| As the lightest metal in engineering application,magnesium and magnesium alloy have the advantages of high specific strength,high specific stiffness,good damping performance,easy processing and recycling,and are known as "green engineering materials in the 21st century".However,its low strength and poor corrosion resistance limit its wide application.Microstructural regulation is a direct way to solve this problem.In the whole process of magnesium alloy preparation,the control of microstructure includes the removal of inclusions in magnesium alloy melt,the control of dendrite and segregation in the solidification process of magnesium alloy melt,the control of precipitates and hydrides in solid magnesium alloy after solidification.As an important means to improve the properties of materials by adjusting the microstructure of materials,pulsed electric current processing has attracted more and more attention due to its high efficiency and energy saving.Taking magnesium alloy as the research object,the microstructure of the whole process of magnesium alloy preparation was studied,and the mechanism of pulsed electric current on inclusions,micro/nano precipitates and hydrides was explored so as to optimize the multiple properties of magnesium alloy.It is very important to improve the cleanliness of magnesium alloy melt to ensure the metallurgical quality and improve the final performance of products.Taking the most representative inclusion(MgO)in magnesium alloy as the research object,the effect of pulsed electric current on the removal of MgO inclusion in magnesium alloy melt was studied.Through the analysis of the number and size distribution of MgO inclusions,it is found that the MgO inclusions after pulsed electric current treatment are driven to the surface(bottom and top)of magnesium alloy melt,and then absorbed by the covering agent.In addition,the pulsed electric current also promotes the aggregation of inclusions,but the large-scale inclusions only exist on the surface of the sample.According to the difference of electrical properties between inclusions and magnesium alloy melt,the driving effect of pulsed electric current on electrically neutral MgO inclusions was determined.The numerical calculation shows that the free energy of the system is reduced by the migration of inclusions to the surface under the action of pulsed electric current,and the aggregation trend between the two inclusions with larger radius is obviously stronger than that of the two inclusions with smaller radius.The numerical results are in good agreement with the experimental data,which indicates that the separation of inclusions in molten metal by pulsed electric current is an ideal new method for purifying molten metal.The segregation behavior of magnesium alloy melt during solidification will greatly affect the mechanical properties of subsequent products.Therefore,the effect of pulsed electric current treatment on the precipitation behavior of dendrite and segregation during solidification was studied.It is found that pulsed electric current treatment can not only break the dendrite,but also promote the solute diffusion in the liquid phase,so as to promote the equilibrium state between different precipitates,and finally avoid the formation of divorced eutectic in the solidification process.In addition,it is also found that pulsed treatment during solidification will lead to the diffusion of the atomic clusters which used to be the crystal nuclei and form more fine atomic clusters.These fine atomic clusters will become the crystal nuclei again.Under the condition of constant growth rate,the fine dispersed precipitates will eventually form,which greatly improves the mechanical properties of as cast magnesium alloy.The amount and size of precipitates in solidified as-cast magnesium alloy are closely related to electromagnetic shielding and corrosion resistance.Based on the observation of microstructure and the statistical analysis of the number and size of precipitates,the effect of pulsed electric current on the precipitates in solid magnesium alloy was investigated,and a new idea to improve the electromagnetic shielding performance and corrosion resistance of magnesium alloy by adjusting the precipitates in external field was proposed.The results show that the number and size of precipitates can be effectively controlled by pulsed electric current.By reducing the number and size of precipitates,pulsed electric current can improve the conductivity of magnesium alloy,and then improve the electromagnetic shielding performance of magnesium alloy,especially for high frequency electromagnetic wave.The shielding efficiency of 1500MHz electromagnetic wave is increased by 109.4%.The improvement of shielding effect for high frequency electromagnetic wave is due to the increase of absorption loss caused by redistribution of precipitates and the increase of reflection loss caused by the increase of conductivity.The improvement of shielding effectiveness for low frequency electromagnetic wave is only due to the increase of reflection loss caused by the increase of conductivity.The electrochemical test results show that the corrosion resistance of magnesium alloy is also improved.The decrease of precipitates leads to the decrease of micro galvanic corrosion.In addition,more uniform composition and microstructure are obtained,which reduces the possibility of localized corrosion.The electromagnetic shielding performance and corrosion resistance of magnesium alloy are improved by controlling the microstructure of magnesium alloy with pulsed electric current,which has the advantages of high efficiency and low consumption.When magnesium alloys are used as hydrogen storage materials,hydrogen storage performance is the most concerned aspect.Based on the control of the hydride phase in magnesium alloy by pulsed electric current,the key problem of the poor dehydrogenation kinetics of magnesium hydride phase in magnesium alloy was solved.The effects of pulsed electric current on the amount and rate of dehydrogenation of magnesium hydride phase,as well as the phase composition and microstructure of magnesium alloy were studied.The results show that both pulsed electric current and heat treatment can cause a certain amount of hydrogen release.The dehydrogenation rate of magnesium alloy was significantly increased after pulsed electric current treatment.Pulsed electric current treatment provides a new method to improve the dehydrogenation kinetics of hydrogen storage materials. |
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