Discoloration on machined surface 7

[Jerry66]

Hey all, I am having a problem with discoloration on the machined surfaces of aluminum die castings. These parts are cast from A383 aluminum alloy. The discoloration is kinda a brownish colored tint, maybe a tarnished look. If I scratch it with a knife blade, it is shiny underneath, so it is on surface only. It shows up randomly on different parts from different cnc machines. Maybe only 1 or 2 parts at a time. What could this be? Thanks for any thoughts on this.

25 years experience in aluminum die casting

 

[dicer]

Something in the coolant causing a reaction? Have you recently cut cast iron? Or some material that is brownish?

 

[Jerry66]

No, the cnc machines have the same parts ran on them everyday.

25 years experience in aluminum die casting

 

[cloa]

Couldn't this be an issue with the transport?

 

[ornerynorsk]

As dicer mentioned, coolant would be my first suspect.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[saberblue]

Do you have any odor with the coolant? If so you might have a problem with sulfide reducing bacteria. As part of their biological processes they execrate weak sulfur containing acids -- these acids create the color described on aluminum. If you decide this is the issue you will need to DCR (dump, clean and recharge) the machines in question. Before you DCR the worst machine get a representative sample and have your MWF supplier run a test for anaerobic bacteria this will take about 5 days.

 

[tomwalz]

Do a pH test on your coolant. You should have a spec sheet giving you the correct pH. I would guess it should be slighlty basic at mqybe 8.0 to 8.5 and yours is maybe 6.5

A "do it yourself" bacteria test takes about 3 days.

You may not have to dump and recharge. A good filtering can solve a lot of problems. Bacteria and fungi are easy to filter out.

Tom



Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[Jerry66]

Thanks everyone. These are all great suggestions, I have thought about a ph test. This discoloration will show up on maybe 2 parts. Then maybe again 2 months later. On random machines.

25 years experience in aluminum die casting

 

[tomwalz]

Many problems start slow.

In my experience processes tend to degrade.

SPC or SPQC for Statistical Process Quality Control. As an example.
Drill bits wear down. As they wear they make smaller holes. Holes that are too small make out of spec parts. So you regularly measure the holes as they go from .125" to .124" and so on. This gives you plenty of warning and allows you to replace the drill bit before you make bad parts and without interrupting the production cycle. (There are other definitions equally valid and, perhaps, better.)

Tungsten carbide is often brazed. Clean tungsten carbide typically brazes well. However most tungsten carbide has to be treated to make the surface wet and braze well. There is a company that does this treatment. When they get really busy they it seems as though maintenance on the treatment tanks slides. We typically see this as maybe 20 or 30 parts that are less than perfectly wettable in a batch of 1,000. When this happens we scream like a banshee to others in the industry. This usually gets the tanks cleaned and parts become good again. If corrective action is not taken then the process continues to degrade to the point where saw tips come off saws like shelling corn.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[saberblue]

You state that you only have this problem on the machined surfaces -- if it was only the fluid you would expect it on all surfaces. Is it on all the machined surfaces or primarily on one? What you might have is a galvanic cell between the Al part and the freshly machined surface..

Alternatively you may be depositing iron (Fe) from the fluid on to the part. Most water soluble MWFs control Fe corrosion by raising the pH - the more alkaline the fluid the less likely to hare Fe corrosion. If the pH of you fluid got low (concentration - bacterial action etc.) or a lot of Fe chips with a lot of surface area can put Fe into solution -- at which point any number of things can cause the Fe in solution to participate out( this is often a problem after some one machines a rusty cast iron (CI) part).

Note- the pH of almost all water soluble MWFs is alkaline meaning that they have a pH of above 7.0. Typically the range is between 8.0 and 10.2 (below that you have begin to have Fe corrosion problems and above that dermatitis) there are notable exceptions. if you are going to check the pH of the working solution either use a pH meter or narrow range pH paper and understand that there are things in some MWFs that can cause "strange" readings with pH paper.

Further note -- the information on the typical data sheet is for the undiluted concentrate not a working solution -- so you will probable need to call your MWFs supplier technical service department or do a series of dilutions your self -- let these dilutions sit and check pH periodically (every24hrs?)as often the pH is significantly different over time.

If you have a pH problem it did not cause the brown staining directly but could have contributed to Fe corrosion which could have cased Fe ions to be deposited on the surface of the parts.

 

[Jerry66]

Thanks Saberblue, Just today I asked my boss to buy a ph meter, based on what tomwalz posted, and I was thankfull for his post he sent me in the right direction. You have given me more detail. Sounds like you know what your talking about, it makes sense. I will look into it.

25 years experience in aluminum die casting

 

[tomwalz]

I'd go with pH strips instead.

I think they are close enough for industrial use. They are much cheaper, don't have to be calibrated and don't dry out.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[saberblue]

Get the factory guy from your coolant supplier to look at the situation

 

[TomDOT]

The other major advantage of pH strips is the low liquid volume needed. With a bit of care, you can readily get a pH measurement with 0.05 mL* of your liquid. Very few pH meters (especially few affordable ones) function at low volumes of test liquid.

Be sure to order a selection of papers with different pH ranges, or at least one which is sensitive in your range of interest. You will be able to achieve better resolution than with a single wide-range general-purpose paper.

*For visualization purposes, a standard glass eyedropper will give you about 20 drops/mL. Different shapes and materials will give you different size drops.