Black Anodizing 7075 Aluminum

[cooblacrouse]

We had 2 lot of parts sent out to be Black Anodized per MIL-A-8625, Type II, Class 2 with a thickness of .0002 to .0004 in. One lot was our tpical material for the part 6061-T651, the other lot the material was swithced to 7075-T651. We allow for 1/2 of the build-up per surface and the parts were measured before coating. When they came back they had less material than when they went out, O.D,'s undersized and I.D.'s oversized. The plater said the material switch was the cause but I don't understand why this should be.

Should there be any difference in the parts other than slight color change?

Thank you,

Bob

 

[alexit]

The anodize process will remove material from some alloys and build-up others. It has to do with the matrix of the alloy.

 

[cooblacrouse]

We had 2 lots of 26 parts each. All 26 of the lot using 6061-T651 came back in tolerance. 6 pieces of the 7075-T651 lot came back in tolerance but 20 were out by .0004 to .0006 in. This led me to believe that the vendor had stripped this portion of the lot and recoated it leaving the parts out of tolerance, we had this problem in the past. When I contacted the vendor he pointed to the issue of the difference of materials stating that 6061 coats better than 7075 and insinuating that the reason for the parts coming back out of tolerance was due to this. But why would 6 pieces be alright?

 

[alexit]

You have the measurements from before? Check those six...maybe they were MMC and now LMC. In my experieince the total feature change is about .0008" (0.020mm) from anodize but some alloy grows, others shrink.

 

[surrf]

I have found that if the Anodize contractor has a problem they will strip the anodize and then re-anodize the parts. This will cause the under/over size condition.

 

[IRstuff]

And if so, the vendor was probably correct in blaming the material

TTFN