材料期刊网

通过实施工业条件下的定向凝固实验, 对复杂形状的高温合金铸件中的雀斑进行了检测和分析, 揭示了这种凝固缺陷产生的几个新特点. 结果表明, 雀斑易于产生在铸件的棱角部位而不是平滑表面上, 称之为棱角效应; 铸件外形台阶式地突然扩张和缩小会分别抑制和促进雀斑形成, 称之为台阶效应; 雀斑易于产生在向内倾斜而不是向外倾斜的铸件表面上, 称之为斜面效应; 叶片曲率为正的外凸曲面出现严重的雀斑缺陷, 而曲率为负的内凹曲面却毫无雀斑, 此现象称之为曲率效应. 实验中还发现雀斑不但出现在铸件的外表面, 插入型芯也会诱导铸件内部雀斑的产生, 这说明糊状区的液体流动具有强烈的附壁效应, 不管这种壁面是处在铸件外部还是内部. 通过分析可以确认, 正是这种附壁效应在各种具体形状特征下发挥作用, 导致了上述关于雀斑生成的各种效应.

Freckles are a detrimental grain defect formed during directional and single crystal solidification of superalloy components leading to a high rejection rate. Based on the experimental and theoretical studies over the past forty years, the occurrence of freckles is generally believed to be mainly dependent on the alloy chemistry and process parameters, while the geometrical factor of castings was hardly taken into account. In the present work, a series of superalloy castings with complex geometry were directionally solidified in a production-scale Bridgman furnace. Some new features of freckle appearance have been observed, indicating new aspects of freckle formation. The freckles are preferably formed on the edges instead of on the plane surfaces of the castings. Correspondingly, freckles were found exclusively on the casting surface having positive curvature, whereas no freckles formed on the surface with negative one. The casting portions having inward sloping surfaces are very freckle prone, while those with outward sloping surfaces are absolutely freckle free. Therefore, as an independent factor the geometrical feature of the castings can more effectively affect the freckle formation than the local thermal conditions. It was also observed that freckles could occur not only on the external surfaces, but also inside the castings where a core was inserted, because both the shell and the core wall can provide very high permeability for freckling convection in the mushy zone. Based on this wall effect, all the important phenomena observed in the present work, such as the edge effect, the step effect, the sloping effect and the curvature effect on freckle formation in complex castings of superalloys, can be reasonably explained.