Aluminum surface 6061 T6 moon craters? 1

[DWilliamA]

Hi, first off - I am not a machinest. I am quality guy trying to chase down an answer. from what my machinest tells me, we are doing this right... here are the details.

Extruded 6061 T6 Aluminum 3/4"x2-1/2" stock basically machined to a block shape, lots of flat shiny surfaces. Approximately 1/8" to 1/4" of material from the sides and top taken off. 4 flute cutter, 3800 rmp nice chip load the surface looks really nice - but when you look real close, really really close when the light is just right you can see what looks like a moon crater surface or orange peel surface... actually it looks like a spackle paint maybe. At a glance you see the nice subtle rainbow sheen and the miniature fly cut lines - looks nice. but upon close examination there is this bumpy spackle characteristic

Any ideas on what causes this and how to eliminate it, if possible with machining technique?

I believe that when milling - the cutter creates a shear zone and just under this shear zone is a compression zone. Its almost as if the aluminum is inconsistent in hardness and certain areas compress more than others. This situation didn't just start btw.. - so I don't think it is just bad lot of material. Its just that I am the first person to come along and try to solve the problem.

that said... this condition is causing problems, or is possibly related to problems we have later when the part is Type III hard black anodized. The same type of mottling shows up in the hard black anodize. Unfortunatley, we could hit these parts hard with the ano and etch heavy and anodize thick, but this dulls the surface. We want a nice shiney surface which takes a light anodize so to speak, which leaves the spackling intact.

different cutter - thicker extrusion and cut through more exterior material, heat treat, lower Iron levels, plate stock (if possible) ... ?????

Thank you,

David A.

 

[saberblue]

Most MWFs have a small amount of yellow metal corrosion inhibitor formulated into them -- these corrosion inhibitors typically work by passivating the surface of the yellow metal and is thus consumed in doing its job - the amount formulated into the typical fluid is enough to handle the incidental Cu based materials found in the machine tool plus a little -- if you have machined Cu with the fluid particularly if you didn't get the chips out ASAP you have probably exhausted the corrosion inhibitor -- if you have Cu in solution you can be sure that this is the case --your fluid supplier supplier should be able to give you a tank side add to replace the corrosion inhibitor you will need to add quite a bit initially to take all the Cu out of solution and then build you the level to the proper amount -- be careful making the add as the materials typically are expensive, have a bad odor and cause foam. So unless the sump if quite large it may be better to DCR (dump clean and recharge) the system.

 

[saberblue]

Another thought after some sleep -- is it possible that you have so much Cu in solution that when the coolant dries on the surface of the product. As the fluid dries the surface tension will cause the drops to contract raising the concentration of the Cu. At this point you might have enough Cu to cause the galvanic corrosion.

In general the Cu content of 6061 is low enough that you don't see the "white rust" issue very often.

Interesting problem good luck.

 

[DWilliamA]

We probably do have 'so much' CU.... the coolant doesn't dry and puddle or contract into drops - the parts stay oily. but if you leave the parts setting wet for a day they turn dirty rainbow colors and places where the coolant was wiped off with fingers from handling remain relatively shiny, i.e., they leave finger prints. Pretty drastic.

We wipe the parts clean now and they stay shiny and look good coming out of hard black Ano. But I still wonder if the very sharp edges still corrode in a small way and this causes Ano problems - because hard anodize has issues with sharp corners as it is on it own.

We had a bunch of parts go through recently and the plan was laid out... we arranged clean and rinse buckets - everybody is on board, clean the parts off NO COOLANT stains... end of the day shift later I stop by and our 2nd machinest wasn't cleaning them - he thought like maybe doing it before the end of the shift was Ok.....cause he was focused on some edge filing instead, so the next machinest wouldn't have to 'work' his parts.

Can I swear out loud now?....

We probably had 20 to 30 parts ($75 a pop) that were corroded for 3 to 4 hours, but they appeared to wipe clean, I am thinking we pushed our luck on these and these were the 20 or 30 that ended up with brown/gold stains on the edges. can't prove this though.

I am going to see if we can't have another shop machine some up with clean coolant and see what we get.

 

[saberblue]

There are products specifically designed to do this kind of parts cleaning etc. Here is a link to one that I am familiar with and I know works particularly well -- it was designed to solve problems with cleaning and protection on some a family of 7075 parts

http://www.masterchemical.com/na_en/fluid_products/show_product.php#