Nie Shijin, Li Lin, Wang Qin, Zhao Rongxia, Lin Xin, Liu Furong
Key Laboratory of Trans-Scale Laser Manufacturing, Beijing University of Technology, Ministry of Education, Beijing 100124, China.
Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China.
Materials (Basel). 2022 Dec 15;15(24):8968. doi: 10.3390/ma15248968.
Complex thermal cycles and stress fields commonly occur in the selective laser melting process for nickel-based superalloys, which are prone to generating cracks and decreasing the performance of forming parts. In this paper, the reasons for cracking were analyzed by combining the experiment with the evolution behavior of the temperature field/stress field during the solidification process of a nickel-based superalloy (FGH96) via a three-dimensional finite element thermo-mechanical coupling model. It showed that a radial temperature distribution of the melting pool led to a similar distributed stress; as a result, the value declined slowly along the scanning direction but declined quickly along the direction perpendicular to the scanning direction. A stress concentration with maximum stress up to 339 MPa was found at the center of the molten pool, easily causing a crack in SLM. It was found that both the initiation and propagation of the cracks were along the grain growth direction and were affected by the epitaxial growth of columnar crystals. For the case of process parameters with relatively high power or low scanning speed, the stress value of the molten pool during solidification was more than 370 MPa so as to form a large area of cracks. The adjustment of the rotation angle between the adjacent layers was effective at avoiding stress accumulation in the building direction and prevent the formation of long grain boundaries, thus avoiding crack propagation. The present study lays a foundation for the wide applications of selective laser melting technologies in nickel-based superalloys.
在镍基高温合金的选择性激光熔化过程中，通常会出现复杂的热循环和应力场，这容易导致裂纹的产生并降低成型零件的性能。本文通过三维有限元热-机械耦合模型，结合实验以及镍基高温合金（FGH96）凝固过程中温度场/应力场的演变行为，分析了裂纹产生的原因。结果表明，熔池的径向温度分布导致应力呈类似分布；因此，应力值沿扫描方向缓慢下降，但沿垂直于扫描方向快速下降。在熔池中心发现了应力集中，最大应力高达339MPa，这很容易在选择性激光熔化过程中导致裂纹。研究发现，裂纹的萌生和扩展均沿晶粒生长方向，且受柱状晶外延生长的影响。对于功率较高或扫描速度较低的工艺参数情况，凝固过程中熔池的应力值超过370MPa，从而形成大面积裂纹。调整相邻层之间的旋转角度有效地避免了在成型方向上的应力积累，并防止了长晶界的形成，从而避免了裂纹扩展。本研究为选择性激光熔化技术在镍基高温合金中的广泛应用奠定了基础。
