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Formable aluminum: Press brake bending 6061 with the 20% rule

Understanding why it works relates to four principal elements

The information in this chart reveals that the best series for press brake forming are 3xxx, 5xxx, and 6xxx.

Figure 1
The information in this chart reveals that the best series for press brake forming are 3xxx, 5xxx, and 6xxx. Series 2xxx and 7xxx are extremely strong and difficult to form. Editor’s note: This chart is for illustration purposes only; for the mechanical properties of your material, refer to material property data from the material supplier.

Basing the radius factor on the tensile strength of the aluminum in relation to cold-rolled steel, my radius factor should be around 13.5 to 15 percent of the V-die opening. This is not so; it seems that it is more like 18 percent. I have since used test pieces to get my numbers dialed in. What am I doing wrong in my figuring? I would like to know so that if we get any other material that we do not usually do, I can have some sort of a baseline for figuring the radius formed without wasting material to find it.

Answer: Your findings are correct: 18 percent of the die opening is approximately equal to the inside bend radius. This is referred to as the 20 percent rule. Note that “20 percent” is only a title; the percentages actually change with the tensile strength of the material.

  • 5052 H32 soft aluminum, the inside radius is between 11 and 13 percent of the die opening
  • Hot-rolled pickled and oiled, 14 to 16 percent
  • 6061 in “O” condition, 18 to 20 percent
  • 304 stainless steel, 20 to 22 percent
  • 60-KSI-tensile cold-rolled steel, 15 to 17 percent

The last mentioned—60-KSI cold-rolled steel—is our baseline material. For material types other than those listed, simply look up the tensile strength and estimate the radius by comparing the 16 percent for the 60-KSI cold-rolled steel.

For instance, say you’re working with a stainless material that has a tensile strength of 73 KSI. In this case, you’d divide this by the 60 KSI of the baseline material: 73/60 = 1.22. You then multiply that result by 16 percent: 1.22 × 0.16 = 0.1952, or about 20 percent—a good estimate.

You may notice, though, that when you apply this baseline comparison to 6061-O aluminum, the math gives you a different percentage than the 18 to 20 percent mentioned previously. This is because it doesn’t take as much force for the punch tip to penetrate the surface of aluminum and dig a ditch along the bend line, turning the bend sharp. To avoid this, you need to lower that force (known as the punching tonnage) by using a larger punch nose radius. This involves what’s known as the minimum bend radius, and you can read more about it by referring to the June 2015 Bending Basics column, “Bending soft, not sharp,” available at thefabricator.com.

Regardless, the results of the 20 percent rule are only an estimate, at best, but they are reasonably accurate. These percentages work well in estimating, but to uncover why they work, we need to dig a little deeper.

Before starting, ask yourself about the material you are going to use. What condition is it (alloy and temper)? What is its thickness, and what is the radius you want to produce? When you review the data for aluminum, you will focus on four areas: (1) formability; (2) thickness and bend radius; (3) tensile, yield, and elongation; and (4) how to deal with bending tempered materials.

1. Formability

This first area is the most important in determining if the material is suitable for press brake bending. Designations within the aluminum family of alloys inform you of the chemical composition of each grade. By looking at these properties you can determine which material is the correct one for the application, or at least what to expect when you form the material.

  • Series 1xxx are the aluminum alloys with 99.00 percent pure aluminum, very ductile in the annealed condition.
  • Series 2xxx are the aluminum-copper alloys. They have limited cold formability except in the annealed condition.
  • Series 3xxx are the aluminum-manganese alloys. These are ductile, have very good formability, and are commonly one of the most preferred alloys for forming applications.
  • Series 4xxx are the aluminum-silicon alloys. Silicon lowers the melting point substantially. This series is commonly used for welding wire.
  • Series 5xxx are the aluminum-magnesium alloys. These alloys exhibit a combination of high strength and formability.
  • Series 6xxx are the aluminum-magnesium-silicon alloys. They are heat-treatable, strong, and have good formability.
  • Series 7xxx are the aluminum-zinc-magnesium or copper alloys. They exhibit extremely high strength, which makes this series difficult to form.

These series are categorized further into non-heat-treatable and heat-treatable groups. Series 1, 3, 4, and 5 alloys are not heat-treatable because of the lack or presence of various alloying materials, including manganese, silicon, and magnesium. With added copper, magnesium-silicon, and zinc, the aluminum becomes heat-treatable.

2. Material Thickness and Inside Bend Radius

Just like all sheet metal, aluminum work-hardens during the forming process; that is, it gets stronger and harder by working (bending) it.

When you are forming aluminum, if the radius is sharp (small) in relation to the material thickness, you will overwork the material, making it harder, more brittle, and much more likely to fail.

Five temper conditions are shown as a hyphenated suffix following the alloy designation number:

  1. -F as manufactured
  2. -O annealed (soft)
  3. -H strain-hardened
  4. -T thermally treated
  5. -W as quenched (thermal treatments such as cryogenics)

Consider aluminum 6061-T6. Here the alloy is 6061 and the temper is T6, meaning the alloy has been thermally treated.

Material suppliers have tables that show the approximate minimum inside bend radius for the various alloys, the temper condition of each, and the minimum allowable inside radius at 90 degrees of bend angle. (For one example, check out the chart, “Approximate Minimum Radii for 90° Cold Bend,” available at www.aircraftspruce.com/pdf/2016Individual/Cat16057.pdf.)

Forcing a radius to less than the recommended minimum may cause the final part to fail in service and may void any warranties or guarantees issued by the material supplier.

3. Tensile, Yield, and Elongation

This third area has a lot to do with why your radius was found to be larger than you expected. Formability of a given aluminum alloy can be stated as the percentage of elongation, an expression of the difference between a material’s yield strength and its ultimate tensile strength.

Yield strength is a material’s ability to resist stress, and the yield point is the point at which the material deforms plastically and remains bent after forming. Until the yield point has been reached, the material will deform elastically; when it’s released from applied stress (forming pressure), the material will return to its original shape.

Ultimate tensile strength, often called simply the tensile strength, is the total amount of stress that a material can withstand. The value is developed by stretching the material to its breaking point.

Another rule of thumb states that the larger the elongation number becomes (that is, the greater the difference is between yield and tensile strength), the better the formability of the alloy is.

When you analyze the data from a material supplier, it should become quite apparent which of the aluminum alloys are best-suited for forming and which are not. For example, as shown in the chart in Figure 1, the best materials for forming on a press brake are those aluminum alloys within the 3xxx, 5xxx, and 6xxx series.

The 3xxx, 5xxx, and 6xxx series have additional digits that represent a specific alloy within the two major groupings, heat-treatable and non-heat-treatable. Following are general descriptions of the most common found in the average sheet metal shop:

  • 3003: This is the best aluminum for most applications. The alloy exhibits medium strength together with high elongation (such as 25 percent). The difference between its yield and tensile strength is the largest—a 14-KSI difference when the material is in the annealed or soft condition (O temper).
  • 5052 is a close second in formability. At an annealed temper it has an elongation factor of 20 percent and a difference between yield and tensile strength of 21.5 KSI. This has the highest strength of the non-heat-treatable grades. In the annealed condition it has better formability than the 3003 or even 1100 alloys (99 percent aluminum, very soft).
  • 6061 is the most versatile series within the heat-treatable family of alloys. In its annealed state, 6061 can be formed since the elongation is up to 18 percent and the difference between yield and tensile strengths is 10 KSI. However, as you move down the list of tempers, from annealed to T4 or T6, formability changes. This aluminum series’ ability to bend tends to decrease with temper. Bending these tempered alloys is not impossible, but it is very difficult and will most likely require larger bending radii to avoid cracking on the outside of the bend or completely fracturing at the bend line.

You should avoid using series 2xxx and 7xxx, which are extremely strong, difficult to form, and should not be considered for parts that require bending. Since they exhibit extreme strength, the forming capabilities of these series are very limited, even in the annealed condition.

4. Bending Tempered Materials

From a forming point of view, it is always best to bend your parts in an annealed state and then temper them to the correct condition. That’s the ideal, anyway. But the reality of the shop floor dictates that some parts will arrive at the press brake in less than suitable condition—something other than annealed.

To avoid cracking or breaking parts manufactured from 6061 in a T4 or T6 condition, you have two options. One, you can have a large inside radius relative to material thickness. If needed, you can perform a three-step bend. For instance, you can perform a 2-degree bend in front of the bend centerline, a 2-degree behind the bend, and then an 86-degree bend in the center.

The second option is to heat the part. Be aware, however, that aluminum does not behave like steel when heated. The heating process is the same, using an oven or torch with a rosebud tip; if you make the material hot enough, it will form like butter. But when you heat aluminum, an outer shell is created when aluminum makes contact with the air, and this causes the metal inside to melt before the outer shell does. If you are not careful, this will leave a hole in the part.

A generic temperature for forming is around 500 degrees F. Still, how hot you need to heat the material depends on the alloy. If it is a heat-treated grade and if you heat it enough to bend it, you could change the temper of the base material, so you may need to retemper it.

Another challenge with heating aluminum is that, unlike steel, it does not change color. To determine if the entire workpiece area is at the temperature you need, try this effective but crude method: Detune (that is, unfocus the flame) of your oxyacetylene torch and coat the top surface of your part (down the bend line) with black carbon soot. Heat the part from the far side and watch for the carbon to burn off.

Bending Formable Aluminum Doesn’t Have to Be Difficult

Forming aluminum does not have to be difficult, especially if the right alloys and tempers are selected for the application. There is a lot behind where and how the inside bend radii are derived and how exactly the bend is produced. Still, the simple 20 percent rule works pretty well as a rule of thumb.

In specific cases you can bend a test piece, measure the inside radius, and calculate the percentage multiplier that you can use. From that day forward, you then can rely on this information for other jobs, as long as the series and temper remain consistent.

About the Author
ASMA LLC

Steve Benson

2952 Doaks Ferry Road N.W.

Salem, OR 97301-4468

503-399-7514

Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association. He is the president of ASMA LLC and conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country.