In-situ Observations of Melting and Solidification Processes of Ni-based Superalloy U720Li

The melting and solidification behaviors of Ni-based superalloy U720Li were investigated by high temperature confocal laser scanning microscopy (HT-CLSM) and differential thermal analysis (DTA). The in-situ HT-CLSM observation of melting process shows that incipient melting takes place at the front of some eutectic (gamma+gamma') particles at about 1122 degrees C. But the molten pools cannot expand rapidly until the temperature is increased above 1173 degrees C. Discrete spotted molten pools occur on localized region of the dendrite area at around 1195 degrees C, and the radius of these molten pools increases slowly with the increase of temperature. The eutectic (gamma+gamma') particles precipitated in the interdendritic region begin to melt at about 1235 degrees C and the molten pools rapidly expand towards the dendrite area as the temperature increases. Finally, the dendrites are completely melted at 1333 degrees C. The in-situ HT-CLSM observation of solidification process shows that the melt starts to solidify at about 1315 degrees C and the solidification ends at around 1180 degrees C. 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 gamma' dissolution in the ingot is about 1047 degrees C, and the ingot is completely melted at 1362 degrees C. By comparison, it is clear that the results of HT-CLSM in-situ observations are about 30 degrees C lower than those of DTA analysis.