研究了强静磁场对定向凝固Al-4.5Cu (质量分数, %)合金微观偏析的影响. 结果表明, 强磁场显著影响了凝固组织中非平衡第二相的形态和数量. 无磁场时, 粗大的第二相为网络状, 连续分布于晶界上; 施加磁场后, 晶界上连续分布的第二相被打断, 其面积分数随磁场强度的增加而减小. 在磁场作用下, 溶质原子的再分配行为发生改变, Cu溶质成分曲线降低, 有效分配系数ke减小. 上述现象主要是由于磁场在熔体中引发热电磁对流以及由热电磁对流驱动的二次流, 在糊状区内产生搅拌, 改变溶质传输行为.
Microsegregation is the unbalanced distribution of alloying element between solid and liquid phases in dendritic scale during solidification. The solute redistribution usually leads to the formation of brittle secondary phase, which is harmful to the workability and final mechanical properties of casting component. It has been accepted that fluid flow plays a critical role in mass transfer during solidification and thus altering the microsegregation pattern. High static magnetic field has been considered as an effective way to control the convection in solidification. In this work, the impact of the high static magnetic field on the microsegregation was investigated. Al-4.5Cu (mass fraction, %) alloy was directionally solidified from <001> seed crystal under various magnetic fields with a constant pulling rate of 50 μm/s and temperature gradient of 101 K/cm. OM and BSE were applied to characterize the microstructure of the solidified samples. The fraction of Al2Cu second phase was obtained by software analysis from the transverse and longitudinal sections. The results show that the Al-4.5Cu alloy solidifies in dendritic morphology. The formation of second phase is significantly affected by the magnetic field. Without magnetic field, the continuous network of second phase is observed at grain boundaries. In the presence of the magnetic field, the second phase is disconnected in the grain boundaries and dispersed in grains. The fraction of the second phase is reduced with the increase of the magnetic field. EDS area scan was carried out to measure the concentration of Cu solute in dendritic scale. Isoconcentration contour maps of Cu in the plane perpendicular to the primary dendrite trunk were drawn. The concentration profiles of Cu were plotted from the measured data and the effective partition coefficient ke was calculated. It is found that the redistribution of Cu solute in interdendritic region is greatly altered by the magnetic field. When the intensity of the magnetic field increases, the concentration profile and the ke decrease. The disturbance of the Cu solute in the plane perpendicular to the primary trunk suggests the existence of fluid flow in the interdendritic region. The above phenomena could be attributed to the dendritic scale thermoelectric magnetic convection (TEMC) as well as the second flow driven by the TEMC. The azimuthal TEMC and meridional second flow will bring about stirring in mushy zone and lead to the modification of solute transport during solidification process.
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