The porous Ni-based superalloy manufactured by customized selective electron beam melting strategies: Pore structure, microstructures, performance, and computational thermal fluid dynamics simulation

Abstract

In this paper, the potential of selective electron beam melting (SEBM) of Inconel 625 nickel-based superalloy powder was investigated with a focus on achieving controlled fabrication of interconnected porous structures with uniform micrometer-sized dimensions. This is achieved through customized manufacturing strategies, such as focus offset, pulsed scanning, and layer-by-layer rotation, based on the foundational factors of powder particle size, beam spot diameter, and the constraints that form the initial pores. The samples' build-direction (BD) and scan-direction (SD) morphology under varied parameters were demonstrated using optical microscopy (OM), X-ray computed tomography (3D-CT), and other methods. The typical microstructure of the samples consists primarily of dendritic γ-Ni (FCC) and interdendritic phases rich in Nb and Mo. Additionally, three types of prior powder boundaries (PPBs) can be observed, which influence the microstructure and result in distinct features. A whole progress of the SEBM using discrete element/computational thermal fluid dynamics (DEM/CFD) composite simulation was performed to investigate the unique molten pool behavior in porous material SEBM which relies on the flow through pores and to revealed the formation process of PPBs. Practical mesoscopic models of porous materials and equivalent pore network models (PNM) were established to characterize key pore characteristics such as bulk porosity, mask porosity, pore size and distribution, average tortuosity, distribution of equivalent pore spheres, and their mean coordination numbers, and were compared with porous materials of the similar bulk porosity prepared by powder metallurgy (PM). Permeability simulations were implemented. The absolute permeabilities of porous Inconel 625 prepared by SEBM in the BD and SD reach 7.37 d and 6.57 d, respectively, representing an improvement of about 95 % compared to PM samples. The directional dependence of this characteristic was elucidated. Finally, the fracture surfaces and fracture behavior of the samples were analyzed, the evolution mechanism of the microstructure was used to explain the formation of regions with weak mechanical properties. This completes the establishment of the relationship between morphology, pore characteristics, microstructure, liquid permeability, and mechanical properties of porous Inconel 625 alloy fabricated by SEBM. This material exhibits enormous potential as a transpiration cooling material for thermal protection systems, boasting high efficiency and performance.