Bending 6061-T6

[sheafromme]

Hi all,
I inherited a sheet metal part design that has some tabs bent on it that allow tensioned stabilizing lines (basically guy wires, but for a dock) to loop through as a tie-off point. I've gathered that the previous designer was very weight-conscious so specced the part as .188" 6061-T6 with .5" radii on the bends. However, I know 6061-T6 is finicky to bend and I heard from our supplier that often the bending process can affect the hardness/strength of the material in the bend region which is exactly where the rope loops through... I have been proposing switching to .25" 5052-H32 but have gotten pushback about the weight increase so I'm wondering two things and wanted to run it by you all: 1. Does bending 6061-T6, even with a huge radius, reduce the strength significantly. 2. Does switching to 5052-H32 as I propose at least sound reasonable? Back of the envelope the lower yield is made up for by the greater material thickness and I believe it'd be cheaper anyways, as well as avoiding any potential fab issues. I'm a mech-e but not usually involved in structural; just often called in to review other parts of the work at my company...
Thanks!
-Shea

 

[drawoh]

sheafromme,

Sheet metal shops in North American like 5052[‑]H32. This is somewhat weaker than 6061[‑]T6, but not a whole lot. Just what are your safety factors? Aluminium 6061[‑]T6 is stronger than 5052[‑]H32 only if the bends have not cracked.

You can order and bend 6061[‑]0[ ](annealed) and then send it out for heat treatment.

Instead of bending the 6061[‑]T6, you can machine your part out of billet, complete with inside corner radii. If your structure is that nasty and critical, I suggest the substantially stronger 7075[‑]T6.

You need your inspector to verify that you have the right material. Vickers hardness anyone?

How critical really is light weight? Is it worth all the hassle? I am in favour of your 1/4" 5052[‑]H32.

--
JHG

 

[Compositepro]

Bend the 6061 before heat treating to T6.

 

[sheafromme]

@drawoh Thank you for the response! 5052-H32 really does seem to be the bread and butter of most sheet metal shops I've dealt with, I've even had some quote flats specced as 6061-T6 in 5052 without even asking if it was acceptable However, if there really is no cracking (and that was somehow guaranteed) is it true that there isn't any other weakening of the 6061-T6 from bending it? Safety factors for intended loading are fairly conservative, but that also relies on the installer not over-tightening the assembly and it's also a single point of failure so one bad part could be critical...

As for the weight, I honestly feel pretty strongly that it should not be a critical factor; increasing from .188 to .250 is really only a 33% increase which, for a singular part of a large assembly, doesn't really seem worth fretting over.

To both you and @Compositepro, unfortunately the parts are actually fairly large since they fulfill more than just the tiedown purpose but those other features are in the flat, so the heat treat would be prohibitively expensive. This also would preclude the machining side of it, unfortunately, as it'd be a huge cnc job.

 

[drawoh]

sheafromme,

Look for charts on aluminium bend radii. According to the one I found, aluminium 6061[‑]T6 requires a bend radius of five to seven times the thickness, in your case, .94 to 1.31[ ]inches. Is this acceptable?

Sheet metal shops bend with a tool that controls the radius. They can do weird radii by bending "in air". I am not sure what this means, but it is less accurate.

I saw a 19"[ ]rack box made from 2mm or 1.6mm 2024[‑]T4. It probably was hacked together in an airplane hangar. The cut-out corners were beautifully stress relieved, and they used quite large bend radii. Most of the edges looked like they were done with a nibbler. It wasn't pretty.

If you must have an edge feature on your part, can you screw something on? If you weld anything to aluminium 6061[‑]T6, it will be weak. If you are working to small safety factors, you had better have a well controlled work process. I hope somebody is going structural analysis.

Light weight design is like styling. Emphasize to your boss that if the thing is to be pretty or lightweight, it is going to cost more.

--
JHG

 

[dvd]

Also see this Eng-Tips post-

Bending of Aluminum Sheet

 

[SnTMan]

I inherited a sheet metal part design...
I know 6061-T6 is finicky to bend and I heard from our supplier that often the bending process can affect the hardness/strength of the material...

Well I'm curious. Have you been having trouble with these parts in manufacturing or in service? If not why change?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[MintJulep]

Tensioned stabilizing lines (basically guy wires, but for a dock) to loop through as a tie-off point.

Load path (with potentially 0 - load cycle) going through a feature that's prone to cracking during fabrication.

Worth reevaluating.

 

[rb1957]

why would anyone be weight conscious for something to go on a dock ? (as mentioned above)

a 1/2" bend rad in 3/16" thick sheet seems very tight ... possibly form in O condition and heat treat (to T62) ? (as suggested above)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[btrueblood]

My thoughts exactly, rb. Unless there are so many of them, on a floating dock, that somebody is worried that they would sink the dock?

 

[IFRs]

The Aluminum Association "Aluminum Design Manual" Table 3-1 gives minimum bend radii for 90 degree cold bends. For 6061-T6 in 3/16" thickness they recommend 3T or in your case 9/16". I'm not too surprised the shop could use 1/2" and not get visible cracking. It was probably the closest die they had to that size. Working the metal may induce some strain hardening. You could do some NDE to see if there is any interior cracking or hardening of the outer surface. The Aluminum Association publishes many extremely useful and authoratative documents well worth acquiring.

 

[3DDave]

3T probably depends on the grain direction from rolling - with the grain vs against the grain. Not knowing the grain, it's possible to get a tighter bend without cracking and not appreciate why.

 

[IFRs]

3DDave has an excellent point.

Additionally, note 1 under the table reads: "The radii listed are the minimum recommended for bending sheets and plates without fracturing in a standard press with air bend dies. Other types of bending operations may require larger radii or permit smaller radii. The minimum permissible radii will also vary with the design and condition of the tooling." They do not clarify if they are bending with or against the grain, which is a very important piece of information to leave out.

My advice is to try it both ways (with and against the grain), do NDE to validate the result, increase the radius as needed rather than the metal thickness, evaluate the risk of metal failure if it comes to you harder than T6. You might also check T651 or other tempers to see if the additional processing helps your situation.