aluminum weld breaking question

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Hello, im in dire need of help.I did a bunch of welding on an aluminum dump truck box with my Miller 30A spool gun.When I was done welding,15% of my finished welds made a huge bang and cracked right down the middle of the weld.I used 4043 and 5056 and both cracked, can anyone help me to correct this problem,Tomorrow or Sunday I need to go back and repair the welds that cracked.I have a Miller Trailblazer 302 and I am running it between 18-24 Volt depending on the various thicknesses of the material.I am in the spray arc transfer mode and am using 100% Argon set at 25-30PSI because of wind.All welds look mint but something is wrong.Please help.Sorry for long explanation.
Reply:A photo of the welds may be helpful in determining the problem.And what is the history on this, a brand new box or repairs on an old box?If a repair, I guess the original welds had problems also?What is the base metal type?Sounds like the welding produced alot of residual tensile stress in the welds, probably because the material is fairly thick and the joints are highly restrained? Off the top of my head I wonder if the weld beads are too wide and shallow, concave maybe, which could make them susceptable to centerline cracking.  Seems like 4043 or 5356 should not be a problem, and 4043 in particular should be very ductile and forgiving to stresses from a constrained weld joint.Aluminum is very susceptable to porosity and oxidation, you mentioned welding in high wind, could the cracking be related to porosity or oxidized welds?
Reply:I think Pulser is onto the answer. It sounds like the piece got too much stress added into it with the weld, and then tore itself apart. As a suggestion, I would grind down clean the weld as best you can. Then, use multiple 1-1/2" welds to get it super tacked in place. Be sure to stagger the welds, one here, one way over there, and fill in as you go. Once you have a good dose of these on, give the sheetmetal some light massaging with a soft mallet, leather would be my first choice, rubber my second choice. Use a wood block on the backside of the areas you are tapping. After you have done this, the material should be a bit relaxed. You could then fill in all the extra weld areas. The whole block thing is for flat areas, for corners, I would just tap from the outside  with not as much concern for the backer.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Preheat with a weed burner.   I would.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:Pics would help a whole bunch here. If it's heavy plate preheat it and you might even want to post heat it as well. What's the ambient air temp like? That could be a lot of the problem as in cooling too fast. I have even seen this happen on steel with both 7018 and 7024 when it is really cold out and the base metal is heavy.The difference between art and craft is the quality of the workmanship. I am an artist.
Reply:Of course pics would be great, but...My suspision is that it could hae been welded with a lot of heat.  With aluminum, if you weld it too hot (or too slow), the magnesium content is depleted.  This causes it to loose ductility, and become brittle.  Think about what it's like trying to fuse aluminum without filler.  The reason it cracks is because the loss of magnesium.  Your filler metals have a higher concentration of magnesium than the base material.  That allows the finished weld to still have an acceptable amount of it, retaining its ductile properties.At that point, it could come down to what Pulser said...  the normal stresses in the weldment then cause the weld to crack.Therefore, I'd look at three things:1)  Heat (amperage) input2)  Welding speed3)  Amount of filler added to the weld.