Hot tears were frequently formed in Aluminum (Al) parts with complex structure. In this study, OM (optical microscope), SEM (scanning electron microscope), EDS (energy dispersive spectrometer), and FDM (finite difference method) were used to reveal the hot tears mechanisms in the casted AC4B Al engine. Shrinkage porosities with the size of 300–500 μm were found in casted specimens near the parts’ surface. Many cracked brittle Fe-bearing phase were also found in crack section. The secondary dendrite arm spacing observation in five typical positions showed that the cooling rate in position 5 was faster than that in position 1. The impropriety solidification sequence in position 5 and position 1 impeded the solidification feeding of position 1, and it might lead to casting defects or even casting cracks. Numerical simulation also showed that solidification sequence promoted casting defects, which was accordant with microstructure observation. Impurity may concentrate at position 5 during filling. It is concluded that large β-Fe intermetallics and improper solidification sequence together contributed to crack initiation in the engine.

中文翻译：

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铸造结构复杂的AC4B铝合金零件的裂纹萌生机理

在具有复杂结构的铝（Al）零件中经常形成热撕裂。在这项研究中，使用OM（光学显微镜），SEM（扫描电子显微镜），EDS（能量色散光谱仪）和FDM（有限差分法）来揭示铸造的AC4B Al发动机的热撕裂机理。在零件表面附近的铸造样品中发现了300-500μm的收缩孔隙。在裂纹区还发现了许多裂纹脆性的含铁相。在五个典型位置的二次枝晶臂间距观察表明，位置5的冷却速率比位置1的冷却速率快。位置5和位置1的不适当凝固顺序阻碍了位置1的凝固进料，并可能导致铸造缺陷甚至铸件裂纹。数值模拟还表明，凝固顺序促进了铸件缺陷，这与显微组织观察相符。填充过程中杂质可能会集中在位置5处。结论是，大的β-Fe金属间化合物和不正确的凝固顺序共同导致了发动机中的裂纹萌生。