Inconel 718 is a high strength, precipitation-hardenable Ni-Fe-Cr-based superalloys, which is widely used in aerospace and nuclear applications in extreme working conditions. Because of the excellent combination of alloying elements, Inconel alloy possesses superior mechanical properties such as high yield (700-1250 MPa) and ultimate tensile strengths (950- 1650 MPa), good creep strength, rupture strengths, and high resistance to fatigue and corrosion at elevated temperatures. However, this alloy has a narrow-forming&nbsp;temperature range, more deformation resistance, and complex microstructures. Thus, an extensive investigation on the high-temperature flow behavior of Inconel alloys is very important. Understanding deformation behavior is a key requirement to optimize process parameters for improving the material processing conditions and conforming safe performance during forming.In this work, the hot deformation behavior of the Inconel 718 alloy sheet has been investigated at 700°C under multiaxial loading conditions. The forming behavior has been analyzed along three distinct deformation modes (uniaxial stress, plane strain, and biaxial stretch regions) and results were quantified using a Gurson-Tvergaard-Needleman (GTN) damage model. Firstly, uniaxial tensile tests have been performed at 700ºC and quasi-static strain rates (0.0001, 0.001, 0.01 /s) to identify the material constants of the damage model. Based on uniaxial tensile test data, the GTN damage model is formulated for Inconel 718 alloy. The model is used to define the adiabatic heating, strain rate sensitivity, and flow softening behavior of a material. Further, the standard Nakazima test has been conducted to characterize the forming behavior under various deformation modes. The tests were performed at 700ºC with fixed blank holding pressure (BHP) of 2.5 MPa and punch speed of 2 mm/min. Different geometry/dimension blanks as per ASTM E2218-15 standard and Hasek specimens (ISO12004-2 standard) were considered to prevent draw bead failure caused by lower width rectangular specimens.Subsequently, Finite element (FE) analysis of Nakazima test coupled with plastic and damage modeling has been carried out to predict the failure behavior of the Inconel alloy. The FE predicted results showed good agreement with the experimental test in predicting the fracture behavior of the alloy. The fracture behavior of Inconel 718 alloy has been examined with the help of a Scanning Electron Microscope (SEM) in the hot uniaxial loading and multiaxial loading conditions. The fracture surfaces of Inconel 718 alloy clearly indicate a ductile–brittle fracture. Nucleation and micro-voids growth are the main fracture phenomena observed in Inconel 718 alloy. The presented results give a basis of optimal hot forming process of Inconel 718 alloy.

Prediction of Hot Deformation Behaviors Under Multiaxial Loading Using Gurson-Tvergaard-Needleman Damage Model for Inconel 718 Alloy Thin Sheet

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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