aluminium motorcycle wheels - pc'ing cause failure?

[LeeParsons]

Although I have never heard of or read about it happening I supose it could happen. I'm with you guys on getting all the information available. At the very least it would be a good idea to make that information available to your customers and have some sort of waiver if they still want them powdered.

[sdold]

I've read about it happening but can't find info on the temps at which it occurs. Thanks for the links, I'll take a look tomorrow night, I'm too tired from driving right now.

I spent a couple of summers ago experimenting with heat treating 2xxx alloy rivets, and testing them after various quenchings, non-quenchings, etc. It was interesting. The critical temp to get them to do what I wanted was 930 degrees, but I suppose something could happen at a lower temp. I just don't know. After treating they needed to be quenched and put in a freezer to hold the treated state. After taking them out and driving them, they were suddenly way harder, it was extremely cool to see it happen. If they were allowed to cool slowly in stead of quenching, they would harden and come up to strength (but be very hard to drive), but I've read that weird things happen that would cause intergrannular corrosion later on from the big grains that form during slow cooling. It's apparently not something that would show up right away or always be seen in a strength test. That's what I read anyway. But none of my books discuss other than the optimum treating temps.

I don't pretend to know much about this, so I thought I'd ask if anyone knew of a source of info. I don't plan to PC any critical structural parts but it would be nice to know anyway, just for the sake of knowledge. I don't THINK the PC temps are a problem, but I don't know why I think that. Thanks for the links and info, I'll be doing a lot of reading soon

Steve

[sswee]

From this link supplied by Caswell:

http://naca.larc.nasa.gov/reports/1956/naca-tm-1419/naca-tm-1419.pdf

As I understand, of the three grades tested, 2014, 6061-T6, and 7075-T6, there should be no problems in curing PC on Al at elevated temps of 300 to 400 degrees. The decrease in tensil and yeild strength was substantial after 100 to 200 hrs but they said the results were cumilative over time. So unless a person was to coat a piece many times, the degradation of strength should not be a concern.

Is this the way you all understand it? Or am I missing something? I'm old enough to know interpretation is different to each person and would like to know if someone has a different interpretation than I?

[Popeye]

I posted the following a while back.... and it seems fitting to post again.
__________________________________________________ ______________

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.

"Hi Larry,

I'm a little concerned about heat treating
6061-T6 at 400°F. I'm trying to find more data,
but what little I have suggests that the alloy
might overage quickly. The result would be a
drop in ductility and some drop in strength. Let
me do some more digging and I'll get back to you.

Hi Larry,

Wanted to get back to you about the 6061-T6. I
looked at some of the heat treating curves for
this alloy. At 400°F the reaction rates are
pretty quick. Starting with the T-4 condition
full hardness (T-6) can be reached in less than
an hour. If you're already in the T-6 condition
the alloy would overage quickly and you could see
a measurable change in mechanical properties.
The big effect would be a loss of ductility.

Things are much better at 340-350°F. If you can
process the coatings at those temperatures the
alloy would tolerate close to an hour. For
example if you start in the T-4 condition a
vendor would normally heat treat for 6-10 hours
at 350°F to get it in the T-6. Sticking another
40 minutes or so onto the time at temperature for
your powder coating wouldn't result in much of a
change.

As for doing the frame of a helicopter...I
wouldn't sign off on the drawings for that one
regardless of the heat treatment temperature.
Complicated stresses, need to accept an
accidental overload, etc. Too much liability on
that job. I would suggest epoxy painting and
leaving the metallurgy alone.

Lenny"

[Popeye]

A good source for material properties.

http://www.matweb.com/search/SearchSubcat.asp

[getastro]

I think that some Japan motorcycle companies already pc their wheels at the factory.

[mtnbiker]

To simpify some and not get too techy... it looks like the problems occur with the higher end wheels when you coat them. They are made of a much lighter alloy/differant alloy than stock and are also a lot thinner than stock wheels.

[PowderVisions]

When putting an alloy wheel in the cure cycle, remember that there is usually more than one cure cycle you may use. Instead of putting the part in the oven for say 12-15 minutes at 400 degrees, try putting the part in the oven for 30 minutes at 350 degrees. It will cure the same.

[rx7carl]

This is what I was looking for. Ive read about horror stories of wheel failures. Now I understand that there are low temp curing powders out there which would be safer on stressed aluminum parts. Thats probably how wheel mfg's do it. They cure at 250 degrees IIRC.

[KZjim]

I am interested in the low-cure powders.... does caswell stock them? if not who?