Abstract
Hastelloy X (HX) is a Ni-based superalloy which suffers from high crack susceptibility during the laser powder bed fusion (LPBF) process. In this work, the microstructure of as-built HX samples was rigorously investigated to understand the main mechanisms leading to crack formation. The microstructural features of as-built HX samples consisted of very fine dendrite architectures with dimensions typically less than 1 µm, coupled with the formation of sub-micrometric carbides, the largest ones were mainly distributed along the interdendritic regions and grain boundaries. From the microstructural analyses, it appeared that the formation of intergranular carbides provided weaker zones, which combined with high thermal residual stresses resulted in hot cracks formation along the grain boundaries. The carbides were extracted from the austenitic matrix and characterized by combining different techniques, showing the formation of various types of Mo-rich carbides, classified as M6C, M12C and MnCm type. The first two types of carbides are typically found in HX alloy, whereas the last one is a metastable carbide probably generated by the very high cooling rates of the process.
Highlights
Laser powder bed fusion (LPBF) has the potential to build near net shape components with complex geometries starting from consecutive layers of loose powders, which are directly consolidated by laser beam irradiation, adding material layer by layer [1,2,3]
The chemical composition of the powder feedstock and processed LPBF solid samples were assessed using an inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis, except for C and O2 evaluated by means of an infrared absorption (IRA) and an inert gas fusion (IGF) analysis, respectively
The formation of these spherical allowed the observation of near-perfect spherical pores, of which the largest ones had a diameter of pores may stem from the entrapped during the± 0.30%
Summary
Laser powder bed fusion (LPBF) has the potential to build near net shape components with complex geometries starting from consecutive layers of loose powders, which are directly consolidated by laser beam irradiation, adding material layer by layer [1,2,3].This technology is a type of additive manufacturing process that allows the fabrication of a wide range of metal alloys, reaching high densification levels, such as titanium alloys [3,4], aluminium alloys [5,6,7], stainless steel [8], and different kinds of composites [9] as well as superalloys [10,11,12,13].In particular, for fabricating materials with complex shapes characterized by high temperature strength such as Ni-based superalloys the LPBF can drastically reduce production costs, taking the place of traditional processes, such as casting and forging [12,14].after an adequate process parameters optimization, LPBF enables the production of different Ni-based superalloys with relative density close to 100%, among which the most investigated ones are Inconel 718 and Inconel 625 alloys [10,11,12,13,15].Notwithstanding this, for Ni-based superalloys, characterized by high crack susceptibility, the occurrence of cracks is difficult to avoid, which reduces the densification level, even when a process parameter optimization is applied [15,16].Materials 2018, 11, 106; doi:10.3390/ma11010106 www.mdpi.com/journal/materials. Laser powder bed fusion (LPBF) has the potential to build near net shape components with complex geometries starting from consecutive layers of loose powders, which are directly consolidated by laser beam irradiation, adding material layer by layer [1,2,3] This technology is a type of additive manufacturing process that allows the fabrication of a wide range of metal alloys, reaching high densification levels, such as titanium alloys [3,4], aluminium alloys [5,6,7], stainless steel [8], and different kinds of composites [9] as well as superalloys [10,11,12,13].
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