Questions: We're having trouble bending 6061-T6 aluminum without cracking and were wondering if you could give us some pointers?
Answer: 6061-T6 aluminum is hardened and known not to bend easily. From a bending perspective, it's always best to bend these parts in the annealed condition and then quench them to the correct condition. This is ideal anyway, but the reality is that many parts arrive at the press brake in suboptimal condition. Unfortunately, that's the life of a press brake operator.
T6 aluminum is precipitation hardening, a form of artificial aging that causes the particles within the metal to become evenly distributed throughout the metal's grain structure. Once dispersed, the particles prevent further grain shifting, thereby strengthening the metal.
To generate these precipitates, aluminum is heated by solution treatment at high temperatures and a prescribed time, and then rapidly cooled. This type of curing is typically done in an inert atmosphere under vacuum at temperatures between 900 and 1150 degrees F. Depending on the material properties, the process can take up to four hours.
General aluminum tips
When bending aluminum, remember that the smaller the inside radius of the bend, the greater the chance that the part will break. Also note that for best results and fewer tears on the outside of the fold, the fold line should cross the grain of the material or run diagonally whenever possible.
Ideally, part designers should know that 3003 and 5052 will flex on aluminum grades, while 6061 will not. This is of course a generalization as there are ways to shape 6061. Aluminum series bendability tends to decrease as you go up the hardness list, from annealed to T4 and T6. Bending these hardened alloys is not impossible, but it is very difficult and will likely require large bend radii to avoid cracking on the outside of the bend. If you're not careful, you can completely break the fold line.
Choosing the right die angle
As with air forming any other material, when casting aluminum you need to choose an appropriate mold width based on the material thickness and radius-to-thickness ratio.
If you look through a tooling catalogue, you will likely find that the number of die widths to choose from is quite large, from 0 to 0.5 inches, and each is typically cut at included angles of 88 to 90 degrees. At die widths from 0.5" to 1" you have fewer options and the included die angle ranges from 90 to 88 and 85. Between 1 and 2" your width options narrow again and the included die angle closes even more, from 78 to 73 and even less (cfillustration 1).
Why the different die angles in air bending? The larger the width of the die, the more springback you will get. Therefore, the included die angle is reduced accordingly to compress the material around the punch tip, which helps reduce springback. Overload the material. When the pressure is released, the material returns to the desired angle.
At some point the die will become too narrow and the punch will no longer have room to bend too much. Reducing the bump might be an option, but not for your .25" 6061-T6. thick, which again is notoriously difficult to shape.
Regardless, with other materials and thicknesses, hitting the bottom of the punch can be a way to "make it work". The punch descends to a "thickness less than stock" position in the die. By reducing the punch and die and applying the additional force, you can achieve the desired angle plus springback, but with significantly more forming force.
Another (and usually better) strategy is to bend a raised cube in the air (seeFigure 2🇧🇷 This die offers the necessary free space so that the punch can penetrate deeper into the die space. Embossed die angles can be very tight, up to 60 degrees in some cases.
If you still want to avoid cracking, stick to the minimum bend radius of 0.25" for 6061-T6. thick, which is quite large (see Figure 3) and avoid pronounced bending angles. Suppose you get a printout that says you need to bend a part with an outside angle of 100 degrees, that is, with an inside angle of only 80 degrees. If you bend 6061-T6 by 0.25 inch. thick, cracking can occur if bent beyond an outside angle of as little as 86 degrees. It will most likely never reach 90 degrees, let alone 100 degrees exterior angles.
In my 40+ years in the trade I have bent a lot of 6061-T6 aluminum. I avoided cracks by having a large radius relative to the thickness of the material. And sometimes I would do a three-step turn: a 2-degree turn before the centerline of the turn, a 2-degree turn after the turn, and then an 86-degree turn in the middle.
I also heated the part. I learned a "easy and messy" way to break aluminum early on, and it's one of the best tricks I know. It involves heating the piece with an oxy-fuel torch as follows:
- Turn off the acetylene torch and soot the area to be bent.
- Turn the O2 back on and hold the tip of your rosebud up to a normal flame.
- Heat the piece evenly until the black soot is gone.
This should anneal the 6061-T6 (or other "T") into a T-0 material. This makes aluminum as flexible as possible.
Remember that aluminum doesn't change color when heated, so burning becomes a real problem. Also, since aluminum comes from the factory, it develops a layer of aluminum oxide as it cools. The mill overlooks this finish because it is a natural coating that protects the aluminum from the elements during shipping and storage.
Although protective, this coating creates another problem for the self-annealed material: aluminum oxide melts at a higher temperature than encapsulated aluminum. You have to be very careful as aluminum melts from the inside out. You can cut a hole in the material before you see any visible signs of melting.
A general temperature for molding is about 500 degrees F. Note that if you heat materials enough to bend them, you can change the temper of the base material, in which case you will need to re-temper it.
Let's consider a challenging job. Let's say you're bending .25" 6061-T6 aluminum. thick at an outside angle of 100 degrees (inside angle of 80 degrees). To make it easier I would start with your 6061 material in the T-0 soft state. It would probably bend in the air with a 3-inch. Matrix port with increased profile. This shape also gives the stamp the necessary clearance and requires much less tonnage than the bottom.
When the job is complete, send the parts to be quenched, possibly arranging the formed components to reduce the chance of them warping during the quenching process.
3003 more... please
For more information on this subject, type “applying the 20% rule to 6061 aluminum” in the Manufacturer.com search bar.
Forming 6061 aluminum is not impossible, but part designers should know that it is not the ideal material for the press brake operator. Good press operators will get the job done, but would prefer a different type of aluminum. When an operator sees aluminum 5052, or better yet aluminum 3003, in printed parts, it's probably going to be a much better day.
Steve Benson is a member and past Chair of the Fabricators & Manufacturers Association International® Precision Sheet Metal Technology Council. He is President of ASMA LLC, firstname.lastname@example.org. Benson also leads the FMA certification program for precision press brakes, which is conducted at locations across the country. For more information, visit www.fmanet.org/training or call 888-394-4362. The author's latest book, Bending Basics, is available now from the FMA Bookstore at www.fmanet.org/store.