Aluminum wheel failure after powder coating?
Aluminum wheel failure after powder coating?
(OP)
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
Thanks.
Dan





RE: Aluminum wheel failure after powder coating?
RE: Aluminum wheel failure after powder coating?
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
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RE: Aluminum wheel failure after powder coating?
RE: Aluminum wheel failure after powder coating?
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!
RE: Aluminum wheel failure after powder coating?
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.
RE: Aluminum wheel failure after powder coating?
RE: Aluminum wheel failure after powder coating?
RE: Aluminum wheel failure after powder coating?
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.
RE: Aluminum wheel failure after powder coating?
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.
RE: Aluminum wheel failure after powder coating?
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.
RE: Aluminum wheel failure after powder coating?
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 °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.
RE: Aluminum wheel failure after powder coating?
RE: Aluminum wheel failure after powder coating?
Thanks, cory.
RE: Aluminum wheel failure after powder coating?
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.