Cutting Fluid for Aluminium: The Staining and Discolouration Problem
Aluminium stains and darkens with high pH and certain additives. The rules for choosing a fluid that lubricates well without harming the surface.
Aluminium behaves differently from steel: it is soft, conducts heat well but is chemically sensitive. The most common problem is staining and darkening (discolouration) on the surface after machining. The cause is usually the fluid's chemistry and pH.
Why does it stain?
- —High pH: Alkaline (high-pH) fluids oxidise and darken the aluminium surface. Aluminium needs pH-controlled, low-alkalinity formulas.
- —Unsuitable additives: Some EP/chlorine additives react with aluminium and leave marks.
- —Long contact: A part left sitting in the fluid stains more; drying after the process matters.
What should the right fluid be?
- —A pH-controlled, aluminium-compatible emulsion or semi-synthetic.
- —Good lubrication (aluminium is soft, high sticking/BUE risk) but low foam.
- —A non-staining, non-discolouring formula; the maker should state "suitable for aluminium".
Operation tips
- —A sharp, polished insert reduces aluminium sticking.
- —High cutting speed + abundant fluid manages heat and sticking.
- —Do not leave the part sitting in the fluid after the process; rinse and dry.
- —Keep the emulsion clean; tramp oil and sludge raise the staining risk.
Casting or wrought/sheet?
Cast alloys (with silicon) and machined sheet/extrusion can behave differently; silicon is abrasive, so tool and fluid choice follow accordingly.
Summary
The key in aluminium is a pH-controlled, aluminium-compatible, well-lubricating fluid and correct cleaning/drying after the process. Ahmet Ceviz recommends a non-staining product for your aluminium application; test it on your own part with a sample.