Scott,
“Can you bend to a tighter radius with heat or should you ever even bend without heat? Years ago I got a flat gear blank from Sig and had to heat it up to be able to bend it. Without heat I thought it would crack.”—Scott Richlen
You can heat aluminum to bend it—but you may not get the result you are trying to achieve. There are a few issues to consider.
First, before we talk about bending it, understand that there is the hot strength issue. The strength of aluminum starts to go away at a relatively low temperature. If I recall correctly, for aircraft the military requires that you progressively derate the handbook values of aluminum for temperatures above 140F. That’s not very hot, in a practical sense. A car hood sitting in the midday sun will get that hot. And it’s why you don’t have the forward surfaces of triple-sonic aircraft made out of aluminum. It would be like rubber as loads are applied. So, for the question at hand, heating up your gear leg will reduce the yield strength, and it will take less force to bend it--but it may still fail while you are bending it unless you have reached a hot enough temperature to cause the precipitates to go back into solution.
Second, for something like a landing gear you are going to want the high yield strength in the finished part so it can do its job without permanently deforming every time you bounce a landing. If you could anneal the material to make it easy to bend or easy to make a tight bend, then what you are left with is a soft gear. The material will work harden some as it is bent, but it will not be close to the original heat-treated properties. For aerospace, if a small bend radius is critical, they would form the part in the O-condition and then send it to a certified heat treater to have it properly brought back to the correct temper. Shops that do a lot of forming work have ovens next to the presses and form tools. They may reanneal it multiple times before achieving the final shape. Then they go back in the heat treat oven for solution HT and aging. You are not going to be able to do that in your kitchen oven. It must be done hotter, for a long period of time, and be quenched.
Third, many alloys will naturally age harden, but they only “come back” so far. Since this is not at a “solution heat treat” temperature, you’re not going to get much back.
Fourth, trying to hit a high enough temperature to cause enough annealing to make the part malleable without overheating and melting it (oxyacetylene) can be tough because the metal does not go through obvious progressive color changes like steel does. Heating too fast, in too localized and area, or on small thin parts can lead to a meltdown.
So while you can take your torch (propane or oxyacetylene) to your parts to make them easier to bend, you are not going to have an optimum result for a lightweight landing gear where strength matters. Instead, if you buy material that is exactly the alloy and temper you want, cut the blanks aligned properly with the material grain, and bend it properly using the minimum (or larger) bend radius, you should not have any problems. If you do not have access to a sheet metal brake and find the blanks are difficult to bend (ie. takes a lot of force) then secure the part in a padded jaw vise (I use a pieces of radiused hardwood). Next, clamp a piece of hardwood to the part to use for leverage. I use a piece of ¾” by 2” by 18” oak that was handy. I use it also when bending titanium gear, which takes much more force but is certainly possible.
Minimum Bend Radius for Likely Gear Applications
For .090” thick stock--
6061-T4, T6 .16” (all dimensions to inside of bend)
2024-T3 .38”
7075-T6 .50”
For .063” stock—
6061-T4, T6 .09” (all dimensions to inside of bend)
2024-T3 .22”
7075-T6 .31”
References:
1. Principles of Heat Treating of Nonferrous Alloys, ASM Handbook Vol.
2. Heat Treating of Aluminum Alloys, ASM Handbook Vol. 4, NIST 1991
https://materialsdata.nist.gov/bitstream/handle/11115/192/Heat%20Treating%20of%20Aluminum%20Alloys.pdf?sequence=3&isAllowed=y3. Minimum Recommended Bend Radius for Maximum Strength, American Machine Tools Co., Chicago IL
https://americanmachinetools.com/bend_radius.htm
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