Using advanced characterization techniques such as synchrotron radiation microdiffraction and transmission electron microscopy, the research team of Shan Zhiwei from the School of Materials Science and Engineering of Xi'an Jiaotong University started with the laws of microstructure evolution and customized a heat treatment system for 3D printed single crystal superalloys, which solved the 3D printing repair of single crystal blade After the recrystallization problem, and proposed a new mechanism of high temperature alloy plastic deformation recovery.
The blades of aeroengines are made of expensive single crystal nickel-based superalloys. Due to the harsh service environment, single crystal blades are susceptible to local damage, and the development of reliable blade repair technology is essential for the life extension and cost reduction of aeroengines. 3D printing shows its attractive application prospects in the field of single crystal blade repair / remanufacture due to its "precise positioning and controllable additive" characteristics. However, due to the rapid cooling rate of 3D printing, it is easy to cause metastable microstructures with high residual stress and high dislocation density. This metastable structure is prone to recrystallization during standard heat treatment or service, resulting in a drop in the high-temperature mechanical properties of the material and potential safety hazards. Therefore, there is an urgent need to develop a new process to meet the current problems of 3D printing single crystal superalloys to satisfy it: no recrystallization, low stress, low dislocation density, and morphology, density and matrix of g ￠ precipitation strengthening phase Consistent with superalloys.
The standard heat treatment system of superalloys generally consists of solid solution and aging. Practice has proved that this process will cause recrystallization of 3D printed superalloys. After systematic literature research and comprehensive analysis, the authors proposed and confirmed that adding a "Recovery" step before solutionization can eliminate the recrystallization driving force. After "recovery-solid solution-aging" treatment, first, the residual stress of the superalloy is eliminated with the directional coarsening of the microstructure γ 'phase, and the dislocation density can be reduced to about 5% before heat treatment, and precipitation strengthening γ' after aging The phase reaches the same level as the as-cast substrate. Because the phenomenon found is similar to the Rafting effect of superalloys under high temperature creep conditions, and in fact has a recovery effect, it is named "Rafting-recovery" effect (Rafting-enabled) recovery). This discovery breaks through the classic concept that "single crystal superalloys do not have the ability to recover", provides a scientific basis for the design of non-standard heat treatment systems for 3D printed superalloys, and shows that the new heat treatment system can fully meet 3D printing The need for single crystal blade repair.
The research is titled "Rafting-Enabled Recovery Avoids Recrystallization in 3D-Printing Repaired Single-Crystal Superalloys" with "Rafting-Recovery" effect in the authoritative journal of materials science Published in Advanced Materials (doi: 10.1002 / adma.201907164). Professor Chen Kai of the School of Materials, Xi'an Jiaotong University is the first and corresponding author of the paper, Professor Shan Zhiwei of Xi'an Jiaotong University, Professor Li Ju of the Massachusetts Institute of Technology, and Professor Mane of Johns Hopkins University are co-corresponding authors of the paper. Huang Runqiu, Lin Sicong, Zhu Wenxin, graduate students of School of Materials Science and Engineering, Xi'an Jiaotong University, Dr. Li Yao of Chang'an University, Dr. Nobumichi Tamura of Lawrence Berkeley National Laboratory, etc. participated in the research. The research work is supported by the National Natural Science Foundation of China and the National Key R & D Program.