Abstract
In the present study, the effect of homogenization and solution treatment times on the elevated-temperature (650 °C) mechanical properties and the fracture mechanisms of Inconel 718 (IN718) superalloy fabricated by laser powder bed fusion (LPBF) was investigated. Homogenization times between 1 and 7 h at 1080 °C were used, while solution treatments at 980 °C were performed in the range from 15 to 60 min. The as-printed condition showed the lowest strength but the highest elongation to failure at 650 °C, compared to the heat-treated conditions. After heat treatments, the strength of the IN718 alloy increased by 20.3–31% in relation to the as-printed condition, depending on the treatment time, whereas the ductility decreased significantly, by 67.4–80%. Among the heat treatment conditions, the 1 h homogenized conditions at 1080 °C (HSA1 and HSA2) exhibited the highest strength and ductility due to the combined effects of the precipitation hardening and sub-structural changes. Further increases in the homogenization time to 4 and 7 h led to a decrease in the strength and significant ductility loss of the LPBF IN718 due to the considerable annihilation of the dislocation tangles and a greater precipitation of coarse MC carbide particles. Furthermore, it was found that the solution treatment duration had a crucial influence on the mechanical properties at 650 °C due to the increase in the grain boundary strength through the pinning effect of the intergranular δ-phase. In addition, the fracture mechanism of the LPBF IN718 was found to be dependent on the heat treatment time. Finally, this investigation provides a map that summarizes the effect of homogenization and solution treatment times on the high-temperature mechanical behavior of LPBF IN718 by relating it to the corresponding microstructural evolution. This effort strives to assist in tailoring the mechanical properties of LPBF IN718 based on the design requirements for some specific applications.
Highlights
In the past decades, elevated-temperature applications have become more extensive, and have come to include different fields, such as aerospace, petrochemical, power plant, energy, and nuclear reactor industries[1,2]
The Laves phase in the as-printed Inconel 718 (IN718) provides a favorable site for microvoids nucleation and crack propagation
After heat treatments, the tensile and yield strength significantly increased by 20.3–31% and 46.1–62.3%, respectively, as compared to the as-printed condition, depending on the solution and homogenization treatment time, while the ductility correspondingly decreased by 67.4–80%
Summary
In the past decades, elevated-temperature applications have become more extensive, and have come to include different fields, such as aerospace, petrochemical, power plant, energy, and nuclear reactor industries[1,2]. The LPBF process presents significant advantages, such as (1) the ability to fabricate complex-shaped components without geometrical limitations, (2) a low buy-to-fly ratio (as little as 2) versus conventional manufacturing methods, where this metric can reach 50, (3) production time and cost savings, and (4) the production of fine microstructures and improved mechanical properties as compared to traditional fabrication methods, such as casting and powder m etallurgy[8,9,10,11]. LPBF-fabricated IN718 components usually require thermal post-treatment to reduce their level of residual stresses, homogenize their microstructure and precipitate the desired phases to enhance their mechanical properties, especially at elevated temperatures. Suitable post-treatments are required to obtain a homogenized microstructure as well as high mechanical properties at elevated temperatures
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