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Old 12-25-2019, 02:02 PM
Sean in CT Sean in CT is offline
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Default Welding 6061 sheet with 1100 rod

Hi - building a fuel tank for my Kurtis inspired rally car. I have some 6061 sheet, 0.062 thick that i brake formed. Its only a 12 gal tank (20x17x8), very well supported. Will Tig welding with 1100 rod cause cracking? I did some test pieces which seemed to weld really well, but figured I would ask the experts before i go any farther.
Thanks
Sean


PS I was in chicago a few weeks ago the Bean is amazing!

Chicago Bean 12 5 2019.jpg
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Last edited by galooph; 12-26-2019 at 03:19 AM.
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Old 12-28-2019, 10:31 AM
crystallographic crystallographic is offline
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Quote:
Originally Posted by Sean in CT View Post
Hi - building a fuel tank for my Kurtis inspired rally car. I have some 6061 sheet, 0.062 thick that i brake formed. Its only a 12 gal tank (20x17x8), very well supported. Will Tig welding with 1100 rod cause cracking? I did some test pieces which seemed to weld really well, but figured I would ask the experts before i go any farther.
Thanks
Sean


PS I was in chicago a few weeks ago the Bean is amazing!

Attachment 54626

Hi Sean,
The 1100 will not have the strength of the 6061, but will join it without cracking, unless you work (hammer or roll) the weld seams a lot. You should be able to bump the welded seams flat and then file them off, but you won't be able to really planish them up to hardness like you would if you used slivers of parent metal or 5356 as your filler. Going frugal on this one?
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Old 12-28-2019, 11:58 AM
Marc Bourget Marc Bourget is offline
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In addition to Kent's advice, see if you can incorporate into your design, 1" or greater radius on the corners. The Lockheed factory had a shake table for fuel tanks for testing fatigue properties (FAR requirement) Square cornered tanks never passed the shake test.


FWIW
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Old 12-28-2019, 04:42 PM
Sean in CT Sean in CT is offline
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Thanks Guys - welded it up this afternoon. Not going frugal - just wanted to get it done. It seemed to weld really nicely. It is 20 x 17 x 8" Im going to weld a pair of baffles across the 17" side (divide into 3rds) so it should be very solid. It sits on the chassis above the rear axle (IRS) which gives it a rigid base. Not planning on grinding or planishing welds.
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Old 12-29-2019, 06:36 AM
Jerry Jackson Jerry Jackson is offline
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Sean

Let us know how the baffles work out. I tried to install baffles using 5356 filler on .05 inch 5052 tank material. I was very disappointed in the amount of distortion caused by welding on the flat tank surface. I welcome any suggestions on how to control that distortion.

Jerry Jackson
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Old 12-29-2019, 11:52 AM
Rick Mullin Rick Mullin is offline
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Jerry,
You are going to get some distortion if you are welding to a flat panel. I am assuming that you are making a lap joint where the baffle has a flange which is being welded to the tank surround. I would use a heat sink on the outside and clamp the tank down firmly. I use a large flat 3/8 thick steel table. The other suggestion would be that perhaps you are putting too much heat into it achieving excessive penetration. Penetration of the weld into the base metal should be about 30%. Beyond that, you are greatly changing the structure of the material at the weld and potentially creating a weakened area as well as, unnecessary distortion.
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Old 12-29-2019, 05:05 PM
Sean in CT Sean in CT is offline
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I ended up with some oil canning on the bottom after i welded the filler "neck" on the side. I am going to flange the baffles to make it all more rigid. Thinking of drilling holes and plug welding them.


I think i would have needed to roll some beads or other shape into the bottom to reduce deformation. I used aluminum blocks as heat sinks whic i think helped. Also used the Fireball Tool Mega Squares to fixture - they work great!
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Old 12-29-2019, 05:52 PM
Jerry Jackson Jerry Jackson is offline
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Rick

Thanks for the advice. My baffles used the design you described. But I took two steps that are in conflict with your advice. I am sure they contributed to my problem.

First, my clamping pressure was not high enough to prevent the base material from forming a raised bump around the fusion area. That bump lifted the baffle and then the fused material solidified with the baffle resting on top of the bump. Then the next tacks just made things worse. I should have used a clamping method that was much stiffer and located much closer to the fusion zone.

Second, I was using a steel backing plate but I placed a thin fiberglass cloth between the plate and the tank wall. My reasoning was that the cloth would thermally decouple the tank from the backing plate and would not interfere with the welding process. And that is what happened, the weld penetrated the entire wall and the weave of the cloth was imprinted onto the solidified weld.
I would have been better of omitting the cloth and allowing the cold backing plate to increase the thermal gradient in the tank, thus limiting the penetration.

Such mis-steps are an inescapable part of trying to teach yourself to weld aluminum! I will try again, following your advice!

Jerry Jackson
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Old 12-30-2019, 11:29 AM
bobadame bobadame is offline
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I suppose that the baffles don't really need to be welded to the outer skin. You could weld horizontal legs to each baffle, 4 per baffle. These legs would need to have some structure to handle the G load of the fuel, probably not more than 1 G total divided by the number of legs. The ultimate G load would depend on the ratio of the area holes in the baffle to the solid area. But disregarding that ratio and figuring 1 G total as a worst case, the load on each baffle would be nearly even fraction of the number of baffles in the tank. Please forgive the stream of semi consciousness. So legs shaped like inverted V or U should work well. Stack them in the tank and weld the ends on.
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Old 12-30-2019, 11:52 AM
sblack sblack is offline
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A friend bought a fuel tank for his bushby mustang II homebuilt airplane from the designer. It was 6061t6, probably .040 or 0.050. It was made in the traditional way with the ends pounded over a form block and the skins wrapped around and flange welded. On that airplane the tank sits on the fwd fuselage behind the panel, so basically in your lap. He finished the airplane and was doing initial engine runs. He had the tank full. After a few minutes of running the tank seams opened up completely, essentially the tank burst and 30 gal of av gas was dumped on his lap, with the engine running and electric system powered. The fuel started draining out the drain holes in the belly, right by the hot exhaust. He pulled the mixture and jumped out, waiting for 12 yrs of work to go up in flames. Luckily it did not. It was a huge mess.

He had a new tank locally made and the welder said that flange welds are no good for tanks and did butt joints instead. I have read all of Kent’s articles on tank construction and I know that flange welds have been widely used for decades on all sorts of containers. So I suspect that there was a problem with the welding or the tank design i.e. not stiff enough etc. Strange as the bushby mustang has been around for eons so you would think the design was proven.

So this us a bit off topic but an interesting story and it reminds us that you really want a fuel tank to be well built.
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