单晶高温合金超薄铸件雀斑形成机制

Formation Mechanism of Freckle of Single Crystal Superalloy Casting with Ultrathin Thickness

采用定向凝固工艺,在不同抽拉速率条件下,制备了厚度0.6 mm不同结构单晶高温合金薄板铸件。利用金相显微镜(OM)、扫描电镜(SEM)、X射线三维成像(XCT)观察了铸件宏观组织及枝晶形貌,采用ProCAST软件模拟了铸件定向凝固过程。结果表明:抽拉速率为1 mm/min时,糊状区从凝固炉中心位置向炉壁方向倾斜,中心位置先凝固,炉壁方向后凝固,后凝固枝晶间的液体对先凝固枝晶间进行补缩,先凝固枝晶受径向补缩流动或其引起的热溶质对流冲击导致枝晶断裂,破碎枝晶在先凝固区域长大形成倾斜枝晶或雀斑缺陷;抽拉速率为6 mm/min时,糊状区虽然从中心位置向炉壁方向倾斜,但枝晶间径向补缩流动被发达二次枝晶阻挡,流动冲击较弱,只有极个别枝晶倾斜,无雀斑形成。

Single crystal plates with a thickness of 0.6 mm and different geometric structures were prepared by directional solidification under different withdrawal rates.The macroscopic morphology and dendrite microstructure of the castings were observed via optical microscopy(OM),scanning electron microscopy(SEM)and X-ray computed tomography(XCT).The experimental results showed that the mushy zone was tilted from the center of the furnace to the wall of the solidification furnace at a withdrawal rate of 1 mm/min.The part of the casting near the center of the furnace solidified earlier than that near the furnace wall region.The liquid located in the interdendritic region near the furnace center fed the shrinkage of the interdendritic region near the furnace wall.Both the radial feeding flow and thermosolutal convection induced by radial feeding impinged the dendrite,and the dendrite arm was tiled or broken.Then,the tiled and broken dendrites continued to grow and form large tiled dendrites or freckles.When the withdrawal rate was 6mm/min,the mushy zone was tilted from the center to the wall of the solidification furnace.However,the feeding flow was blocked by the developed second dendrites and the intensity of the flow weakens.Therefore,only one or two tiled dendrites and no freckles could be found in the castings.