Welding fuel cell: aluminium or Stainless?

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Hey guys!I will be building/welding a fuel tank for one of my brothers friends who are going to use it on his Locost build.The choice of materials is narrowed down to 2mm aluminium or 1mm (0.040") stainless. He prefers stainless since he wants the option to be able run the car on alcohol based fuels in the future and wants it thin to keep the weight down.I already have 0.040" tungsten and 0.040" filler rods for SS but I'm seeing problems with welding thin SS like distortion and the need of back purging.What would you do? Best regardsErik
Reply:Look at ZTFab's (I think) Mig like Tig thread. Although you obviously aren't Miging this, he has some good pointers on welding sheet metal in that thread. Tack, Tack, Tack...
Reply:Why not use mild steel and powdercoat it?
Reply:Corrosion, Especially with alcohol. Personally ive used both materials for tanks, .040" is pretty heavy for a stainless tank, Im used to .025". But a lot of it depends on configuration and size, so more detail would be helpful. As far as avoiding distortion, its no secret, its all in geometry. Butt welds and corner welds??? Ever wonder why you dont see them much on production tanks? Flange welds avoid the warpage issue, and are the most commonly seen on light fuel and oil tanks. Geometry, geometry, geometry.-Aaron
Reply:Originally Posted by makoman1860Corrosion, Especially with alcohol. Personally ive used both materials for tanks, .040" is pretty heavy for a stainless tank, Im used to .025". But a lot of it depends on configuration and size, so more detail would be helpful. As far as avoiding distortion, its no secret, its all in geometry. Butt welds and corner welds??? Ever wonder why you dont see them much on production tanks? Flange welds avoid the warpage issue, and are the most commonly seen on light fuel and oil tanks. Geometry, geometry, geometry.-Aaron
Reply:Originally Posted by EjdepalmThanks for the replies!I will try to make a CAD drawing tomorrow and post it up.Did you weld your tanks with enough penetration to justify backpurging?
Reply:Yes full penetration, thats a given. Tanks are very dynamic, and poor penetration or poor bead profile ( no atmospheric protection ) is just begging to form cracks with either material. www.tinmantech.com has a few nice tank pictures to show construction ideas. Aluminum has been the material of choice for years, and with the proper alloy should be no issue with alcohol based fuels. Avoid the "cube" tank syndrome, put some thought and geometry into it and you should be fine. Heck a riveted together aluminum tank with the seams brazed up would work well too. Just think of the aerobatic pilots, and all the forces their tanks go through. .080" is not a bad thickness for something thats going to see a pounding. History has shown, as far as aluminum tanks go, thicker material from a softer alloy holds up better to heavy dynamic loading.
Reply:I'm not sure of the exact reason but the Coast Guard really frowns on stainless steel fuel tanks.  There must be a reason 99% of metal fuel tanks on small boats are aluminum.  If you make it thin, it will vibrate and crack.  I think that is a primary reason for the lean towards aluminum.  Stainless tanks will always be thinner than aluminum tanks.
Reply:Originally Posted by makoman1860 As far as avoiding distortion, its no secret, its all in geometry. Butt welds and corner welds??? Ever wonder why you dont see them much on production tanks? Flange welds avoid the warpage issue, and are the most commonly seen on light fuel and oil tanks. Geometry, geometry, geometry.-Aaron
Reply:Originally Posted by clearchrisWhat is a flange weld?  Is that like a lap weld, with one piece offset so that the interior has a flat surface?  Thx.  BTW, I googled, didn't find anything.
Reply:Thanks!  I could see how that could prevent warping at the outer edge, while the hidden seam can twist.  I'd assume for tanks you would want all flange seams running vertical so they don't trap any fluid.Would it be acceptable to also weld the outer edge for applications that require a flat, unbroken surface, or would that be defeating the purpose?
Reply:The flange goes towards the outside typically. And they dont have to be as tall as the ones shown, many times the flange is melted down almost flush.  Yes welding it on both sides for this application would not be ideal.
Reply:Originally Posted by makoman1860more detail would be helpful.