Formation mechanism of microporosity in complex nickel-based single crystal casting

Abstract

Microporosity in the body and platform of the three different compositions complex nickel-based single crystal castings solidified by high rate solidification (HRS) process was investigated by the X-ray tomography (XRT). The results show that the solidification range and the dendrite tortuosity have a strong effect on the formation of microporosity. In the body of the casting, the arrangement mode of dendrites among three different compositions castings is the same. Due to the largest solidification range of the first generation single crystal SX1, its porosity volume fraction is the largest. The difference of solidification range in the second generation single crystal SX2 and the third generation single crystal SX3 is not obvious, thus, their porosity volume fraction is mostly influenced by the eutectic volume fraction. With the increase of single crystal generation, the level of refractory elements will be increased, also the eutectic fraction of the body increases. Therefore, the size of interdendritic zone in the final solidification stage is increased, and the pressure drop in liquid phase is reduced, leading to the decrease of porosity volume fraction. The porosity in the platform is mostly decided by the dendrite tortuosity compared with that in the body. In the platform of the first generation single crystal SX1, there is no obvious difference in the dendrite tortuosity, thus, the volume fraction of porosity is similar to that in the body. Due to the larger dendrite tortuosity in the second generation single crystal SX2 and the third generation single crystal SX3, the corresponding porosity volume fraction is increased.

Keywords

• single crystal superalloy
• microporosity
• platform
• high rate solidification