Fabrication And Properties Of Mg₆₅Zn₃₀Ca₅ Amorphous Layers By Laser Remelting

Mg-Zn-Ca amorphous alloys have been considered as the promising orthopedic implant materials because of their excellent mechanical properties,biocompatibility and biodegradability.At present,the research of of Mg-Zn-Ca amorphous alloys mainly focus on their glass formation ability,mechanical properties,degradation behavior and biocompatibility.Mg-Zn-Ca amorphous alloys are mainly prepared through copper mold casting and single roller melt-spinning,whose shape and size could not meet the clinical requirements,which seriously limits their application in orthopedics.In view of the inherent limitations of the traditional manufacture process of Mg-Zn-Ca amorphous alloys,Mg₆₅Zn₃₀Ca₅ amorphous layers were prepared by laser remelting in this study.Furthermore,the microstructure of Mg₆₅Zn₃₀Ca₅ amorphous layers were optimized through crystallization treatment,and their degradation behavior and biocompatibility were systematically and deeply investigated.The main research work and results are as follows:Mg₆₅Zn₃₀Ca₅ alloys were remelted with different laser linear energy density,and the effect of laser linear energy density on the microstructure of the remelted layers were studied.The results indicated that the remelted layers were composed of?-Mg and Mg₅₁Zn₂₀ phase with high laser linear energy density,while of amorphous phase and Mg₅₁Zn₂₀ dendrites with low laser linear energy density.With the decrease of the laser linear energy density,the volume fraction of amorphous phase in the remelted layers increased firstly and then decreased,resulting in the same trend of the corrosion resistance.When the laser linear energy density is 6 J/mm,the volume fraction of amorphous phases in the remelted layer is the highest with the most excellent corrosion resistance.In order to explore the forming condition of Mg₆₅Zn₃₀Ca₅ amorphous alloy during laser remelting process,the temperature field of the molten pool was simulated using finite element method,and its temperature distribution and cooling rate were obtained.The results indicated that the peak temperature of molten pool decreased gradually while the cooling rate increased with the decrease of laser linear energy density.The critical cooling rate of Mg₆₅Zn₃₀Ca₅ amorphous layers prepared by laser remelting was between 512.7 K/s and 1159.4 K/s.The precipitation phase of Mg₅₁Zn₂₀ in the Mg₆₅Zn₃₀Ca₅ remelted layers mainly formed in the process of laser overlapping.Increasing the laser heating rate and reducing the residence time(i.e.reducing the laser linear energy density)could reduce the structural relaxation of amorphous phase and inhibit crystallization.The crystallization behavior and non-isothermal crystallization kinetics of Mg₆₅Zn₃₀Ca₅ amorphous alloy were studied by differential scanning calorimetry(DSC).The results indicated that the initial crystallization stage of Mg₆₅Zn₃₀Ca₅ amorphous alloy belonged to typical primary crystallization,and the precipitated phase was Mg₅₁Zn₂₀.The local crystallization activation energy and local Avrami index decreased with the increase of crystallization volume fraction,in which the activation energy of nucleation was 190.04 KJ/mol and the activation energy of growth was 161.83 KJ/mol.The nucleation and growth process of Mg₅₁Zn₂₀ phase could be divided into three stages:in the early stage of crystallization,the growth was controlled by three-dimensional diffusion with gradual increase of nucleation rate;in the middle stage of crystallization,the growth was controlled by three-dimensional diffusion with gradual decrease of nucleation rate;in the late stage of crystallization,the growth was controlled by one-dimensional diffusion with gradual decrease of nucleation rate.In order to meet the clinical requirements of uniform corrosion,the microstructure of Mg₆₅Zn₃₀Ca₅ amorphous remelted layers were optimized through crystallization treatmen,and the degradation behavior of Mg₆₅Zn₃₀Ca₅ amorphous remelting layer was studied systematically and comprehensively.The results indicated that the corrosion resistance of Mg₆₅Zn₃₀Ca₅ amorphous remelted layers could be improved first and then reduced after crystallization treatment.In addition,T453K remelted layer exhibited the optimized corrosion-resistance property.The results of immersion test indicated that the deposits of Mg₆₅Zn₃₀Ca₅ remelting layer were mainly consisted of Mg(OH)₂,Mg/Ca carbonates,Mg/Ca phosphates and other compounds during the degradation process in the Hank's solution.With the extension of immersion time,the relatively stable passivation layers were formed,which could significantly inhibit corrosion.The biocompatibility,osteogenic function and antibacterial ability of Mg₆₅Zn₃₀Ca₅remelted layers before and after crystallization treatment were evaluated through cell tests and antibacterial tests.The results indicated that the cytotoxicity-gradation of As-remelted,T373K and T453K was 0,which induced non-cytotoxicity to MC3T3-E1cells,while that of T553K was 2 with slight cytotoxicity to MC3T3-E1 cells.As-remelted could promote the proliferation of MC3T3-E1 cells to a certain extent,but had little effect on the differentiation process.T373K and T453K could promote the proliferation and differentiation of MC3T3-E1 cells,while T553K could inhibit the proliferation and differentiation of MC3T3-E1 cells.As-remelted,T373K,T453K and T553K had strong sterilization effect on Staphylococcus aureus and exhibited good antibacterial property.