Damaging 6061-T6 by PCing

[KurtXP]

Hi All,

New to this PCing but I just finished building a 2' x 3' x 9.5' oven to powder coat an ultralight helicopter frame I built out of 6061-T6 aluminum. Unfortunately, people at Transprot Canada have told me that the PCing of 6061-T6 may damage the strength and hardness of the aluminum. Supposedly, you are not supposed to heat the aluminum (6061-T6) over 275 degrees. Now I have no real details on this, but if this is true I was wondering if I could get around this by curing the coat at a lower temp for longer? Or are there other types of powder that can cure at lower temps. I have purchased tgic super mirror powders for this job.

Thanks very much for any info on this. If I can't find a work around I've just spent a $1000 or so dollars for no good reason.

Kurt

this is one for dale..... i would say alum. is ment to be polished not coated .. but that is my 2cents ... i am not sure if the lower temp longer time will work .. i always went at 400 for 20 minutes

[bzer1]

Kurt,

Sorry to tell you, but they are in fact correct about the heating limitations of 6061-T6 aluminum. The higher temps of powder curing will cause the airframe to become brittle and potentially cause a major failure.

[Noob2PC]

Originally Posted by bzer1

Kurt,

Sorry to tell you, but they are in fact correct about the heating limitations of 6061-T6 aluminum. The higher temps of powder curing will cause the airframe to become brittle and potentially cause a major failure.

UMMM THAT WOULD BE BAD! What about the Aluminum parts for cars such as manifolds, Valve Covers, and Brackets?

[pdeinc]

Originally Posted by Noob2PC

UMMM THAT WOULD BE BAD! What about the Aluminum parts for cars such as manifolds, Valve Covers, and Brackets?

They are non-tempered components for the most part. It's the "T6" designation on the 6061 that's the issue. And reheating it does in fact relieve the temper. It doesn't become brittle, rather it becomes ductile, but most importantly, it does loose it's strength.

And I'm certainly no expert on PC, but it's unlikely a longer cure time at a lower temperature will work. Thermoset and thermoplastic resins must reach a certain temperature to flow.

[jtagger19]

you can use low cure powders that work at 250 for 15 mins
I got black and white by mistake so there should be other colors too.

[Popeye]

I though I’d pass along some information we acquired… We do a fair amount of aviation related items. For fear of dorking something up I got a hold of a metallurgist to see what we could and could not work with.
I believe the 6061 T6 is similar to the 7075 T6, I sent him an email today and asked about that specifically. The response will be posted.


His earlier information
Sorry to ask so many questions but the aluminum alloys come in a lot of flavors and their response to heat treatment is very varied. In general, 400F for 20 minutes would give you some small changes in mechanical properties. For many of the aluminum alloys the changes should be barely noticeable. A couple of alloys, however, could be a problem.

If you want to get a general text for your own reference there are a few from the American Society for Materials http://www.asminternational.org/ The "ASM Metals Reverence Book" covers just about any alloy, aluminum, steel, copper that you'd ever run across. The ASM "Aluminum: Properties and Physical Metallurgy" goes into more depth (could be overkill) and has a lot of heat treatment curves. The ASM also has a several volume set called the "Metals Handbook". The volume for general properties of aluminum would be “Volume 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials.”

First, regarding the Alloy 3003-T3. I can’t find a reference to this alloy in the T-3 condition. There are plenty of references to 3003 in the O condition (full anneal or real soft) and also in the H12, H14, H16 and H18. The H conditions refer to stain hardening…basically cold working. H12 is 1/4 hard and H18 is full hard. H18 is the equivalent of reduction by rolling mill to 75% of the original thickness. I’m not sure it is available in the T-3 condition.

If the alloy is strain hardened it can be annealed by heat-treating. Ignoring all the painful details that means the yield and ultimate strengths will drop while the ductility will go up. Typically 3003 is annealed by going to 775F with no hold time. Get to that temperature and the job is done. Obviously at lower temps you need to hold for a bit. Your 400F heat treatments are a long way from 775, so I’m confident that your 10-minute heat treatment doesn’t have much effect. Of course, if you furnace cools so slowly that the time at temperature is effectively very, very long, then you can get some annealing. If you heat-treat and cool reasonably fast you’re okay. If in doubt, heat-treat open the furnace door or pull the part out and put in on the shelf to cool.

As for the 2024 T-6, that alloy is a heat treatable alloy. Heat it up and hold for a while and the yield and ultimate strengths should go up. Conversely ductility drops. The T-6 heat treatment is usually at 365-385F for 11-13 hours. Your 400F processing is above the high end, but what the heck…you do it for 10 minutes as compared to 10 or more hours to get peak aging. I think you have almost no effect on the properties. The heat treating curves are not straight lines and are exponential, but your times are so short and the temps so close to actual heat treating conditions that you don’t make a dent in the properties.

As for 7075 T6, this is a bit tricky. Nominally the heat treatment is 245-255F for 24-28 hours. 400F for powder coating is a big overshoot and you could substantially change the properties in a few minutes. In the T-6 the yield strength is on the order of 75-80 ksi (ksi = thousands of pounds per square inch). From the limited data in my handbooks I don’t see any way this can be achieved at 400F. The alloy that starts in the T-6 almost immediately overages and softens. In fact, if you start with a 7075 part that is fully annealed (O-temper) it will harden to 75 ksi when heat-treated at 325F for only 1 hour. Past 1 hour it starts to soften to values below that of the T-6 condition.

Bottom line – 3003 and 2024 alloy…you’re okay at 400F, the kinetics of aging are very forgiving at those temps. What you’re doing now couldn’t hurt these alloys. 7075, especially for something stressed and subject to fatigue, like a spinner…I wouldn’t do any heat-treating. Even if you start with a spinner in the O temper (if you could get it) it might warp when heat-treated at 325F and be out of round (just a worry, maybe not true). Not a big problem for an instrument panel, but an out of round spinner would be exciting.

As for the Mg alloys: Can you tell me which alloy the yokes are made from? Mg is out of my normal realm of knowledge and I need to do a bit of reading and literature searching before I can even render a sensible opinion.
**
Short story...10 minutes or so at 400F will not affect Alloy 40E.

Longer story...alloy 40E is now known as aluminum alloy 712. 40E is
the old and probably a trade name (712 is also known as SAE 310).
The unified numbering system came into being a number of years back
and replaced the trade and SAE numbers. Metallurgy is a confusing
mess of trade names, SAE, government specs and the like, many of
which are still around. The unified numbering system covers all the
aluminum alloys, but most (old) people still refer to the old trade
names.

712 is used for sand casting and is usually used in the as cast
condition with no heat treatment. It has a higher melting point
than most of the other casting alloys, so it is useful in
applications where some sort of brazing assembly is required. It is
also more easily machined than some of the other casting alloys.
Downside is that it can be difficult to cast and care must be taken
to avoid cracking or defects during solidification.

712 is a natural aging alloy. The properties change during the first
few days after casting and generally stabilize after 21 days at room
temperature. Typical yield strengths are about 25000 psi. The alloy
can be furnace aged at 315F for 6-8 hours to achieve the needed
properties more quickly. 400F for 10 minutes won't do anything
significant. Maybe a slight overaging occurs, but the small reduction
in yield strength would not be detectable unless you did some sort of
sophisticated analysis...even then you might not detect any changes.


Not responsible for the accuracy of the above information

[KurtXP]

Hi Popeye,

Great info. The kind I was looking for. Eagerly awaiting the response to your query on the 6061-T6. One thing I thought though was that the 6061-T6 was a closer cousin to the 2024-T6 than the 7075-T6? If this is the case maybe the 6061-T6 could slip through on the low temp longer cure work around? I have much of the channel and pieces built from plate aluminum in 2024-T6 as well. Even so at this point I'm not sure where to get such low temp cure powders. My bottom line is that I'd like to protect the tubing from corrosion and PCing seemed a great way to do that very well. Also it seemed to me to be much more durable than most other methods of coating. The other thing is it would be quite a bit harder for a newbie to do a good job at standard spray painting. Originally I was just going to polish up the aluminum tubing and leave it un-coated. I don't know if there is a quick and easy way to coat this polished state?

Thanks again guys.

Kurt

[11111111]

i have a couple hurricanes and a j-3, what brand of copter are you building. jeff

[KurtXP]

I'm building a plans/scratch build helo called the Skytwister. Ben Showers Aero sells the plans. I chose this one because it is relatively inexpensive and is fairly easy to build. It has very little to weld and is mostly a bolt together frame. I felt this made it safer for the homebuilder. Some production models have failed due to undiscovered crack in a welded frame. Harder to preflight a welded frame. Anyway, I have a ways to go as I will be buying the main and tail rotor heads from Ben Showers Aero and a Hirth H-37 specifically designed for helos which alone is running about 12 to 13 K Canadian by the time it gets here. I've put i quite a bit of time building the frame and now I'd like to protect it for the long term. I'm still hoping to get info that says PCing will be ok for the type of alloys I'm using. There are some parts of the frame that are 4130 steel. I have assumed that the steel would be no problem for PCing but I have been told it too may suffer from warping or decreased strength. Personally this doesn't make sense to me as the welding of same doesn't seem to be a problem and it's at much higher temps. (MIG)

Kurt