Preparation Of Bismuth Lead-free Cutting Brass And Corrosion, Cutting Performance Study

The purpose of this study is to develop a new kind of easy cutting leadless bismuth brass, which is to substitute for the widely used toxic leaded brass. In this work,the easy cutting bismuth brass was fabricated by melting, casting and extruding processes. The effect of adding Ti, Al and Ce on the microstructure and corrosive properties were studied by X-ray diffraction,optical microscope, scanning electron microscope , potentiodynamic polarization and electrochemical impedance spectroscopy, The mechanical property and machinability were tested by electron universal test machine and metal stock removing machine. The corrosion process and fundamentals of the cutting process were mainly discussed. The following results were obtained:1) By adding Ce to the Cu-Zn-Bi alloy,the as-cast micro-structure of Cu-Zn-Bi-Ce alloy was refined. The grain size of as-extruded and annealed Cu-Zn-Bi-Al alloy were smaller than Cu-Zn-Bi, Cu-Zn-Bi-Ce and Cu-Zn-Bi-Ti alloys at the same condition. The ultimate tensile strength and yield strength of Cu-Zn-Bi-Al were much higher than the other three alloys, and also Cu-Zn-Bi-Al alloy has good plastic property.2) The Bi exist as film along phase bounaries in Cu-Zn-Bi alloy , but the Bi film changed to particles with the addition of Ti, Al and Ce. The average dezincify layer of Cu-Zn-Bi and Cu-Zn-Bi-Al alloys are about 300μm,but the corrosion layer of Cu-Zn-Bi-Ce is more than 1000μm.3) The microstructure of the Cu-Zn-Bi and Cu-Zn-Bi-Al alloys in 1.0%CuCl₂ solution were examined. Also examined the mechanism of crack formation process on the corrosion surface , and it is a result of effects of corrosion and residual stress. The open potential of Cu-Zn-Bi and Cu-Zn-Bi-Al are -40.72mV and -144.67mV.4) The typical Nyquist plots with different immersion time of Cu-Zn-Bi and Cu-Zn-Bi-Al alloys were examined, and the equivalent circuit were obtained. The formation of protection film of the alloys were studied, and it was found the film was formed before immersing 6h, and the film was destroyed by Cl^-. The adding of element Al accelerate the formation of the protection film.5) According to the depth of the corrosion layer, the diffusion coefficient of Zn were calculated , and the order of magnitude of it is 10^-10～10^-9. The diffusion coefficient of Cu-Zn-Bi-Al is much lower than that of Cu-Zn-Bi, but the diffusion coefficient of Cu-Zn-Bi is lower than that of Cu-Zn-Bi-Ce.6) With the increasing of Bi content, the machinability of Cu-Zn-Bi-Ti alloy improved. The chip of Cu-Zn-Bi-Al is needle like, and the cutting force is lower than that of HPb59-1. The chip of Cu-Zn-Bi-Ce is helical, but the cutting force is much higher than HPb59-1.7) The effect of cutting heat on the deformed area was also studied. It was found a slim change of the temperature can lead to the heat stress of Bi, which can result in the deform of Bi. According to the fracture theory of Griffith, the critical dimension of Bi was calculated, which is 0.3446μm.8) Cu-Zn-Bi-Al alloy has good mechanical properties,corrosive nature and machinability. It can substitude for the widely used leaded brass and have a pratical application.