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6061 aluminum turns black after machining and chemical polishing

I AM SUPPLYING 6061 (HIGHER PERCENTAGE OF Mn and Cr) ALUMINUM EXTRUSION FOR MY CUSTOMER. NOW, THEY ARE FACING "BLACK STAIN" AFTER THE MACHINING (COOLER ADDED) AND CHEMICAL POLISHING. FIRST, THEY CAN'T SEE ANY "BLACK STAIN" AFTER MACHINING. BUT, AFTER THE CHEMICAL POLISHING THE "BLACK STAIN" APPEAR. THE "BLACK STAIN" IS JUST LIKE FOLLOW THE TRACK OF CUTTER (CARBIDE). UNDER THE SEM, WE CAN'T SEE ANY CARBON. WHY IT SHOW BLACK?

P.S.LIM
ALUMINUM EXTRUSION - MALAYSIA
2005

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2005

Hi there :
please talk with your customer and check what kind of machining coolant/lube are they using.
Make sure to check if they are removing the first layer of aluminum (oxide/anodized).
Aluminum form oxide in the exterior very easily and very fast , and is also capable to absorb some amount of carbon coming from the coolant.
coolant/lubricator liquids have a hidrocarb base , so , you can find big amounts of carb on it an usally leave a little stain ( "black smoke") on some materials.
Good luck with that!

Patricio Lopez
- Charlotte , NC

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BLACK SPOTS MAY BE DUE TO THE PRESENCE OF MAGNESIUM SILICIDE Mg2Si on the surface of aluminium. This coarse precipitation formed due to different cooling rate after the extrusions from presses. It can be avoided by keeping the contact time between the hot extrusions and the run out table to a minimum untill the section has cooled to about 250C.

DESMUT STAGE DURING THE ANODIZING COULD REMOVE THIS BLACKING BUT IF THE PROBLEM APPEARED AFTER THE ANODIZING SO YOU HAVE TO APPLY THE PREVIOUS RECOMMENDATION.

Ahmed Fouad Serag Eldin
Ein Shams - Cairo - Egypt
2005

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I see that on chemical polishing you are getting black spot. I want to know what solution you are using for chemical you are using for polishing. I you are using alkaline soluiton you get black smunt, which you need to remove with dip in nitric acid.
you could try

Dr. H B Rudresh
metal finishers - Bangalore, India
2006