| With the increasing demand for high-end equipment manufacturing industries such as aerospace,rail transit,and weapons equipment,the development of large-sized high strength aluminum alloy ingot with high quality and large cross-section for DC casting are vigorously promoted.However,with the increase of ingot size,the internal heat transfer,melt flow,and other behaviors become more and more complex,unavoidably resulting in more serious defects such as coarse and uneven microstructure and macrosegregation,which is very difficult to be eliminated by the following heating and deformation processes,and greatly limits the subsequent processing and comprehensive performance of final products.At present,China’s high-end aluminum materials are in heavy dependence on imports,which seriously restricts the independent development of major strategic fields.Although electromagnetic DC casting process has been applied in the production of small and mediumsized ingots,the complicated temperature and flow fields increase the control difficulty of ingot homogenization,and little systemic research has been done to reveal the homogenization mechanism of large-sized ingot.In this paper,by combining theoretical analysis,numerical simulation and experimental research,the solidification behavior including heat and mass transfer,melt flow of large-sized high strength aluminum alloy ingot in DC casting was studied,and the homogenization mechanism of large-sized 2219 aluminum alloy ingot under electromagnetic field was revealed.A fine-grained and homogeneous 2219 ingot with a diameter of 880 mm was prepared,which provides a new way for solving the common technical problem of uneven composition of large high-strength aluminum alloy ingot.The main research results are as follows:(1)An electromagnetic stirring DC casting model coupling with electromagnetic,temperature,and flow fields for φ880 mm 2219 aluminum alloy ingot was established.The influences of parameters such as the structure and size of the electromagnetic inductor on the electromagnetic,temperature,and flow fields in the melt were investigated.The results show that the outer size and the cooling-end shape of the electromagnetic stirring processor are the key factors to ensure that strong shear stirring and consequent uniform solidification can be achieved in the large volume melt.The optimized key technical parameters are as follows:The processor has an external dimension of 350 mm,nine groups of induction coils.The cooling end is spheroidal in shape.Based on this,an electromagnetic stirring processor for the φ880 mm 2219 aluminum alloy ingot was made.(2)Based on the electromagnetic stirring DC casting model coupled multi-physical fields,the effects of the casting process parameters(casting speed,casting temperature,and cooling strength)and the electromagnetic process parameters(core cooling strength,working position,current strength and frequency)on the temperature and flow fields were examined,and the relationship between liquid sump depth and process parameters was fitted.The research shows that the casting speed is the main factor of the liquid sump depth,and the reasonable match between core cooling and working position is the key to control the uniform temperature field and melt flow pattern.The process parameters are optimized as follows:casting speed is 0.00036m/s,the casting temperature is 700±5℃,the cooling water flow is 26 m3/h,the core cooling intensity of the electromagnetic stirring processor is 400 W/(m2·℃),the working position of the electromagnetic stirring processor is at 0 mm,the frequency is at 10-20 Hz,and the current intensity is at 2000-4000 At.(3)The electromagnetic stirring casting on the φ880 mm 2219 aluminum alloy ingot was tested to study the influences of different process parameters on the microstructure and macrosegregation of the ingot.The results show that,compared with the conventional DC casting,electromagnetic stirring melt treatment(EMSMT)process can effectively improve the temperature uniformity of melt in DC casting,and the depth of liquid sump decreases by 20.8%from 480 mm to 380 mm;The microstructure in ingot center is refined greatly,and the average grain size changes from 892 μm in conventional ingot to 203 μm and the lowfold grain size of the EMSMT ingot is superior to grade 2;The size of the second phase decreases obviously and the distribution is more uniform,and the area fraction of the second phase in ingot center decreases by 20.48%from 9.13%in conventional ingot to 7.26%;The macrosegregation degree of the ingot is reduced,and the relative segregation rate of Cu element in the center ingot is reduced from 5.64%to-2.12%.The mechanical properties of large-size ingot prepared by EMSMT are significantly improved,and the tensile strength at the edge,1/2 radius,and center of the ingot are 166.26 MPa,159.32 MPa and 160.21 MPa,which increased by 2.5%,16.5%,and 56.5%compared with conventional ingot,respectively.(4)Application of φ880 mm 2219 aluminum alloy ingot in the preparation of φ7.3 meters forged ring was conducted.The results show that the tangential,axial and radial tensile strengths are 473.25 MPa,458.5 MPa,468.25 MPa,and the yield strengths are 365.25 MPa,342 MPa,349.5 MPa,and the elongations are 13.86%,9.63%,12.13%,respectively.The comprehensive performance of φ7.3 meters forged ring is superior to AMS-4144F standards,which has better mechanical properties of consistency and no obvious anisotropy.(5)The influencing mechanism of EMSMT on microstructure refinement and homogenization was disclosed.The EMSMT has little effect on the Gibbs free energy difference and interfacial energy,and importantly change the thermodynamic and kinetic conditions of non-homogeneous nucleation during solidification,leading to microstructure refinement and homogenization.The large-volume melt can be undercooled rapidly and uniformly under EMSMT,which benefits for the explosive nucleation.Meanwhile,a large number of floating grains can be rapidly dispersed into the melt,which increases the heterogeneous nucleation substrate.Besides,the alleviated compositional undercooling can help to inhibit the dendrite growth.(6)The mechanism of EMSMT to improve the segregation of large-sized ingots was revealed.The refinement,homogenization,and non-dendrite of microstructure reduce the micro-segregation to some extent.The uniform temperature and composition field by the forced melt flow importantly changes the flow pattern of the solute-enriched melt from the edge to the center of the ingot at the solidification front resulting from the thermo-solute convection.Meanwhile the fine floating grains generated at the cooling end will be helpful to reduce the solute content in the ingot center under EMSMT.Unbefitting parameters such as higher core cooling intensity or lower working position will result in negative segregation indeed,therefore it is of key importance for parameter configurations to precisely control the segregation at the ingot center. |
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