高温合金单晶铸件中杂晶缺陷的试验研究

Experimental Investigations on Stray Grain Defects in Single Crystal Castings of Superalloys

分析了高温合金单晶叶片铸件中杂晶缺陷的特征和形成机理。指出杂晶主要产生在铸件横截面大幅度扩张的部位,如缘板和叶冠处,原因是边角处冷却过快,熔体过冷超过了合金的临界形核过冷度。通过试验验证了叶片形状、凝固工艺和合金过冷能力等因素的影响,并提出了这种几何性杂晶形成的条件判据。在生产试验中还发现杂晶也会由其他原因引起,如在型壳内壁的裂纹处和型壳与芯头的间隙中,浇注时渗入的金属液在凝固时形成杂乱晶向的金属披缝,有可能长入铸件形成杂晶。另外,铸件在定向凝固中若产生雀斑缺陷,则雀斑链中的细小等轴晶粒可能会继续生长,扩展为粗大的杂晶组织。条纹晶也可发展成为三维的大尺度杂晶缺陷,但与基体的晶向偏离不会太大。

The most serious structure defect in single crystal (SC) castings is stray grain, which cannot be tolerated due to the random crystal orientation and the high-angle grain boundaries. In this work, the stray grain defects and their formation mechanism in the SC turbine blades were investigated. It was found that the stray grains mainly occur at the platform portions because of the abrupt transition of the cross-section. Due to the rapid cooling of the local melt at the platform's extremities, an isolated, thermally undercooled region of melt is formed. When the geometry-related undercooling is sufficient to exceed the undercoolability of the used alloy, stray grains will nucleate before the arrival of the primary SC growth from the airfoil. Based on the experimental results, an analytical criterion for the occurrence of the geometry-related stray grains was proposed, indicating the influence of alloy property, blade geometry and solidification condition. Although the geometry of the castings becomes increasingly a pronounced factor to affect the grain defects during SC process, the stray grain formation can be initiated by the mould defects. In the cracks of the mould surface and in the gaps between shell mould and core, the melt could be quickly cooled, leading to the nucleation and growth of stray grains into the SC components. The stray grains could also be formed from the equiaxed grains of exciting freckles which was previously formed in the mush zone during directional solidification. In addition, the slivers could develop into 3-dementinal stray grains of large size, whose crystal orientation is not very different from the matrix.