Laser-directed Energy Deposition (laser-DED) is an additive manufacturing process where a 3D structure is fabricated layer-by-layer with powder that is directed and melted using a high power laser. The processing parameters of laser-DED have a strong influence on the microstructure of the samples that are fabricated. Due to a very high cooling rate, laser-DED produces some unique microstructures like columnar, dendritic grains, and macro-micro grains. The objective of this study is to understand how these microstructures will influence the ultrasonic properties of Inconel 718 (IN718). This work focuses on frequency-dependent parameters such as ultrasonic phase velocity, attenuation coefficient, backscatter and absorption, since they can be related to microstructural features spread over multiple wavelengths: grain size, grain orientation, phases, etc.

Nonlinear ultrasonics has been shown to be sensitive to small scale damage and microstructural changes in solids. However, measuring the nonlinearity parameter of a solid requires complex calibrations and point-by-point measurements using contact transducers. This poses a challenge when it comes to applying this technique for imaging. The present work explores the use of immersion based nonlinear ultrasonics to perform nonlinear harmonic imaging of solids. A solid sample immersed in liquid (water) will form a “3-layer” structures (water-solid-water) when two ultrasonics transducers are used in through transmission mode. This work will explore the analytical modeling of such a structure using the KZK equation to model the combined effect of attenuation and diffraction of the three layers. The objective of the modelling effort is to develop an inversion model to invert the acoustic nonlinearity parameter from experimental results. The nonlinear model was further validated using experimental measurements carried out on a Steel sample with localized damaged.

Additive manufacturing of nickel based super alloys such as IN718 is highly desirable since they have a wide range of applications in high performance structures. Compared to conventional methods, laser processing allows for near net shaping of complex geometries. However, laser processing can result in very complex microstructures including meta-stable phases, grain boundary segregation of precipitates, dendritic grains and cellular microstructure. Describing elastic properties of such structures can be quite challenging due to these features. This article explores the use of resonant ultrasound spectroscopy (RUS) to characterize the elastic properties of IN718 samples fabricated using Laser Directed Energy Deposition (DED). For initial estimates of the elastic constants, ultrasonic wave (longitudinal and shear) velocities measured at 5MHz and 2.25 MHz respectively. The initial assumption was that the eventual structure will be orthotropic and the 9 elastic constants were determined using a combination of RUS and propagating wave experiments. A finite element approach was adopted to model this system and to minimize the values of elastic constants. The results seem to suggest that the secondary phases such as Laves will influence the eventual anisotropy of the bulk structure.