Discoloration on machined surface
Discoloration on machined surface
(OP)
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





RE: Discoloration on machined surface
RE: Discoloration on machined surface
25 years experience in aluminum die casting
RE: Discoloration on machined surface
RE: Discoloration on machined surface
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.
RE: Discoloration on machined surface
RE: Discoloration on machined surface
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.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Discoloration on machined surface
25 years experience in aluminum die casting
RE: Discoloration on machined surface
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.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Discoloration on 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.
RE: Discoloration on machined surface
25 years experience in aluminum die casting
RE: Discoloration on machined surface
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.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Discoloration on machined surface
RE: Discoloration on machined surface
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.