K439B是一种服役温度可达800 ℃以上的新型镍基高温合金，由于铸件结构向着薄壁化方向发展，有必要开展K439B合金薄壁铸件的微观组织特征研究。为此设计了壁厚分别为1 mm和2 mm的薄壁铸件，进行了精密铸造实验和数值模拟。铸件的铸态组织对比分析表明，壁厚1 mm和2 mm薄壁的枝晶生长方向均为沿型壁指向中心，区别在于1 mm薄壁的枝晶生长方向与型壁夹角更接近垂直，并且1 mm和2 mm薄壁的平均一次枝晶臂间距分别为60.64 μm和46.23 μm；平均二次枝晶臂间距分别为19.31 μm和22.69 μm；并且1 mm薄板的平均晶粒尺寸为216.61 μm，2 mm薄板的平均晶粒尺寸为239.11 μm。结合数值模拟分析表明，枝晶臂间距与温度梯度和冷却速率的关系趋势基本符合已有的经验公式，但是当壁厚减小到某临界厚度时，一次枝晶臂间距与温度梯度和冷却速率的关系不再简单符合该经验公式。实验和模拟分析结果为合理制订K439B合金薄壁件的铸造工艺提供了参考和借鉴。

K439B is a new type of nickel-based superalloy with a service temperature up to 800 ℃. In the face of the demand for mass reduction of aircraft， the structure of its components is developing in the direction of thin-wall. Thus， it is necessary to study the microstructure of the K439B alloy thin-walled castings. For this purpose， the thin-wall castings with wall thicknesses of 1 mm and 2 mm are designed， and gravity investment casting experiments and numerical simulations are conducted. Comparative analysis of the as-cast microstructure of the castings shows that the growth directions of the dendrites in both 1 mm and 2 mm thin-wall are along the shell wall pointing to the center， the difference is that the growth directions of the dendrites in the 1 mm thin-wall are closer to the vertical angle with the wall. The average primary dendrite arm spacings （PDAS） are 60.64 μm for 1 mm thin-wall and 46.23 μm for 2 mm thin-wall， respectively. The average secondary dendrite arm spacings （SDAS） are 19.31 μm for 1 mm thin-wall and 22.69 μm for 2 mm thin-wall， separately. Meanwhile， the average grain size of the 1 mm thin-wall is 216.61 μm， and the corresponding size of the 2 mm thin-wall is 239.11 μm. Combined with numerical simulation analysis， it is shown that the relationship trend between dendrite arm spacings， temperature gradient， and cooling rate basically matches the existing empirical formula， but the relationship between PDAS and temperature gradient and cooling rate no longer simply matches the formula when the wall thickness is reduced to a certain critical thickness. These results of the experimental and simulation analysis could provide a reference to rationally design the casting process for the K439B nickel-based superalloy thin-walled casting crafts.