Welding 6061 sheet with 1100 rod

[Sean in CT]

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

[crystallographic]

Originally Posted by Sean in CT

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?

[Marc Bourget]

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

[Sean in CT]

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.

[Jerry Jackson]

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

[Rick Mullin]

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.

[BTromblay]

Originally Posted by Jerry Jackson

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

Hi,

I made a smoke oil tank for a North American F-86, this may held answer some questions, here is a link. http://allmetalshaping.com/showthread.php?t=18066

To control distortion from welding in a flat section of the tank can be done with some pre-planing. A flat sheet of material has relative low stress from the cold rolling process. The stress is uniform and that is why the panel stays flat. Welding of the edges or rosette welds to hold in bulkheads causes higher stress concentrations from several factors including, material shrinkage from the weld, re-crystallization from exceeding the anneal temp and internal material changes to crystallized structure from the high heat of welding.

It is important to isolate the high stress area of the weld, from the low stress area of the flat panel. This is done by tipping all edges several degrees 3/16" - 1/4" in from the edge. Dimple all holes that is the same size or slightly larger than your weld. The broke edge creates a mechanical lock and defines the edge from high stress to low stress. The weld is large enough that it covers the broke edge so it is not seen once welded. Because a steel weld is smaller than an aluminum weld, plan the broke edge accordingly.

Flange welded tanks have been around for many years with millions o flight hours flown on this seam design. With that being said, it is possible to have them fail due, more times than not due to bad welding. Lack of penetration, oxide development and filler rod selection all play a roll in the failure. Pressure testing should always done, once the tank is made. I fill all of my tanks with water, then use compressed air to 5 psi.

I'm welding up a tank right now, will see i I can post some pictures in the next few days.

Bill