Different sources recommend differing times for heat treating 356 aluminum to a T-6 temper. The first step is solution heat treating, where the casting is raised to between 980-1000F, and held there for a period of time. Several sources mention that heat treating process at this temperature only begins at 6 hours, and the generally recommended time at this temperature is between 6-12 hours, followed immediately by a hot water quench (water at boiling temperature, or slightly less). I think this first step brings the metal to a T-4 temper. Cool water can also be used, but the boiling water is suppose to minimize distortion of the part. The second step is called aging, and it brings the aluminum to a T-6 temper. A variety of temperatures and times can be used for the second stage of heat treatment, such as one hour at 400 F; five hours at 350F; and eight hours at 325F. There is no quench after the aging step. The properties of 356 aluminum are improved considerably with tempering. There can be some distortion of the parts, although I have not noticed a significant problem with the parts that I successfully treated. Treated 356 aluminum is harder, stronger, and machines more cleanly than untreated 356. That being said, my untreated 356 parts also machined and worked well, so I guess it is just a matter of how valuable you find the heat treating properties. If you don't have close temperature control during solution heat treating, you can melt your aluminum parts (been their, done that), so be careful. Edit: For my aluminum parts, I use 8 hours at 980F, followed by a cold water quench (hot water quench is too much trouble), and then one hour at 400 F. I don't temper all my aluminum parts because 356 also works well without a temper, but is a bit softer and a little bit gummy when machining untempered.
Pat I new I ad read this but it took some looking today, I had another client ask about heat treating. again thanks for sharing your knowledge with us.
When heat treating aluminum you start with forgetting everything you know about hardening steel. These are two completely different processes that don't have much in common. Heat treatable alloys (not all of them are) relies on precipitation hardening where alloying elements are allowed to precipitate out and modify the crystalline structure. The alloys act like "solid solutions", especially at temperatures closer to the melting point, where alloying elements can dissolve and diffuse. Both these two processes are temperature dependent, with elevated temperatures increasing both mobility and solubility. The first step is homogenization, the slow and uneven cooling of the casting will cause segregation of the alloying elements. This is done at temperatures reasonably close to the melting point, and as PatJ writes this is a slow process. It also doubles as the solution treatment step, if performed on pre-homogenized metal an hours soak would probably suffice. The treatment is finalized with a quench, "freezing" in the alloying elements in a supersaturated solid solution. Due to the low melting point aluminum alloys will age somewhat even at room temperatures. If it's necessary to retain an annealed state for extended periods one might have to refrigerate the parts. But for the higher tempers somewhat elevated temperatures are needed. The goal is to increase mobility without increasing solubility too much.
There's theory and practice and for home foundry guys, my advice would be to forget T6 and accept T5. Why you say? Do you really know what alloy you have? If casting with scrap, likely no, and if so, the heat treat schedule may be meaningless and furthermore, no schedule may be able to achieve a T6 condition. A couple of years ago I just started buying certified ingot because it was almost as cheap as scrap if I bought 500lbs or more at a time. I also bought a life time supply of grain modifier and refiner, but no need to worry about use of these unless you know you are using an alloy with silicon content. You'll get most of the improvement in mechanical properties with T5 as with T6. The published figures for 356 vary but comparing yield strength would be 20kpsi vs 24kpsi for a sand casting. Do you really need the extra 4kpsi? Hardness will vary about the same and although T5 doesn't machine quite as well as T6, it's much better than as cast.....not nearly as gummy. T5 is easy and practical compared to T6. It can be done in a kitchen oven. 6-8 hrs at 440F, but 4-5hrs is probably sufficient depending upon your starting point. You cant over age it and cooling is non-critical. You'll never achieve T6 in a home made gas fired furnace. Even with an electric kiln, it is very difficult because the radiant heat from the heating elements almost always elevates the castings to melting or hot short and the casting slumps and/or distorts. Even so, it's not at all unusual to have several hundred degrees F temp difference from bottom to top of the kiln, which with solution process temp of ~1000F, is enough to ruin your day. I use a sealed metal bucket with 1/4" ceramic paper insulating liner, place the castings in the bucket, and then the bucket in the kiln. This acts as a buffer, blocks radiant heat transfer, and averages the temperature variations within the kiln and due to cycling. Even with all of that, depending upon the casting shape/design, it can be hard to prevent distortion, especially when quenching, and in commercial settings, the use of heat treating fixtures is common. There are so many other things that have a more direct and pronounced affect on the properties and quality of your castings compared to the difference in temper between T5 & T6.......alloy, casting design, H2 gas porosity, iron contamination,......etc. PatJ is not around any more, but he has a picture of a whole batch of his castings slumped over in the bottom of his kiln on one of his T6 attempts. Still want to T6 your aluminum alloy castings? Best, Kelly
I need to find a USA cross reference of the alloy for the rivets I'm using at the moment but (hopefully in keeping with the topic) I'm hot soaking at 500C for 1Hr then warm water quenching. If they are left more than 15 minutes before setting they go back to being as hard as hell, if you don't do the set in one stroke they work harden instantly! I'll check today the alloy but it has really surprised me the change in properties. I use 2024 sheet which has a soak and cold quench to T3 and that is very brittle cold worked.
