Analysis of As-Built Microstructures and Recrystallization Phenomena on Inconel 625 Alloy Obtained via Laser Powder Bed Fusion (L-PBF)

Thibaut De Terris,Halim Haddadi,Vincent Michel,Patrice Peyre,Zehoua Hadjem-Hamouche,Olivier Castelnau

doi:10.3390/met11040619

Abstract

The microstructures induced by the laser-powder bed fusion (L-PBF) process have been widely investigated over the last decade, especially on austenitic stainless steels (AISI 316L) and nickel-based superalloys (Inconel 718, Inconel 625). However, the conditions required to initiate recrystallization of L-PBF samples at high temperatures require further investigation, especially regarding the physical origins of substructures (dislocation densities) induced by the L-PBF process. Indeed, the recrystallization widely depends on the specimen substructure, and in the case of the L-PBF process, the substructure is obtained during rapid solidification. In this paper, a comparison is presented between Inconel 625 specimens obtained with different laser-powder bed fusion (L-PBF) conditions. The effects of the energy density (VED) values on as-built and heat-under microstructures are also investigated. It is first shown that L-PBF specimens created with high-energy conditions recrystallize earlier due to a larger density of geometrically necessary dislocations. Moreover, it is shown that lower energy densities offers better tensile properties for as-built specimens. However, an appropriate heat treatment makes it possible to homogenize the tensile properties.

Highlights

The first condition is a standard one in terms of Volume Energy Density (VED) and is equivalent to the manufacturing condition proposed by the machine manufacturer (L-PBF-2)
The determined using optical microscopy carried out out on asThesample sampleporosity porositywas wasfirst first determined using optical microscopy carried on polished samplesetching) and image analysis binarization thresholding) as-polished samples
The results obtained here for as-built test specimens manufactured horizontally (LPBF-1, 2, and 3 H) were slightly lower than those obtained by Nguejio et al [52]. This difference can be explained by the fact that these authors worked with less energetic manufacturing parameters than us (P/V ratio = 0.247 J/mm [52] vs. 0.307 J/mm here), and we have previously shown that when the energy density supplied for manufacturing decreases, the tensile properties are improved

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The laser powder bed fusion (L-PBF) process is widely recognized as the most powerful and efficient additive manufacturing process for building complex 3D shapes with a high degree of precision and combined with satisfactory metallurgical properties. The building chambers have recently been enlarged (e.g., from 850 to 1000 mm in building height) and some of them offer up to four lasers, which are used simultaneously. Many authors have investigated the classical triad between process optimization, microstructures and mechanical properties for a large range of classical L-PBF metallic alloys, such as 316L or maraging steels [1,2], superalloys

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Conclusion