Research On Uniform Cooling Annular Electromagnetic Stirring Casting Technology For Large-sized 7 Series Aluminum Alloy Billets

The 7 series high strength aluminum alloy,characterized by light weigh,high strength,high toughness,corrosion resistance,has been widely used in aerospace,defense industry,rail transportation and other fields.However,the large-sized high strength aluminum alloy billets prepared by normal direct chill casting have defects such as coarse strcuture,segregation and hot crack,which seriously affect the following processing and service performance of the final products.In this paper,based on annular electromagnetic stirring(AEMS)technology,the uniform cooling annular electromagnetic stirring(U-AEMS)casting was invented for preparing large-sized billet with uniform and fine structures by imposing the intercooling and electromagnetic stirring to large volume melt during the direct chill casting.Based on the combination of numerical simulation and experimental methods,the prototype of U-AEMS casting technology is designed and developed.The effects of U-AEMS casting process on the solidification behavior,the microstructure and property,the mechanism and the hot deformation behavior of the 7 series aluminum alloy billets are systematically studied,which lay a solid theoretical and technological foundation for the engineering application and industrialization of this technology.The main research works and results of this paper are as follows:A three-dimensional numerical model of U-AEMS casting coupling multiphysical fields such as electromagnetic field,temperature field,flow field with solidification process was established.The effects of U-AEMS on the temperature field,flow field and solidification in the preparation of 7 series aluminum alloy billets were studied by this model.It was found that U-AEMS could increase the heat convection and heat conduction of the melt,improve the melt cooling rate and reduce the sump depth during the direct chill casting.The key parameter ranges of the in-mold cooler cooling strength,the stirring current,the stirring position and so on were determined by numerical simulation,which provides a design basis for experimental research.Based on numerical simulation,a prototype equipment of U-AEMS casting was designed and established.The experimental study on the preparation of large-sized 7 series high strength aluminum alloy billets was carried out.The effects of intercooling heat transfer coefficient,electromagnetic stirring current and the electromagnetic stirring position on the solidification behavior and structure of the billets were studied systematically.The technical barriers of-preparing large-sized 7 series high strength aluminum alloy billets are broken through.7005,7050 and 7055 aluminum alloy billes with diameter more than 500mm have been successfully prepared.Compared with the billets produced by normal direct chill casting,the billets structure is obviously refined and homogenized,the average grain size is less than 200 μm,and the relative segregation rate of the alloy elements is less than 5%,macrosegregation has been effectively improved.The effects of the U-AEMS casting on the solidification behavior,effection mechanism of large-sized aluminum alloy billets and its mechanism were studied.The application of U-AEMS can significantly improve the uniformity of the melt temperature field and composition field,which is beneficial to the realization of the explosive nucleation of large volume melts and also can significantly increase the effective nucleation rate.From the point of view of thermodynamics,the effect of the U-AEMS on the nucleation was analyzed.The U-AEMS casting can increase the undercooling,the Gibbs free energy difference at the unit volume,the magnetic Gibbs free energy difference and electric Gibbs free energy dif’ference during the nucleation,therefore,both the critical nucleation radius and critical nuclear energy of the nucleation are reduced.The U-AEMS casting could effectively change the flow direction of the solidification front melt,so that the macrosegregation were improved.The as-cast mechanical properties and hot deformation behavior of large-sized high strength aluminum alloy billets prepared by U-AEMS casting were studied.Compared with normal direct chill casting,it was found that the tensile strength and elongation of the as-cast 7005 aluminum alloy billets prepared by U-AEMS casting were improved.The hot deformation simulation indicates that the hot deformation processing property of the 7005 aluminum alloy billets prepared by U-AEMS casting is obviously improved,the flow stress is reduced,the uniformity is greatly improved,and the window of hot deformation is expanded.These studies will provide a powerful theory and process guidance for the subsequent hot deformation processing.