Ultrasonic Methods For Characterization Of Additively Manufactured Materials PDF Download

The goal of this research is to find new noninvasive methods to certify the quality of safety-critical additively manufactured (AM) metallic parts for use in industries such as aerospace and defense. Additive manufacturing facilitates rapid prototyping, building, and repairing of custom components with increased agility, production rate, and reduced waste. A recognized barrier to the wide adoption of additive manufacturing is the lack of new approaches for AM part qualification. Our research objective is to exploit the material's linear and nonlinear ultrasonic response - which represents the measurable changes and distortion in elastic waves encountering macroscopic and microscopic defects - to establish links between microstructure and macroscale mechanical properties of AM metals. We measure linear and nonlinear ultrasonic parameters for a series of AM and wrought 316L grade stainless steel samples and compare the obtained parameters against mechanical properties of the samples measured on corresponding coupons. The samples are heat-treated to different temperatures to induce microstructural changes which alter their mechanical properties and ultrasonic response. Two sets of specimens are manufactured, one from the additive manufacturing method Laser Powder Bed Fusion (L-PBF), and the second from a traditional wrought method. Using the nonlinear ultrasonic method of Second Harmonic Generation (SHG), the acoustic nonlinearity parameter is estimated. SHG has been shown to offer a highly sensitive response to microstructural heterogeneities such as dislocations and grain boundaries. A linear ultrasonic parameter, wave speed, is also recorded with pulse-echo testing. Alongside these ultrasonic measurements, mechanical testing parameters including elastic moduli and yield strength are evaluated for the specimens. To accompany the experimental testing, a series of numerical simulations were conducted using commercial finite-element software to study the effects of randomly distributed heterogeneities on wave distortion in a controlled environment. In these simulations, randomly generated heterogeneities are scattered throughout a 2D plate with materials properties different from the bulk material. Ultrasonic wave propagation is simulated within this heterogeneous medium to investigate the effects of the heterogeneities' elastic properties, geometry, and distribution on ultrasonic signals, including distortion measured in terms of higher harmonic generation (HHG). Experimental results indicate correlations between the nonlinearity parameter and both ultimate tensile strength and yield strength, where nonlinearity generally decreases as these mechanical parameters increase, particularly in the AM samples. We hypothesize that microstructural changes in grain size and distribution through the heat treatment process influence these trends in measured nonlinearity. Additionally, substructures at even smaller length scales, such as nanoscale precipitates and dislocations affect the ultrasonic and mechanical behavior. Measurements of elastic moduli and total elongation do not exhibit trends with the nonlinearity parameter. The linear parameter, wave speed, does not correlate well with the mechanical parameters, which is attributed to its lack of sensitivity to detect changes in microscopic features. These results show promising evidence for the feasibility of AM parts qualification using nondestructive nonlinear ultrasonic testing. Results of the simulations indicate that changes in heterogeneity size, volume fraction, and material property deviations from the bulk material affect HHG to varying degrees. As expected, heterogeneities of smaller sizes and volume fractions have a less significant effect. However, at increasingly large values, changes in HHG are more pronounced, and material density and stiffness deviations from the bulk material are shown to have a larger effect on HHG. Future work includes continuing nonlinear ultrasonic testing, as well as comparing results to nonlinear resonant ultrasound spectroscopy (NRUS). New geometries and materials will be tested to expand the dataset. Microstructures will be imaged using scanning and transmission electron microscopy (SEM, TEM) and evaluate our hypotheses, and further complexity in numerical simulations will be implemented to isolate microstructural features and explore their effects on material behavior.