Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments
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JMEPEG https://doi.org/10.1007/s11665-020-05338-z
Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments Giulio Marchese, Eleonora Atzeni, Alessandro Salmi, and Sara Biamino Submitted: 14 July 2020 / Revised: 13 October 2020 / Accepted: 7 November 2020 The current work aimed to study the influence of various heat treatments on the microstructure, hardness, and residual stresses of Inconel 718 processed by laser powder bed fusion process. The reduction in residual stresses is crucial to avoid the deformation of the component during its removal from the building platform. Among the different heat treatments, 800 °C kept almost unaltered the original microstructure, reducing the residual stresses. Heat treatments at 900, 980, and 1065 °C gradually triggered the melt pool and dendritic structures dissolution, drastically reducing the residual stresses. Heat treatments at 900 and 980 °C involved the formation of d phases, whereas 1065 °C generated carbides. These heat treatments were also performed on components with narrow internal channels revealing that heat treatments up to 900 °C did not trigger sintering mechanisms allowing to remove the powder from the inner channels. Keywords
additive manufacturing, hardness, laser powder bed fusion, microstructure, phase state, residual stress, superalloys
1. Introduction Inconel 718 (IN 718) is a Ni-based superalloy characterized by high mechanical stability up to around 650 C, including elevated oxidation resistance and good fatigue life. This alloy is one of the most considered materials for aeronautics industries, such as parts of aircraft turbine engines and land-based gas turbine engines (Ref 1-7). Nowadays, near-net-shape complex components made of IN 718 can be fabricated employing additive manufacturing (AM) processes. Among these processes, laser powder bed fusion (LPBF) enables the production of components starting from powder melted by a laser beam layer by layer, based on a 3D CAD model (Ref 8-10). The high weldability of this alloy allows the fabrication of L-PBF IN 718 components with extremely high densification levels (close to 100%) (Ref 1114). However, the material is subjected to extremely high cooling rates (around 105–106 C/s) during the melting and solidification, generating extremely fine microstructures composed of dendritic/cellular structures with nanometric Laves phases, and micrometric segregations chiefly located into the interdendritic areas. Moreover, high cooling rates lead to the formation of high residual stresses within the material (Ref 11, 15-17). These high residual stresses can lead to distortion of the component as well as to reduce its fatigue life (Ref 18). It is Giulio Marchese and Sara Biamino, Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy; and Eleonora Atzeni and Alessandro Salmi, Department of Management and Production Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy. Contact e-
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