| Magnesium alloy has good biocompatibility,specific strength and degradability,so it is of excellent application potential in the new generation of internal fixation materials.However,the poor corrosion resistance of magnesium alloy limits its application on clinic treatment.Amorphous magnesium alloy has homogeneous single-phase structure and no grain boundary,which improves its corrosion resistance.The size of amorphous magnesium alloy prepared by traditional methods is small,and it can not be directly processed.Therefore,it is of positive significance to explore a new method to construct a amorphous structure on magnesium alloy surface for promoting the clinical application.In this paper,the temperature distribution of laser surface amorphization of Mg67Zn28Ca5 alloy was studied by numerical simulation.Meanwhile,and the formation conditions and influencing factors of laser surface amorphization were analyzed.Amorphous coatings were prepared on the Mg67Zn28Ca5 alloy surface by fiber laser.Then the effects of laser scanning speed and organic protective coating on the microstructure,phase composition,microhardness and corrosion resistance of the amorphous coating were investigated.The main results are as follows:The three-dimensional transient temperature field analysis model of Mg67Zn28Ca5 alloy during laser amorphization was established by ANSYS Workbench.The effects of latent heat,heat transfer mode and thermophysical parameters on the temperature field were considered in the model.The result of numerical simulation showed that the maximum temperature distribution of the molten pool is related to the laser moving speed,which is slightly behind the physical position of the laser spot center.Increasing the output power of laser can improve the cooling temperature gradient of molten pool.When the laser scanning speed was 500 mm/min,100 mm/min,2000 mm/min and 4000 mm/min respectively,the corresponding average cooling rate of molten pool was 426.0 oC,1003.2 oC,2239.7 oC and 4798.3 oC.The residence time above Tg was 0.8 s,0.35 s,0.16 s and 0.07 s,respectively.Increasing scanning speed of laser can improve the cooling rate of the molten pool and significantly shorten the residence time,which is conducive to the formation of amorphous on the Mg67Zn28Ca5 alloy surface.The amorphous layer can be formed on the Mg67Zn28Ca5 alloy surface when the laser power was 200 W and the scanning speed was 2000 or 4000 mm/min.The average microhardness of amorphous layer increased from 200 HV to 470 HV,which is 2.35 times higher.The amorphous layer is mainly composed of amorphous phase and a small amount of crystal phase.The crystal phase is?-Mg and Mg51Zn20 phases,and the existence of crystal phase is due to the thermal effect of the amorphous layer in the subsequent laser processing process.The higher the laser scanning speed was,the less thermal effect would be on the amorphous layer.The oxidation phenomenon on the Mg67Zn28Ca5 alloy surface was obviously inhibited by organic coating.The depth of molten pool was 780?m and the width of was 2121?m when the laser scanning speed was 2000 mm/min.While the scanning speed increased to 4000 mm/min,the bath depth and width were 530?m and 1689?m,respectively.The error of bath depth and width values between measurement and simulation was less than 10%.The mass loss rates of the three samples(2000 mm/min,S1;2000 mm/min supplemented with organic coating protection,S2;4000 mm/min,S3)were 0.192%(S1),0.175%(S2)and 0.125%(S3)after soaking in simulated body fluid for 168 h.Compared with as cast magnesium alloy,it decreased by 6.0 times,6.6 times and9.2 times respectively.The corrosion potential of S3 was-1.15 V,which was 0.07 V,0.05 V and 0.16 V higher than that of S1,S2 and as cast alloy.The corrosion current density of S1,S2 and S3 was 1.91×10-6 A/cm2,1.53×10-6 A/cm2 and9.2×10-7A/cm2,it decreased by 6 times,8 times and 13 times respectively compared with the as cast alloy. |
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