镍基高温合金U720Li的熔化和凝固过程原位观察

In-Situ observations of melting and solidification processes of Ni-based superalloy U720Li

摘要:

本文采用高温激光共聚焦显微镜（HT-CLSM）和差热分析（DTA）研究了U720Li镍基高温合金的熔化和凝固行为。HT-CLSM熔化过程原位观察表明，升温至1122 ℃时在部分(γ + γ′)共晶颗粒前沿发生了初熔，但温度超过1173 ℃时该熔池才开始迅速扩大；升温至1195 ℃时枝晶干处开始出现离散的斑点状熔池，且该熔池随温度的升高缓慢扩大；升温至1235 ℃时(γ + γ′)共晶开始熔化且随温度的升高该熔池向枝晶干迅速扩展，最终在1333 ℃枝晶完全熔化。HT-CLSM凝固过程原位观察表明，降温至1315 ℃时熔体开始形核凝固，并在1180 ℃左右凝固结束；固相转化速率随温度的降低先缓慢增加，再迅速增加到最大值，然后迅速降低并在凝固后期基本为零。DTA分析表明，铸锭中γ′相开始发生溶解的温度约为1047 ℃，铸锭完全熔化的温度约为1362 ℃。将HT-CLSM原位观察结果与本文及我们前期的DTA分析结果进行对比，发现前者的测试结果较后者低30 ℃左右。

Abstract:

The melting and solidification behaviors of Ni-based superalloy U720Li were investigated by means of high temperature confocal laser scanning microscopy (HT-CLSM) and differential thermal analysis (DTA) in this study. The in-situ HT-CLSM observation of melting process shows that incipient melting takes place at the front of some eutectic (γ + γ′) particles at about 1122 ℃. But the molten pools cannot expand rapidly until the temperature is increased above 1173 ℃. Discrete spotted molten pools occurs on localized region of the dendrite area at around 1195 ℃, and the radius of these molten pools increases slowly with raising temperature. The eutectic (γ + γ′) particles precipitated in the interdendritic region begin to melt at about 1235 ℃ and the molten pools rapidly expand towards the dendrite area as the temperature increases. Finally, the dendrites are completely melted at 1333 ℃. The in-situ HT-CLSM observation of solidification process shows that the melt starts to solidify at about 1315 ℃ and the solidification ends at around 1180 ℃. As the temperature decreases the transformation rate of the solid phase initially increases slowly and then increases rapidly until a maximum value, afterwards it quickly decreases and at the final solidification stage it is about zero. The DTA analysis indicates that the onset temperature of γ′ dissolution in the ingot is about 1047 ℃, and the ingot is completely melted at 1362 ℃. By comparison, it is clear that the results of HT-CLSM in-situ observations are about 30 ℃ lower than those of DTA analysis.