| During the solidification processes of most commercial alloys,the primary solid solution phases formed from the melt commonly exhibit dendritic morphologies.The dendritic microstructure dominates not only the characteristic of grains,but also the distribution of secondary phases in matrix.The formation of the dendritic microstructure has notable effect on the final properties of casting products.As a result,it is essential to understand and manipulate the evolution behavior of the dendrite in order to further improve casting qualities.It is well-known that the substance transfer during phase transformation would be altered by magnetic field.The introduction of high static magnetic field to material processing has been accepted as an effective way to control the microstructure evolution.The dendrite growth is governed by the heat and mass transfer in solid-liquid transition.Consequently,the dendritic microstructure would be modified by the application of high static magnetic field during solidification.The present work aims to investigate the influence of high static magnetic field on the columnar and equiaxed dendritic solidification of ?-Al primary solid solution phases.Al-4.5wt.%Cu and 2024 aluminum alloys were selected and directionally solidified without and with Al-5Ti-1B grain refiner under various solidification condition in the presence of the high static magnetic field up to several Tesla.The following results are obtained:The morphologies of ?-Al columnar dendrite are significantly affected by high static magnetic field during directional solidification of Al-4.5wt.%Cu alloy seeded by <001> crystal.The primary dendrite spacing is increased with the increasing of the magnetic field.It is observed that tertiary dendrites of Al-4.5wt.%Cu alloy grow asymmetrically on secondary arms in the plane perpendicular to the primary trunk,and a pinwheel-like pattern forms.The area fraction of the second phase is reduced with the increasing of the magnetic field for Al-4.5wt.%Cu alloy.Without magnetic field,the continuous network of second phase could be founded at grain boundaries.In the presence of magnetic field,the second phase is disconnected in the grain boundaries and dispersed in matrix.The formation of stray grains during solidification process is promoted by the magnetic field.In addition,the application of magnetic field makes the unfavored grain overgrow the favored grain during directional solidification by altering the competition behavior of ?-Al columnar dendrites.Numerical simulations were performed using finite-element code COMSOL~TM software to investigate the thermoelectric magnetic convection(TEMC)induced by the magnetic field.The result shows that the TEMC is a swirling flow and flows around the columnar dendrite.The velocity of the TEMC increases with the increasing of the magnetic field.On the other hand,the natural convection in melt would be damped by the magnetic field.Comparison between the experimental and numerical results proves that the development of the pinwheel-like appearance is in accordance with the flow pattern of the TEMC.The adjustments of primary dendrite spacing and microsegregation for ?-Al could be attributed to the combination effect of the magnetic damping and TEMC.It is implied that the detachment of secondary arms contributes to the emergence of the stray grains.The TEMC gives rise to the pinch-off of the sidebranches.When the fragments form,they will become the source of the stray grains.Moreover,the sidebranches grown from the unfavored grain can be boosted by interdendritic TEMC so that the favored grain would be blocked and surpassed by unfavored one.As a result,the columnar dendrite array is tilted to the direction of magnetic field.Equiaxed dendritic grains are obtained by adding Al-5Ti-1B grain refiner into Al-4.5wt.%Cu alloy.The interface between the mushy zone and the bulk liquid is flattened by magnetic field.As opposed to the area fraction of second phase,the size of ?-Al equiaxed grains is firstly decreased and then increased as the magnetic field enhanced.Numerical simulation performed by COMSOL~TM reveals that the TEMC also flows around the equiaxed grains.The grain size and microsegregation are determined by the interaction between the natural convection and TEMC.The ?-Al equiaxed grains can rotate in melt and tend to align the <310> crystallographic axis along the direction of magnetic field.The <310> crystallographic direction is the easy magnetic axis of ?-Al crystal.As measured by Faraday method,the doping of Cu atoms into Al lattice makes the magnetic susceptibilities of equivalent planes different.Consequently,?-Al can be rotated by magnetic torque induced by the anisotropic susceptibility of ?-Al crystal.The primary dendrite spacing of directionally solidified 2024 alloy is decreased and then increased as the magnetic field enhanced.Without magnetic field,the equiaxed grains of 2024 alloy are almost spherical;with a 5T magnetic field,the grains are solidified with elevated size and rosette morphology.After T6 heat treatment,the tensile strength of 2024 alloy solidified under magnetic field is increased.The defects in ?-Al matrix are augmented by thermoelectric magnetic effect during solidification process.As a result,more secondary phases would dissolve into the ?-Al matrix after solution heat treatment.The increase of solution atoms content in the alloy matrix leads better solid solution strengthening of 2024 alloy.Furthermore,the modification of grain morphology also contributes to the enhancement of tensile strength. |
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