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Aluminum Sheetmetal Warping after Anodizing

Quickstart:
     In the anodizing of aluminum the components are immersed into a tank of acidic solution and connected to the anodic (+) positive pole of a power supply. The current separates some of the water in the solution into positively charged hydrogen and negatively charged oxygen. The oxygen is attracted to the positively charged components and converts the skin of the aluminum to aluminum oxides.
     Readers new to anodizing of aluminum may wish to view our "Intro to Aluminum Anodizing".
     The coefficient of thermal expansion of the anodized layer is far lower than that of pure aluminum, so even if you successfully reduce stress in the deposit to near zero, thin aluminum sheets with heavy anodizing are very prone to warping if they have unequal anodizing thicknesses.

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
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Q. First time posting, i hope i find you all doing well.

I have an aluminum (6061-T651) plate measuring approximately 5 x 5 inches. The plate features a 0.188-inch-thick perimeter frame providing structural rigidity, while the central region is pocketed down to 0.0435 inch thick to accommodate a high-density array of holes. Prior to anodizing the part stays flat around 0.0004". The part warps after anodizing reaching 0.005" thickness.

More detail on the part:
The inner recessed area contains a dense pattern of precision-drilled holes. Multiple through-holes are distributed along the perimeter for mounting and alignment to the mating hardware or fixture.

Here is the current anodizing process followed by vendor:
Anodizing Process Details:

Control Method: Voltage-based process, ramped from 10 V up to over 30 V

Bath Parameters:

Soap cleaning: 30-60 seconds at 80 °F

Caustic etch: <1 minute at 120 °F

Hard anodising: 90-120 minutes at 29 °F

Black dye: <10 minutes at 100 °°F

Sealing: 10 minutes at 180 °F

Final hot DI rinse: <2 minutes at 140 °F

Racking: Four contact points along the perimeter on the thicker section

I'd really appreciate insights from anyone who's dealt with similar thin-part distortion during hard anodizing.

Could the voltage ramp or current density be a factor here?

Would switching to a current-controlled process help?

Could adding more rack contact points or supporting the part differently reduce flexing?

Any recommendations on bath parameters or fixturing approaches for parts with large thin areas?

Thanks in advance for your suggestions! ⇦ Answer?

-AB

⇩ Similar, related, Q&As -- oldest first ⇩

Q. Hi, I'm a manufacturing engineer for a family run CNC machine shop in Brea, CA. We're having trouble conforming to specifications on this troublesome part, and my local anodizer sent me here to see if any of you could help.

My company has manufactured some parts for a customer from 1/16" thick 6061-T6 plate. The customer requires Type III anodize and a flatness tolerance of .010" The parts range in size from 9" x 11" to 14" x 18". There are a few small (.070") holes along one edge, but other than that they are featureless. We CNC milled the periphery on a vacuum chuck, deburred, finished with an oscillating sander and Scotchbrite 7447 pads ⇨
and mechanically straightened them to within .005" - .008" flatness. After returning from Type III hard anodize a significant percentage of parts were up to, and some over, .020" flatness. We've been unsuccessful in mechanically straightening most of them. The hard anodize seems to be making the parts spring back, much more so than they did before processing.

I'm looking for help or suggestions on this. Is there a way to get the hard anodize to "relax"? I'm thinking the harder skin is putting some stress on the thin substrate. Would a thermal treatment help, maybe heating, then sandwiching between flat heavy plates to cool? Vibratory stress-relief? Cryo treatment? We're grasping at straws trying to get these parts flat enough for our customer. I realize this might be as much metallurgy related as it is finishing, but perhaps somebody out there has had some experience with what we've encountered here?

Thanks in advance for any help anyone can provide.

A. Your material is a bit thin for the size of the plate to hold 0.010. If you are near the limit before anodizing, it does not seem unusual that it could regain some of the original deflection or memory. A stress relief cycle would help before it is sent to the anodizer. You could boil them in DI water for a few minutes, cool to room temp and recheck for flatness.
I know that in stainless steel that the removal of the highly stressed outer layer of metal would cause thin material to warp severely. I used to flatten parts that were machined and grit blasted by selective grit blasting.

James Watts
- Navarre, Florida

A. My guess ( give it 90% confidence rating! ) is that the parts were compression racked from one side to the opposite and in doing so, in order to assure good electrical contact and prevent burning (especially important on thin 0.062" thin parts), bowed the parts. Once they are hardcoated in the bowed state, they'll likely return to that same bowed state unless something dramatic happens to redistribute the stress - perhaps heat and pressure (but I'd give that ~ 10% confidence). The only sure thing is to strip them, then re-flatten them and have them re-hardcoated. It sounds to me that this might not be possible given that this will likely shrink your part and open holes by 0.001". Hate to be the bearer of bad news but this is reality as I see it.

Milt Stevenson, Jr.
Plating shop technical manager - Syracuse, New York

A. Neil

I have seen cases where the anodizer (me) racked the part so tightly that the part anodized in a flexed state, and retained some of that shape afterwards. I have also seen parts with significant material removal during machining, warp after anodizing where racking was not a factor but stress was.

On a part 14" x 19", it is reasonable to expect the anodizer to be pumping 30 ASF or about 100 amps to the part. To do so safely, the part must be racked securely, or the contact points will burn/arc. If your anodizer is racking on the perimeter and compressing the part in a set of 'Vees' to maintain a secure electrical contact, it would not surprise me to see such a part lose it's flatness.

Other racking options would be to provide tooling holes where the anodizer can sandwich the part between 2 nuts on a threaded rod, or sandwich/clamp along an edge. Both will leave a significant rack mark, but by compressing across the thickness of the part, racking induced warpage is prevented.

Willie Alexander
- Colorado Springs, Colorado

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A. Getting the current spread evenly on a thin sheet is very difficult. Have your anodizer ramp up slowly over 15 minutes to no more than 24 amps per square foot (later try only 20 ASF), have him make contact in several places - at least 4, preferably 6 or 8. Using an eddy current thickness tester, check the thickness corner to center, if not uniform, then lower the current to 20 ASF and raise the temp from 32 °F to 45 °F.

Heating after anodizing will not help, above 80 °C will cause cracking, however, you should do something to relieve the "work hardening" before anodizing.

There is nothing in my book that addresses this problem.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

A. 6061 aluminum contains silicon and copper, and small amounts of other alloying constituents. If the etching of the aluminum prior to anodizing is too severe, some of the these constituents are removed. The purpose of the alloy metals is to strengthen and harden the aluminum. Copper and possibly other materials are usually removed using nitric acid, and/or tri-acids. Aluminum is removed in caustic solutions. The removal of these alloying constituents will change the hardness and strength of the item. I suggest that you use a very minimum etching prior to anodizing.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics"
           and "Plating ABS Plastics" eBay or AbeBooks affil links])

 Gentlemen,

Thank you all very much for your thoughtful replies. I might be out of luck with my current parts, but will take your recommendations to my anodizer for future runs.

Best Regards!

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

You'll love this book. Finishing.com has sold a thousand copies without a return request  🙂

Q. We are doing sheet metal fabrication of aluminium AA5052 H32 sheets. After punching & bending process, the aluminium sheet metal part goes to chromating & powder coating process. After chromating, while curing the part in water dry off oven at 130 °C, the parts get slight bow. After powder coating, while curing at 200 °C, the parts get more bow. We checked the chemical & mechanical properties of aluminium sheet material. We even checked the part by processing chromating & powder coating process from another source. Still we could not sort out what went wrong?

A. Hi Mahendra. I would suggest putting a bare unprocessed part in your dry-off oven and then in your curing oven and observing whether there is bowing. If there is not, then Don Baudrand's advice seems applicable.

If you succeed in solving that problem, I would suggest re-evaluating the temperature of your dry-off oven as it seems to be hot enough to destroy the chromate protection. Good luck.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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