Effect of a Novel Thermally Controlled Solidification Process on Filling and Defect Control in Complex Thin-walled Superalloy Castings

To address the casting challenges of poor mold filling and severe shrinkage porosity in complex thin-walled castings of IN718 nickel-based superalloy, a novel thermally controlled solidification (TCS) process equipped with directional withdrawal was proposed. This process realizes stable sequential solidification by raise the temperature of shell mold combined with directional withdrawal. Comparative experiments consisting of conventional casting, TCS-1 process with pouring temperature of 1360 ℃, mold-shell temperature of 1260 ℃ and TCS-2 process with pouring temperature of 1400 ℃, mold-shell temperature of 1290 ℃ were carried out on specimens with variable wall thicknesses. In-situ solidification observation, metallographic analysis and quantitative defect characterization were combined to investigate the effects of the process on casting filling, internal defects and grain structure, and engineering verification was performed on bearing casing castings. The results reveal that the conventional casting process exhibits inferior filling capacity for thin-walled regions accompanied by severe shrinkage porosity. Both thermally controlled solidification processes substantially enhance the mold-filling performance of thin-walled sections and drastically mitigate shrinkage porosity. Specifically, the higher-temperature TCS-2 process enables full mold filling of the casting with extremely low residual shrinkage porosity. The fundamental mechanisms accounting for improved filling capacity and mitigated shrinkage porosity lie in three aspects: high-temperature shell molds alleviate chilling effect at mold walls, directional withdrawal induces progressive sequential solidification, and unobstructed feeding channels are maintained throughout solidification. However, the thermally controlled solidification processes inevitably trigger grain coarsening, and the grain size disparity between regions with different wall thicknesses is further aggravated as pouring temperature and shell mold holding temperature rise. Engineering verification on bearing casing castings demonstrates that the TCS-2 process eliminates misrun defects in thin-walled areas and shrinkage cavities at hot spots. The resultant castings are free of columnar grains and possess homogeneous equiaxed grain structures. This process enables synergistic regulation of forming integrity and internal defect suppression, offering a viable technical reference for the precision manufacturing of thin-walled superalloy castings.