高温合金单晶铸件中形状因素对雀斑缺陷的影响

Effect of Casting Geometry on the Freckle formation during Single Crystal Solidification of Superalloys

摘要:

通过对单晶高温合金铸件中雀斑缺陷的观察研究,发现铸件横截面和纵截面的形状都对雀斑的形成产生很大的影响。铸件横截面形状的影响表现为棱角和曲率效应,即雀斑容易出现在外凸形的棱角(夹角<180°)和曲面(曲率为正)上,而不会出现在内凹形的棱角(夹角＞180°)和曲面(曲率为负)。其原因在于糊状区液体流动的附壁效应会在铸件外凸和内凹部位分别发生叠加和发散,从而分别促进和减弱雀斑的生成倾向。另外,铸件纵截面形状变化对雀斑的影响表现为收扩效应,即沿凝固方向呈收缩型和扩张型的的外表面对雀斑的影响完全相反,分别起到强烈的促进和抑制作用。研究结果表明,上述的铸件形状因素对雀斑的作用远远超过了凝固条件的影响。

Abstract:

Freckles are presently one of the main casting defects in the single crystal(SC)components made of superalloys. In the present work,a series of SC components with complex geometry, including turbine blades, were directionally solidified. It was found that the casting shape has more significant influence on the freckle formation than the local thermal condition. On the transverse sections of the components with curved contour, freckles were exclusively found on the outward curving surfaces having positive curvature, because the surface effect zones of the neighboring sides are overlapped, providing more favorable convection condition. In comparision, the surfaces with negative curvature remained freckle free, because the surface effect of the neighboring sides is divergent from each other. In the longitudinal direction, the freckle formation can be promoted by contracting contour and suppressed by expanding one, respectively.