| In recent years,the study and application of wrought magnesium alloys have attracted considerable attention.Compared with cast magnesium alloys,wrought magnesium alloys has better mechanical properties and wider application prospect.The preparation procedure of wrought magnesium alloys ingot is an important prerequisite for the production of deformed magnesium alloys.The metallurgical quality of ingots directly affects the deformation behavior and the properties of the materials.The casting ingots of magnesium alloys prepared by ordinary DC(Direct Chill Casting)semi-continuous casting process has some problems,such as coarse grain size,inhomogeneous microstructure,macro-segregation and micro-segregation,serious oxide inclusion and so on.How to improve the metallurgical quality of magnesium alloys ingot through technical improvement is always the goal of people's efforts.The magnetic forceinduced convection in the low-frequency electromagnetic semi-continuous casting process can effectively regulate the shape of liquid sump and mushy zone by changing the flow and temperature fields in the solidification process,so as to achieve remarkable refinement of the solidification structure,restrain macro-segregation,and reduce cracking trend,improving the surface quality of ingots.However,the practical production results show that there are some problems in the process,such as poor refining effect of large-sized ingots and poor matching between electromagnetic parameters and cooling conditions,and it only has significant refining effect under the appropriate electromagnetic parameters.Therefore,to explore the new electromagnetic field and its generating technology,and to study the influence of the new electromagnetic field on the semi-continuous casting process will provide a technical reference for upgrading the electromagnetic casting process of magnesium alloys.At the same time,it will be possible to improve the metallurgical quality and stability of magnesium alloy ingots.Related researches are significant to the theoretical and practical applications.For the problems of low-frequency electromagnetic casting,in this paper,the semicontinuous casting of AZ80 magnesium alloy ?300mm ingots was investigated,and the effects of PMF and DPMF on the magnetic field distribution of magnesium alloy in semi-continuous casting process and its effect on the melt flow field and temperature field were studied by numerical simulation.Simultaneously,the results of ordinary semi-continuous casting and the application of HMF and DHMF were compared.Finally,the effect of PMF on the metallurgical quality of AZ80 magnesium alloy ?300mm semi-continuous ingots and DPMF on AZ80 magnesium alloy ?85mm ingots was studied.The main conclusions are following:(1)Numerical simulation results show that,whether PMF is applied or DPMF,the pulsed magnetic field has higher magnetic flux density and greater electromagnetic force than the equivalent HMF and DHMF;Compared with PMF,when the DPMF is applied,the magnetic flux density and the electromagnetic force will decrease,but the equivalent HMF will decrease significantly.Under the same electromagnetic conditions,the relationships of the magnetic flux density and electromagnetic force in the metal melt with the four types of electromagnetic field are BPMF>BDPMF>BHMF>BDHMF and FPMF>FDPMF>FHMF>FDHMF,respectively.(2)Compared with the equivalent HMF,the electromagnetic force of the PMF has a larger angle with the axis,that is,the horizontal component is larger and the direction is directed to the axis,and the magnitude of the electromagnetic force changes periodically.Therefore,the effect of melt oscillation is easily caused.From the distribution of the electromagnetic force of the pulsed magnetic field in the metal melt,it can be seen that the effect of melt oscillation gradually decreases from the top surface to the inside,but the axial direction is basically consistent,which can reach deep below the liquid surface;Whether the DPMF or the equivalent DHMF,both of them have oscillation and forced convection at the same time,but they all have stronger convection effect than PMF and HMF,the oscillation effect is poor.(3)Within the parameters of this study,the magnetic flux density and electromagnetic force of PMF increase with the increase of current intensity and duty cycle,and the electromagnetic force of DPMF is similar to that in PMF,however,the magnetic flux density fluctuates when the duty cycle is high.The magnetic flux density of PMF and DPMF increased with the decrease of frequency and the peak value increased in 0.4T(period),and the peak value appears,at the same time,the difference in the peak value of the two is increased.After 0.4T(period),the magnetic flux density is opposite and no peak appears.The electromagnetic force increases with decreasing frequency and the sensitivity is higher when the frequency is lower than 25Hz,but the frequency has little effect on the electromagnetic force when the DPMF is applied.(4)In ordinary semi-continuous casting,the transverse temperature gradient of melt is large and the liquid sump is deep.When the PMF is applied the radial temperature distribution of the melt tends to be uniform near the center,but the temperature gradient near the surface of the ingot is higher.When the DPMF is applied,the radial temperature distribution of the liquid sump becomes more uniform,the liquid sump becomes shallower.With the increase of the current intensity,the melt flow velocity increases obviously,the transverse temperature gradient decreases,the liquid sump becomes shallower,and the velocity decreases obviously with the increase of the frequency,but the temperature distribution does not change much.As the increase of duty cycle,the melt flow velocity increases first and then decreases,and the depth of the liquid sump increases slightly.(5)The results of PMF semi-continuous casting AZ80 magnesium alloy ? 300mm ingots show that the solidification structure is coarse and the macro-segregation and micro-segregation are large when no magnetic field is applied.When the PMF is applied,the solidification structure of the ingots is significantly improved and macro-segregation and micro-segregation of elements decreased.At low frequencies,the macro-segregation of Al and Mn elements is reduced,but Zn distribution does not differ much.Within the parameters of this experiment,when the electromagnetic parameter is I=70A,f=30Hz,D=20%,the fining effect is better.(6)The results of the study of ?85mm die casting AZ80 magnesium alloy ingots show that the DPMF can significantly improve the uniformity of the solidification structure and gradually decrease the grain size of the ingot with the increase of current intensity.When the electromagnetic parameter is I=70A,f=30Hz,D=20%,the grain size is uniform and fine.The grain size range is reduced from 400?m?600?m to 350?m?450?m in die casting.If the frequency is too low,the grain coarsening will occur,and if the frequency is too high,the effect of refinement of the electromagnetic field will gradually weaken. |
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