Using advanced characterization techniques such as synchrotron radiation micro-diffraction and transmission electron microscopy, Shan Zhiwei’s research team at the School of Materials Science and Engineering of Xi’an Jiaotong University started from the law of microstructure evolution, customized a heat treatment system for 3D printing single crystal superalloys, and solved the problem of 3D printing repair of single crystal blades. After the problem of recrystallization, a new mechanism for plastic deformation recovery of superalloys was proposed.
The blades of aero-engines 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 aero-engine life extension and cost reduction. With its “precise positioning and controllable additive” characteristics, 3D printing has shown attractive application prospects in the field of repair/remanufacturing of single crystal blades. However, due to the fast cooling speed of 3D printing, it is easy to cause metastable microstructure with high residual stress and high dislocation density. This kind of metastable structure is prone to recrystallization during standard heat treatment or service, resulting in a decrease in the mechanical properties of the material at high temperatures, and posing safety hazards. Therefore, it is urgent to develop a new process for the current problems of 3D printing single crystal superalloys to satisfy: no recrystallization, low stress, low dislocation density, and the morphology, density and matrix of the γ′ precipitation strengthening phase The superalloy is consistent.
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 solid solution can eliminate the driving force for recrystallization. After the “recovery-solution-aging” treatment, the residual stress of the superalloy is first eliminated with the directional coarsening of the microstructure γ′ phase, and the dislocation density can be reduced to about 5% before the heat treatment, and precipitation strengthening γ′ after aging The phase reaches the same level as the as-cast substrate. Because the discovered phenomenon is similar to the rafting effect (Rafting) that occurs under high temperature creep conditions of superalloys, and in fact it has a recovery effect, it is named “Rafting-enabled” effect. recovery). This discovery breaks through the cognition that “single crystal superalloys do not have the ability to recover” in the classic concept, provides a scientific basis for the design of non-standard heat treatment systems for 3D printing superalloys, and shows that the new heat treatment system can fully satisfy 3D printing The need for repairing single crystal blades.
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