平板・円柱鋳物の凝固速度と凝固時の冷却速度のシミュレーション及び中心線引け巣発生との関連

<p>  The behaviors of the distributions of solidification velocity and cooling rate during solidification in pure metal and alloy castings were examined by casting simulation, and their mechanisms and relations with actual phenomena such as the generation of the shrinkage cavities in pure Al castings as an example were investigated. For the 2-dimentional solidification simulation of plate castings, the acceleration zone of solidification was observed out of the end and riser effective ranges, irrespective of the casting and mold materials and/or the critical solid fraction. This zone corresponds to low temperature gradient and low NIYAMA parameter region, where the shrinkage cavities along the zone was observed on the cross section of a pure Al plate casting. For the cross-sectional observation and 3-dimentional flow and solidification simulation of cylindrical pure Al castings, neither the acceleration zone of solidification nor centerline shrinkage cavities were observed, irrespective of the mold materials. These findings may be due to the formation of the longitudinal temperature gradient with decreasing melt temperature during mold filling. Meanwhile, high cooling rate zones in and around the end effective ranges of the plate castings were observed in 2-dimensional solidification simulation under the critical solid fraction of 0.99. This high cooling rate zone was caused by the cooling of the unsolidified mesh by the surrounding solidified mesh and end effect. Although shrinkage cavities could be prevented by the high temperature gradient in the high cooling rate zone, these results suggest that there exists a correlation between the zone and morphology of the solidification macro- and microstructures.</p>