Effect Of Zinc Equivalent Regulation On Chip Deformation-fracture Mechanism Of Lead-free Silicon Brass

As a traditional engineering material,lead brass is widely used in many industries,such as electrical engineering,household hardware,instrumentation and sanitary ware,due to its good casting properties,mechanical properties,corrosion resistance and cutting performance.However,lead brass is pron to precipitate Pb element during use,which is harmful to the environment and human health.Therefore,the development of a new type of environmentally friendly lead-free silicon brass has become an inevitable trend.At present,the research on the cutting performance of lead-free silicon brass mainly focuses on chip breaking performance and tool wear,while the research on dynamic mechanical properties and chip deformation fracture mechanism of lead-free silicon brass is less.In this paper,the lead-free silicon brass is used as the research object,and zinc equivalent is used as the main variable to regulate the composition and structure of lead-free silicon brass.Firstly,the influence of zinc equivalent on the dynamic mechanical properties of lead-free silicon brass is analyzed.On this basis,the chip serration and localized fracture of lead-free silicon brass were studied by experimental methods.The relevant analytical models are used to analyze the physical quantities in the cutting process of lead-free silicon brass.Finally,the effect of zinc equivalent on chip deformation-fracture mechanism of lead-free silicon brass was revealed.The main research contents and conclusions are as follows:(1)The microstructure and quasi-static and dynamic mechanical properties of four lead-free silicon brasses with different zinc equivalent were analyzed.It is found that with the increase of zinc equivalent,the phase structure of lead-free silicon brass changes from?+?phase to single?phase,and then into?+?phase.Moreover,within the range of 39.0%?49.0%zinc equivalent,the elongation of lead-free silicon brass decreases with the increase of zinc equivalent,while the tensile yield strength increases with the increase of zinc equivalent.In addition,in the strain rate range of 5×10²s^-1?1×10⁴s^-1,the flow stress of lead-free silicon brass increases with the increase of zinc equivalent and strain rate,which has obvious strain strengthening effect and strain rate strengthening effect.(2)The effect of zinc equivalent of lead-free silicon brass on the chip deformation mechanism was studied.Firstly,the turning experiment is used to analyze the chip morphology of lead-free silicon brass from low speed to high speed cutting conditions.The experimental results show that,under the same cutting conditions,the critical cutting speed of lead-free silicon brass for chip serration decreases with the increase of zinc equivalent.For four types of lead-free silicon brass,the critical cutting speed of chip serration decreases with the increase of feed rate,and the decrease of tool rake angle.Meanwhile,a prediction model of critical cutting conditions for chip serration based on the Williams model was constructed,and lead-free silicon brass with different zinc equivalents was predicted and verified.It is found that the higher the zinc equivalent,the earlier the inflection point of the critical dynamic yield stress of the lead-free silicon brass,which makes the critical cutting speed of the chip forming adiabatic shear band relatively small,and the chip enters the serration deformation stage in advance.(3)The effect of the zinc equivalent of lead-free silicon brass on the localized fracture mechanism of serrated chips under high-speed cutting conditions was studied.Firstly,experimental methods are used to analyze the chip morphology of lead-free silicon brass under high-speed cutting conditions,and a chip localized fracture prediction model is constructed based on the adiabatic shear saturation limit theory.On this basis,according to the experimental results and analytical model results,the influence mechanism of the zinc equivalent of lead-free silicon brass on the localized fracture of serrated chips is analyzed.The experimental results show that the larger the zinc equivalent of lead-free silicon brass,the faster the formation and propagation of cracks in the deformation zone of the generated serrated chips,and the higher the degree of energy accumulation in the adiabatic shear zone,which makes the critical cutting speed for chip localized fracture decreases with the increase of zinc equivalent.In addition,the critical cutting speed for chip localization fracture decreases with the increase of the feed rate and the decrease of the tool rake angle.