Role of lead in free-machining brass

Abstract

The role of lead in free-machining brass has been studied by calculating the temperatures formed in the primary and secondary shear zones of chip formation and by examining, in a scanning electron microscope, the structure of chips from cutting experiments. The calculations show that the temperatures in the primary zone do not exceed 200°C for workpiece speeds up to 200mmin^−l. The discontinuous chip segments are formed as ductile fracture occurs along a shear instability in the primary zone. Scanning electron microscopical observations of the cuplets on the free surface of the chips suggest that voids occur at the lead inclusions so that the void–sheet mechanism of crack propagation gives rise to the discontinuous chip. A possible cause of void initiation at the lead inclusions is discussed. In the chip/tool contact region the major function of the lead is to provide layers of low shear strength within the secondary shear zone. Lead is also drawn out of the many voids to be deposited on the tool faces. At high cutting speeds the maximum temperatures, occurring at the end of the contact length, exceed the melting point of lead. However, in the speed range investigated this has only a marginal injluence on the chip/tool interaction. Thus, the ‘internal lubrication’ effects, arising over the first part of the contact length, remain the domi/Ulnt function of the lead inclusions in the secondary shear zone.