Oxide coarsening and agglomeration during melt-based additive manufacturing of dispersion-strengthened alloys

Abstract

Dispersion-strengthened alloys densified with laser powder bed fusion, a melt-based additive manufacturing technique, have coarser dispersoids, lower dispersoid number densities, and greater tendency to form slag compared to conventional wrought dispersion-strengthened alloys. These differences degrade creep and fatigue resistance, and mitigating their extent is critical to printing high-performance components for demanding high-temperature structural applications. In this work, experiments and modeling were used to assess how printing parameters, alloy chemistry, and powder feedstock collectively affect dispersoid evolution and slag formation. Laser powder bed fusion parameter studies were used to assess the effects in Ni-20Cr-Y₂O₃ feedstock produced via resonant acoustic mixing then consolidated with systematic variations in laser parameters (power, speed), Y₂O₃ concentration, and Al content. Dispersoid structure was subsequently characterized using small angle neutron scattering. The finest dispersion achieved among fully dense (>99.5 rel. density) specimens has mean dispersoid diameter 21 nm and number density 230 μm-3. Dispersoid diameter was shown to decrease with the following adjustments: decreasing laser power, increasing scan speed, decreasing Y₂O₃ concentration, and keeping Al content below 0.3 wt%. Model predictions for dispersoid diameter were consistent with experimental values, and several key factors which influence the evolution of dispersoids were identified: convection-influenced thermal excursion, Y₂O₃ solubility, reaction with Al, nucleation, and diffusion-driven growth. The model also considers oxide dissolution over multiple melt cycles to establish bounds for slag-free printing of ODS alloys, showing a tradeoff between build rate and the quality of the oxide feedstock.