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Reworking anodized Al alloy sheet

Reworking anodized Al alloy sheet

Reworking anodized Al alloy sheet

We have a problem with aluminum sheet forming.
We have a part manufactured of 2024-T42 Clad sheet. Is an L-shaped angle manufactured bending the sheet in –O condition. Minimum radii requirements are largely met. After this process, no cracks are observed in the part. This part is protected with a chromic acid anodizing and a layer of primer. The problem comes when installing this part. It is a complex design and doesn’t fit properly, so, they are being cold reworked to a maximum angle of six degrees. Theoretically, with these angles of reworking there would be no problem if we were talking only about the clad sheet with no subsequent finishing, but, as this part is already anodized and painted, we are not sure that we are not damaging and cracking the more brittle anodizing layer.
Does someone know if reworking part in that way will damage the anodizing layer?. I have been looking for some literature about this subject and minimum bed radii for anodized parts like these and I have not been able to find anything. If someone can tell me something about the subject or where can I find something that can help I would be really thankful.
Thank you very much.

RE: Reworking anodized Al alloy sheet

Hi mcolmena,
You have 2 problems:
1) Physical; basically, will the part perform as required?
2) Contractual – will you be in legal violation of blueprint, contractor, FAA or other requirements?

The first cannot be answered unequivocally.  Perhaps, with more anodize details and the performance specs.  What is the thickness of the anodic layer?  Chromic anodize on Al 1100 (comparable to the cladding on 2024) per MIL-A-8625F, Type I, may be anywhere from 0.7 to 8 microns thick. Flexibility is inverse to hardness ~ thickness.  What is the 6o bend in terms of radius of curvature? A. W. Brace and R. Peek, Trans. Inst. Met. Finishing, 34 232-252 (1957) performed bend testing on 6 & 8 micron chromic anodize films (anodizing parameters: 10% CrO3, 30 V, 54oC).  Results: no visible damage when bending test coupon over a 1.5” diameter mandrel, very slight crazing on compression surface only when bent over a 1” mandrel, some crazing on both surfaces when bent over a 1/2” mandrel.  If the bend in your part is large in comparison to these mandrel diameters, you may be OK.
Another reference (for which I don’t have results) is H. N. Hill and R. B. Mason, Metals and Alloys, 27 972-975 (1942).

Corrosion resistance is presumably required for the part (Alclad is used) and is part of all anodizing specifications, e.g., 336 hours in a 5% salt spray per ASTM B117 by MIL-A-8625F, Types I (and II).  I won’t guess how the bent part would perform (after removal of paint, of course) except that it couldn’t be any better, only worse.

Fatigue resistance is a common requirement for aircraft parts.  Cracks in anodize are a good initiator of fatigue cracks.  How does the radius of the post-anodize bend compare to bending during a fatigue test?  If less, you may be OK.

Second, contractual obligations.  Presumably too, the parts are produced to approved procedures and specifications – who signed off on bending them in the first place?

MIL-A-8625F requires all forming be completed prior to anodizing:
“  Anodizing of parts.  Unless otherwise specified in the contract, purchase order or applicable drawing (see 6.2), parts shall be anodized after all heat treatment, machining, welding, forming and perforating have been completed.”

At the very least, you would need an Engineering Change Order from the contractor – no small feat.  Also, you would need to have corrosion testing done on test coupons of equivalent material with the same post-anodizing bend. Similar fatigue testing may perhaps be required by contract.

Do you have any scrapped parts from which the paint could be chemically stripped without harm to the anodize (possibly, the entire lot)?  Have a metallurgist examine for cracking of the anodize.  

Hope this helped without raising too many further questions.

RE: Reworking anodized Al alloy sheet

Hi Ken!
Thank you very much for everything, it was great help. Unfortunately, it raised one more question.
Our part is an L-shaped profile manufactured bending the sheet. The 6 degrees rework refers to the increase or decrease in the opening of the angle of the profile. Interior bending radius is 0.27 inch and exterior radius is 0.35 inch. We are trying to relate data in the test results of the bending test over the 1.5”, 1” and 0.5” mandrels you mention (A.W. Brace and R. Peek Trans. Inst. Met. Finishing) with real part with but we don’t have this bibliography available. We want to calculate the maximum strain in the bending, we think we are in the permanent yielding area. To relate the strain in both cases we need to know which was the thickness of the test coupon for the bending test you mention (not only the anodizing layer but the test specimen itself).¿Is it mentioned in the test this data?
Thank you very much

RE: Reworking anodized Al alloy sheet

Life is never so simple.  
The results were given in Anodic Oxidation of Aluminium & Its Alloys by V. F. Henley, p. 129 (1982). This author maakes the statement
"Even under the most favourable conditions for producing flexible coatings the maximum elongation before cracking occurs does not exceed 0.3%."
Perhaps you can compare that to your geometry.
I tracked down the same Brace and Peek reference in The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn. by P. G. Sheasby and R. Pinner. These authors  comment "The flexibility of the coating is markedly superior to that of sulphuric acid coatings..."
(p. 497).

Hope this helps,

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