Evaluation of Fracture Properties of Additively Manufactured IN-718 Under Quasi-static and Dynamic Loading Conditions

Abstract

Inconel 718 is a Nickel based alloy with many applications in the aerospace and automotive industries. Due to its high strength under intense heat, IN-718 is used

in extreme environments such as rocket engine manifolds and automobile exhaust systems. The focus of this work is on the fracture behavior of additively printed IN-718. Edge notched three-point bending specimens are additively manufactured using laser powder bed fusion (LPBF) and mechanically tested to evaluate the fracture performance under quasi-static and dynamic loading. Several combinations of laser process parameter, heat treatment, and shielding gas are used during manufacturing which influence the microstructure. Two different heat treatment procedures are used along with two different shielding gases, namely argon (A) and nitrogen (N) gas. The laser process parameters can also introduce defects into 3D printed parts. An underpowered laser causes lack of fusion (LoF) defects and an overpowered laser causes keyhole (KH) defects in printed parts. Three laser process parameters, one to induce more LoF defects, one to induce more KH defects, and a middle powered “recommended” (R) laser parameter not meant to induce either kind of defect, are used. Electrical discharge machining (EDM) is used to cut a crack-like notch into the edge of the specimens. A pair of EDM cut side grooves is added to the front and back surfaces along the uncracked ligament to constrain the crack growth to occur self-similarly, mitigate crack tunneling effects, and suppress shear lip formation. A random speckle pattern is applied to the specimens to allow for implementing digital image correlation (DIC)—a full- field optical technique—to measure surface deformation fields during fracture.

The fracture behavior of the specimens are evaluated by conducting quasi-static and dynamic experiments. Under quasi-static loading, notched three-point bend specimens are slowly loaded until the crack initiates and grows. The surface deformation are simultaneously measured by DIC. To evaluate the fracture behavior of IN-718 under dynamic loading, similarly notched three-point bend specimens are tested using a split-Hopkinson pressure bar (SHPB) apparatus to rapidly load the specimen while implementing DIC for full-field deformation measurements. The fracture event is recorded using an ultrahigh-speed camera at 400,000 frames per second. In both quasi-static and dynamic experiments, images are analyzed using DIC to quantify surface displacement fields. The energy release rate (ERR) is extracted at each load-step or time-step by computing the J-integral using a hybrid DIC-Finite Element (DIC-FE) method. The methodology is applicable to both quasi-static and high strain-rate experiments and accounts for elastoplastic stress-strain behavior in the material. A modified least-squares analysis of the measured displacements is also used to evaluate stress intensity factors in the elastic range to further validate the DIC-FE approach. In this case, the DIC-FE method is found to be more robust than the elastic least squares method and allows for the evaluation of fracture properties beyond the point of fracture. Results for the quasi-static specimens are validated with a complementary finite element solution. It is found that the most favorable manufacturing conditions are those which used argon shielding gas and heat treatment 1. The highest quasi-static critical ERR is 163.1 N/mm for the A1K (shielding gas of argon, heat treatment 1, keyhole defects) condition. The lowest is 46.9 N/mm for the N2K (shielding gas of nitrogen, heat treatment 2, keyhole defects) condition. This pattern is found to be the same under high strain-rate loading, with the highest critical energy release rate of 140.1 N/mm for the A1 condition. The lowest energy release rate is 50.2 N/mm for the N2 condition.