Metal finishing Q&As since 1989

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Preventing yellow/tan discoloration of molded aluminum

Q. We want to NOT colorize our metal! We make alloyed aluminum ingots intended for our customers to remelt and make product castings. These are casting alloys with high silicon, somewhere between 7% and 17% Si, and other elements.

We pour molten aluminum alloy into a continuous conveyor of ingot shaped molds. The conveyor passes water cooling sprayers to rapidly freeze the molten metal. The ingot molds are then turned upside down where the frozen ingot falls out of the mold. A couple of times a year our ingots have a yellow to tannish brown colored surface. This is probably due to a thin film at the atomic level diffracting the light shining on it at a certain wavelength. It usually goes away upon remelting at the customer's foundry.

Telling our customers this sometimes doesn't do much good. Some of them think that there's something wrong with metal. We have seen this appearance on the ingots of some of our competitors.

We think we know how this is happening, we just want to prevent it from happening. Any ideas?

Ron Zakrzewski
Laboratory Manager - City of Industry, California, USA
August 14, 2023