Abstract
The structural evolution of oxides in dispersion-strengthened superalloys during laser-powder bed fusion is considered in detail. Alloy chemistry and process parameter effects on oxide structure are assessed through a parameter study on the model alloy Ni-20Cr, doped with varying concentrations of Y2O3 and Al. Small angle neutron scattering measurements of the dispersoid size distribution show the dispersoid size increases with higher laser power, slower scan speed, and increasing Y2O3 and Al content. Complementary electron microscopy measurements reveal reactions between Y2O3 and Al, even in nanoscale dispersoids, and the presence of micron-scale oxide slag inclusions in select specimens. A scaling analysis of mass and momentum transport within the melt pool, presented here, establishes that diffusional structural evolution mechanisms dominate for nanoscale dispersoids, while fluid forces and advection become significant for larger slag inclusions. These findings are developed into a theory of dispersoid structural evolution, integrating quantitative models of diffusional processes – dispersoid dissolution, nucleation, growth, coarsening – with a reduced order model of time-temperature trajectories of fluid parcels within the melt pool. Calculations of the dispersoid size in single-pass melting reveal a zone in the center of the melt track in which the oxide feedstock fully dissolves. Within this zone the final Y2O3 size is independent of feedstock size and determined by nucleation and growth kinetics. If the dissolution zones of adjacent melt tracks overlap sufficiently with each other to dissolve large oxides, formed during printing or present in the powder feedstock, then the dispersoid structure throughout the build volume is homogeneous and matches that from a single pass within the dissolution zone. Gaps between adjacent dissolution zones result in oxide accumulation into larger slag inclusions. Predictions of final dispersoid size and slag formation using this dissolution zone model match the present experimental data and explain process-structure linkages speculated in the open literature.
| Original language | English |
|---|---|
| Article number | 104554 |
| Journal | Additive Manufacturing |
| Volume | 96 |
| DOIs | |
| State | Published - Sep 25 2024 |
Funding
The authors from MIT gratefully acknowledge support from ONR through contract no. N00014\u201322\u20131\u20132036. The authors from Monash University gratefully acknowledge funding from Australia Research Council grant no. LP220100400. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The beam time was allocated to GP-SANS (CG2) on proposal number IPTS-31543. The authors acknowledge the use of instruments and scientific and technical assistance of Dr. Nilanjan Chatterjee at the Department of Earth, Atmospheric, and Planetary Sciences, MIT. The authors from MIT gratefully acknowledge support from ONR through contract no. N00014-22-1-2036. The authors from Monash University gratefully acknowledge funding from Australia Research Council grant no. LP220100400. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The authors acknowledge the use of instruments and scientific and technical assistance of Dr. Nilanjan Chatterjee at the Department of Earth, Atmospheric, and Planetary Sciences, MIT.
Keywords
- Laser-powder bed fusion
- Neutron scattering
- Nickel-based superalloys
- Oxide dispersion strengthening
