Acceptable amount of bending for 6061-T6 shop fabrication 2

[sunstructures]

Hello, we have a structure that will require some connection brackets to join members that meet at odd angles. There are corner and intermediate joints where the 8" 6061-T6 channels or I-beams meet at 85 degrees, and in one location, 78 degrees. The brackets will be fabricated from excess length material by removing the flange from a channel section, and cutting to achieve an angle bracket with 3.75" and 7" legs.

Can 6061 angle bracket as described above be bent in the shop, by cold work, so to modify the 90 degree angle to an 85 degree angle? Also, can the reverse be done, to modify the 90 degrees to 95 degrees (for the opposite side connection)? Finally, will a 12 degree bend to create the 78 degree angle bracket exceed what can be done in cold work?

The steel shop manager is planning to cut and re-weld each angle bracket to the required angle. I would rather avoid the welding, as it is costly, and the shop I am working with has limited experience with Al welding.
Thanks.

 

[unclesyd]

It is going to be a little hard to bend. You are going to have to keep a minimum radius on the bends and preferably form over a radiused die.

If you can give the thickness of the material I can give you numbers for the recommended bend radii for a cold bend on a particular thickness.

 

[sunstructures]

The part to be bent is a cut section of CS8 channel. Basically, taking a 6" long section of CS8, and cutting off the bottom flange, leaving the top flange and the web connected. Then, hoping to take the top flange and the web and bending the web "inwards" so that the 90 degree angle between the flange and the web becomes an 85 degree angle. On the opposite part, the web would be bent "back" and "outwards" so that the 90 degree angle between the web and the flange becomes 95 degrees.
A CS8 has a flange thickness of 0.41 inches and a web thickness of 0.25 inches, with a fillet radius of 0.35 inches at the inside. The web would be the most desirable to bend for the needed 5 degrees of sweep inwards or outwards.
If 5 degrees is possible, can a 12 degree bend be formed in the same manner? Five additional brackets with a 12 degree sweep are needed.
Thanks.

 

[unclesyd]

I'm a little confused with your description of your bends. From this description this got a little more trickery. The radius makes it even more complicated.
If I understand correctly you want the bending of the web to twist the flange either up or down depending on whether you are bending the web the hard way or easy way.

Here is the bend data
According to ALCOA data the min. radii for 1/4 " 6061-T6 is 3t-4t and 4t-6t for 1/2".
The only problem I see is the flange want to wrinkle as you are attempting to bend it.
All the bending I've seen on channel or angle is been in guides.

 

[sunstructures]

Sorry about the confusion. I will see if I can post a diagram on the web if I cannot clarify it here.

I think you have it right. The entire flange is rotated "up" or "down" by 5 degrees by bending along a line parallel to the web and the flange. This would make an "L" shaped angle bracket with an 85 or 95 degree "L" instead of the usual 90 degree "L".

From the ALCOA min radii requirement, it appears that in the metal shop, the part will have to be clamped so that the flange remains planar (no twist). Then for a 1/4" thick web material, the bending radius will be 3t = 3 x 0.25 = 0.75" . The bend should begin at the tangent between the fillet and the web. So the bend would actually begin 0.41" + 0.35" = 0.76" from the outside edge of the web-flange joint, continuing until the 5 degree bend is acomplished.

Mainly I am concerned about creating stress cracks, and the potential for SCC or actual fracture. If I keep within the ALCOA bend radii requirement, then this should be OK, it appears.

The 12 degree bend sounds trickier.

Would cutting and rewelding the piece be easier? My shop manager seems to think so, I think he wants to work with as familiar work as he can. I'm concerned that welding a small piece will weaken the bracket unnecessarily.

Thanks again for the help.

 

[MikeHalloran]

It would be simpler to just weld two sheared plate rectangles at whatever angle you like.

The weld and the HAZ will end up in the -0 temper, so size the part accordingly or heat treat the bracket after welding.



Mike Halloran
Pembroke Pines, FL, USA

 

[sunstructures]

Welding two plates to form the angle bracket is what the steel shop manager suggested. On the few (5) 78 degree angle brackets that are needed, I gave them a welding alternate design, using 1/4" 6061-T6 Al plate and 5356 electrodes, with a 3/16" fillet weld on the inside joint of the bracket, and a beveled groove weld on the backside of the joint. Stresses are low ( ~2 ksi ), but I want a uniform stress distribution in the bracket in case they get banged around in the field (they will).

I figured the 5 degree bending on the majority of the brackets would speed up fabrication, and be cheaper overall. I need about 280 of these brackets fabricated for this project.

Am I wrong?

Thanks.

 

[unclesyd]

You might try doing a little hot forming by doing the trim work on the channel the heating the end to 400°F and immediately form it to the desired shape. Hold at temperature no more than 30 minutes. If this helps lower the temperature to 350°F and try it. If it works at this temperature you can hold the part for 8 hours with no more than 5% loss in strength. All temperatures are ± 5° F and if you err do it on the low side.

I would get me a little jig setup to do the bending before attempting to heat the channel. 6061 responds very well to hot forming in most cases.