The Research And Development Of The Environmental Friendly Unleaded Free-cutting Brass And Its Property Investigation

Lead-brass has been widely applied in various fields because of its excellent mechanical and working properties. However, element lead in the lead-brass is easy to pollute the environment. Nontoxic Bi is the best alternative to Pb for the machinability of brass and environmentally friendly materials. the purpose of the study is to produce the quality Bi-brass bars, which can used to submitted the traditional HPb59-lbrasses.The combining process of semi-continuous casting, hot extrusion and cold-drawing and the combining process of semi-continuous casting and hot-rolling were used to successfully produce the quality Bi-brass bars, whose composition is according to the HPb59-1, and the brass is alloyed with 0.8-1.5%Bi and 1.5%-2% Mn. The content of Pb element in the brass is less than 0.01wt %(immerse quantity of the Pb to water is less than 0.004mg/L).The X-ray analysis, SEM, mechanical properties test, machinability property test, dezincification test and anode polarization test were used to character the properties of the unleaded brass and the effect of the alloy element Bi on the property of the unleaded brass.The microstructure of the brass is composed ofα(Cu_0.64Zn_0.36) ,β(CuZn) and Bi phase, the element Mn is solid soluted in the. matrix. And the morphology is varied with the different processes. the morphology and distribution of alloy element Bi in the brass under different processes is as follows: The grain size of the Bi phase in the semi-casting brass after air cold is about 10μm×20μm, and distributed on a phase and the boundary of a phase in globe-like form. While after water cold, the grain size of the Bi phase is much smaller only about 2μm and uniformly distributed on a phase and the boundary of a phase in both strip-like and particle-like form.The grain size of the Bi phase in the hot-rolling brass is about severalμm, and seriously assembled on the boundary ofαandβphase is block-like form.The grain size of the Bi phase in the hot-extruding brass is about 2-10μm, and ununiformly distributed on the phase boundary ofαandβphase in strip-like form with a little film-like form.The cold-deformed unleaded brass has the poor corrosion resistance because of the distribution of the b phase which is along the process direction and its mass of dislocation and retained stress. The annealing treatment can improve the corrosion property of the cold-deformed unleaded brass and annealed at 250°C the brass has the best corrosion resistance. The corrosion property of the hot-extruding and hot-rolling unleaded brass is comparative to the semi-casting unleaded brass due to the dynamic recovery and recrystallization.The corrosion rate of the unleaded brass in the tap water is increased with the temperature in the binary function, and in the tap water the unleaded brass has the good corrosion resistance. The passivation of the unleaded brass occurred in the solution containing Cl^- and SO₄^2- ions. The brass is more sensitive to the Cl^- ion, and the corrosion rate of the brass in the solution containing Cl^- is about 5-6 times of that in the solution containing SO₄^2-.The corrosion density of the brass in acid solution is larger than in neutral solution. the unleaded brass has excellent passivity property in NaOH alkaline solution, and the active dissolve occurred until the polarization potential is up to 700Mv (vs.SCE).The unleaded brass developed has the excellent mechanical property, machinability property and corrosion resistance compared to the traditional HPb59-1 and it's the environmental friendly materials, so, the brass can used to replace the HPb59-1 brasses.