Thermal effects on microstructural heterogeneity of Inconel 718 materials fabricated by electron beam melting
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Kinga A. Unocic Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Ryan R. Dehoff Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA; and Manufacturing Demonstration Facility, Oak Ridge National Laboratory, Knoxville, TN 37932, USA
Tapasvi Lolla Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210, USA
Sudarsanam S. Babu Manufacturing Demonstration Facility, Oak Ridge National Laboratory, Knoxville, TN 37932, USA; and Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN 37996, USA (Received 5 June 2014; accepted 9 June 2014)
Additive manufacturing 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 (d, c9, and c0) are discussed.
I. INTRODUCTION AND BACKGROUND
Powder bed based additive manufacturing (AM) processes using both lasers and electron beam have demonstrated the ability to fabricate complex geometric components with significant performance improvement through weight reduction and design optimization at reduced costs and time to market. Among all powder AM processes, the electron beam melting (EBM) process is considered attractive for several reasons.1 Due to the inherent nature of vacuum processing, the impurity pick up level is low from the atmosphere, which is critical when fabricating materials such as Ti–6Al–4V where the oxygen content strongly influences mechanical properties. The electron beam can be moved across the powder bed without any inertia. As a result, it is possible to control the molten pool size, depth, and velocity. Additionally, the electron beam can be defocused and rapidly scanned over the surface of the powder bed, resulting in nearly planar heat a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.140 1920
J. Mater. Res., Vol. 29, No. 17, Sep 14, 2014
http://journals.cambridge.org
Downloaded: 11 Mar 2015
input over the surface. This scan strategy is utilized to preheat t
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