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Trace Elements and their Impact on Surface Finishing Characteristics

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Trace Elements and their Impact on Surface Finishing Characteristics of Aluminum Extrusions

Jude Mary Runge, Ph.D., Comprehensive Metallurgical Consulting and
CompCote International, Inc., Lombard, Illinois, USA


ABSTRACT—Because of its face center cubic (fcc) structure, aluminum readily mixes with other metals
in the liquid state; however, the solid solubility of various metals is limited to a few percent. Since no
element has complete solid solubility with aluminum, alloying elements form phases with the base
aluminum, and/or intermetallic compounds with each other. While main alloying additions develop
primary mechanical properties, which designate the various aluminum alloy classifications, trace
elements are what bring the alloy composition together, refining the characteristics of tensile strength,
ductility, corrosion resistance, fatigue resistance, and even surface quality to the finished product. These
small amounts make a big impact on alloy performance; shifts in trace element ratios can drastically
change the alloy microstructure. This paper addresses the impact that variations in trace elements can
have on the extruded component surface, even within the specification limits for the alloy composition.
Changes in the surface chemical potential, caused by these small, but important shifts in composition alter
corrosion resistance and finishing characteristics of the extruded component. By characterizing the
finishing performance of the alloy with corresponding trace element ratios, lots of material can be
identified as “finishing quality”. Key Words: Trace alloying elements; Aluminum extrusions; Surface
finishing; Anodizing.

© Extrusion Technology for Aluminum Profiles Foundation (ET Foundation). All rights reserved. No part of The Proceedings may be reproduced in any form without the express written permission of the ET Foundation.