利用X射线同步辐射影像技术对Al-15%Cu (质量分数)合金凝固过程的枝晶生长和破碎现象进行了实时观察. 通过加入脉冲电磁场和改变枝晶生长方向获得了大量不同实验条件下的动态影像. 利用Matlab软件对实验结果进行定量化分析, 开发了统计测量程序, 统计了枝晶破碎数量随不同实验条件的变化, 测量了枝晶破碎数量沿糊状区深度、糊状区固相率的分布关系. 结果表明, 加入电磁场、逆重力方向生长和生长速度快的枝晶会产生更多的枝晶破碎; 枝晶破碎数量沿着糊状区深度、糊状区固相率呈一定的Gauss分布, 且在固相率为0.45左右达到峰值. 最后分析了速度场造成的缩颈断裂、重力场造成的溶质富积以及电磁场造成的晶间对流对上述定量化结果的影响程度.

Alloy solidification is an important process to control the mechanical properties of engineering products. During solidification, dendrite fragmentation occurs commonly as a key phenomenon to determine the microstructure and to obtain fine grain size. Recently, in situ synchrotron X-radiography technique was developed and applied to observe thermodynamic behaviors such as dendrite growth and fragmentation during solidification. External forces such as mechanical and electromagnetic stirring, and thermal shock were added into the solidification process to investigate their effects on the fragmentation behavior. However, most work conducted in literature focused on qualitative aspects e.g. morphology transition or solute distribution and quantitative investigation such as determining the specific relationship between fragmentation and solidification conditions was rather limited. In this work, the third generation synchrotron X-radiography technique was used to observe the solidification process of an Al-15%Cu (mass fraction) alloy. Experimental conditions including the strength of the pulsed electromagnetic fields, dendrite growth direction and the temperature gradients were varied and the subsequent effect on fragmentation was studied and quantified. A computer program was developed based on Matlab to perform the image processing and measurement. The fragmentation number according to experiments was counted and correlated to the mushy zone depth and local solid fraction. Results showed that a stronger electromagnetic field, growing against gravity and growing at higher velocity would significantly increase the fragmentation number. Furthermore, the fragmentation number followed a Gauss distribution as a function of either mushy zone depth or local solid fraction, and the maximum fragmentation occurred when the solid fraction was about 0.45. In the end, the extent to which caused those statistic results above were analyzed as the necking process due to the velocity field, the cumulative solid due to the gravity field and the liquid flow due to the electromagnetic field.