Speaker
Description
This study aims to assess the fabrication route of cobalt-based superalloys using directed energy deposition (DED) process. Compared with nickel-based superalloys, the precipitation kinetics of the γ’-strengthening phase leads to moderate hardening at high cooling rate, which prevents or limits strain aged cracking during fabrication.
The DED process induces a complex oriented grain structure and a strong crystallographic texture along the build direction. Moreover, segregations occur during dendritic solidification causing low melting point phases. These particular features result in a solidification and liquation crack sensitivity. The study of several deposition strategies may decrease the crack occurrence by a reduction of High Angle Grain Boundaries (HAGB) proportion.
The as-build microstructure features several types of γ’-precipitation size gradient. Normal to the build direction, size heterogeneities occur due to the solidification segregation. Along the build direction, size gradients arise due to the thermal cycling and the specific thermal path of each layer. Furthermore, the chemical segregation in the interdendritic regions leads to the precipitation of complex multi-phase particles. An automated crystal orientation method (ASTAR), electron energy loss spectroscopy, energy and wavelength dispersive spectrometry are used to identify the different phases.
Specific heat treatments allow the modification of this complex as-build microstructure, in order to obtain more isotropic properties. Different aging and homogenization heat treatment enable the evolution of the microstructure and the transition from columnar to equiaxed grains. In addition, detailed analysis on the heat treated materials reveals the elimination of the chemical heterogeneities, and, consequently, a homogenization of the γ’-precipitation size distribution.
| Speaker Country | France |
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