Heat treating 6061

Hi,
I have a decent supply of 6061 for casting small projects. The problem is that it is pretty soft after casting does anyone know the recipe to heat treat alluminum or know of a good book. I aready have "Heat Treatment, Selection, and Application of Tool Steels" by William E. Bryson. The book covers tool steels very indepth, but doesn't say anything 6061.
I have an open atmosphere electric heat treat oven without fancy heat ramping controls. I read somewhere that it is to complicated to do, so leave it to the professional heat treaters. When somebody says I can't I want to do it even more. I just hope alluminum isn't like M2 because you need an expensive vacuum furnace. Can anyone help me.

I'm not sure about all aluminums, but it age hardens. Just let it sit around for about 50 years then machine it. Anybody who has ever messed with rivets can tell you, anneal them, and in about 3 days at room temp, you need to anneal them again.

If I recall, and I very well may not, you just need to cook it at a fairly low temp, 400 range? (again, its been a while) for X length, this may not apply for all series of aluminum, but it should be pretty simple low temp stuff.

50 years

The trouble with the books is they won't tell you what you want to know because 6061 was never intended to be a casting alloy. You could do some research on chemistries of the casting alloys and see if any were close to having what 6061 has in it. If there were such a thing, you could apply the heat treat specs for that casting alloy.

John

Alum is not hard to heat treat. I find it more forgiving than steel.Most alum is hardend through pricipitation hardening this can be done by natural ageing which takes about three days or through artifical ageing. Go to www.key-to-key metals for more info. If you would like let me know what temper you want for example 6061T6 and I will give you exact cook times.Dont forget to say how thick it is.

This should read www.key-to-metals. Also I just noticed the part that you are useing this for casting and as stated before it was not really intended for this purpose.

I went to the key to metals site and found you need to subscribe so I guess I will ask you. I want to treat a 3in cylinder by 4in long to T6.

My guess is that part of your problem is that your alloy is 6061 when you throw it in the pot, but not necessarily when you pour it out. Aluminum is funny that way- it takes some pretty precise chemistry and casting tricks to keep those itty bitty amounts of trace elements in the right proportions when you get it hot.

But my ASM Metals handbook says that heat treating 6061 goes as follows- somewhere between 250 degrees and 500 degrees for somewhere between 1 hour and 1 day.
The time and temp vary depending on thickness, and yield strength desired. The higher the temp, the shorter the treatment, but my guess is that 350 for a good 8 hours might work.
Commercially, they do this in solution- that is, in heated liquid.
I am sure there is a reason for this, maybe you will find out.

Hey- when I put that book away, I found another one- "Aluminum Heat Treating". I forget sometimes what I have-
Now this book is written as a factory manual, not a home shop guide- it says they usually heat treat in salt baths, and that you should water quench after your heat treat.
My guess is that this being and old book, (1960) that in industry they probably dont use salt baths any more, as OSHA would tend to frown on them. Molten salt doesnt like water much-I have a friend who is a knifemaker who uses a salt bath, and he has some pretty impressive scars from it.
Nowadays they probably have very precisely controlled hot liquids, though, with immediate cold water quenching. On bigger parts, they use high pressure cold water sprays, as just dipping forms a steam blanket around a hot piece, impeding further cooling.

Thanks a lot about the heat treating advice. The reason I want to know about the heat treating is to know everything about my particular alloy. In some of my application that I cast require a little more tensile strength. I have absolutly no problem casting it. Actually it is the best alloy I have worked with so far and I have a ready supply for free. Thanks again
Clint

ClintJ,

Heat treating 6061 is not exactly complicated. The bulk of commercial treating is done in air furnaces, no salt baths, no liquids. There is a need for fairly tight temperature control, so the commercial furnaces have forced air convection to improve uniformity and reduce heating times.

Step one: heat to 975 - 1000F and hold until the entire piece reaches 975F. In a non-circulating air furnace this could take up to three hours for a 3" thick part.

Step two: remove from heat and cold water quench. The transfer needs to be made quickly, before the surface of the part can cool below 900F. General practice is to go from furnace to water in under 15 seconds.

Step three: heat to 340 - 350F for eight hours.

This process gives the highest strength possible at the cost of reduced ductility. You can skip step three and almost double the ductility with around half the yield strength.

The last post sounds about right but I think I would use room temp water. I will look in my book tommorow if I can remember it is a USAF tech manual. I mostly do 2024 alloy so I will have to look.I will also give you a rockwell B-scale reading for 6061-T6 if you like.

Remember the need to use non conducting tongs to move part from furnace to quench. Metal tongs can draw a tremendos amount of heat out of the local area of contact.

Most of these stronger alloys are 'hypereutectic'. Long story short, they have to be solidified under special conditions or else some of the alloying metals come out of solution causing big inclusions and very poor grain structure. Not good. It will no longer be 6061.

Jimbo

how does heat treating effect the anodizability (new word...)? in my application, hard anodize type 3 is the prefered process.

Hey- I said my books were OLD-
They used to use salt baths- I have pictures.

how does heat treating effect the anodizability (new word...)? in my application, hard anodize type 3 is the prefered process.

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Heat treating shouldn't affect the "anodizability". It's still the same alloy after heat treat.

I ran a rockwell test of 6061 T6 today and got 55 on the b-scale 1/16 ball 100kg load while 7075 T6 made 83.Try and use the recepie stated before the reason I say to use slightly warm water is its supposed to prevent intergranural corrosion when doing thick peices but im sure this may take some time to set in.No more than ten seconds from furnace to quench, keep part moveing or water circulating avoid large footprint tongs.Then back in the oven for the artifical ageing.I think you will be lucky to reach T-4.I dont know what these parts would be used for but dont bet your life or saftey on them.

To to all of you that are wondering what I am doing I like to make pulsejets in various shapes, sizes and design. I don't stake my life or safty on the parts or anything.
I think my parts are a little on the soft side because if I bump them on anything they will dent.I will use most of the advice and I will give everyone a update as soon I complete my heat treat.
Thanks,
Clint

Ries said:

Hey- I said my books were OLD-
They used to use salt baths- I have pictures.

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OLD? we still use a salt bath for 6061.