Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components
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JMEPEG DOI: 10.1007/s11665-017-2547-4
Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components K. Sajun Prasad, Sushanta Kumar Panda, Sujoy Kumar Kar, Mainak Sen, S.V.S. Naryana Murty, and Sharad Chandra Sharma (Submitted August 11, 2016; in revised form November 17, 2016) Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (r-FFLD) and effective plastic strain-stress triaxiality space (gEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using abovementioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal. Keywords
electron microscopy, finite element modeling, fractography, fracture forming limit diagram, Inconel 718, limiting dome height
1. Introduction Inconel 718 (IN718) is a precipitation-hardenable nickelchromium-iron-based superalloy extensively used for aerospace applications such as fuel tanks, heat exchangers and several engine components under cryogenic (250 C) to elevated temperature conditions (705 C) (Ref 1-3). Recently, IN718 sheets are found to be suitable for manufacturing of gas bottles to store high-pressure oxygen for space missions (Ref 4). Also, this alloy is used in outer casings of nickel-hydrogen (Ni-H2) cells to store energy for the satellite power systems (Ref 5). IN718 possesses high fatigue resistance to sustain pressure variations during the charging and discharging cycles of the Ni-H2 cell. Also, this material is compatible with alkali (KOH) and offers strong resistance to hydrogen embrittlement under operational environment. Apart from the high strength, its ductility is very high due to continuous strain-hardening ability of this complex alloy system during uniaxial deformation (Ref 6). Hence, the
K. Sajun Prasad and Sushanta Kumar Panda, Department of Mechanical Engineering, I.I.T. Kharagpur, Kharagpur, WB 721302, India; Sujoy Kumar Kar and Mainak Sen, Department of
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