Need HELP TIG welding aluminium

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I've always though of my self as a very good tig welder, but I'm having difficulty welding a simple part. I have some 1/8 aluminium plate that I bent into  C channels. I'm trying to butt weld 2 of the C channels togeather to make a rectangle tube. I'm have problems with the weld cracking at the start of the weld. I'm useing 4043 rods. The only way I have got it to not crack is to really pile on the beads which does not look real good. i need these peices to look real good. I've tried preheating the metal, up and down on the current and still no good results. I'm not sure what grade aluminium plate I have, but it's suppose to be plain ol sheet aluminium. I assumed it was 6061. I tried to weld a bead on the side of one of the channels to practice and it still cracked on the flat surface. But if a really pile a bead on it seems to not crack. Any ideas???thanksRandy
Reply:I am guessing wrong filler.  Try 5356.  I too am tigging sheet metal and the pure doesn't weld as nice as the alloyed.Other guess More gas?  Backhand or forehand?DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:My guess is you are going to crack at the end also. 5356 will do you better but most likely will not solve your problem. It is technique. A little thicker bead to start with, then when you move to the second bead look to see if a crack is developing. If so, drop a little more filler on it and then move along.Good luckBill
Reply:If it came out of sheet it more than likely is 5052 or similar, pretty sure the filler is 5183
Reply:Are you v preping the edges?Regards from sunny Cornwall__________________www.iancarey.co.uk
Reply:I'm another guy with a similar ?-I had a lot of cracking trouble with 1/8" wall 6061 square tube recently also.I was making a wire spool rack and they all looked good, but the next day the rack was falling apart!!!which flller would be reccomended for this?  4043 or 5356?
Reply:I'm have problems with the weld cracking at the start of the weld. I'm useing 4043 rods.
Reply:I did a butt weld on  1/8" X 5" X 2" sheet aluminum.  Pass with filler on one side, no filler on the other side.  The no filler side cracked the first few inches.I did the same thing with 1/4" X 1" X 8" Hot rolled. Real purdy weld with no filler, but it cracked too. Getting a digital camera next week.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:aluminium expands and (more importantly) contracts a lot when welded. solidification cracking can occur if there is insufficient metal (penetration/filler) to resist the contraction forces. any stress risers in the weld bead (poor starts/stops, insufficient filler/poor bead spacing) can become the start of a crack which will usually grow.some alloys cannot be welded without filler. aluminium has a crack sensitive structure when certain amounts of copper, silicon, and/or magnesium are present. 6000 series for example contain both Mg&Si (as magnesium silicide) within this crack sensitive range- to be welded successfully the magnesium silicide must be diluted with filler (4043- 5% Si or 5356- 5%Mg are the most common recommended). depending on material thickness suitable root gaps/preps will help. 4043 has a lower melting temp making it less prone to cracking- filler freezes slightly later, after most of the solidification shrinkage has occured. 5356 is more prone to solidification cracking but used correctly will give a less brittle and more fatigue resistant weld. 5356 is the filler to use if anodising is required (4043 turns grey). 4043 is the one to use if the part is to be used at elevated temp (above 150F and more than 3%Mg stress corrosion cracking can occur)
Reply:thank you very much hotrodder.
Reply:no problem, glad to of help
Reply:Maybe I didn’t catch it? or I am missing what’s going on.With a butt weld (w/Aluminum) after your tacks (ends & centers), shouldn’t you start your weld 3/4"-1" IN from the end of one side of the butt joint, then weld towards the outer edge.   Turn the work piece around then start to weld the other direction (overlapping the previous weld 1/4")Weld Direction:>>>>>>>>>>>>>>>>>>>>>>||Bead(|((((((((()))))))))))))))))))))))))))))))))))))))  |)Also you would want "Fall-through Penetration" right?  So then on both welds, when your start your arc you would wait to add filler rod after you see the weld puddle fall through a little-repeat the steps.never delt with cracking using this method. Fellas am I on the right  path with this?