Aluminum tubing breaking while bending. 5

[Compositepro]

I have an application where we make 90 degree bends, 1" bend radius, in 5/16" O.D. aluminum tubing with 0.035" wall. The alloy is 6061-T6. We make several thousand parts per year. These are not for fluid flow but a non-critical structural application. I've been getting tubing through distributors at a cost of about $1.50 a foot (this always seemed high). Recently we had an urgent need and I couldn't find any distributors that had tubing in stock. I did find an extrusion company that could make it quickly and the price was only $0.25 per foot. Their minimum order was twice what I needed but that was still much cheaper than the alternatives.

However when we tried to bend the extruded tubes they snapped at very small bend angles (high yield strength, low elongation). When I contacted the extrusion company they didn't think that 6061-T6 could be bent. They suggested heat treating the tubing at 400F for one hour. This did make the tubing bendable but we still got about 5% breakage and, of course, the yield strength was reduced, but not to unacceptable levels. Also, on the tensile side of the bend I could see a pattern that looked like there were coarse grains in the aluminum.

Can anyone explain what is going on? I don't know how Alcoa makes their tubing which is bendable. I imagine that it, too, must be extruded to start with. I would really like to stay with this extruder, not only because of price but because I can also specify a minimum I.D. The extruder has suggested a T4 temper. I wonder why "6061-T6" can be so different in exactly the same shape.

 

[MikeHalloran]

6061 is not a common choice for formed parts.
Crack a book and work out the strain required at the outside of the bend. That should tell you whether 6061-T4 could work.
You were probably not _getting_ 6061-T6 before, no matter what you ordered.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Here is a link to the McMaster-Carr catalog that lists 6061-T6 tubing and describes it as bendable with a tool and flareable. I've gotten this through various distributors and it often comes printed with Aloca or Kaiser as the manufacturer and the grade of 6061-T6. I can verify that it is bendable to a 1" radius.

http://www.mcmaster.com/#catalog/117/132/=ay4afv

 

[TVP]

Compositepro,

The difference between the two products can be explained by the different manufacturing processes used. For the McMaster product, it is a drawn seamless tube, meaning that it started life as a hollow extrusion ingot (either cast hollow or pierced), then it was extruded into a tube, then it was drawn to its finished size and heat treated. ASTM B210 or MIL-T-7081 or SAE AMS-WW-T-700/6 are appropriate standards, which are listed on the McMaster page that you provided. For the extruded tube, it starts life as a direct chill cast billet, then it is hot extruded, then it is likely solution heat treated & quenched at the extrusion press (ASTM B 807).

The key difference will be in the grain size of the end products, which you have essentially confirmed by stating "on the tensile side of the bend I could see a pattern that looked like there were coarse grains in the aluminum". A proper metallographic examination would identify this and more. You might be able to work with the extruder to optimize the chemical composition, extrusion process, and heat treating parameters to more closely replicate the drawn tubes, but most likely you are stuck with an inferior product as far as cold formability in the T6 condition is concerned.

 

[MikeHalloran]

The developed length of your bend at the centerline is 1.57"
The developed length of your bend at the outer fiber is 1.82"
So the tube at the outer fiber needs to grow in length by 0.25" during the bend.
That's an elongation of 15.63 percent.
No permutation of 6061-T6xx will do that, per

http://www.alcoa.com/adip/catalog/pdf/Extruded_Alloy_6061.pdf

... but -T4 will.

When you annealed the tube, did the appearance of the tube or the markings change significantly? I'm guessing no.
I'm also guessing that whoever cuts the tube for Macarco buys -T6 and anneals it to -T4ish during their processing. Macarco wouldn't know.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Well, after a lot of searching and reading it appears that What I need is T4 temper. My understanding so far is that the tubing coming out of the extruder would be in the W temper (basically an unstable O or annealed condition). It then ages naturally at room temperature to a T-4 temper, which has a higher yield strength, due to precipitation of Mg2Si particles. This can be brought to a T6 temper by aging for 18 hours at 320F followed by 8 hours at 350F. Over aging T6 will give a T7 temper which has similar strength to T4 but the grain structure is different and it can't be brought back to T6 without first going through another solution heat treat to redissolve the precipitates. Any comments?

I'm amazed that material is sold and labeled as T6 by many distributors, when it is not. Perhaps the intent of Alcoa is that the tubing is supplied in T4 condition with the intent that it be heat treated to T6 after bending?

 

[Ron]

Agree with Mike...you probably didn't get T6 and you certainly can't do what you want with T6....T4 or possibly T5 would be more in the range.

 

[ornerynorsk]

6063-T52 will give you excellent bending charachteristics, better corrosion resistance, and better results for anodizing, if necessary. Mechanical properties slightly less than 6061-T4. Just my 2 cents.

 

[metman]

Compositepro

You wrote,

"...My understanding so far is that the tubing coming out of the extruder would be in the W temper (basically an unstable O or annealed condition)..."

You might well understand correctly but the wording leaves me in doubt.

If the tubing coming out of the extruder is quenched, then yes it would be in the W or metastable condition. Whereas if it is not quenched it would be in essentially O or annealed (not metastable). Either way it will be soft or maleable but if in the O condition it will not naturally or artificially age harden.

I hope this helps.

cheers

 

[Compositepro]

I read that 6061 alloy is not extremely sensitive to quench rate and that for small extrusions air cooling provides an adequate quench rate.