Investigation on the Hot Crack Sensitivity of a Nickel-Based Single Crystal Superalloy Fabricated by Epitaxial Laser Metal Forming

Abstract

In this study, the epitaxial laser metal forming(E-LMF) process was used to deposit a nickel-based single crystal superalloy under different cooling conditions, and its microstructure and formation mechanism were investigated using optical microscopy (OM), scanning electron microscope (SEM), electron backscattered diffraction (EBSD), and electron probe micro-analyzer (EPMA). The results revealed that under air-cooling, a epitaxial microstructure with an almost crack-free is formed under air-cooling, while due to the thermal stress accumulation, water-cooling increases the hot crack sensitivity of the deposited due to the thermal stress accumulation. In the case of water-cooling, almost all cracks appear in the overlap zone, with a few cracks appearing at other region. The element distribution analysis and grain boundary angle analysis show that micro-segregation, as well as high angle grain boundaries, result in typical solidification cracks. In the case of air-cooling, the crystal orientation of the deposited zone is consistent with that of the substrate, although there are some stray grains in the top zone. As the bottom layers were annealed for a longer time than the upper layers, the γ' phase is more fully precipitated at the bottom than at the top.

Keywords: Epitaxial laser metal forming(E-LMF), Nickel-based single crystal superalloy, Cooling conditions, Crack mechanism, Deposition strategy