Overview

The major uses of pure, unalloyed copper are based on its high electrical and thermal conductivity as well its good corrosion resistance. Almost all alloying elements are detrimental to the electrical conductivity of copper, making the purity of the metal an important issue. Commercially pure copper is represented by UNS numbers C10100 to C13000. The various grades of unalloyed copper differ in the amount of impurities and therefore do behave differently. Oxygen free coppers are used in applications requiring high conductivity and exceptional ductility.

The pure copper or high copper alloys are made from copper ores that are obtained from the mines as sulfides, which contain zinc, lead and other sulfur. The ores are crushed and milled until they becomes a powder. A technique known as flotation separates the metal from the non-metal components of the powder. The next step is a concentrating stage where minerals are concentrated into a slurry that is about 15% copper. The copper is then melted and purified in several stages until it is 99% pure copper. At this point it is cast into anodes. Oxygen remains in the structure as cuprous oxide, Cu2O. The majority of the structure is pure copper. The copper metal solidifies from the liquid state by the growth of crystals. The crystals grow in preferred directions and form open, tree like structures called dendrites. The dendritic structure is very typical of cast metals. A lower melting point mixture of pure copper and cupprous oxide, called a eutectic, forms in the open spaces between the dendrites. The eutectic particles are usually dark, globular bodies dispersed in a copper background. The cuprous oxide particles form a network, outlining the dendritic cells. Pores, seen as dark spots in the microstructure, are also present in the as-cast material.

The copper anodes are then refined electrolytically to 99.9% purity. Copper melted under non oxidizing conditions is called oxygen free copper. The most popular form of pure copper is the standard electrical wire grade of copper (C11000) contains 99.95% Cu, 0.03% O2, and less than 50 ppm metallic impurities. It has a high electrical conductivity, in excess of 100% IACS. In the as cast form it is called electrolytic tough pitch (ETP) copper. The structure of the as-cast material is similar to that described above. When the as-cast ETP copper is hot rolled the eutectic structure is completely destroyed. The microstructure of the hot rolled copper contains many small grains. Parallel straight lines extending across many of the grains are called annealing twins. They appear after a metal has been mechanically worked at a high temperature, called annealing, and deformed. The interdendritic network of cupprous oxide particles was destroyed by hot rolling. After hot rolling, cupprous oxide particles changed form, and are present as stringers or aligned rows of dark particles. The oxide particles are much larger and fewer in number than in the as cast microstructure.

NOTE: The file size of the Larger and Largest View of the Micrographs are substantially larger than the thumbnail shown. The Larger View images range in size from 11K to 120K depending on the image. The Largest View images range in size from 125K to almost 500K.