Hardcoat Anodizing Halos or ghosts

[aidrock]

I am working on a project for a client where we have turned a cylinder (AL 6061-T6) to a certain diameter ~2" and must have it "black hardcoat anodize, teflon impregnate, .0008/.0010 thick buildup". We do not do the hardcoat anodizing (Type III, Class 2) or teflon coating in house.

The first batch we made and had coated passed inspection perfectly. The second batch has had an ~50% rejection due to discoloration of the hardcoat in certain areas. These areas look like dried puddles - concentric rings of varying offset located in a random pattern on some of the pieces. We spoke to the anodizer who walked us through his process showing that there is no time where the pieces are placed on a surface that might cause this discoloration. They told us that the discoloration is due to the grain within the AL and that it was created that way at the mill. They said they have seen this before where this grain structure (like a knot in a piece of wood) runs entirely through a piece and other times where it is only surface marring.

I have added a photo that somewhat shows this pattern.

Any thoughts or explanation on this issue would be greatly appreciated as the pieces are being rejected for cosmetic reasons only - no structural or functional flaws.

Thank you for your time.

Aidan

 

[MintJulep]

Amazing though is it, the anodize guy not entirely lying.

Anodizing chemically alters the oxide layer that naturally and spontaneously forms on the surface of aluminum exposed to the atmosphere.

Grain structure, hardness, temper, contamination etc. can and do affect the oxide layer, and in turn the appearance of the anodizing.

 

[aidrock]

ok - just seems odd that the first run of 75 pieces yielded 0 rejects and the second run yields 50%!!
Are there any ways to combat this issue?

You say the "anodize guy is not entirely lying" . . . what is meant by entirely? Is he partially pulling my leg?

Really need some good help/information here. This is a big customer and would like to keep this work and keep them happy.

 

[SincoTC]

Hi aidrock,

I'm pretty certain that what's giving you problems are what's known as Back-end, Butt-end or Bar-end Defects; we had problems with this a few years ago on both round and flat extruded bar, it didn't show up on the machined part, but after anodising, some would show a distinctive "woodgrain" pattern. Apparently, it's an unavoidable part of the extrusion process as the bloom is squeezed into the die and folds/shear lines occur, but they should cut off the affected portion, however, I fear that the "bean counters" grumble if too much is discarded and I suspect that in these hard economic times, suppliers are using "cheaper cuts" of material from dubious sources who aren't so careful about supplying really good quality material

It can occur all the way through a length if the extrusion speed and temperature conditions are not correct for the grade of material.

We had a word with our suppliers and now insist on certificates with each delivery to prove that they originate from a reputable source. I know one company who uses millions of pounds worth of bar/flat extrusions all of which have to have a beautiful black finish and they insist on their supplier submitting an identified slice off each end of every length, which they then machine and acid etch to reveal any defects before they will accept delivery of every order. But then they have the financial clout to make such demands!!

I'd speak to your supplier if you haven't already and see what they have to say.



Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2013x64 SP2.0 Intel Core i7 2.94Ghz, 12Gb Ram, NVIDIA Quadro FX3700 Driver: 6.14.11.9100
SW2013x64 SP2.0 Intel Core 2 Duo 3.17Ghz, 8Gb Ram NVIDIA Quadro FX3700 Driver: 6.14.11.9100

 

[MintJulep]

So far the anodizer is focused on convincing you that "it's not his fault" - which mostly it isn't.

However he hasn't offered any constructive help on how the end result could be improved - by more careful material selection, altered machining processes or perhaps some form of treatment prior to anodize.

You need to get the entire supply/process chain working together.

 

[aidrock]

Thanks for the information all!!
SincoTC:
These markings are occurring along the sides of the round stock, not the ends. I am not sure if Back/Butt/Bar-end defects you are referring to are only at those locations of if they occur throughout the bar (including sides)?
You are correct - we unfortunately do not have that much clout regarding our supplier of material. We are looking back at the material certs to see if there are any noted differences between the material received on the first run and the second run. As far as I/we can tell this seems to be completely random even within the same lot of material!

MintJulep:
Agreed. Although they did offer the potential to do a "dyed Teflon" coating that would be baked on, but we are not sure of the comparable properties. We have offered this as a possible alternative to our client and are waiting for their Engineering to approve/disapprove. However with this coating I believe the dimensional tolerances will be quite difficult/impossible to hold so we also have to ask for leniency on them. Unfortunately none of this (that we know of) is due to the machining process as it is identical for every piece yet the issue is random. I do wonder if a type of heat treatment prior to anodizing might realign these grains to eliminate or minimize the halos???
Getting everyone to work together is paramount to the success of all! Working towards that goal.

Thank you all again.

 

[SincoTC]

Hi aidrock,

The word "end" in these defects is used to refer to the nominal length at each end of the bar that likely contains defects and should really have been cut-off and binned before we got it. Some defects such as Pipe or Funnel are limited to the core of the extrusion and while most of the defects are clearly visible on the ends (after etching/anodising) just like a tree stump, most will extend to become visible on the sides as they are seldom truly concentric and so show up as longitudinal streaks or "grain" patterns like on planks sliced off a log. There are certainly surface only defects, due mostly to die and operating conditions and sometimes impurities in the pre-extrusion billet. We had a whole load of 2" x 1/2" that has streaks and little black hard spots in it that instantly destroyed the edges of TC cutters.

There's quite a lot of info in this file regarding both material and finishing problems.

http://faculty.kfupm.edu.sa/EE/sbaiyat/events/SEC2002/vol5/P137.pdf

Loads more to be found on the Net! Good luck





Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2013x64 SP2.0 Intel Core i7 2.94Ghz, 12Gb Ram, NVIDIA Quadro FX3700 Driver: 6.14.11.9100
SW2013x64 SP2.0 Intel Core 2 Duo 3.17Ghz, 8Gb Ram NVIDIA Quadro FX3700 Driver: 6.14.11.9100

 

[TVP]

Lots of good stuff on the internets:

Etching effects and the formation of Streaking Defects on Al Extrusions

http://www.aluminiumtoday.com/contentimages/features/zhu_web_30_AIT_0110.pdf

Microstructure Investigations Of Streak Formation In 6063 Aluminum Extrusions By Optical Metallographic Techniques

http://onlinedigeditions.com/articl...allographic+Techniques/1352751/0/article.html

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8864250

 

[aidrock]

SincoTC - Thank you very much for that link. Great article.

TVP/SincoTC - Agreed that there is lots of information on the Internet, problem is using the correct search words and/or knowing the good sites to go to.
I will definitely have enough reading to keep me busy this weekend!! Thank you both for the help.

 

[swall]

Aidrock--Is it possible that your first batch of parts was machined from rolled bar rather than extruded? I have seen hardcoat appearance variations on the same part, but rolled bar vs extruded. Rolled did not show the pattern; extruded did.