Here's a Aluminum Tig question

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Got a little question for you guys.  100 gallon aluminum tread plate diesel tank came in for a little repair.  Sorry no pictures.  Guy bought it from scratch and dent store.  Welds looked good, except one weld bead had a crack right down the center of it.  About 4". Cleaned everything up, put about a 6" bead on it.  Overlapped existing welds.  Came out nice.  Good penetration, no porosity etc...Got a call tonight saying it cracked in the exact same spot.  My weld this time.  Right down the center.   Going to try and patch it this weekend.  I'm scratching my head at whats causing the crack.  Granted I never saw how it was mounted.  What do you guys think?Esab Caddy Mig C200iThermal Arc 186HTP Microcut 625
Reply:Sounds like a stress crack, I'm no expert by any means . I would try preheating before and maybe a little heat after the welding and slowly bring the temperature down .
Reply:Did you cut out the bad weld first?  Likely did, just thought I'd ask.Dave J.Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~ Syncro 350Invertec v250-sThermal Arc 161 and 300MM210DialarcTried being normal once, didn't take....I think it was a Tuesday.
Reply:Yeah I cut it out, put the torch to it , to burn any junk out, wiped with acetone, wire brushed, then welded.  Only thing I'm coming up with is that it's got a pretty good sized dent on one end of the tank creating a stress point at the crack.  That only gets magnified when it's bouncing down the road.Esab Caddy Mig C200iThermal Arc 186HTP Microcut 625
Reply:I would put a dollar on it being fused without filler seen it a few times on those style of tanks  welds look good but have zero structure to them. Is there any baffling in it and is where is the weld located top side bottom?
Reply:What filler did you use?Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Clean new 4043 fillerEsab Caddy Mig C200iThermal Arc 186HTP Microcut 625
Reply:No baffling, and it's on the bottom.Esab Caddy Mig C200iThermal Arc 186HTP Microcut 625
Reply:Originally Posted by jwm8830Clean new 4043 filler
Reply:I built a tank w/o baffeling and the first time I bolted it down it flexed the bottom and cracked a few spots , I cut the top off added baffeling and welded a few spots inside also and haven't had any problems.
Reply:If it's checker plate it's probally a 3000 series I'd bump up to 5356 filler 100g tank should have some baffling though that's a fair bit of weight slamming around
Reply:At a weld most aluminum is going to be annealed. The stuff is pretty soft, Outside corner welds, were you to cut out a coupon and inspect in cross section, are often very thin at the weld bead. Extra cross section, even at the cost of aesthetics, might be in order. Hot metal expands, as it cools it shrinks. Usually outside corner joints just shift toward center harmlessly as they cool. Is something like an internal baffle , or even cool weld bead adjacent to your repair restricting this necessary movement? I might try preheating not where you intend to weld, but on the extended seam past each end of the crack. If the metal at the crack isn't as hot as that surrounding past each end of the crack. Expanding this metal should open the crack leaving room for more filler. This should offer less stress as it cools. Annealed, it is pretty ductile if you can keep it in one piece until cool.
Reply:Imagine a plate with no stresses in it 2' square 1/4" aluminum. At the center of it cut with a saw a kerf 9" long. With no preheat, you weld up the kerf. The weld will at first be exactly the size as the kerf. As it is hot it is in an expanded state, it quickly falls below the 1220 degrees F turning to a solid. As it cools further to ambient temperature, it wants to shrink. At this time it must stretch, pucker the surrounding metal, or crack. Had you before welding, extended the line of your cut in the center of your plate and preheated it to expand it the cut kerf would widen. You could fit more filler inside the cut. Then as it cools it need not shrink as far.
Reply:Originally Posted by jwm8830Got a little question for you guys.  100 gallon aluminum tread plate diesel tank came in for a little repair.  Sorry no pictures.  Guy bought it from scratch and dent store.  Welds looked good, except one weld bead had a crack right down the center of it.  About 4". Cleaned everything up, put about a 6" bead on it.  Overlapped existing welds.  Came out nice.  Good penetration, no porosity etc...Got a call tonight saying it cracked in the exact same spot.  My weld this time.  Right down the center.   Going to try and patch it this weekend.  I'm scratching my head at whats causing the crack.  Granted I never saw how it was mounted.  What do you guys think?
Reply:Thanks for all the tips.  Cut out the bad spot and got a patch on it now.Esab Caddy Mig C200iThermal Arc 186HTP Microcut 625
Reply:just curious, metallurgy on this, does the properties of AL change after soaking in diesel for a long time?
Reply:Originally Posted by welding_kidjust curious, metallurgy on this, does the properties of AL change after soaking in diesel for a long time?