Cracking plug welds

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'lo - building aluminum fuel tanks - 3/16" 5052 plate, using 5356 filler, and having problems with just the plug welds the top plate landing on aluminum angle.    Originally migged the plug welds - didn't do a great job, so when my new tig arrived, I went over the welds, just refloating with the tig. no filler - and it then looked like the welds are tearing apart - so I went over again, but used filler - same tears. I'm a aluminum tig welding noobie - is there a specific technique for plug welding aluminium?running 150a with pulse, 3/32" electrode and filler, 9 lph argon. Puddle looks clean/bright while welding, and I'm being careful about tapering off arc when finishing the weld - this is not crater cracking.
Reply:Your amps are too low for 3/16" alum. to start with. You probably need to be up around at least 200 amps minimum if not higher.Alum is very heat sensitive. Too much heat and you weaken the base metal. That's why you almost always need filler with alum, to offset the weakness in the weld due to the heat. With the amps too low and alum being such a great conductor of heat, you are weakening a large area around the weld. It sounds counter intuitive, but more amps equals less heat, not more in the part as a whole. The other thing is because of the excess heat, once you finally get the area hot enough to form a puddle, a large area goes molten all at once. Chances are you aren't adding enough filler to off set the large puddle.Picts would help, but My guess is a big part of your issues is simply coming from lack of practice and understanding running alum..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:ditto what DSW sez.Ditch the pulse--that limits the heat output1/8" sharp point tungsten--anything but pure, thoriated or ceriated(I'm not impressed with lanthiated)Type, model of machine? A/C frequency?Balance setting?Really good ground and ground cable for min. resistance?You also may be creating a form of stress cracking, in your effort to makethe MIG plug welds look better by washing over them with TIG. IOW--the heavy parent sheet cools down, contracts, then shears the existing weld. Solution--stop trying to do cosmetics...or if you insist--then carbidebur out some of the plug weld, TIG and add filler.     If, if you've overheated the parent sheet--that's a prescription forfuture stress cracking--in use. (Fuel tanks really aren't a learner project.)Last edited by dave powelson; 06-28-2012 at 04:04 PM.Reason: additionBlackbird
Reply:Everlast PP205 - tig/stick/plasma- running 110 hz AC freq, and 50/50 on balance- ground is excellent- using all ceriatedSince the spider cracking started, I blew away going for pretty - happy just to get to no leaks.After DSW post, I used a router to remove the effected area, and rewelded at 200A - looks decent, there is no cracking and no leaks. Thanks gents.Building boat - pic attached. 50ft Steel hull - the entire thing has been a learner project! Just new to aluminum.Norm Attached Images
Reply:a good rule of thumb is 1 amp per .001 of an inch of base metal is generally a good starting point.Dynasty 200DXPassport plus w/ spoolmate 100victor 315c oxy/(act and prop)Miller digital elitemilwaukee power tools
Reply:Originally Posted by nofacey'lo - building aluminum fuel tanks - 3/16" 5052 plate, using 5356 filler, and having problems with just the plug welds the top plate landing on aluminum angle.    Originally migged the plug welds - didn't do a great job, so when my new tig arrived, I went over the welds, just refloating with the tig. no filler - and it then looked like the welds are tearing apart - so I went over again, but used filler - same tears. I'm a aluminum tig welding noobie - is there a specific technique for plug welding aluminium?running 150a with pulse, 3/32" electrode and filler, 9 lph argon. Puddle looks clean/bright while welding, and I'm being careful about tapering off arc when finishing the weld - this is not crater cracking.
Reply:5052 is kind of crack sensitive to begin with, and although 5356 is an 'OK' filler for the 5052 it is more crack sensitive in itself than the slightly lower strength (but less ductile) 4043 filler.  If you didn't add enough filler when you made the weld and ended up with mostly the original parent material 5052 in the weld puddle then you probably had your cracking problem right there.  You have to make sure you add enough filler when you do the weld.Also, you mentioned that the plate was 5052 but didn't specifically mention what alloy the aluminum angle was.Going over the 'iffy' MIG plug welds with the TIG and not quite having enough amps to  get in and quickly melt a puddle was probably also 'iffy' as well.  The best laid schemes ... Gang oft agley ...