Lots of people are surprised when they hear that aluminum can be annealed. Since it’s already a soft material, why do you need to make it softer?
Actually, not all kinds of aluminum can be annealed. It’s only certain grades. In this article, I’ll go over what grades can be annealed, what you gain by annealing them, and an awesome trick that will let you get it right every time.
How to Get Annealing Right Every Time by Hand
Might as well start with what you’re probably the most interested in.
If you already know that the grade you’re working with can be annealed, and it’s worth doing, here’s how you can nail the annealing (almost) every time:
Get a Sharpie.
Yeah, that stupidly cheap little permanent marker. You’ve probably got 3 in your toolbox right now. It doesn’t matter which kind. It doesn’t even really need to be Sharpie brand (although I don’t know why you wouldn’t want to use them).
Scribble all over the piece of aluminum. Don’t worry about covering it 100%, just enough to leave a mark on every general area.
Now get out your torch. Start heating up the part. Don’t heat it up to fast, and keep the flame moving over the whole part to keep the heat even. You don’t want hot spots. It’s actually really easy to overheat and melt the material (annealing happens at a temp just a hair under the melting point), so be patient and take your time.
You’ll see the sharpie marks changing color. Then, at a certain temperature, they’ll pretty well disappear. That’s the sweet spot that you’re looking for. Quench the aluminum in water and you’re done!
The cool thing about this method is that it gives you a lot of options for heating up the aluminum, based on what you have. You can use an oxyacetylene torch, a propane torch, whatever’s on hand that can get the material up to around 775 F. This is the ultimate DIY way of annealing aluminum.
If you’re one of the strange people that doesn’t have a Sharpie in your toolbox, there are a few other ways to achieve the same result:
- If you’re using an oxyacetylene torch, light it up without a really low amount of oxygen. Then run the smoky flame across the part. It’ll cover it with black carbon. Then turn on the oxygen to a normal amount and heat the part up until the carbon disappears just like the Sharpie marks do.
- In a pinch, you can use bar soap. Just dry rub it on the surface of the part and heat it up until it turns dark brown. Not my favorite approach but it does work. I don’t know if every brand of soap works for this, but I know that Dove will do the job.
To be clear, this isn’t technically considered a full anneal. Fully annealing aluminum requires very controlled ovens and specific soak times. Partial annealing like this though will give you the ability to form it more easily without it cracking.
This technique also only works on sheet metal or very thin parts. The heat needs to soak through the metal. If the metal is too thick, the surface temperature will be correct but the core temperature will be too cold to have been annealed.
Note on carbon soot method: It will take a little bit of skill to know how thick to apply the carbon soot. How much you need to put on will change based on how thick the metal is. If it’s only around 0.020″ thick, you’ll need to put on a really light, not even fully black layer. Otherwise the soot will take too long to burn off an you’ll end up melting through the part. Try it on a practice piece a few times to get a feel for it.
If you’re working with something around 0.020″-0.030″ thick, the soot shouldn’t be totally black. You should be able to easily see the metal underneath. If it’s closer to 0.040″-0.060″ thick, the soot should applied to the surface to make it fully black.
Pro tip: If you really want to nail the temperatures, you can pick up special marking sticks that will change from opaque to clear within 1% of the intended temperature. Just look up “temperature indicating sticks”.
Applications Where Hand Annealing Works Great
There are a few areas where annealing aluminum this way makes absolute sense. Here’s a quick list of some examples:
Pipe Bending: It can be tricky to bend the aluminum tubes without them cracking, or at least turning white. Annealing them is a great way of having tube that’s retained its strength.
Tube Flattening: If you’re making something structural with tubes, you might want to flatten the ends so you can drill them to pass a bolt through securely. Annealing is a great way to get a nice clean form. I’ve done 2″ diameter tube with a 1/16″ wall, but I’m sure that you could make it quite a bit larger than that. Annealing will allow the flat to be squashed without deforming an excessive amount of the tube – the bends will be nice and crisp.
Flat Bar Bending: I’ve done this with flat bar as big as 1/2″ x 4″ quite successfully. Flat bar can be a pain to bend since it’s really prone to cracking. I definitely recommend torch annealing it for forming whenever possible.
The cool this with torch annealing is that it’s so easy to localize. Just anneal the section to be bent, you don’t need to touch the rest of it. Once it’s formed, the bent areas will likely work harden and regain a good amount of their strength, and the parts that you didn’t anneal will be untouched.
How to Anneal Aluminum with a Heat Treating Oven
Obviously, this is going to really be dependent on the grade of aluminum. That said, the basic process is generally the same. Heat it up to a certain temperature, hold it at that temperature, then cool it down. After you’re done working on it, it’s pretty easy to restore the temper of the aluminum with a little bit more heat treating.
So here are a few common grades of aluminum that you might want to heat treat, along with the oven annealing instructions:
|Anneal at 650 F, make sure it’s heated through, then air cool.
|If working from a heat treated condition, soak at 750 F for two hours then slow furnace cool. Between cold working, anneal at 650 for 2 hours then air cool.
|Anneal at 775 F, air cool.
|Anneal at 650 F then air cool.
|Anneal at 775 F, hold for 2-3 hours at temperature, then air cool.
|Anneal at 775 F for 3 hours, then cool to 500 F at a rate of 50 F per hour, then air cool. This is fussy stuff to work with.
Ok so after you anneal and finish working with the aluminum, you might want to restore the previous heat treat. If you don’t, you might be fine, or you might have some cracking later on. It just depends on how important the part you’re working on is.
I won’t get into all the different ways of heat treating every grade of aluminum and how to achieve every temper, that would easily make up an entire article of its own. Since this one is specifically about annealing, just be aware that it might be worthwhile to restore the heat treat and temper on anything that you’ve annealed.
What Annealing Does to Aluminum
Since aluminum is already such soft stuff, what can you gain from annealing it?
The main reason to anneal aluminum is to improve malleability. Any involved forming can lead to cracking or fatiguing the material, but annealing can reduce these problems.
Quick note: Avoid machining aluminum in its annealed state – it’s really sticky, and it doesn’t cut cleanly any more. It’ll be harder to get a nice surface finish that’s not all galled. It’s also reasonably likely that the material will gum up, weld itself on to your cutting tools, and cause catastophic failure that everyone else in the shop will hear. Try to re-heat treat any annealed parts before machining if possible.
If you bend aluminum cold and without annealing it, you’ll probably see that the stretched side turns kinda white. These are small little cracks that are forming on the surface of the material.
It’s usually not a big deal, but under some conditions, this might be a problem. For example, if it’s a high performance part on which you’re relying to have maximum strength, the cracking with be a big issue. Applications where the bent aluminum is exposed to vibrations can be another time when annealing makes sense. Or if it’s exposed to hot/cold (even summer and winter in Northern areas).
Whenever you’re doing some real forming with the metal, when you’re trying to stretch it, shrink it, or deep draw it, you should at least consider annealing it.
Keep in mind that you may need to anneal it several times as you’re forming it, since aluminum will work-harden and become stiff again. It can also lose the anneal over time – some grades will only hold it for an hour, some might hold it for a year. Aluminum can be funny stuff once you get to know it.
What Kinds of Aluminum can be Annealed
Here’s a list of wrought grades and information as to whether they’re practical for annealing as well:
|1xxx – Pure aluminum (99% or better), also known as electrical grade
|Non heat-treatable. Technically annealing is possible, but it’s really hard and best avoided entirely unless absolutely necessary. To be honest, this stuff is so soft that it’s really unlikely that you’ll need to anneal it unless you’re really stretching and compressing it.
|2xxx – Copper alloy
|Heat-treatable. Generally, though, this grade is susceptible to hot cracking, so don’t try annealing it unless absolutely necessary.
|3xxx – Manganese alloy
|Non heat-treatable. This is really formable stuff, and it’s common to see used in applications like cookware. You can anneal it fairly easily.
|4xxx – Silicon alloy
|Both heat-treatable and non-heat treatable, depending on the digits that follow the 4. This can have a silicon content of up to 21.5%, which makes the melting point very low. It’s commonly used as weld filler. Annealing it is a pain.
|5xxx – Magnesium alloy
|Non heat-treatable. This is common to see in sheet metal form, and it’s malleable stuff. Annealing works fine for complicated and deep forming, but to form simpler stuff it’s unlikely that you’ll need it.
|6xxx – Magnesium-silicon alloy
|Heat-treatable. This is one of the most common kinds of aluminum (especially 6061) and it anneals very well. You might need to anneal it a few times as you form it, since it’s prone to work hardening.
|7xxx – Zinc alloy
|Heat-treatable. This is a very stiff aluminum, and it’s commonly used in aerospace applications for things like wing spars. Annealing is possible, but it’s extremely tricky and best avoided, especially if you’re trying to do it by hand. It’s really prone to stress-cracking, too.
|8xxx – “other elements”
|This is a total random mix of alloying elements, and usually pretty specialized stuff. For annealing, your guess is as good as mine. See if you can get information from the supplier.
Practical Information about Aluminum Designations
Here’s an explanation of some of the extra codes you’ll find on the grade designation that will help you understand what condition the aluminum is currently in:
Basic Aluminum Heat Treat Designation:
|“As Fabricated” – no controls have been placed on the material after the forming process of casting, hot working, or cold working.
|“Annealed” – very low strength but extremely ductile.
|“Strain Hardened” – this is only for wrought products. This designation will be followed by a number which will be explained in a table below.
|“Solution Heat Treated” – You’ll almost never see this – it’s untempered and therefore unstable. The internal stresses are high and this type is usually best avoided if at all possible.
|“Solution Heat Treated” – strengthened by heat treatment, possibly strain hardened, but a temper code (described below) will explain what else might have been done to adjust the properties of the aluminum.
Aluminum Temper Codes:
|Cooled from a high-temp shaping process (like extruding) and naturally aged to a stable condition
|Cooled from a high-temp shaping process, cold worked, and naturally aged to a stable condition
|Solution heat treated, cold worked and naturally aged to a stable condition
|Solution heat treated and naturally aged to a stable condition
|Cooled from a high-temp shaping process and artificially aged
|Solution heat treated and artifically aged
|Solution heat treated and overaged
|Solution heat treated, cold worked, then artificially aged
|Solution heat treated, then artificially aged, then cold worked
|Cooled from a high-temp shaping process, cold worked, then artificially aged
Aluminum Strain Hardening Codes:
|Strain hardened then partially annealed
|Strain hardened then stabilized
|Strain hardened then painted or lacquered
Ok, that’s about everything you’ll need to know about annealing. Hopefully this helps you out.
Keep in mind that you might need to try the process out a little before you risk messing up something valuable. See if you can get a little piece of the exact aluminum you’re working on so that you can see exactly how it responds.
Any questions? Other tips? Write them in the comments!