复杂镍基单晶铸件显微孔洞的形成机理

Formation mechanism of microporosity in complex nickel-based single crystal casting

采用高分辨透射X射线三维成像技术研究高速凝固法(high rate solidification,HRS)制备的3种成分复杂单晶铸件叶身和缘板部位的铸态显微孔洞分布情况。结果表明:凝固温度范围和枝晶排列曲折程度对孔洞的影响很大。铸件叶身部位,不同成分单晶的枝晶排列方式相同。其中,一代单晶的凝固温度范围最大,其孔洞体积分数也最大。二代单晶和三代单晶的凝固范围差异不大,孔洞体积分数主要受共晶体积分数的影响。随着单晶代次的增加,合金中难熔元素的含量将会增加,继而引起叶身部位共晶体积分数的增加。因此,合金最后凝固阶段枝晶间的空隙尺寸增加,液相压降降低,导致形成孔洞的体积分数减小。相比于叶身部位,缘板部位的孔洞主要由枝晶曲折程度决定。一代单晶缘板部位的枝晶曲折程度变化不明显,其孔洞体积分数与叶身部位的孔洞体积分数差异不大。二代单晶和三代单晶缘板部位的枝晶曲折程度逐渐增加,其孔洞体积分数增加。

Microporosity in the body and platform of the three different compositions complex nickel-based single crystal castings solidified by high rate solidification(HRS)process was investigated by the X-ray tomography(XRT).The results show that the solidification range and the dendrite tortuosity have a strong effect on the formation of microporosity.In the body of the casting,the arrangement mode of dendrites among three different compositions castings is the same.Due to the largest solidification range of the first generation single crystal SX1,its porosity volume fraction is the largest.The difference of solidification range in the second generation single crystal SX2 and the third generation single crystal SX3 is not obvious,thus,their porosity volume fraction is mostly influenced by the eutectic volume fraction.With the increase of single crystal generation,the level of refractory elements will be increased,also the eutectic fraction of the body increases.Therefore,the size of interdendritic zone in the final solidification stage is increased,and the pressure drop in liquid phase is reduced,leading to the decrease of porosity volume fraction.The porosity in the platform is mostly decided by the dendrite tortuosity compared with that in the body.In the platform of the first generation single crystal SX1,there is no obvious difference in the dendrite tortuosity,thus,the volume fraction of porosity is similar to that in the body.Due to the larger dendrite tortuosity in the second generation single crystal SX2 and the third generation single crystal SX3,the corresponding porosity volume fraction is increased.