Machinability and microstructure of some common non-ferrous metals and alloys

Abstract

The most common machinability parameters used for non-ferrous metals – tool life, surface finish, and swarf size and shape – are reviewed. The microstructure of ‘free-machining’ non-ferrous alloys is controlled primarily by manufacturing procedures, e.g. casting technique, extrusion conditions, heat treatment, and aging, and by alloy composition. Variations in machinability so produced, particularly in relation to copper-base alloys, are discussed. Recent BNF work showed conclusively that the size and distribution of the lead-rich phase in leaded brass and the CU₂S phase in sulphur-copper alloys controlled the tool wear rate in single-point turning operations. However, in drilling, composition alone was the controlling factor. The presence of hard particles (precipitates) and inclusions in either type of alloy was detrimental to tool life, as illustrated by iron in brass and oxygen and phosphorus in dilute copper alloys. The effects of variations in production methods for aluminium alloys are discussed, particularly in relation to the size and distribution of the various phases that can precipitate from the common free-machining alloys containing lead and bismuth. A brief review of the magnesium alloys is also included.