T6 aluminum fatigue 2

[Durableguy]

Hello all. First of all, I apologize for asking what I expect may be a simple question to you. I am not an engineer, but would like to ask those who are and versed in what the issue is

I am a powder coater. I have a set of motorcycle wheels cast in T6 aluminum. Since they are cast, SOP says they should be outgassed above the powder cure temp, in this case 400* F, prior to powder application.

The wheels have been outgassed and saw a PMT of 430* F for about an hour. My customer is very uneasy about the wheels seeing these temps in this grade of aluminum. Monday I hope to have them Rockwell tested.

If their integrity has been compromised, is it possible to anneal them and return them to where they should be?

Apologies for coming here like this, but you go where the water is!

Thank you.

 

[Metalguy]

Assuming they are the very popular 356-T6, you have most likely reduced the ultimate, yield and fatigue strengths somewhat, perhaps ~10%-20% They are overaged, so they would require both a solution heat treatment and an aging HT to restore the correct properties.

They will probably not be very round after the SHT.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[Durableguy]

I believe they are the 6061 variety. Does that make any difference?

 

[Metalguy]

6061 isn't usually used for castings. It is a wrought alloy designation. Please explain why you believe the wheels are cast 6061.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[Scheffers]

I'm with Metalguy, about the 356-T6.

take a look over here, there is some discussion, :

http://slcsubaru.freeforums.org/painting-vs-powdercoating-for-rims-t830-15.html

found some more discussions without a real answer... and another quote; "The discussion usually ends up with the general consensus that The coating process will not harm the aluminum enough to matter, as most any aluminum structure is over engineered by enough of a margin that the heat treating process is not compromised to a degree of eminent failure. "

I think i have to agree with that quote.

 

[Durableguy]

Sorry its been a few days, thanks guys. Really appreciate the link. Like you said, no clear answer.

I believe you are right to question the 6061, owner believes they are 365 after all. Going tomorrow to have them Rockwell tested to get some clear data as to what has happened. Owner also has some unlomested factory wheels he is having tested on his end. As long as teh wheels are from the same few years of peoduction, is is safe to assume they are of similar alloy? 90's Honda motorcycle?

Really appreicate the info and help here. As messed up as this has become, its a little comforting to get educated info and leads such as that link.

 

[Metalguy]

While you can probably assume the old wheels are 356 also, why not have a met. lab. run a simple check to be sure?

Riders depend on those wheels to stay alive.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[Durableguy]

Hope to get the Rockwell test done tomorrow!

 

[Scheffers]

Let us know!

 

[Durableguy]

Hit another snag. Took the wheels into the local university to get them Rockwell tested. This is the machine they had.

http://www.brystartools.com/servlet/the-71/4TT-Wilson-Rockwell-Hardness/Detail

Not large enough to get any kind of a good reading on the wheels as they really could not be held still. Professor has a PhD in Metalurgy. Smart guy. We talked for ~30 minutes and his general concensus was that the wheels have not been rendered useless but only a hardness test could validate anything. Spent a good amount of time pouring through ?ASM? books and charts.

I am going to look into buying 365-T6 bar stock, doing some controled bakes at varying temps and times and have those tested. This seems to have piqued his curiosity a bit and is eager to help out and get concrete data. I can report back with results if anybody is intereted.

 

[Scheffers]

Yes, cool!

 

[Metalguy]

Can you sacrifice one wheel and cut it up? Then find a Brinell hardness tester and do the 500kg test on a cut-out piece.

The Brinell test uses a 10mm ball, which measures the hardness over a far larger area than the 1/16" ball of an Rb test. This can be a factor on castings.

BTW, the alloy is 356, not 365. If you buy any for testing, make sure you do not get 355, which is stronger than 356.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[swall]

Durableguy--it would behoove you to obtain the publication,
"Standards for Aluminum Sand and Permanent Mold Castings" from the Aluminum Association if you contemplate continued business powder coating aluminum wheels. The standard costs $60 and lists the ageing times/temperatures for all of the common cast alloys.

 

[gadkinsj]

Does anyone have any experience with Eutectic Seperation. As I understand it (which is to say I only vaguely understand it and found this a good question with which to clarify it.) it is a matter of silicon based aluminum alloys being heated and having silicon crystals form out of the solution. When this happens, the crystals will not break apart during re-treating and go back into solution thus rendering the alloy permanently degraded.

Is this actually the process of eutectic seperation?

There are some situations where this is desireable but I would like a better understanding of which alloys are susceptible to this, and see if someone with knowledge of this process will comment on these wheels.

I have run into this issue while coating a cast aluminum piece that is not too different than this wheel in size/mass and found that it could not be re-treated. But the temperatures involved were over 700 degrees.

 

[Durableguy]

I apologize for the 365 reference, realized a short time ago what I was doing.

~Metalguy, we are looking into obtaining a sacrificial wheel right now. Of course no one wants to put up a good wheel but bent/unusable ones should pop up before too long. That would be the ideal candidate for testing.

~swall, That book sounds like a perfect addition to what I do, but with ym very limited knowledge, I'd have next to no idea what alloy I am dealing with. What little I have gathered so far, 356 seems to be the alloy of choice in an application like this?

90% of the aluminum I powder coat does not bear a load, i.e. motorcycle valve covers, trim castings, etc.

The general standard in custom powder coating is to outgas for 60 minutes 50* F above the intended powder cure temp. In my case, 400*. This to ensure any pockets of gasses are released and oils/chemicals are burned to the surface. All to ensure this doesn't happen during the cure process ruining the finish mandating a complete strip and recoat. The professor I spoke to mentioned that 300* for 60 minutes should be sufficient to accomplish this. Opinions?

 

[cloa]

gadkinsj start a new thread!

 

[gadkinsj]

I only put it here because I think it could have implications for this situation.

To share my experience. I heated a circular aluminum casting to apply a coating and it completely lost functional strength. This result surprised the engineer we consulted on material selection and the company applying the coating, as well as me. So my company tossed three highly finished, highly machined pieces.

But to just post that information to this thread without some explanation of how it may be different from his wheel situation would be irresponsible. So the person who can answer my question might have some insight as to whether this wheel might run into a similar problem.

 

[swall]

Durableguy--The idea of having the standards is that you can look up the particular alloy you are planning to coat and see what temperature range you need to stay below to keep from overageing the part. As for 300F 1 hour bake out, you should be OK for A356-T6. The ageing schedule for A356, is 2-5 hours at 320-350F. For standard 356-T6, it is 3-5 hours at 310F. Those figures are for permanent mold castings.

 

[Durableguy]

Swall, could you define permanent mold casting for me? I really don't know what that means.

I really appreciate the time/temp specs on the aging. Reading that, the problem begins to be time/temp requirements for powder. Typically 400* for most colors. Primers can require 460*. Soem can get as low as 325*. Its possible to 'slow bake' powders at 10%-15% lower temps fro longer times but then its possible to run into adhesion issues.

Fortunately, time requirements at those temps are usually just 10-15 minutes once the part reaches temperature.

 

[swall]

Permanent mold casting involves casting aluminum into metal molds either at ambient pressure or at low pressure. Not to be confused with aluminum die casting, which is casting aluminum into metal molds at high pressure.The aluminum solidifies much more rapidly than with sand casting and this gives rise to enhanced mechanical properties.