The effect of lead on the friction and machining of brass

Abstract

Experiments have been carried out on two αβ-brass alloys of similar zinc content, one containing 2·9% lead and the other being lead free. The experiments were of three types: (i) metal-cutting experiments, (ii) sliding friction experiments and (iii) mechanical-property measurements. The effect of lead was found to be very marked in all three cases. In metal machining, the lead addition caused the rake-face friction force to be reduced by a factor of about 5. Similarly, the coefficient of friction in sliding experiments, using an unlubricated steel hall, was reduced from 0·62 without lead to 0·32 with lead. Chip fracture was found to occur at larger depths of cut when machining the leaded alloy. This feature is related to a large reduction in ductility when lead is present. In a torsion test the shear strain to fracture was reduced by a factor of approximately 3 and a similar reduction was found for the fracture energy in a three-point notched bend test. The results of tensile tests showed that the presence of lead had no significant effect on U.T.S. or percentage elongation. The hardness was also unchanged. The reduction in friction in both cutting and sliding is shown to be due to the presence of a lead-rich film with a low shear strength at the interface. The large reduction in ductility is postulated to arise from the low cohesive energy of the lead-brass interface.