Silicon bronze is what I used. Good reinforcement with a lot less warping than you'd get with mild steel filler. That's the advantage.
this is what i do when i'm TIG welding aluminum, Don't like the weld go back and run the puddle over it again
Use these Alignment Weld Sleeves from Hooker. They align the tubing and have a ring of fill material. Lots of sizes.
Too much heat is the enemy. As a rule of thumb the weldment material (the actual bead) is on average 10x stronger than the parent metal (what you're welding to). When steel or aluminum get over heated they become annealed (softened). Now throw lots of vibration into the mix and the softer over heated metal flexes around the edge of the weld metal which is substantially harder and Walla you have a crack. As mentioned a series of tack welds is a great way to weld thin material with a mig welder, no gap necessary as you will create more heat compensating for the gap. I prefer using the tig for headers as you can control your heat with the foot petal as you go. I usually just run out some .030" mig wire and use that for filler this works well for torch welding too. Also filling a poor weld with brazing rod to fill the holes is no good either. As mentioned before it will cause porosity and aggravation if future repair is needed (although if you have to re-weld you probably didn't do it right the first time). Going over and re-puddling your weld is also a no no. You are over heating the material and substantially weakening the weld/joint. The bottom line on welding is its a 20 minute lesson and twenty years of practice. On the contrary if it looks good it is good. In my experience one of the biggest under estimated factors is being comfortable. If you're in an uncomfortable position your going to be fighting yourself and the welder and will likely make a mess of things. Don't rush and if it gets too hot back off! Don't gap thin metal, keep it clean and throw away your flux core mig welder that you bought at the shang-hi tool company!!!
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