Abstract
In the present study, Inconel 718 (IN718) superalloy fabricated by laser powder bed fusion (LPBF) has been characterized focusing on the effect of both homogenization and solution treatment time on grains structure, crystallographic texture, precipitates formation/dissolution and material hardness. For this purpose, a heat-treatment time window with a wide range of soaking times for both treatments was established aiming to develop the optimal post-treatment conditions for laser powder bed fused IN718. It was found that the as-printed IN718 is characterized by very fine columnar/cellular dendrites with Laves phase precipitating at the grain boundaries as well as inter-dendritic regions, which differs from the microstructure of wrought and cast materials and requires special heat-treatment conditions different from the standard treatments. The results reveal that the relatively short homogenization treatment at 1080 °C for 1 h was not enough to significantly change the as-printed grain structure and completely dissolve the segregates and Laves phase. However, a completely recrystallized IN718 material and more Laves phase dissolution were obtained after homogenization treatment for 4 h. A further increase in time of the homogenization treatment (7 h) resulted in grain growth and coarsening of carbides precipitates. The solution treatment time at 980 °C did not cause noticeable changes in the crystallographic texture and grain structure. Nevertheless, the amount of δ-phase precipitation was significantly affected by the solution treatment time. After applying the heat-treatment time window, the hardness increased by 51–72% of the as-printed condition depending on the treatment time due to the formation of γ′ and γ″ in the γ-matrix. The highest material hardness was obtained after 1 h homogenization, whereas the prolonged time treatments reduced the hardness. This study provides a comprehensive investigation of the post heat-treatments of the laser powder bed fused IN718 that can result in an optimized microstructure and mechanical behavior for particular applications.
Highlights
Inconel 718 is a nickel-based superalloy which was developed by the International NickelCompany in 1959 [1]
Microstructure examinations of the Inconel 718 (IN718) in the as‐printed and heat‐treated conditions were carried out using a MEIJI TECHNO optical microscope (OM, San Jose, CA, USA) equipped with a carried out using a MEIJI TECHNO optical microscope (OM, San Jose, CA, USA) equipped with a 3.0
The temperature difference between the upper level of the powder bed, where the local laser heating is at its maximum, and the lower level which is close to the building platform results in high heat dissipation and thermal gradient along the building direction
Summary
Inconel 718 is a nickel-based superalloy which was developed by the International NickelCompany in 1959 [1]. Materials 2020, 13, 2574 in different industrial applications such as aerospace and energy industries [2] This is due to its outstanding properties such as high strength, high resistance to corrosion, wear and oxidation at both low and elevated temperatures up to 650 ◦ C [3,4]. IN718 is a multiphase superalloy that consists of primary face-centered-cubic (FCC) γ-phase matrix along with other secondary phases such as FCC γ0 (Ni3 (Al,Ti)), body-centered-tetragonal (BCT) γ” (Ni3 Nb), orthorhombic δ-phase (Ni3 Nb), hexagonal Laves phase (Ni,Fe,Cr) (Nb,Mo,Ti) and (MC, M23 C6 , M6 C) carbides [7] Both γ0 and γ” are the primary strengthening phases of the γ-matrix, whereas δ and Laves phases are known to degrade the mechanical properties in this alloy [7]. The microstructure and mechanical properties of IN718 superalloy are sensitive to the types, contents, distribution, and size of these precipitates [8]
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