Aluminum Workshop: How hot is too hot for aluminum?

How hot can aluminum get?

According to The Welder:

Q: I am building a low-pressure piping system from 6061-T6 and gas tungsten arc welding (GTAW) it for a customer who has designed the system. It will operate at 300 to 400 degrees F steady-state temperatures, but will spike to 600 degrees F. Which filler should I use? Someone has recommended that I avoid 5356. Can you explain why?

A: There are actually two questions here—the one you asked and the one you didn’t. So let’s answer the question you asked first. Whoever advised you not to use 5356 filler is correct. This is because any aluminum alloy that contains more than 3 percent magnesium, as 5356 does, can become sensitive to stress corrosion cracking after long-term exposure to temperatures above 150 degrees F, which is pretty low. Since significant cracking problems can occur, neither 5356 nor 5183 nor 5556 should be used if long-term temperatures above this are expected. So although it may be counterintuitive, the answer to your question is that 4043 is the correct choice for this application.

Now to the question you didn’t ask, which is whether it’s a good idea to use aluminum at service temperatures of 400 degrees F (or worse, 600 degrees). Even if the maximum temperature is 400 degrees, the answer is probably not. Just like steel, aluminum alloys become weaker as the service temperature rises. But aluminum melts at only about 1,260 degrees, so it loses about half of its strength by the time it reaches 600 degrees.

This means the strength of welded 6061-T6, which is 25 KSI at room temperature, is only about half of that (12 KSI) at 600 degrees. Even at only 350 degrees, its strength is only 17 to 18 KSI. Most codes do not give allowable stresses for aluminum alloys for service temperatures above 350 degrees. So aluminum pressure vessels and piping systems are usually restricted to a maximum service temperature of 350 degrees. Trying to use aluminum at a service temperature of 600 degrees is probably a very bad idea.

I know a few of you reading this will probably dispute this considering we use aluminum alloys for cylinder heads and cylinder blocks, where the combustion chamber gets a lot hotter than 350 degrees. While that is true, it doesn’t mean that the entire head or block gets very hot. Remember that there is cooling water is constantly flowing in the heads and block, keeping the service temperature at 200 degrees or less for the bulk of the material. It is only a very thin layer of aluminum immediately around the combustion chamber that is exposed to high temperatures, while the bulk of the aluminum stays at fairly low temperatures.

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