Study On Multi-Annular Electromagnetic Stirring Melt Treatment Method And Its Application For Large Volume Aluminum Alloy With High Strength

As a kind of deformed aluminum alloy material,high strength aluminum alloy has been widely used in aviation,aerospace and military industry.However,due to its high degree of alloying,coarse and nonuniform solidification structure,and serious macrosegregation,the defects such as porosity,shrinkage cavity and hot crack occurred in the direct casting process,which seriously restricted its use and development.The results of previous studies show that AEMS(Annular Electromagnetic Stirring)can effectively promote the forced convection of the melt and obtain the temperature field and composition field with uniform distribution,which is conducive to refining the grains and reducing the segregation.However,the existing AEMS melt treatment method is only suitable for the treatment of small volume(? 5kg)high-strength aluminum alloy melt,which can not meet the requirements of large-size high-strength aluminum alloy casting for large volume(? 20kg)melt treatment quality,which limits the application of AEMS technology in the field of direct casting of large-size high-strength aluminum alloy parts.Therefore,based on AEMS,a new technology(M-AEMS,Multi-Annular Electromagnetic Stirring),which is suitable for the treatment of large volume melt(? 20 kg)is proposed in this paper.The typical 7075 aluminum alloy was selected as the research material to study the melt treatment of high strength aluminum alloy and the direct forming of large castings.Based on the AEMS model,the electromagnetic shielding module and the central cooling module are added by UG software,and the M-AEMS model suitable for large volume 7075 aluminum alloy melt treatment is established.The electromagnetic field,flow field and temperature field of 7075 aluminum alloy melt in the M-AEMS melt treatment were simulated.The numerical results show that the optimized electromagnetic generation module and electromagnetic shielding module can effectively improve the magnetic field strength at the middle and bottom of 7075 aluminum alloy melt,and reduce the magnetic field strength at the top of the melt,so that the middle and bottom of the melt can be stirred violently while maintaining a relatively stable melt surface.The electromagnetic field is changed from horizontal rotating electromagnetic field to spiral electromagnetic field,which improves the shear strength and turbulence degree of the flow field.The central cooling module improves the uniformity of temperature field and cooling rate of 7075 aluminum alloy melt,and improves the processing efficiency of M-AEMS model.Therefore,the M-AEMS model can realize the uniform distribution of temperature field and composition field in a short time.According to the optimization results of the structural parameters of the M-AEMS model by numerical simulation,a 25 kg melt treatment experimental device was developed.The electromagnetic field,temperature field,flow field,liquid surface morphology,microstructure and chemical composition distribution of 7075 aluminum alloy melt with a mass of 20 kg were studied by M-AEMS device.The experimental results show that the electromagnetic field strength distribution measured by Gauss meter is in good agreement with the numerical simulation under the condition of stirred crucible under no load condition.After 21 seconds of M-AEMS treatment,the macrosegregation rates of Zn,Mg and Cu decreased from 4.4%,5.9%,6.5%to 2.1%,3.3%and 4.2%,respectively.The average grain size of ingot decreased from 170 ?m to 118 ?m,the grain shape factor increased from 0.37 to 0.56,and the equiaxed degree of grain morphology increased.Furthermore,the effect of M-AEMS on the solidification behavior of 7075 aluminum alloy was analyzed theoretically.The application research of 7075 aluminum alloy track plate liquid die forging was carried out by m-aems melt treatment method.The comparative experiments of M-AEMS method,Sc,Zr chemical refining method and M-AEMS+Sc,Zr chemical refining method were carried out,the results show that:?after M-AEMS treatment,the microstructure of 7075 aluminum alloy track plate is finer and more uniform,the average grain size of the casting is reduced from 151 ?m to 110 ?m,the grain size nonuniformity factor is reduced from 3.3 to 2.5,and the macrosegregation rate of Zn,Mg and Cu in the casting is reduced from 2.4%,4.5%,5.2%to 1.3%,1.8%and 2.2%,respectively·?after chemical refining by Sc and Zr,the average grain size of 7075 aluminum alloy track plate is reduced to 61 ?m,the grain size is 3.1,the macro segregation rate of Zn,Mg and Cu is 2.6%,4.3%and 5.0%,respectively,and there is no significant change in the grain size nonuniformity factor and the macro segregation rate of Zn,Mg and Cu·?after M-AEMS+ SC and Zr chemical refining treatment,the average grain size of 7075 aluminum alloy track plate was reduced to 53 ?m,the nonuniform factor of grain size was 2.3,the macrosegregation rate of Zn,Mg and Cu elements was 1.1%,2.2%and 1.9%,respectively,and the grain size,the nonuniform factor of grain size and the macrosegregation rate were significantly reduced.After T6(738 K × 8 H+748 K × 4 h,water cooling,393 K x 24 h)heat treatment,the yield stren:th.tensile strength and elongation of the track plate castings are 481 MPa,568 MPa and 9.4%,respectively.The mechanical properties of the track plate castings are close to that of 7075 aluminum alloy forgings.