Aluminum wheel failure after powder coating?

[bleucamaro]

I've heard rumors of aluminum wheels failing (cracking) after being powder coated. I've heard cases of both cast and forged wheels failing. Unfortunately I do not have any alloy details. According to AMS-H-6088(SAE specification for the Heat Treatment of Aluminum Alloys) most alloys will not anneal until apprx. 650°F. Powder coatings typically cure at or below 400°F and cure for less than 1 hour. Is this heating significant enough to alter the properties of the wheel. Please include a reference if possible.

Thanks.
Dan

 

[swall]

In order to provide a sound metallurgical opinion on this, one would need to know the casting alloy and heat treat condition (i.e. T6 or whatever). Many common casting alloys, such as 356, are aged at 325F . If exposed to a powder coat cure schedule of 375-400F for 10 minutes or so, some overageing could occur and the fatigue life of the wheel could be reduced. It all depends.

 

[CoryPad]

Powder coating is the method used to coat the majority of Al wheels used in the auto industry. As swall said, overaging can occur with the wrong time/temp combination, which could reduce strength properties.

The common alloys used are 356-T6 for cast wheels and 6061-T6 for forged wheels. The common coatings are epoxy, polyester and acrylic powder coatings.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bleucamaro]

Assume a 356-T6 for cast wheels, and 6061-T6 for forged wheels, is it possible to age past these tempers? Is there a spec. or reference that shows this?

 

[NickE]

bluecamaro- yes as swall told you A356-T6 is aged at 325F. Since aging is far more sensitive to temperature than time heating it at 400+/-??? for any time coudl overage the alloy and cause a reduction in strength.

My ASM reference book gives the T6 aging temp for 6061 as:
350F. So again the treatment at 400F+/-??? could cause overaging and weakening of the material.



Nick
I love materials science!

 

[VaBeachZ]

I don't doubt NickE's response at all. I plan on doing this powdercoating to a set of Alcoa wheels.

I called them and asked and their reply was to proceed, the process would not be a problem.

However, I do admit, this was one person. Probably, if I were to call next week and get a different person, I'd probably would get a different answer.

But I do remember of stress relieving alum, 6061, to 280*F for min of 8 hours, then let it cool naturally. So, maybe 400*F would be too much.

 

[swall]

Another item that needs to be considered when powder coating aluminum is the coater's pre-treatment system. In out plant, we powder coat steel items and our cleaning stage is a caustic cleaner at 140F. I asked our tech rep about coating aluminum and he told me the caustic cleaner was not compatible with aluminum. Got to check out these little details.

 

[NickE]

note the question marks in my last post. I was trying to indicate that the temperature variation (hot spots and overall setpoint control) at most commercial powder coating places is probably not all that well monitored.

 

[bleucamaro]

Swall, When I was in the powder coat industry, the typical aluminum cleaning process was as follows: remove any grease with solvent based cleaner (MEK), remove any coatings with media blasting (thin components may recieve alternative method), outgas cast aluminum at 225°F for 45min, degrease with solvent or vapor degrease, then coat.

NickE, our oven control used a single thermostat, the oven was a convection type with furnace box on top and a large blower. heat surveys typically showed a maximum deviation of 10°F from nominal. As far as hot spotting the metal, we'd let heavier materials soak longer (up to an hour as stated in my first post) as the cure time is typically only 15 minutes once material has reached cure temp.

I guess I phrased my question wrong before, if an A356 casting is already at the T6 condition would an hour at 400°F age it even partially to another more brittle condition? which condition would this be? My ASM spec only shows it going to the T6 aging.

Also, how would 400°F for 1 hour affect forged 6061-T6? Would it cause any grain restructuring? My understanding is that strain relieving occurs pretty close to the anneal temperatures, but then again, I'm not exactly a materials wiz.


On a semi side note: I found that BBS uses both a powder coat primer and clear coat for their cast wheels.

 

[CoryPad]

bleucamaro,

ASM Handbook Volume 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials has the following information:

356-T6: aging at 305 to 315 °F for 2 to 5 hours
Yield stress = 185 MPa
Ultimate stress = 262 MPa
Fracture strain = 0.05

356-T7: aging at 435 to 445 °F for 7 to 9 hours
Yield stress = 165 MPa
Ultimate stress = 220 MPa
Fracture strain = 0.06

Based on these data, a 400 °F treatment for less than 1 hour shouldn't reduce the fatigue strength significantly.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

It is my opinion that without testing there is no way to tell if 400F for 1hour would overage the alloys in question. Also it is my opinion that there is a good chance that the alloys would become overaged and weaker.

There is no Tn-condition for overaged at the wrong temp. There are overaging treatments but they use different temperatures and different times.

The aging treatments performed to increase the strength of these alloys are precipitation treatments. These type of treatments depend on a VERY small precipitate called GammaPrime. This precipitate is coherent with the matrix and induces strain into the lattice. This precipitate is at the optimum size in the T6 condition. Coarsening of this precipitate is generally accompanied by weakening of the material.

 

[CoryPad]

Nick,

Gamma prime is Face-Centered Cubic Ni₃(Al,Ti) found in superalloys.

The strengthening precipitate in Al-Si-Mg alloys is Mg₂Si, also an FCC crystal.

I think the data I posted from the ASM Handbook can be trusted, so 400 [&deg;]F for < 1 hour shouldn't be a problem. As always, care should be exercised, and testing is prudent.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

I digress slightly, but in lean manufacturing, one would try to fine tune the powder cure cycle so that it could serve as the ageing cycle for the aluminum. I.e., you would take a solution treated wheel and skip the normal ageing cycle, relying on the powder cure cycle to age the part. Alternatively, you would start with a permanent mold cast aluminum part and skip the solution cycle and go right to the paint cure for ageing, resulting in a T5X condition. This would all have to be proven with appropriate mechanical and fatigue testing, of course.

 

[NickE]

Cory- I need a good reference for the whole precipitate systems. (If you can call it that.) I keep having trouble with the Gunier Preston zones, (Is that even close?) the Gamma Prime, and then theres the TiAl precipitate (17-7?) that TVP got on me about years ago. Can you reccommend any? I'll have to pull out the Mechanical metallurgy book, and maybe my phase X-form book, but that was more theroy.

Thanks, cory.

 

[CoryPad]

Nick,

Guinier-Preston (yes, you were close).

17-7 PH stainless steel is strengthened by Ni₃Al.

As for references:

1) ASM Handbooks, of course

2) I.J. Polmear, Metallurgy of the Light Metals, Halstead Press (John Wiley & Sons, Ltd.), 1996



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.