The solidification microstructures of Pb-45% Sn hypoeutectic and Pb-85%Sn hypereutectic alloys were studied in rotating magnetic field (RMF). A transition of primary phase from dendrite to spherical growth was caused by the RMF, which simultaneously fractured and fined grains. The fracture and refinement increased first and then decreased with the increase in RMF intensity. When magnetic intensity exceeded a critical value, the size of the primary phase became bigger on the contrary. Therefore, there existed an optimum value of magnetic intensity in fracture and refinement of grain. Moreover, the rotating frequency determined the skin depth of magnetic field, and further affected the homogenization of temperature and solute. The rotating frequency and magnetic intensity were key factors affect- ing refinement and uniformity of solidification microstructures. The introduction of RMF not only changed the solidification thermodynamics, but also led to a reduction in Gibbs free energy associated with the formation of critical crystal nucleus and atom diffusion activation energy, thus enhancing the rate of nucleation.
The influence of the rotating magnetic field (RMF) on the solidification proc- ess of Pb-Sn binary alloys is studied by comparing the solidification microstructures under the common condition and RMF condition. It is found that the RMF can completely elimi- nate the gravity induced macrosegregation, and result in dendrite fragmentation and promote solute diffusion velocity. These differences are regarded as the effect of compli- cated melt flow caused by RMF. Moreover, when the content of the primary phase is small, many spherical microstructures form under the RMF condition. The analyses indicate that these special microstructures are likely the conjunction action of melt flow and concentra- tion and temperature field uniformity caused by RMF.
MENG Xiaohua CHEN Changle HONG Zhenyu WANG Jianyuan
