Based on the observation of the microstructural evolution using advanced characterization methods such as synchrotron X-ray microdiffraction and TEM, researchers from Xi’an Jiaotong University (XJTU) developed a novel heat treatment protocol for the 3D printed superalloy single crystals, and in the meanwhile a new mechanism of deformation recovery in superalloys is discovered.
Superalloy single crystals are of great importance for modern aero-engine. Due to the high cost of cast single-crystalline superalloy turbine blades, it is for long expecting whilst challenging to repair the damaged ones. Additive manufacturing (as known as 3D printing) is a promising route to this end, but a formidable obstacle is that the residual stress, dislocation density, and /′ microstructure in the single-crystalline fusion zone after 3D printing are unacceptable, prone to recrystallization that degrades the high-temperature creep properties.
In this study, a post-3D-printing recovery annealing was designed to eliminate the driving force for recrystallization, prior to standard solution treatment and aging. Conventional wisdom believed that recovery was almost impossible in Ni-based superalloy single crystals, but the authors found that at a temperature close to the typical creep testing temperature rafted microstructure formed, assisting occurrence of recovery. The recovery is rendered possible by the rafting of ′ particles that facilitates dislocation rearrangement and annihilation. The rafted microstructure is removed after solution treatment, leaving behind a damage-free and residual stress free single crystal with uniform ′ precipitates that is indistinguishable from the rest of the turbine blade.
This discovery offers a practical means to keep 3D-printed single crystals from cracking due to unrelieved residual stress, or stress-relieved but recrystallizing into a polycrystalline microstructure, paving the way for additive manufacturing to repair, restore and reshape any superalloy single crystal products.
The result is published on Advanced Materials, titled as Rafting-Enabled Recovery Avoids Recrystallization in 3D-Printing Repaired Single-Crystal Superalloys (doi: 10.1002/adma.201907164). The work was designed by Prof. Kai Chen and Prof. Zhiwei Shan from CAMPNano, State Key Laboratory for Mechanical Behavior of Materials, XJTU, Prof. Ju Li from MIT and Prof. Evan Ma from Johns Hopkins University. Graduate students Runqiu Huang, Sicong Lin, and Wenxin Zhu conducted the experiment and characterization, assisted by Dr. Yao Li from Chang'an University and Dr. Nobumichi Tamura from Lawrence Berkeley National Lab, US. The work was supported by the National Natural Science Foundation of China (Grants No. 91860109, 51901026), and the National Key Research and Development Program of China (Grant No. 2016YFB0700404).
For more information: https://onlinelibrary.wiley.com/doi/10.1002/adma.201907164
