grain direction detection in aluminium tubing 4

[Carl84]

hi all , could someone please advise me how to easily find the grain direction of aluminium tubing for bending , i am making a bike frame and have to bend some 25mm diameter tubes through 45 degrees but am experiencing cracking before i get to 45 degrees,. it is a 90mm radius bend what sort of magnification is required, hoping to purchase a microscope if its relatively inexpensive , thanks in advance
Carl

 

[EdStainless]

A tube would have to be drawn lengthwise.
This makes any bend transverse.
What wall thickness?
What alloy and temper?
A r=4d bend should be doable.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

OP
It has to do with the hardness condition of the tubing.
Material , common practice is to bend in the annealed condition, then heat treat.

 

[3DDave]

What Ed and mfgengear said.

Almost always the problem is the material alloy and temper.

I'm laying odds on 6061-T6, which is a very popular choice for availability and strength and not very good for bending. If it is 7000 series, that's worse for bending.

One source suggests a 5T radius for around 12 mm 6061-T6; the table doesn't go to 25 mm.

See this table:

https://clintonaluminum.com/wp-cont...or-90-degree-cold-bend-of-aluminum-alloys.png

 

[Carl84]

thanks for replies

ed stainless, now you put that does make sense and has jogged my memory seeing this before somewhere , i was getting confused with flat plate bending and tubes bending , no w i know.
3.25mm wall , 6063t6 which my supplier said was better suited for bending compraed to 6082t6 which i was using before

mggengear i tried annealing using the garden chimenea, i read some where i need 100 c to get a bend without cracking , i wasn't successful reaching this temp so back to the drawing board

i am puzzled by one thing , i have done half a dozen bends at room temp ( i know now this to be wrong practice ) and one was perfect , it had no orange peeling affect what's so ever on the inside or outside of the bend , all the other attempts have this and as it suggest the the material is stressed and near / on its it limit is this coect ? ,also why would this one seeamilgy good bend happen under simlilar circumstanes ?

again appreciate your help thanks
cheers Carl

 

[mfgenggear]

Op
Once upon a time , some where in Archives , pull the certs and view the
And analyze the yt, ut, and the percentage of elongation.
And the material must be annealed condition. N I w there is a tip to insert liquid rubber I believe it's been a while. To support the tube while bending. Ahh pour two part mix rubber in the ID then let set and form.
Back to the material the percentage of elongation must be a minimum to bend.
I stacks of books written by old timers
With thickness if material and minimum bend thickness for materials

 

[3DDave]

A 90 mm radius circle has a circumference of 2*90*pi.
If the material is equally on each side then the outer circumference is 2*(90+25/2)*pi

The ratio is 2*(90+25/2)*pi/2*90*pi = 1.13, or 13% elongation (helpfully the pi's cancel and the 2's cancel.

The typical elongation limit for hardened aluminum is 8-10%; check the properties sheet.

For 6082-T4 the elongation limit is around 14%, just outside the requirement.

If that is the inside bend radius the elongation required is (90+25)/90 = 27%

An elongation limit for 6082-O is around 27%; that would be annealed condition.

I expect, like for 6061, that O condition is unstable and will age harden spontaneously, so it cannot be put on a shelf and expected to remain annealed indefinitely. If you order annealed material, it may remain soft for a usable amount of time for you to bend it. The supplier should have information about that. Then it is artificially age hardened, by heating to a particular temp for a particular amount of time that varies with the alloy and desired temper.

I would recommend finding a heat treater that can do the annealing and the artificial age hardening as it is really easy for lack of experience or proper facility to produce garbage.

Typical and not specific to your case:

To anneal a work hardened aluminum alloy, the metal must be heated to somewhere between 570°F (300C) to 770°F (~400C) for a specific period of time, typically between thirty minutes and three hours. The actual time and temperature required depends on two factors: the size of the part that is being annealed and the composition of its alloy.

 

[mfgenggear]

OP
I google this
The minimum percentage of elongation required to bend tubing depends on the specific material and bend geometry, but a general guideline is around 25% for most applications, which is calculated using the formula: Elongation Required = [(OD/2)/CLR] x 100, where OD is the tube's outside diameter and CLR is the centerline radius of the bend; essentially, a larger bend radius requires less elongation.

 

[EdStainless]

Dave, the strain is actually a bit less sense there will be some ovality in the bend.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[3DDave]

Ed,

We used ball mandrels in our tube bender - any ovality was minimal. Still, yes, allowing the tube to collapse will lower the tensile strain.

 

[Carl84]

hi all, my apologies I've been unwell and burnt out this week hence the absence on this thread, i am grateful for your guidance on my problem as this has been playing on my mind for some time ,

3d dave , just to add and yes 6061 is the preferred material for bike frames, but for some reason we cant get hold of it over here in the uk and shipping from us would not be cost affective for a Fred in the shed small order .a google search has come back with a slightly higher tensile strength and tensile yield strength which is reassuring to know, a trial's bike manufacture uses the 6082 grade and these bikes are designed to be subject to high levels of stress so also good to know that im not a Guinea pig.
before i started the project
i have done some research on the what the different types of temper of of the aluminium mean but don't claim to be an expert, however i have learned how welding can seriously weaken the material around the weld area when welding aluminium, this lead me to a belt and braces approach and double the material thickness as i didn't at the time have any means of calculating what thickness i needed for the application for now, but hopefully will have someone to do this in the future for me


i have e been packing the tubes with sand and capping the ends with pieces of wood which has produced a good bend with minimal ovality, so just need to get the tubes annealed and find what radius/ angle i can work with

[(OD/2)/CLR] x 100, where OD is the tube's outside diameter and CLR is the centreline radius of the bend; essentially, a larger bend radius requires less elongation.
thanks for finding this for me edstainless, ill be honest equations are not my strong point can you please simplify this for me
i get the od clr and x100 part so half way there

here are some pics of the mcck up , i said bike but really it is a trike if we are splitting hairs. i had a slight tumble bike racing and am now a paraplegic


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1726835889/tips/seat_assembly_dd200e.IGS[/url]

 

[Compositepro]

The orange peel surface you see after bending is caused by large grains in the metal structure. Yielding occurs between grains. T6 temper is created by "solution anealing" the aluminum at about 1000F, which dissolves all the grain structure, then quenching. Grains will start to grow slowly at room temp by natural aging. Heating to 350-400F will grow the grains and increase strength. If you go hotter and longer, the grains get too large and the metal softens again. You have to go through the solution annealing and aging again to get the strength back.

Don't get confused with steel heat treatment, which

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uses quenching to get hardness and strength. Quenching aluminum is done to keep it soft.