Leaded Coppers

Overview

Lead is frequently added to copper alloys to increase their machinability. The role of lead in copper alloys is two fold, it acts as a lubricant and, in the free machining grades, the lead assists in chip break up. Lead is added to many copper alloys, making all types of free machining alloys. Lead does not affect the structure and properties of copper because it is practically insoluble in solid copper. The pure copper solidifies first, leaving the lead to solidify last as almost pure lead globules at the grain boundaries or in the inter dendritic regions. The size and concentration of lead particles depends upon the concentration of lead in the alloy. Leaded coppers are categorized as low lead alloys, or free machining alloys and high lead alloys. In the free machining alloys, the lead acts as chip breaker and lubricant making these alloys easier to machine than their non leaded counter parts. The high leaded copper alloys are used in bearing applications. In the bearing materials, the lead acts as a solid lubricant and the copper is the load bearing support. Lead is added to many of the copper alloys producing free machining brasses, bronzes and other copper alloys. The free machining brasses and other alloys are presented in the sections with the specific alloy types. The cast, high leaded copper alloys used for bearings are presented in this section. They are designated by UNS C98200 through C98840.

The microstructure of the leaded copper alloys is similar to the structure of the unalloyed copper materials with the addition of almost pure lead particles in the grain boundaries. The size and amount of lead particles in the structures depends on the concentration of lead in the alloy. The microstructure of the as cast copper lead alloys consists of pure alpha copper dendrites, with lead globules in the boundaries between the dendrites. The higher the lead content of the alloy the more lead globules present in the structure. In the wrought structures, the lead is present as discrete particles between the alpha copper grains.

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.