Internal Electromagnetic Stirring Method For Preparing Large-sized Al Alloys Billet With High Strength And High Toughness

The aerospace and rail transit industries are developing rapidly in global,requiring a large amount of high performance materials.Among the many high performance materials,the large-sized A1 alloys billet with high strength and high toughness plays an important role,which should be deformed before application.The properties of the deformed alloys are determined by two factors.One is deformation process,the other is billet.The latter is the fundamental of the deformation.However,the coarse microstructure and severe macrosegregation always occur in the large-sized A1 alloys billet,which decreases the properties of the product.What's worse,these defects become more serious with the billet size increasing.Normal electromagnetic stirring is utilized outside the melt.However,because of the skin effect,it can alleviate the abovementioned defects only in small-sized billet.When the billet size is larger than 600 mm,the normal electromagnetic stirring become ineffective.Hence,a new electromagnetic stirring method,known as internal electromagnetic stirring,has been proposed in this paper.This method is coupled with intercooling and utilized inside the melt.In this study,the numerical simulation and experimental research on the preparation of large-sized A1 alloy billet with high strength and high toughness by internal electromagnetic stirring direct chill casting have been carried out.The specific research contents and research results are as follows:(1)A coupling numerical model of temperature field,velocity field and electromagnetic field for internal electromagnetic stirring direct chill casting was established.The basic law of magnetic field generated by internal electromagnetic stirring was investigated by numerical simulation.The feasibility and efficiency of the internal electromagnetic stirring method was verified.The effects of current frequency,central cooling position and cooling intensity on the velocity field and temperature field during the direct chill casting process were also investigated.Finally,the appropriate direct chill casting and internal electromagnetic stirring processes parameters were determined,which provided references for the subsequent experimental research.(2)Based on the numerical simulation,the prototype of the internal electromagnetic stirring equipment was designed and manufactured,and applied to the preparation of the large-sized high strength and high toughness Al alloy billets.The results show that the internal electromagnetic stirring can significantly refine and homogenize the microstructure of the 2219 Al alloy billet with a diameter of 508 mm.Its average grain size from the edge to the center of the billet is between 124 ?m and 135 ?m.The internal electromagnetic stirring also greatly reduces the number of porosity and improves the macrosegregation of Cu element.The relative segregation rate of the billet edge and center are decreased to 4.0%and 2.3%,respectively.The internal electromagnetic stirring can also refine the microstructure and alleviate the macrosegregation in the 7050 Al alloy billet with a diameter of 508 mm and in the larger-sized 2219 A1 alloy billet with a diameter of 630 mm.Compared with the normal electromagnetic stirring,the effects of the internal electromagnetic stirring on the microstructure refinement and elements homogenization are more significant.(3)Based on the numerical simulation and experimental research,the mechanism for the microstructure refinement and composition homogenization of the billet under the internal electromagnetic stirring were investigated.Internal electromagnetic stirring increases the number of nuclei,improves their survival rate,and promotes them disperse homogeneously.The magnetic field reduces the system free energy;hence the critical nucleation radius and critical nucleation energy decrease.The internal electromagnetic stirring reduces the temperature gradient G and increases the total number of heterogeneous substrate particles originally available per unit volume N0,promoting the transition from columnar to equiaxed.The Peltier effect and intercooling affects the undercooling of the columnar growth front.When ATP<AT,,the columnar to equiaxed transition can be promoted.The effects of ?N0,ATP,?T1 and ?G on the transition from columnar to equiaxed are complicated.Especially when ?TP>?T1,the columnar to equiaxed transition may be restrained.Hence,appropriately increasing the distance between the induction coil and the solidification front and appropriately increasing the intercooling intensity contribute to the transition from columnar to equiaxed.Meanwhile,the intercooling intensity should not be much higher.The improvement of macrosegregation under internal electromagnetic stirring can be attributed by three aspects.First,the homogeneous temperature field decreases the thermo-solutal convection.Second,the melt flow is accelerated,promoting the solute-rich liquid flow from the center to the edge of billet.Third,the permeability decreases because of the grain refinement.Consequently,the negative macrosegregation at the edge and the positive macrosegregation at the center of the billet are alleviated.(4)The hot simulation of the internal electromagnetic stirring direct chill casting 2219 Al alloy billet(fine microstructure and homogeneous distributed elements)and normal direct chill casting 2219 Al alloy billet(coarse microstructure and inhomogeneous distributed elements)was implemented.The effects of internal electromagnetic stirring on the hot deformation of billet were investigated.The constitutive equation and processing map were established;the appropriate hot deformation parameters were determined.Compared with normal direct chill casting billet,the safe region for hot deformation of the internal electromagnetic stirring direct chill casting 2219 Al alloy billet is larger;its appropriate process window is wider and strain rate is faster.