Relationship between lead content of Cu–40Zn, machinability, and svvarf surface composition determined by Auger electron spectroscopy

Abstract

The surface composition of swarfs from leaded Cu-40Zn has been determined by Auger electron spectroscopy and ion etching as a function of bulk-lead content and machining conditions. The lead additive is distributed at the swart surface as a continuous thinfilm of monatomic thickness and as varying proportions of thick (p.m) lead striations along the cutting direction. The surface lead concentration of the chip is much higher than in the bulk metal. There is also a strong enrichment of zinc at the swart surface. It is considered that the lead surface layers are formed by a process of extrusion of lead owing to the high compressive forces which are developed during cutting. One complete lead monatomic layer is formed at bulk contents≥ 0·88 wt-% and this is associated with the absence of a secondary-shear zone, pointing to the lubrication of the tool/swarfinterface by the lead. A lead content in excess of 2·76 wt-% reduces power consumption, produces discontinuous chips, and is sufficient to maintain a lubricant monatomic layer of lead and 30–50% coverage of thick lead at the swarfsurface over a very wide cutting speed range of 2·5–200mmin⁻¹. The swarf-surface lead concentration is unaffected by the presence of external oil–water lubricants except at the lowest speed.