Ultrasonic Measurement of Microstructural Anisotropy in a Ni-Based Superalloy Fabricated by Direct Laser Deposition

In this article, the anisotropic microstructure of a nickel IN718 sample fabricated on a forged nickel alloy base plate by the Additive Manufacturing (AM) method using the Direct Laser Deposition (DLD) technique was characterized by the mode converted (Longitudinal to Transverse, L-T) ultrasonic scattering. The L-T ultrasonic measurements were conducted using two same focused transducers in a pitch-catch configuration on both forged and AM regions in two perpendicular directions. The ultrasonic scattered signal was converted to the two-dimensional scalogram through the Continuous Wavelet Transform (CWT). The energy distribution of ultrasonic scattering over the whole traveling path was obtained in the Time-Frequency (TF) domain. The apparent integrated scatter (AIS) was calculated to represent scattering energy by selecting the time window over the focal zone and the right frequency range. The AIS ratio of L-z- T-y/L-z- T-x measured in two perpendicular directions was used to quantify the microstructural anisotropy at each measurement spot. Then the variation of the microstructural anisotropy can be imaged over the whole sample, from which it can be seen that the microstructural anisotropy increased progressively towards the interface between the AM region and the forged base plate, and reached the maximum near the interface. The dependence of the characterization of microstructural anisotropy on the selection of the time window was also investigated by choosing several different time windows around the focal zone. The results show that the characterization of microstructural anisotropy was barely influenced, a result which indicates the measurement stability of this method. The mode-converted diffuse ultrasonic scattering technique demonstrates a strong sensitivity to anisotropic microstructure, an outcome that can be applicable for quality control during additive manufacturing.