Ultrasonic Nondestructive Characterization and Monitoring in Metal and Hybrid Additive Manufacturing

Metal additive manufacturing (AM) technologies promise the ability to produce complex geometries, reduce assembly steps, and reduce waste amongst other advantages. Hybrid AM fully couples AM with additional processes or energy sources during manufacturing. In this manner, the making of intricate components with multiple materials, high customization, and locally tailored properties for critical, high-performance, and/or highly-specialized applications is enabled. A material undergoing AM experiences a thermomechanical history unlike that of conventional manufacturing processes, particularly in terms of high cooling rates and repeated thermal cycles. These differences result in heterogeneous microstructures and large statistical variations in material properties. Consideration of these effects is crucial in engineering design for AM. Methods to measure, map, and quantify these effects nondestructively during and after processing are necessary. Ultrasonic scattering has been extensively used to quantify the elastic properties and microstructure of polycrystalline media. Sound velocity and attenuation, from coherent propagation, and diffuse backscatter amplitudes have been related to the elastic constants, crystallographic orientations, characteristic microstructure length scales (i.e., grain size), morphology, and stress state in polycrystals. Assumptions of statistical homogeneity and uniformity limit the application of these relationships for spatially heterogeneous metal AM parts. In this dissertation, in situ/real-time and ex situ coherent and diffuse ultrasonic backscatter measurements are used to map and quantify the heterogeneity, microstructure, and elastic properties of AM and hybrid AM samples made of 420 stainless steel and Ti6Al4V. For this purpose, a nonpermanent flexible in situ ultrasonic measurement accessory is designed and demonstrated in an open-platform Directed Energy Deposition (DED) system. The thermomechanical history and destructive measurements of the samples are used to interpret the nondestructive results with good agreement. In this manner, ultrasound-based methods enable rapid and accessible nondestructive evaluation (NDE) of heterogeneous metal AM samples