I'll try to make this short. A friend has a 4" FP FMIC. He cracked it, so he had a local place weld it. They hit it with a propane torch and said it was easier to weld that way then welded it. 2 weeks later it cracks on the other side. Brings it elsewhere and they did the same with the propane torch. I'm guessing they are cooking the impurities out of it??
He rewelded the entire thing! And guess what, it cracked yet AGAIN.So he asked if I could try. Figured why not. It's junk now anyway. I looked and the crack went through both welds and is about 10-12" long!
I lay it out and wire brush and acetone it. I heated it up with the torch because I am on 110v and can only do 125amps. Figured preheating would help me out. I cranked up the argon to like 30cfh, used 3/32 (red and tried blue) and used 1/16th 4043 filler. It took a few seconds for a puddle to forum but it did and I dipped and moved on. A few places seemed to weld like it was cast. Just didn't want to weld very good at all.
Anyway I get a bead and take the torch away and my brand new shiny bead has a crack right down the center!

Ok Let me try again on the other end of the crack. No matter what this thing was hot cracking. As soon as I was done welding my new weld was cracked from start to finish. How/Why does this happen and how do you prevent it?

Bastarddsm, - Yep it's joes. It's back on his car now until he gets a new one.

Did you by any chance drill a hole at each end of the crack? From the sound of it this defiantly requires drilling, as should any crack, then grinding the problem area out and starting fresh. Mainly due to the metal being attempted to be repaired multiple times and who knows the kind of impurities that were left in the metal.

Any pictures ?

Sorry no pics.

I was going to mention stop drilling the crack too. But either way, it shouldn't immediately crack once welding is finished. I don't know enough about alumnum to know whats going on though.

It could be cracking because it was too hot and crystallized, or the aluminum is junk from being welded so many times and crystallized. You should use 4043 3/32 wire on it. If you use 5356 wire on it, I could believe it cracked. It has a tendency to crack. The thicker wire can help it not crack because the puddle won't be as hot.

Might be time to have someone who knows how to actually weld try it?

92awdgsx said:

It could be cracking because it was too hot and crystallized, or the aluminum is junk from being welded so many times and crystallized. You should use 4043 3/32 wire on it. If you use 5356 wire on it, I could believe it cracked. It has a tendency to crack. The thicker wire can help it not crack because the puddle won't be as hot.

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I was using 1/16th 4043. I've heard nothing good about these fp cores. Even if I did pin it on each end that still doesn't make sense on why the new weld would of cracked right away.

Where is Glenn? I know he would have the answer/

Burnett03 said:

Might be time to have someone who knows how to actually weld try it?

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Anytime you decide to not be an asshole then I'll take your comments seriously. And it was offered to me, so why wouldn't I try it?? It's going to be mine when he gets his new one anyway.

I feel confident that it's just very contaminated aluminum and should be ground out where the crack is. You don't need to grind everything away to the point of having a gap though. A little trick would be usinging a cutoff wheel and run it down the crack but not all the way through the material. Next run your bead and watch to see if any of the contaminants "float" to the top of the weld and if so stop and grind those out. This will help get the impurities out and give you the best chance at getting a solid bead down. Alway remember it should usually take you longer to prep the piece you're going to weld then it does to actually weld it out.

I cleaned the sh*t out of it with acetone and wire brush.

Still anyone know why the last two places would heat it with a propane torch? I doubt it's to cook the impurities to the top, the tig torch can do that much faster.

Preheating does a few things like reduce the cooling rate of the metal, reduce shrinking stress and weld distortion, helps the metal fuse, and removes any moisture from the material. Although if you overheat some metals you can risk cracking it as it cools. On top of that even though you cleaned it the best you could there is a good chance that oils were "absorbed" into the aluminum. This is very apparent it you ever have the luxury of fixing someones cracked oil pan. When you add heat and melt the metal you'll see the impurities float to the top and if they are bad enough it almost seems like they reject any metal you add to that spot.

If you were using 1/16 filler and not feeding it enough, it would get too hot and possibly crack. It's hard to explain.