Is powder coating really making wheels weaker?

never heard that before and I dont believe that little heat will affect anything.

What I have heard is that with powder, it can hide cracks under the powder that would be seen if it was painted. And I have also heard that is B.S. so I don't know which to believe, I will stick with paint.

 

Some magnesium alloys have melting points as low as 660 F compared to about 865 F for some aluminum alloys. Melting points of pure aluminum or magnesium are similar at 1220 F and 1200 F respectively.

None of those would seem to be low enough to be bothered by powder coating, but magnesium alloy wheels would be more suspect than aluminum alloy wheels. You can set magnesium alloy wheels on fire from sliding friction, but it is much more difficult to do so with aluminum alloy wheels.

 

this...

besides...I have been racing superbikes and cars for years on powder coated wheels.

 

I don't think that they will melt but to know the answer we need to look up is the annealing temperature for the material that the wheel is made from. For every metal there is a minimum annealing temperature. Exceed that temp, even for the briefest of moments, and the metal becomes softer.

I have never looked up the temp for aluminum so I can't say for sure if it has an impact.

 

This is absolutely the case. Not only does this need to be factored in but the "stupid" factor of your vendor. We all think we use the best guys but do we really know who is turning on the oven? When was the last time it was calibrated? Did jose hear 250*F and set it to 350*F? This bad stuff just happens and you will never know.

 

We heat treat aluminum every day and we also powder coat some aluminum parts, so we've looked into whether powder coating temperatures can affect/alter the temper. The alloys we use have a two step heat treat. Powder coating temps are lower than the initial heat treat step, but quite a bit higher than the second step. However, the time is short and it doesn't affect the strength of our parts.

If the powder coating temperature was continued for several hours, depending on the alloy and temperature, there might be some annealing, or, less likely, it could even make the aluminum harder (not necessarily good as it becomes more brittle).

The alloys used for wheels are different from what we use, but probably are not significantly affected by proper powder coating. The very best way to tell would be to do a hardness test on the wheel before coating and then again after. But probably not needed; my 430 came to me with powder coated wheels and I'm not worried.

 

IMHO just steer clear of it for older magnesium wheels such as Campagnolos.

I decided NOT to use it on race mag wheels for the track car which are modern because it was suggested to me that a thin coat of paint that doesn't hide early signs of cracking is a better idea.

Interesting and informative posts in this thread. Thanks guys.

 

OK, I took a moment and found this on the A to Z of materials site. They are talking about aluminum and aluminum alloys. There is a long section on the subject but this should address the issue. As FBB pointed out 250 would be fine but if Jose hears 350 your wheels will be softer than they started.

"Annealing is carried out in the range 300-410°C depending on the alloy. Heating times at temperature vary from 0.5 to 3 hours, conditional on the size of the load and the alloy type. Generally, the time need not be longer than that required to stabilize the load at temperature. Rate of cooling after annealing is not critical."


Found this for magnesium:

"Annealing. Wrought magnesium alloys in various conditions of strain hardening or temper can be annealed by being heated at 290 to 455°C (550 to 850°F), depending on alloy, for one or more hours. This procedure usually will provide a product with the maximum anneal that is practical."

 

No danger of "melting" any of the aluminum alloys ---- powder coating cure temps are not nearly high enough.

As for annealing, and exceeding the annealing temp, different temp ranges exist for different alloys and different types of annealing processes. The general range for the typical (as any that would be used for a car wheel) aluminum alloys is --- 250F to 985F. Also, bear in mind that the material generally needs to be held at those elevated temps for sustained periods of time --- much longer than would occur with powder coating processes.

However, it should be understood regardless that annealing does NOT weaken aluminum alloys or magnesium alloys --- rather it actually INCREASES temper / strength --- that is precisely why it is done.

Therefore, even if the powder coat process reached the range of annealing temps, it would not have any adverse effect on the alloy's strength ---- in fact, it could possibly improve it. The only caveat here is that care should be taken to ensure that the material (wheel) is allowed to cool gradually on its own (by air only at room temp) and not accelerated, as cooling too quickly could cause potential problems. And, I doubt that any powder coating shop would do anything other than air cool, anyway.

So, at the OP ---

As for aluminum alloys, unfortunately, you are "hearing" the concerns from very uninformed sources ---- sources that know just enough about metallurgy to be stupid .

As for magnesium alloys, don't mess with them ---- powder coating is not appropriate for them. Not because of the temperature concerns either, but because of the corrosion susceptibility of the material --- they need to be surface-prepared and painted with special, multi-step processes in order to be properly protected and to ensure good adherence of the paint. Powder coating does not provide adequate corrosion protection (in fact, it can sometimes accelerate the corrosion process), or enduring adherence, on magnesium alloys.

 

Good post. I am by no means an expert in metallurgy. Not my field but I know a little. Having said that, to be fair annealing generally reduces the Rockwell hardness. Yes, it raises temper but at the expense of hardness. From my understanding strength is a non-specific term so if you are equating strength to temper then yes annealing increases that parameter, if you are equating strength to hardness then no annealing reduces that parameter.

And yes, you have to be exposed to the temperature long enough to raise the temp of the metal to the annealing temp so the process isn't instantaneous. Plus the alterations in crystalline structure and/or migration of the alloy elements happens slowly so again, not instantaneous. Oh yea, if you quench you aren't annealing and hardness goes up.

I don't know cure time on powder coating so I can't even make an educated guess as to the impact.

 

EDIT ---

To avoid confusion, I should add something here...

While annealing does not weaken the alloy, I suppose there could be a concern that while the material (wheel) is at its elevated temperature (before cooling back down), it is potentially (depending on temp, alloy composition, and initial temper) in a vulnerable state because it is temporarily "softer" (weaker) only at that point --- once cooled, it is back at full strength (or more).

Therefore, it could perhaps be possible to inadvertently impart a distortion to the wheel geometry (if it is hung / supported or manipulated improperly), and if allowed to cool in that condition, that would likely result in a permanent deformation ---- thus "ruining" the wheel for proper use. Perhaps this is the concern to which your sources were alluding ???

If that is the concern, that damage is easily avoided by simply handling / supporting the wheel correctly and carefully during the point of the powder coating process when it is at the elevated temperature ---- I see know reason why a knowledgeable, competent powder coat shop could not accomplish this safely and without risk using a diligent approach. But, that is something you may indeed wish to vet of your proposed list of potential powder-coaters before letting them do the work .

 

You keep saying strength and implying that it goes up. Unfortunately strength is a non-specific term. Are you talking about shear strength, tinsel strength, ??? If you are going to call us out and say that we don't know anything you need to be specific.

In my opinion the jury is out on the powder coating issue. A low temp polyester coating has the below type of cure temps and times. This could probably be used depending on the wheel alloy.

Cure Schedule, Peak Metal temperature: 30 minutes substrate temperature at 250 degrees F or 25 minutes substrate temperature at 300 degrees F

Other coatings have higher temps 350-400 deg substrate temp is common. Any of those could be a problem with a 30 min cure time.

 

Material strength (for elastic materials obeying Hooke's Law, such as the metals we are talking about here) actually has a very specific derivation and definition ---- it equates precisely to the area under the Stress-Strain curve, and it represents the total amount of energy a material can absorb before plastic yielding. FTR, both hardness (which is directly proportional to Ultimate Yield Stress) AND temper (which is directly proportional to stiffness / Elastic Modulus) effect that curve ---- hence effect the material's "strength".

A very hard material may not be as "strong" as a softer, yet more elastic (strain-tolerant) material ---- for example, a very hard material which is also brittle is not necessarily stronger than a softer material with more elasticity (e.g., glass vs. steel) ---- nor vice versa (e.g., steel vs plastic).

Generally speaking, annealing (for aluminum alloys) is desirable because it increases temper (stiffness) dramatically while having only a small reduction on maximum yield stress ---- therefore, the area under the Stress-Strain curve is larger, and the annealed material typically has greater "strength".

 

With that definition of strength I would agree that there is some optimal temper, not fully annealed and not fully hardened, that maximizes the area under the curve. Where in the spectrum does any given wheel sit and how will even minor annealing alter that area is pretty much a guessing game.

From what I can tell the cure time & temperatures on powder coating is generally high enough to cause some annealing. If that is that positive or negative would be dependent on the material and how it was cured to begin with. You would also want to consider use. Probably not much of an issue for a street car or if you use it for show. Could be a big issue if you track the car.

To clarify for those reading, temper is stiffness but increasing temper doesn't mean making it stiffer. It means giving up some stiffness in favor of a higher elasticity. That way it is less likely to fracture when it sustains a sudden impact.

 

As one of the "stupid" people here there are many factors that keep your wheels together. Engineers uses specific terms like "ultimate tensile" that lay people might just call strength. Engineers design wheels in specific ways for specific loads and when we impart unforseen stresses into that wheel unforseen things can happen. So there is no disputing Finnerty's physics but I do dispute his selective application of it. There is no reason to hear an argument from that stupid guy skidkid or the other stupid guy...Me.

Here are just 2 quotes from HRE wheel and a noted wheel vendor who represents several brands of wheel. Go ahead and ask your wheel maker what powder coating does to your warranty? Want more evidence do a search. google is your friend.

<<Quote:Originally Posted by babymd The heat treating process in powder coating can change the compound composition of the wheel making it weaker. this is as per my consultant at champion. painting is the way to go. good luck.

You have to ensure the time/temp cycle is kept low and short enough otherwise you will affect the heat-treatment of the forgings. __________________

Alan PeltierHRE Performance Wheels
The Best Forged and Flow Formed Motorsport, Street and Luxury Wheels | HRE Performance Wheels >>

<<With the greatest respect, this is an engineering question. It is not for refinishers or chrome platers to decide. Many forged wheels have been weight optimized for a specific load on a specific vehicle. That is, the weight is reduced to the absolute minimum required by the vehicle load, otherwise why bother? Therefore, any loss of strength compromises the reliability of the part and its load carrying capability as well. And potentially, the safety of the driver.The chroming process reduces the ductility and strength of the forged alloy to varying degrees, depending upon the methods used and the specific plating alloy used. This phenomenon, known as hydrogen embrittlement, is well documented scientifically and carefully tested for in aircraft/aerospace components.Wheels can certainly be designed with chroming in mind. Excess material and mass can be added during manufacturing to compensate for losses in the chroming tank. This thread, however, deals with REFINISHING.It is our opinion that there are simply too many unknowns in these processes to risk weakening the alloy. Therefore, for forged, heat treated alloys, we do not recommend powder coating except by trained expert facilities with quality systems and Brinell hardness testing systems.And we do not recommend chroming forged wheels, unless the wheel was designed for this finish. A cure temp of 375 degrees fahrenheit for 30 minutes will overage the alloy and begin the annealing process. This is not recommended by us, HRE, OZ, Champion Motorsports (who forbids powder coat entirely), BBS and most other forged wheel companies. This also includes the 50 or so forged wheel companies worldwide for which we supply the blank forgings.Many of these companies will void the warranty for any refinish not done under their control. We don't blame them.As far as the OEs are concerned, if you refinish or plate a factory wheel and ANYTHING goes wrong, you are on your own: ZERO warranty.Safety and liability are the issues. We don't know anything about your company, its processes, quality systems or technology. But if a wheel is refinished and then fails in service, or someone gets hurt, we all know who gets served with a lawsuit. The USA has 4% of the world's population and over 50% of the world's lawyers.

Mike
Tech1 wheels>>

 

And, you keep confusing "strength" with "stress" --- they are different ---- one is a material property, the other is a characteristic of a material performance...... and each has its own unique physical / mathematical definitions.

I'll just assume you mean tensile....

As for "call us out and say that we don't know anything ", it is unfortunate if that is the inference you take from my comments. The only people I referred to as not knowing anything (actually implying that with respect to metallurgy, they are "stupid") are those who have told the OP that the powder coating process will weaken aluminum wheels ---- to my knowledge that does NOT refer to ANYONE (including yourself) posting in this thread. And, I am further making the assumption that they are powder coating suppliers with whom the OP has discussed the topic ---- or worse, yet another abysmal collection of "internet wisdom", aka bull****.

 

Again, my criticisms were not directed at anyone here.

.......and it has been answered by one. It is your prerogative if you wish to discount it or not.

(sometimes I can't believe how f-ed up this site has become ---- these days it seems everyone is a know-it-all / critic, except those who actually know something)

 

In mechanical engineering, some words used have some pretty specific meanings.
"Strength" refers to either the yield or rupture stress of the material.
"Stiffness" refers to the modulus of elasticity, aka Young's modulus, which is a measure of how much a material will (elastically) strain for a given stress.
The measure of how much a material can physically deform without breaking is usually referred to as its ductility, and is often given as % elongation.
"Elasticity" used alone is very ambiguous.

Whether heating an aluminum alloy strengthens or weakens it depends on the heat treatment it received before being put in service. Some alloys are used as-cast, but often they are aged.

Aging is a two-heat cycle heat treatment; first the alloy is solution heat treated, (heating to a temperature somewhat close to melting), letting the alloying elements disperse and dissolve completely in the base material. Then the material is quenched to room temperature, creating a now supersaturated solid solution. The material is then heated again, but not nearly as high as the first step, more like 300F or less. The alloying elements now start to 'clump together' (precipitate out). When this happens, these 'clumps' somewhat disrupt the structure of the base material. An analogy to what happens is adding diagonal braces to a timber frame.

Now, if the aging process goes on for too long, you will get large clumps spread far apart, and the 'bracing' effect goes away, and you end up with a soft and weak material again.

So will powder coating affect this? It depends.

 

You are lucky. I powder coated a set of magnesium motorcycle wheels only to have them crack during a race.
Every magnesium wheel manufacturer (Marvic, Marchesini, etc.) now specifically forbids powder coating wheels.

You are at the mercy of the powder coater when you have anything coated. Do you think they have the temperature of the coating booth set to lab standards so the heat treatment and metallurgy of the wheel is not affected?

They just heat it hot enough to melt the coating, but maybe too hot for the part. We have sent steering gear boxes and idler arm boxes to be powder coated, only to have the bronze bushings that were inside them destroyed. Now we only paint those components.

You have no control over how hot the part will get and if if has been changed in any way when you send it out for powder coating. It is great for some stuff and death to other stuff.

 

I painted my Mag challenge rims simply for the reason powder coating will seal in corrosion causing it to travel like a cancer under the powdercoat. Anything that carries a high load I would not powder coat for safety sake unless I was going to xray them on a regular schedule.
I have had a few early power coated and chrome plated suspension, frames come in rotted out from the inside including a set of F car wheels.

Yeah yeah its a tough coating but we use imron in its place with proper preparation.

 

So...

The basics.

Thermal treatment of Aluminum:

For most alloys, there are 4 steps. temps depend on what the aluminum is alloyed with... typically, main alloy elements are copper (2xxx alloys) , silicon (4xxx alloys), magnesium (5xxx alloys), silicon and magnesium (6xxx alloys) and zinc (7xxx alloys).

Annealing, - this is done prior to and during forming and machining steps to reduce or eliminate build up of stresses in the part during manufacture

Solution heat treatment - heating of aluminum alloys to 800-1000F to put some of the various alloying elements into solution (as in partially melt the alloying elements)

Quenching - rapid or slow cooling of the material from the heat treat stage in air or liquid to lock in the heat treat properties

aging - can be natural aging as in it happens on its own at room temp or artificial as in heated to artificially age the Al alloy. This improves some of the strength properties over the quenched state for some alloys but can make them harder and more prone to cracking.

I dont know what alloys the aluminum wheel folks are using so this is pretty general but most artificial aging happens in the 300-400 temp range. For many of the alloys, the temps used for powder coat wont have too much affect on the basic strength of the wheel. All heat above 250 or so though does have some effect on the grain structure of the aluminum. Depending on alloy etc and whether or not the wheels get painted or coated those grain structure effects may not be noticeable.

if you concerned and want to powder coat, use some of the low temp versions and don't heat over about 250F and limit the time at heat.