On the microstructural strategies adopted for achieving improved machinability in commercial brass alloys

Commercially available lead-free brasses with improved machinability were investigated to elucidate the microstructural mechanisms responsible for their enhanced interrupted chip formation. The study examines the microstructure and chip characteristics of alloys belonging to the Cu–Zn–Si (CW724R) and Cu–Zn–Mg (CW732R) lead-free systems and compares them with the standard CuZn42 (CW510L) and CuZn39Pb3 (CW614N) brasses. The results indicate that improved machinability in these lead-free alloys is achieved either by increasing the amount and hardness of the β phase through precipitation strengthening, as in the Mg-bearing CW732R alloy, or by promoting the formation of hard constituents capable of replacing the β phase, as observed in the Si-bearing CW724R alloy. Chip analyses from rough and fine turning tests revealed that CW724R develops a deformed chip microstructure characterized by periodic deep cracks and localized shear bands traversing the entire chip thickness, resulting in a discontinuous morphology comparable to that of the lead-bearing CW614N alloy. In contrast, the Mg-bearing CW732R alloy, despite its higher hardness and larger β phase fraction, displayed a chip morphology similar to the standard lead-free CW510L alloy under the machining conditions adopted here. These findings appear to contradict earlier reports showing superior machinability in similar Mg-bearing brasses, highlighting that the microstructural design of free-machining alloys must be carefully tailored to the specific chip-removal technology and machining parameters.