Performance Characterization of Laser Powder Bed Fusion Fabricated Inconel 718 Treated with Experimental Hot Isostatic Processing Cycles

Abstract

Inconel 718 alloy fabricated by selective laser melting (SLM) (or laser powder-bed fusion (LPBF)) has been post-process heat-treated by stress-relief anneal at 1065 °C; stress-relief anneal (1065 °C) + solution treatment (at 720 °C) + aging (at 620 °C); hot isostatic pressing (HIP) (at 1120–1200 °C); stress-relief anneal + HIP; and stress-relief anneal + HIP + solution treatment + aging. Microstructure analysis utilizing optical metallography revealed primarily equiaxed grain structures (having average diameters ranging from ~30 to 49 microns) containing annealing twins, and a high concentration of carbide precipitates in all HIP-related treatments in the grain boundaries and intragrain regions. However, no precipitates nucleated on the {111} coherent annealing twin boundaries because of their very low interfacial free energy in contrast to regular grain boundaries. The mechanical properties for the as-fabricated Inconel 718 exhibited a yield stress of 0.64 GPa, UTS of 0.98 GPa, and elongation of 26%. Following stress-relief anneal at 1065 °C, the yield stress dropped to 0.60 GPa, while the elongation increased to 43%. The associated grain structure was an irregular, somewhat elongated, recrystallized structure. This structure was preserved at a stress anneal at 1065 °C + solution treatment + aging, but grain boundary and intragrain precipitation resulted in a doubling of the yield stress to 1.3 GPa and a reduced elongation of 12.6%. The results of HIP-related post-process heat treatments involving temperatures above 1060 °C demonstrated that the yield stress and elongations could be varied from 1.07 to 1.17 GPa and 11.4% to 19%, respectively. Corresponding Rockwell C-scale hardness values also varied from 33 for the as-fabricated Inconel 718 to 53 for simple post-process HIP treatment at 1163 °C.

A Review of Post-Processing Technologies in Additive Manufacturing

“…The sub-grains can be observed more accurately by electron backscatter diffraction (EBSD) or transmission electron microscopy (TEM) analysis. Similar grain refinement observations are also documented in [51]. As the laser peening process is completed, it is expected that the grain refinement degree of the lower surface of the sample can reach tens of microns on the upper surface [106].…”

“…During the manufacturing processes, internal defects such as balling, porosity, cracks, powder agglomeration, and thermal stress would appear between different printing layers. These defects have serious influences on the internal microstructure and mechanics of the final parts [49][50][51][52][53][54][55]. Therefore, after the parts being manufactured, post-processing operations are usually required to improve the mechanical properties and the surface quality, achieving their intended utilization [56][57][58][59][60][61][62].…”

Influence of Postprocessing on Wear Resistance of Aerospace Steel Parts Produced by Laser Powder Bed Fusion

“…The direct laser manufacturing of the proposed complex parts from the powder takes their production to a new level [52][53][54] since it allows direct production of the parts from the powder layer-by-layer on a substrate in a vacuum chamber or in a chamber with a neutral atmosphere [55][56][57][58] by selective remelting of granules with a coherent and monochrome light beam [59,60]. Laser powder bed fusion was carried out on an EOS M280 industrial unit (EOS GmbH, Krailling, Germany) and an ALAM experimental setup (MSTU Stankin, Moscow, Russia) [61][62][63] equipped with the laser source of continuous radiation LK-200 (IPG LASER GMBH, Fryazino, Russia) with a wavelength of 1070 nm, a beam divergence of 0.2 • , and maximum power of 200 W. Corrosion-resistant steel of the martensitic class, grade 20Kh13 (DIN 1.4021), and corrosion-resistant chromium-nickel steel of the austenitic class, grade12Kh18N9T (DIN 1.4541), were chosen for additive production.…”

Defect and Distortion in Powder Bed Fusion of Metal Additive Manufacturing Parts

“…Furthermore, discontinuities in printing and other external factors often cause defects in built components (Chen et al 2019;Bahnini et al 2018). Several internal defects, such as porosity, powder adhesion, cracks, balling, and powder thermal stress during printing, also affect the quality of the final part, such as internal structure and mechanical properties (Leung et al 2019;Cerniglia and Montinaro 2018;Gisario et al 2019;Di Angelo et al 2020;Tino et al 2020;Varela et al 2020). This paper reviews the defects caused by various factors in metal AM and presents a benchmark for defect mitigation methods, including control of L-PBF.…”