welding. That said: What about welding and tacking on different sides of the joint? When welding coupons, I find that if I tack and weld on the same side I get lots of distortion. If I tack, then flip the coupons over and run a bead along the "back", I get much less distortion. Same thing if I tack both the front and the back before welding. Don't know if it's possible to get to both sides of the metal in this application, but it might be something to consider... if you're not already doing it. I think I've also read that faster travel speed will minimize the HAZ and thus the amount of distortion?