Microstructural evolution of directionally solidified DZ125 superalloy castings with different solidification methods

The properties of Ni-base superalloy castings are closely related to the uniformity of their as-cast microstructure, and different solidification methods have serious effect on microstructural uniformity. In this paper, the influences of high rate solidification (HRS) process (with or without superheating) and liquid metal cooling (LMC) process on the microstructure of DZ125 superalloy were investigated. Blade-shape castings were solidified at rates of 40 μm·s-1 to 110 μm·s-1 using HRS process and a comparative experiment was carried out at a rate of 70 μm·s-1 by LMC process. The optical microscope (OM), scanning electron microscope (SEM) were used to observe the microstructure and the grain size was analyzed using electron back scattered diffraction (EBSD) technique. Results show that for the castings by either HRS or LMC process, the primary dendrite arm spacing and size of γ' precipitates decrease with increasing the withdrawal rate; the dendrites and γ' precipitates at the upper section of the blade are coarser than those in the middle, especially for the HRS castings without high superheating technique. When the withdrawal rate is 70 μm·s-1, the castings by HRS with high superheating technique have the smallest PDAS with fine γ' precipitates; while the size distribution of γ' precipitates is more homogenous in LMC castings, and the number of larger grains in LMC castings is smaller than that in the HRS castings. Moreover, high superheating technique yields smaller grains in the castings. Both the LMC method and HRS with high superheating technique can be used to prepare castings with reduced maximum grain size.