Influence of tin and bismuth on machinability of lead free 6000 series aluminium alloys

This paper presents the microstructural characteristics of an extruded AA6012A-T6 (AlMgSiBiSn) alloy and the microstructural changes occurring during turning operations, analysing the mechanism involved in chip breaking. An experimental investigation has been conducted to determine the effects of different cutting speed and feed rate on the machinability of the alloy. The machinability of the AA6012A-T6 alloy, where Pb is substituted by Bi and Sn, has then been compared to the standard AA6012-T6 (AlMgSiPb) and AA6082-T6 (AlSiMg) alloys. The results indicate that the extensive plastic deformation induces a preferred orientation of the grain structure and secondary phases along the shear plane, and a local increase in the alloy temperature. Low melting point compounds, such as the Sn- and Bi-bearings particles, transform into a soft or liquid state, change their initial compact shape to assume a needle-like morphology. The β-Mg2Si and α-Al(FeMn)Si particles are not influenced by the working temperature and keep the initial shape. The AA6082-T6 alloy shows a very poor machinability, with long and continuous strips, while the AA6012A-T6 alloy reveals a good chip formation with small and discontinuous C-shaped chips, similar to the AA6012-T6 (AlMgSiPb) alloy. In particular, a feed rate higher than 0.2 mm/rev provides short and suitable chips, independently of cutting speed.