Effect of the Spacial Dimensions on Solidification Defects in Rejoined Platform of Ni-Based Single-Crystal Superalloy Castings Under Different Withdrawal Rates

The formation of solidification defects in rejoined platforms with different dendrite developing distances in horizontal direction and solidification heights in the directional solidification direction of DD413 Ni-based single-crystal (SX) castings were investigated under different withdrawal rates. The microstructure was investigated by a high-resolution image scanner and an optical microscope, and the crystallographic orientation of rejoined platforms was characterized by electron backscatter diffraction (EBSD). The thermal and stress fields during the solidification process were simulated by ProCAST finite element software. The experimental results show that the solidification defects including freckles, slivers, and low-angle boundaries (LABs) formed in the rejoined platforms can be affected by the span size and the solidification height. Findings reveal that solidification defects are prone to form in the rejoined platform with 100 mm span size and 220 mm solidification height under the withdrawal rates of 1 to 6 mm/min. In addition, the temperature gradient decreases and the thermal stress increases in the rejoined platform with the increase of solidification height, which reduces the stability of dendrite growth and further promotes the formation of solidification defects. The sliver defect was observed in the rejoined platform with different span sizes at 40 mm solidification height under the withdrawal rate of 6 mm/min, and the reasons are discussed.