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Title: Thermal Effects on Microstructural Heterogeneity of Inconel 718 Materials Fabricated by Electron Beam Melting

Abstract

Additive manufacturing (AM) technologies, also known as 3D printing, have demonstrated the potential to fabricate complex geometrical components, but the resulting microstructures and mechanical properties of these materials are not well understood due to unique and complex thermal cycles observed during processing. The electron beam melting (EBM) process is unique because the powder bed temperature can be elevated and maintained at temperatures over 1000 °C for the duration of the process. This results in three specific stages of microstructural phase evolution: (a) rapid cool down from the melting temperature to the process temperature, (b) extended hold at the process temperature, and (c) slow cool down to the room temperature. In this work, the mechanisms for reported microstructural differences in EBM are rationalized for Inconel 718 based on measured thermal cycles, preliminary thermal modeling, and computational thermodynamics models. The relationship between processing parameters, solidification microstructure, interdendritic segregation, and phase precipitation (δ, γ´, and γ´´) are discussed.

Authors:
 [1];  [2];  [2];  [3];  [4]
  1. Texas A & M Univ., College Station, TX (United States)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. The Ohio State Univ., Columbus, OH (United States)
  4. Univ. of Tennessee, Knoxville, TN (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1224156
DOE Contract Number:
AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Materials Research; Journal Volume: 29; Journal Issue: 17
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Sames, William J., Unocic, Kinga A., Dehoff, Ryan R., Lolla, Tapasvi, and Babu, Sudarsanam Suresh. Thermal Effects on Microstructural Heterogeneity of Inconel 718 Materials Fabricated by Electron Beam Melting. United States: N. p., 2014. Web. doi:10.1557/jmr.2014.140.
Sames, William J., Unocic, Kinga A., Dehoff, Ryan R., Lolla, Tapasvi, & Babu, Sudarsanam Suresh. Thermal Effects on Microstructural Heterogeneity of Inconel 718 Materials Fabricated by Electron Beam Melting. United States. doi:10.1557/jmr.2014.140.
Sames, William J., Unocic, Kinga A., Dehoff, Ryan R., Lolla, Tapasvi, and Babu, Sudarsanam Suresh. Mon . "Thermal Effects on Microstructural Heterogeneity of Inconel 718 Materials Fabricated by Electron Beam Melting". United States. doi:10.1557/jmr.2014.140.
@article{osti_1224156,
title = {Thermal Effects on Microstructural Heterogeneity of Inconel 718 Materials Fabricated by Electron Beam Melting},
author = {Sames, William J. and Unocic, Kinga A. and Dehoff, Ryan R. and Lolla, Tapasvi and Babu, Sudarsanam Suresh},
abstractNote = {Additive manufacturing (AM) technologies, also known as 3D printing, have demonstrated the potential to fabricate complex geometrical components, but the resulting microstructures and mechanical properties of these materials are not well understood due to unique and complex thermal cycles observed during processing. The electron beam melting (EBM) process is unique because the powder bed temperature can be elevated and maintained at temperatures over 1000 °C for the duration of the process. This results in three specific stages of microstructural phase evolution: (a) rapid cool down from the melting temperature to the process temperature, (b) extended hold at the process temperature, and (c) slow cool down to the room temperature. In this work, the mechanisms for reported microstructural differences in EBM are rationalized for Inconel 718 based on measured thermal cycles, preliminary thermal modeling, and computational thermodynamics models. The relationship between processing parameters, solidification microstructure, interdendritic segregation, and phase precipitation (δ, γ´, and γ´´) are discussed.},
doi = {10.1557/jmr.2014.140},
journal = {Journal of Materials Research},
number = 17,
volume = 29,
place = {United States},
year = {Mon Jul 28 00:00:00 EDT 2014},
month = {Mon Jul 28 00:00:00 EDT 2014}
}
  • In this project, Avure and ORNL evaluated the influence of hot isostatic pressing (HIP) and thermal cycling as standalone post processing techniques on the microstructure of electron beam powder bed deposited Ti-6Al-4V and Inconel 718 alloys. Electron beam powder bed deposition is an effective technology for fabricating complex net shape components that cannot be manufactured with conventional processes. However, material deposited by this technology results in columnar grain growth which is detrimental for many applications. For Ti-6Al-4V, it has been found that thermal cycling alone is not sufficient to breakdown the columnar microstructure that is typical of electron beam powdermore » bed technology. HIP, on the other hand, has the potential to be an effective technique to break down the columnar microstructure of Ti-6Al-4V into a more equiaxed and refined β grain structure, and provide a more homogeneous microstructure compared to the thermally cycled samples. Overall, the project showed that hot isostatic pressing reduced/eliminated porosity in both Ti-6Al-4V and Inconel 718 However, based on the unique thermal cycle and the application of pressure in the HIP vessel, Ti-6Al-4V e-beam deposited microstructures were modified from columnar grain growth to equiaxed microstructures; a significant outcome to this collaboration. Inconel 718, on the other hand, shows no change in the macrostructure as a result of the current HIP cycle based on the thermal history, and would require further investigation. Though the results of HIP cycle were very good at changing the microstructure, further development in optimizing the post heat treatments and HIP cycles is required to improve mechanical properties.« less
  • Residual stress profiles were mapped using neutron diffraction in two simple prism builds of Inconel 718: one fabricated with electron beam melting and the other with direct laser sintering. Spatially indexed stress-free cubes were obtained by EDM sectioning equivalent prisms of similar shape. The (311) interplanar spacing examined for the EDM sectioned sample was compared to the interplanar spacings calculated to fulfill force and moment balance. We have shown that Applying force and moment balance is a necessary supplement to the measurements for the stress-free cubes with respect to accurate stress calculations in additively manufactured components. Furthermore, our work hasmore » shown that residual stresses in electron beam melting parts are much smaller than that of direct laser metal sintering parts.« less
  • In this study, powder bed based additive manufacturing technologies offer a big advantage in terms of reusability of the powders over multiple cycles that result in cost savings. However, currently there are no standards to determine the factors that govern the powder reuse times. This work presents the results from a recyclability study conducted on Inconel 718 and Ti-6Al-4V powders. It has been found that the Inconel 718 powders are chemically stable over a large number of cycles and their reuse time is limited by physical characteristics of powders such as flowability. Ti-6Al-4V, on the other hand, finds its reusemore » time governed by the oxygen pick up that occurs during and in between build cycles. The detailed results have been presented.« less
  • A novel technique was developed to control the microstructure evolution in Alloy 718 processed using Electron Beam Melting (EBM). In situ solution treatment and aging of Alloy 718 was performed by heating the top surface of the build after build completion scanning an electron beam to act as a planar heat source during the cool down process. Results demonstrate that the measured hardness (478 ± 7 HV) of the material processed using in situ heat treatment similar to that of peak-aged Inconel 718. Large solidification grains and cracks formed, which are identified as the likely mechanism leading to failure ofmore » tensile tests of the in situ heat treatment material under loading. Despite poor tensile performance, the technique proposed was shown to successively age Alloy 718 (increase precipitate size and hardness) without removing the sample from the process chamber, which can reduce the number of process steps in producing a part. Lastly, tighter controls on processing temperature during layer melting to lower process temperature and selective heating during in situ heat treatment to reduce over-sintering are proposed as methods for improving the process.« less
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