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Toyman01
Toyman01 UltimaDork
9/28/13 8:50 p.m.

Your powder coating thread has cost me money. Namely a powder coating gun and an oven. I didn't realize a regular oven would cure the powder until your thread. I had it in my head you needed a higher temperature than a regular oven would reach. A little research showed me the error of my ways. I have a set of wheels I was going to have done for the princely sum of $150 each. Now I get to do them myself for about $150 for all four. I guess you actually saved me a bundle.

I ended up picking up the HF gun. It seems to do a pretty good job for what it is. It was cheap and in stock. I picked up a jar of black and white powder while I was there. No pictures of it, sorry.

Next I needed an oven. I hit the Habitat for Humanity ReStore. They had this for $50.

Naturally, I carefully measured to make sure it was wide enough for a 17" wheel. Too bad I didn't check the depth. I had to do a little modifying to the door to get enough room.

I just can't buy a new toy and leave it in the box, so I had to play with it. Then I realized I had nothing that needed coating other than the wheels. I now have a white wrench. It worked like a charm after a little testing. Maybe I should have read the directions.

I've already started stripping the wheels. The PO had spray bombed them silver on top of the original finish. I started out with some paint stripper. Next will be the sand blaster.

nicksta43
nicksta43 SuperDork
9/28/13 8:58 p.m.

I blame you too Dr. Boost...not for anything in particular...just know that I blame you.

In other news being able to powder coat at home is

Sine_Qua_Non
Sine_Qua_Non Reader
9/28/13 9:55 p.m.

Great! You are local. Give me your address and I will show up with 4 sets of wheels in needing powder coating.

Toyman01
Toyman01 UltimaDork
9/29/13 6:58 a.m.
Sine_Qua_Non wrote: Great! You are local. Give me your address and I will show up with 4 sets of wheels in needing powder coating.

Give me a chance to figure it out and we can probably do that. I'm going to have to do something different with my sandblasting setup.

Shoot me a PM.

The0retical
The0retical HalfDork
9/29/13 7:10 a.m.

Following your progress with great interest Toyman.

Just a curiosity question. I know there's been some discussion before regarding powder coating wheels. The negative effects previously discussed only apply to the cast aluminum ones due to what's basically equivalent to annealing them. Are you doing steel wheels? If not how concerning is it, to you, to powder coat the aluminum ones?

Woody
Woody MegaDork
9/29/13 9:29 a.m.

You know he had a lift thread a while back too...

DrBoost
DrBoost PowerDork
9/29/13 10:22 a.m.

Sorry Toyo, or you're welcome depending on your perspective. And the thread Woody is referring to... Don't go blaming ME if you end up with a lift in your garage..... BTW, I plan on doing what you're doing; experementing with stuff before I do something big. Somebody learn me about this annealing issue with aluminum wheels. Thats scary

DrBoost
DrBoost PowerDork
9/29/13 10:36 a.m.
nicksta43 wrote: I blame you too Dr. Boost...not for anything in particular...just know that I blame you. In other news being able to powder coat at home is

Did my wife sign in to your account somehow?

bgkast
bgkast HalfDork
9/30/13 10:36 a.m.

I also picked up a Harbor Fright powder coating gun this weekend too. $55 with tax and a tub of black powder.

Autolex
Autolex Dork
9/30/13 10:49 a.m.

doesn't powder coating aluminum wheels weaken them signifigantly?

DrBoost
DrBoost PowerDork
9/30/13 11:00 a.m.
Autolex wrote: doesn't powder coating aluminum wheels weaken them signifigantly?

From the limited research i did, no. Eastwood (the supply company, not Clint) says 400 degrees will not weaken aluminum wheels. Since I've not looked into this real in-depth, I'd love to hear from others who have.

turboswede
turboswede PowerDork
9/30/13 11:25 a.m.

Is this the thread where we blame DrBoost for things?

Um, yeah you might want to avoid the Pacific Northwest for, like, ever. Lets just say the loggers aren't happy with you and neither is PETA.

slefain
slefain UltraDork
9/30/13 11:29 a.m.

Wait, you bought a working double oven for $50? My wife has been dreaming of that setup for years and I'm trying to figure out how to pull it off. The cost for a double oven ALONE pretty much kills my project budget.

Sorry, back to powder coatings.

DrBoost
DrBoost PowerDork
9/30/13 12:14 p.m.

Slefain, you can just tell your wife that there are plenty of $50 double ovens but I've not allowed you to buy one.

benzbaronDaryn
benzbaronDaryn Dork
9/30/13 12:45 p.m.

That's a neat setup. I don't see how heating the aluminum up would weaken it, I figure when they cast/forged the rims it was over 400F. Maybe if you heated the rim then threw it in a pool of cold water or something. I should ask my old man the metallurgist.

DaveEstey
DaveEstey UltraDork
9/30/13 12:47 p.m.

Pretty sure aluminum wheels in racing conditions see similar temps from the brakes, so I wouldn't be concerned.

Toyman01
Toyman01 UltimaDork
9/30/13 1:10 p.m.

Here is a copy/past from another forum.


bleucamaro (Aerospace)
29 Mar 06 20:07 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 (Materials)
30 Mar 06 7:40 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 (Materials) 30 Mar 06 8:40 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 (Aerospace)
30 Mar 06 10:31 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 (Materials)
30 Mar 06 12:21 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 (Mechanical)
30 Mar 06 13:58 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 280F for min of 8 hours, then let it cool naturally. So, maybe 400F would be too much. swall (Materials)
30 Mar 06 14:24 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 (Materials)
30 Mar 06 14:31 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 (Aerospace)
30 Mar 06 15:24 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 (Materials) 30 Mar 06 16:01 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 (Materials)
30 Mar 06 16:07 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 (Materials) 31 Mar 06 8:26 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 °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 (Materials)
31 Mar 06 9:21 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 (Materials)
31 Mar 06 9:23 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 (Materials) 31 Mar 06 10:13 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


Toyman01
Toyman01 UltimaDork
9/30/13 1:10 p.m.

The above is from here.

http://www.eng-tips.com/viewthread.cfm?qid=151053

Toyman01
Toyman01 UltimaDork
9/30/13 1:14 p.m.

I also found this while digging.

http://www.targamiata.com/tags.php?no=8&tag=wheels

For those who want more details on the actual problem: as part of the T6 heat-treating process, these wheels are aged at 350F for about 18 hours. Powdercoating takes the wheels up to 400F for 30 minutes, which can "over-age" the alloy and make it more brittle.

DrBoost
DrBoost PowerDork
9/30/13 3:07 p.m.

Toyman, I just read the thread from eng-tips today on lunch. Heck, based on your post time, I bet we were reading that at the same time haha. I'm no longer afraid to do aluminum wheels. Thanks for posting.

Ojala
Ojala HalfDork
9/30/13 3:26 p.m.

In reply to slefain:

Seriously, find if there is ReStore near you. I bought our stove, the kitchen tile, our wood flooring, and a window ac unit for the garage there.

Ok, back on topic: Is there a recommended level of finish for items to be powdercoated? I understand the need for de-greasing and cleanliness, but how smooth or rough is best to end up with a good looking and tough powder coated wheel?

Toyman01
Toyman01 UltimaDork
9/30/13 3:29 p.m.

In reply to DrBoost:

Probably, I was eating lunch as I was reading it.

If I was coating them for an all out race car, I'd probably have some concerns. I did find some information about powder coating being able to hide cracks since it's more flexible than paint.

These wheels are going on my E24 DD. There is a slim chance it might see an autocross, but that's probably it.

Toyman01
Toyman01 UltimaDork
9/30/13 3:32 p.m.
Ojala wrote: In reply to slefain: Seriously, find if there is ReStore near you. I bought our stove, the kitchen tile, our wood flooring, and a window ac unit for the garage there. Ok, back on topic: Is there a recommended level of finish for items to be powdercoated? I understand the need for de-greasing and cleanliness, but how smooth or rough is best to end up with a good looking and tough powder coated wheel?

It probably needs to be pretty smooth. The wrench I tested, was a cheap on that came with my Chinese mill. It was fairly rough. The powder coat finish is not very smooth. It's glossy, but the grain of the cast came through.

A thicker coat of powder might help with that. I'm going to have to do some testing. Unfortunately I ended up spending most of the weekend helping my son with his car and figuring out how to get a double oven in my already packed shop.

Keith Tanner
Keith Tanner MegaDork
9/30/13 4:58 p.m.

I have powdercoated wheels on my Vanagon. I have painted wheels on my race car. That's how I feel about it

Toyman01
Toyman01 UltimaDork
9/30/13 5:12 p.m.
Keith Tanner wrote: I have powdercoated wheels on my Vanagon. I have painted wheels on my race car. That's how I feel about it

That's the direction I'm leaning.

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