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I recently had a machine shop machine some aluminium panels (3mm thickness) which I then sent to an anodizing shop where the panels were bead blasted and anodized. When they came back they were seriously bent (dimensions of the panel are about 310x130mm).

I did specify 6061-T6 for the alloy but I suspect the machine shop might have used a cheaper non-tempered aluminium.

Is my thinking correct or does this problem come from the anodisation process?

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  • $\begingroup$ Did you specify tempered when you placed the order? $\endgroup$ – Solar Mike May 24 '18 at 17:30
  • $\begingroup$ @SolarMike I specified -T6 so yes $\endgroup$ – b20000 May 24 '18 at 18:12
  • $\begingroup$ Age hardening ( as T 6) looks a lot like tempering of steels. $\endgroup$ – blacksmith37 May 24 '18 at 19:12
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Very difficult to think that anodizing could cause distortion. I think there is a possibility that bead blasting could cause distortion with high pressure and large beads. Bead blasting is similar to shot peening which is used to put residual stresses and distortion into metals. Lower strength materials would be more likely to distort under extreme bead blast conditions. However , I would look for another cause.

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  • $\begingroup$ so you are essentially saying that the cause is that they did NOT use tempered material? $\endgroup$ – b20000 May 24 '18 at 19:25
  • $\begingroup$ I looked in a reference: In the anodizing section there is no reference to distortion . In the blast cleaning section they caution that a too high pressure can cause distortion of thin stock. As far as material, a basic hardness test like Rockwell B scale will determine if the aluminum has been age hardened ( or as you call it -tempered). It will not determine the specific alloy , but I think you are only interested in the strength ( hardness) level. $\endgroup$ – blacksmith37 May 26 '18 at 2:05
  • $\begingroup$ CORRECTION :The hardness of 6061 T-6 is Rockwell E 85 to 97. However if it is clad, which I think most is , the hardness can be a little lower. $\endgroup$ – blacksmith37 May 26 '18 at 16:50
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Assuming the pieces were formed to shape before anodizing:

If the aluminum alloy was a high-strength variant, then bending it would result in a significant amount of strain energy being stored in the bend zone- and the sharper the bend, the more concentrated will be that energy.

Anodization is a chemical etching process in which the deposition of reaction products on the surface of the workpiece exceeds their dissolution rate, and it requires that the reaction product film be porous enough to support the transport of charge and reactants through the film in order to sustain the reaction until such time as the film has grown to the intended thickness. After that, the anodized workpiece is removed from the chemical bath and baked in an oxidizing atmosphere furnace long enough for the porosity in the film to be closed off by the creation of fresh oxide in those pores.

Your parts are probably being stress-relief annealed during the post-anodization bake, and warping as a result. To prevent this, they should be stress-relieved after bending, and then anodized.

Assuming instead the pieces were flat when they went into the bath, then any anisotropic stresses left in the sheet by the rolling & finishing process would get released during the bake, and leave the parts warped out-of-flat. Same fix: stress-relieve before anodize.

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  • $\begingroup$ I don’t think there is any intentional bending going on. I think the question is about warping. $\endgroup$ – Eric S May 25 '18 at 2:34
  • $\begingroup$ I see. will edit my answer. $\endgroup$ – niels nielsen May 25 '18 at 5:05
  • $\begingroup$ the aluminium was machined on a CNC router (and possibly laser cut as well) and was not intentionally bent in any way $\endgroup$ – b20000 May 25 '18 at 7:33
  • $\begingroup$ @nielsnielsen so is the conclusion that 6061-T6 was not used, but instead a cheaper non tempered material, which caused the warping? $\endgroup$ – b20000 May 25 '18 at 7:35
  • $\begingroup$ no, only that there were unrelieved residual manufacturing stresses in the parts. but there are easy ways to tell if the wrong alloy was used: try bending a strip of the material by hand and compare the result to the response of a known sample. $\endgroup$ – niels nielsen May 25 '18 at 16:55
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Your plate is relatively long and thin, and Aluminum (even with T6 temper), is not particularly stiff or strong. That said, typical bead blasting or shot peening should not deform it.

When anodizing, a bunch of electrical current is being pumped into your part, since it is relatively large surface area. In order to prevent arcing of the electrical current, it needs to be clamped or otherwise held securely to the rack used for the anodizing process. Depending how it was clamped (and what kind of warping you're seeing), this could be the cause of your problem.

I'm not sure how badly warped it is, because I'm not sure what your tolerances on the flatness of the plate were, and therefore by what margin it is warped. If you're holding relatively tight tolerances for flatness (less than, say, .010"), shot peening or bead blasting prior to anodizing is a good idea.

In summary, I'm thinking that whatever non-flatness was in place prior to anodizing, plus any additional warpage that was incurred in retaining the plate to the rack caused some bending or warping. When the anodizing was applied, it 'locked in' this warpage, and now your part is more warped than before.

Aside from the obvious possibility of them simply mishandling your parts, I don't think the machine shop or the anodizing shop did anything out of the ordinary. You may just have to make your plate a bit thicker if possible, or control the flatness tighter to begin with (again, not sure how badly out of tolerance we're talking here), or have a discussion with the anodizer about your problem to see if they can process it differently next time.

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