Weld Strength of Aluminum 6061-T6 - Flange to Pipe Connection

[Mario1o1]

Hi all,

I am looking to determine the maximum tensile force I can put on a pipe that has a welded flange. The pipe is 7.5" OD, 3/8 Wall thickness and the flange is 1" thick with its ID matching the pipes OD and an OD of 12".

I have called out a full penetration weld that will require grooving on top for a flat surface and fillet weld on bottom with the filler material being 4943 whose yield strength will come back to 31 ksi after artificial aging.

The surface area of contact is 23.68 IN^2.

Can I simply multiply its surface area by its shear strength to determine the force required to fail?

Thanks,
Mario

 

[weldstan]

The 3/8" thickness governs the area.

 

[BUGGAR]

The Aluminum Association Design Manual covers structural design of welded aluminum. You will have a strength reduction in the heat affected zone at welds. AWS also writes codes for welding aluminum.

 

[drawoh]

Mario1o1,

7.5" OD pipe? Could this be tubing? There is no pipe rated at 7.5"[ ]OD, or that actually is 7.5"[ ]OD.

What BUGGAR is telling you is that aluminium 6061-T6 is heat treated. The material in the vicinity of your weld is the much weaker aluminium 6061-0.

--
JHG

 

[Mario1o1]

Ok I snagged a AA Design Manual.

Base Metal

Rn=FnBM*ABM
Rn = Nominal Strength
Weld Metal
Rn = Fnw*Awe

Since the this load case is in Shear both equations end up being having the same variables

FnBM= 0.6*Ftuw
ABM = Sw*Lwe


Ftuw = 24 KSI as defined by the manual

Sw = .375 as the thickness of the minimum part as a full penetration groove weld
Lwe = Effective length -> Weld parallel to shear -> perimeter = 2*Pi*r

ABM = 8.835 IN^2

FnBM = 14.4 KSI

Rn = 127,224 lbs

 

[Mario1o1]

The manual calls out ultimate strength dropping down to 24 ksi as welded. I will be artificially aging the material to bring the weld back up to 42 ksi ult. I don't exactly know what the base material will be since its probably a combination of the filler and base but the as welded number should give plenty for an FOS.

Yes it is tubing.

 

[BUGGAR]

Check AWS for appropriate electrode and filler specs.
Do you need to artificially age the material? When we weld structural aluminum structures, we design around the as-welded strength with no post treatment. How do you artificially age your parts?

 

[Tmoose]

Are the loads really purely tensile, or bending?

 

[Mario1o1]

The process requires it to be be elevated to a temperature of 350 degrees for a few hours. It will see a 55 kip tensile force. So roughly a 2.3 FOS.

The only bending that will occur is due to the bolt heads being an 1.5 inches away from the weld but the bolted connection should be stiff enough that it will act very much in tension.

 

[Sparweb]

"...The only bending that will occur is due to the bolt heads being an 1.5 inches away from the weld but the bolted connection should be stiff enough that it will act very much in tension. "

That depends on the bolt pattern doesn't it? For your assumption to be true, the bolts should be closely spaced... ...more than 12 bolts required.


STF

 

[Tmoose]

"It will see a 55 kip tensile force."

Is that loading steady state/static? Or is there some variation that will make fatigue resistance important?

 

[BUGGAR]

Tmoose, good point. What does artificial aging do to the endurance limit? Is it the same as T6 from the mill?

 

[Mario1o1]

55 kip is the maximum it will ever see and will be a rare load condition. 20 kip would be normal. What effect does heat treatment have on fatigue resistance?

 

[moltenmetal]

Our experience with welding 6061T6 has been poor at best, and I am not a welding expert- but I do know this to be an issue from experience. Be careful, as this material is known to have a hot cracking risk. The right filler metal is essential, as is using enough of it, and that choice of filler metal may affect the strength of the joint (negatively) in order to reduce the hot cracking risk. A weaker but crack-free weldment is likely to give you better service.

 

[boo1]

attached is a 6061-t6 fatigue chart in air and salt water

 

[Tmoose]

"20 kip would be normal."
Is that loading steady state/static? Or is there some variation that will make fatigue resistance important?

ESAB offers some suggestions when welding 6061

http://www.esabna.com/us/en/education/blog/how-to-avoid-cracking-in-aluminum-alloys.cfm

 

[tbuelna]

boo1- that fatigue data is for a forged 6061-T6 material. Those s-n values are much higher than you will get in the HAZ of fusion welded 6061-T6 pipe that was likely continuous cast.

By the time you apply a knockdown factor for the weld joint and allow for fatigue life, the max tensile stress you should have is probably less than 20ksi.

 

[boo1]

Fatigue life of the 6061-T6 welded is controlled by the geometry of the welding profile, whereas tensile properties rely
on the degree of microstructural transformations that occur in the HAZ.

 

[boo1]

MMPDS-04, Para 3.6.2, Figure 3.6.2.2.8. Bet-Fit S/N curves for unnotched 6061-T6 aluminum ally

 

[boo1]

see fig 1