[nothingman]

I have been tossing around the idea of powder coating my stock wheels for some time, and just recently started looking for places in my area. I noticed the place I was going to take my wheels indicated they would not coat them because they are forged aluminum. This led me to research information detailing the dangers powder coating forged aluminum wheels. I am by no means an expert in this information, so anyone with more knowledge please chime in.

Our stock 437M wheels are made from T6061 forged aluminum. Forging is the process of forcing a solid billet of aluminum between the forging dies under an extreme amount of pressure. This creates a finished product that is very dense, very strong and therefore can be very light. To increase strength, the wheels are subjected to a high temperature heat treatment between 155-200 deg C (310-400 deg F) in a furnace for several hours - a process called aging or annealing. Manufacturers optimize the annealing process for maximum strength and hardness without degradation.

When powder coating a wheel, the wheel is first cleaned and then typically sandblasted to remove the existing finish (a controversial subject itself regarding whether it weakens the wheel) and prepare it for powder coating. The coating is then typically applied electrostatically and cured in a furnace at approximately 200 deg C (400 deg F) for 20-30 minutes to allow it to flow and form a "skin" finish that is tougher than conventional paint. The concern involves the curing or baking of the coating, which continues the annealing / aging process of the forged aluminum wheel and can actually weaken it.

The three stages of the annealing process that proceed as the temperature of the material is increased are: recovery, recrystallization, and grain growth. The first stage is recovery, and it results in softening of the metal through removal of primarily linear defects called dislocations and the internal stresses they cause. Recovery occurs at the lower temperature stage of all annealing processes and before the appearance of new strain-free grains. The grain size and shape do not change. The second stage is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has completed, then grain growth (the third stage) occurs. In grain growth, the microstructure starts to coarsen and may cause the metal to lose a substantial part of its original strength.

This topic has caused a lot controversy amongst the powder coaters themselves and I found it very interesting. They particularly point out to avoid powder coating forged wheels if you are using them for racing due to the added stress you place upon them.

For every post I read saying don't do it, there were easily 10 people saying they had successfully powder coated their forged aluminum wheels without issue. I guess it's one of those things where you take your chances.

Resources:
Forged Aluminum Wheels post in powder coating forum
Heat Treatment of Aluminum Foundry Alloys
Evaluation of the Effects of Powder Coating Cure Temperatures on the Mechanical Properties of Aluminum Alloy Substrates
Powder Coating Aluminum Wheels

[Raptor911]

Good info BUT I have a question. Don't some manufacturers sell Forge wheels that come with a powder coat? How do these manufacturers avoid the same issues? Or perhaps they apply the powder coat during the initial annealing process?

[untzklee]

Good read op

[nothingman]

Originally Posted by Raptor911

Good info BUT I have a question. Don't some manufacturers sell Forge wheels that come with a powder coat? How do these manufacturers avoid the same issues? Or perhaps they apply the powder coat during the initial annealing process?

Good question. Manufacturers powder coat wheels during the initial process before the wheel has been annealed / aged the first time. It's the re-heating process the second time (when powder coating aftermarket) that changes the grain structure of the wheel and can affect it's integrity.

[chip_dip19]

The other option would be to have the wheels painted instead of powder coated

[chris719]

I thought all OEM wheels were actually painted? Or am I wrong.

[nothingman]

Originally Posted by chris719

I thought all OEM wheels were actually painted? Or am I wrong.

I tried researching this question but couldn't find a definitive answer

[mpavon]

The temperatures required for the powder coating are not very high, so in my opinion, you can bake them for 20-30 min at 200 and will not debilitate the strength of your wheels. Check the graphs for this temperatures, strength either stays the same or increases. There is no graphic for 200 degrees, but if the others ones serve as indication will be somewhere in between the 170' and 205'C at 30 min.

[Got f1?]

Originally Posted by mpavon

The temperatures required for the powder coating are not very high, so in my opinion, you can bake them for 20-30 min at 200 and will not debilitate the strength of your wheels. Check the graphs for this temperatures, strength either stays the same or increases. There is no graphic for 200 degrees, but if the others ones serve as indication will be somewhere in between the 170' and 205'C at 30 min.

I believe it is being referenced as 200 degrees C or 400 F during the powder coat that could be the issue.

[VisualEcho]

Originally Posted by mpavon

The temperatures required for the powder coating are not very high, so in my opinion, you can bake them for 20-30 min at 200 and will not debilitate the strength of your wheels.

I've been told by a few high-end powder coating facilities that it all has to do with the consistency of the heat. If the wheel is subjected to low heat, very evenly then all is well. The moment hotter air touches one part of the wheel vs another, then the issues start. So it has to do with the oven itself, how the heat comes and goes. At least that's what I've been told.

[DriverDaily]

Back in 07 when powder coating was starting to become very popular I had my OEM 350z wheels done in black. They showed hairline fractures a few week later.

It's really common
https://my350z.com/forum/wheels-and-...ed-06-rim.html

[Megator]

I had a set of cast aluminium Rotiform wheels that were powder coated and in the 10-15k km I had them they showed no cracks.

However these were heavy as shit and over engineered + not my track wheels (only used 1-2 hours on track).

I would not track powder coated wheels and I would keep a close eye on them vis a vis cracks.

[vafan13]

sorry for digging up an old thread, but this one caught my eye as I was doing my own research on powder coating and aluminum wheels. I read through the Loyola article and I'm curious how the results of their tests do anything but provide evidence that the curing process for powder coating should have no negative impact on the strength of 6061-T6. Typical curing times would be between around 1-1.5 of this tests 12-minute "cycles", and there were no negative effects until 5 cycles. Even then it looks minimal. What am I missing?


and from an old rennlist thread:

"Right now I am looking at the ASM Aluminum Handbook at a table that gives maximum permissable re-heating times for a number of different 2XXX, 6XXX and 7XXX alloys (pg. 318). These times are that for which the particular alloy will not lose more than 5% of it its strength. At the 400 degree F level, the times range from 5 minutes to one half hour (6061-T6 which has a bake time of 8 hours to peak conditions is listed as having a max re-heat time of 1/2 hour on this table). Yes, these are all wrought alloys, not the same as the cast wheels, but I think it is illustrative in that it does show that even short re-heats can have measurable effects. 5% may not be significant most of the time."


P.S. I'm also pretty sure aging and annealing are not the same thing, and it's aging that we're talking about.