Research On Cryogenic Milling Technology Of Medical Magnesium-Calcium Alloy

Magnesium-calcium alloy has great potential to be a medical metal material due to its good biocompatibility and degradability.However,Mg-Ca alloy has some shortcomings,such as fast corrosion degradation rate in human body,chip burning and other phenomena for its high linear expansion coefficient and low ignition point during the process of cutting.In recent years,"green manufacturing" has gradually become research focus.Which use liquid nitrogen as cooling liquid to spray into the cutting area to reduce the cutting temperature and improve the surface quality.Meanwhile,after liquid nitrogen came into use,it would be vaporized into nitrogen and not cause environmental pollution.It is an effective and green processing method for processing medical magnesium-calcium alloy.The thesis researched the processing technology of medical Mg-Ca alloy in liquid nitrogen cooling environment,aiming at solving the problems,such as workpiece deformation,chip burning and burned machined surface in the cutting process of magnesium alloy,improving the cryogenic cutting process of magnesium alloy,and providing theoretical and practical guidance for the processing and application of medical Mg-Ca alloy.The mechanical properties of Mg-Ca alloy at low temperature were studied experimentally in order to clarify the relevant properties of materials at low temperature.It is found that the tensile strength and brittleness of Mg-Ca alloy are enhanced with the decrease of temperature.Then,the clean cutting characteristics of Mg-Ca alloy under different cooling modes were studied,which is found that the cutting fluid and chips remained on the surface when water or oil based cutting fluid was used,while there were fewer chips on the surface using dry cutting and cryogenic cutting.The XRD diffraction analysis on the machined surface found that cryogenic cutting would produce more second phases on the machined surface.When there are enough second phases,the corrosion resistance can be achieved.The effects of cutting speed,cutting depth and feed rate per tooth on surface roughness and surface hardness were analyzed by single factor cutting experiment of Mg-Ca alloy.It is found that as the cutting speed increases,the surface roughness and surface hardness tend to decrease.With the increase of cutting depth and feed per tooth,the surface roughness and hardness show an increasing trend.Through cutting orthogonal test of Mg-Ca alloy,discovered that the influence degree on surface roughness is feed per tooth,cooling mode,cutting depth and cutting speed.The prediction models of surface roughness under dry cutting and cryogenic cutting conditions are established respectively,and the significance of the prediction models is analyzed and verified by experiments using MATLAB.Occurred that the regression effect of the models is remarkable,and the numerical error between the predicted values and the experimental results is small.Causative performance of the Mg-Ca alloy surfaces machined by dry and cryogenic cutting was studied by friction and wear tests and electrochemical corrosion tests.Found that the surfaces machined by cryogenic cutting have better wear resistance and corrosion resistance than dry cutting.