Among additive manufacturing (AM) techniques, fused filament fabrication (FFF) is the most widely adopted. Parameter optimization strategies are commonly implemented to provide some control over print quality, while optical and thermal measurements aim to identify defects in the printed parts. However, challenges regarding both quality and repeatability, and FFF parts are notorious for their variability across prints and printers in terms of both defect content and material properties. Thus, there is still a gap in understanding of the relationship between printing operating conditions and part quality. The purpose of this study is to nondestructively quantify the effect of cooling via forced convection on the material properties and geometrical accuracy of FFF samples. Samples with varying cooling fan activation layers were produced using polylactic acid (PLA) filament. Ultrasonic scanning, X-ray computed tomography (X-ray CT), and optical profilometry, were leveraged to quantify the change in acoustic properties, porosity content, and deformation due to the different cooling conditions. Spatial maps of acoustic properties, porosity, and deformation qualitatively showed property variation trends throughout the build that were associated to the cooling conditions. The relationships between porosity, deformation, and acoustic properties were also quantitatively assessed to identify dominant factors on the acoustic property trends.

Laser powder bed fusion (LPBF) is a metal additive manufacturing technique (AM) used to manufacture complex geometries with high resolution. However, LPBF process parameters lack standardization and lead to microstructural heterogeneity, causing high ultrasonic scattering amplitudes, which limit the detectability of internal features. The objective of this work is to determine the extent to which porosity-driven ultrasonic scattering can be reliably distinguished from microstructure-driven scattering in LPBF SS316L and to understand how background scattering interferes with locating internal features. For this purpose, cubic samples were fabricated using different process parameter combinations to produce distinct microstructures while maintaining comparable nominal energy input. Ultrasound measurements were used to quantify the experimental scattering baseline and define threshold-based bounds, which were then used as a denoising framework to suppress microstructure-driven contributions. Porosity and internal geometry, as well as grain morphology, were quantified via X-ray computed tomography and electron backscatter diffraction, respectively. The results show how detectability limits are affected by process-induced micro and mesostructured and the improvements resulting from thresholding methods. This work highlights the need for microstructure-aware ultrasonic NDE protocols for AM parts and provides a practical pathway to improve internal feature characterization under realistic microstructural variability.

Ultrasonic inspection plays a critical role in nondestructive material characterization. Contact-based methods have been used in industry and field inspections for decades due to their portability. Although immersion testing provides improved spatial resolution, its adoption has been limited to commercial or research facilities due to its cost and footprint. Here, we present a frugal design for a custom immersion system based on a repurposed three-dimensional printer and show that ultrasonic wave speed measurements collected with the custom system and a commercial system are in statistical agreement. This work enables broader adoption of immersion ultrasonics for industry, education, and workforce development.

Polymer additive manufacturing (AM) is critical for the production of products for many applications. However, qualification of polymer AM products requires following standards that are specific to an AM process and documenting process-centric print parameters. However, this qualification approach is tedious and does not address the problem, which is to understand the properties of the printed part. One way to avoid developing extensive qualification standards for each AM process is to characterise the polymer itself. However, the path towards process-agnostic qualification requires process monitoring and nondestructive characterisation techniques to measure local and bulk polymer properties. To expedite polymer-centric qualification of AM products, this review highlights nondestructive evaluation and monitoring techniques in-situ and ex-situ to offer a promising path to evaluate polymer relevant properties. Furthermore, this paper proposes a path to create a unifying framework that is polymer-centric, rather than process-centric, for future standard development and model validation.

Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) process where a laser is used to sinter and fuse powder particles together on a baseplate which acts as a heatsink. This heat transfer results in undesirable residual stresses and potential part failure. A proposed method to study the process of sintering powder without a baseplate is acoustic levitation. A type of acoustic levitation uses a standing pressure wave generated between a set of transducers and a reflector to trap particles at the nodes of the wave. The purpose of this work is to leverage and optimize current standing wave levitation techniques to enable the levitation of metal powder for sintering, i.e., to achieve “bed-less” LPBF through acoustic levitation. Common acoustic levitators are limited to very small or low-density particles. Thus, the first step of this work was to create a simulation tool for the pressure field of an acoustic levitator. The program architecture prioritized input flexibility, including phase, voltage, frequency, number, and position of transducers. The program also outputs the lift capacity to facilitate the following stage of this project which will involve finding optimization solutions for the transducer inputs and acoustic levitator setup suitable for bed-less LPBF.

Immersion cooling systems for data centers provide improved energy efficiency. However, the mechanisms of degradation of thermal interface materials (TIMs) over time and their interactions with coolant fluids have not been thoroughly studied, and there are no current nondestructive/noninvasive degradation monitoring methods. While high-frequency ultrasonics and scanning acoustic microscopy are ubiquitous in metrology and defect detection for electronic packaging, the ability to characterize with these modalities has been underutilized. This presentation will discuss the use of ultrasonic nondestructive evaluation to detect changes in the material properties of coolant fluids and TIMs. For this purpose, combinations of various commercially available coolants and TIMs were degraded and evaluated under simulated immersion cooling conditions (in accordance with JESD22-A103C test standard). Due to the limited quantities and various form factors of the TIMs and coolants, creative experimental solutions were implemented in the ultrasonic data collection process. The relative changes in sound wave speed and attenuation in coolant fluids and TIMs were used to quantify the changes in material properties, which reveal important information about the compatibility between coolants and TIMs. This demonstrates that ultrasonic nondestructive evaluation can be leveraged to periodically or continuously monitor degradation.

One of the most popular additive manufacturing (AM) techniques for producing thermoplastic polymers is fused filament fabrication (FFF). Nonetheless, there are still a lot of issues with the printed parts' quality. Most of the research being done to improve the quality of parts is limited to process parameter optimization and part quality analysis utilizing imaging techniques. These attempts are insufficient to define and forecast local material properties within the parts because of the inconsistent quality among printers and printed parts. Additionally, imaging techniques reveal how print parameters affect part quality; yet the majority of research in this field has been print-focused, leaving a knowledge gap about the connection between real printer operating conditions and final part quality. Polylactic acid (PLA) cubes with different fan activation layers were printed in the present study. The cubes underwent observable and controllable deformations as a result of shifting in situ cooling conditions. The mechanical characteristics and defect content were measured using ultrasonic and X-ray CT nondestructive evaluation techniques, respectively. We used this information to better understand how cooling history and material properties are related.

Process parameter selection in additive manufacturing (AM) is not standardized. Linear energy density (EDL = Power/Scanning speed), while commonly used, is non-unique which leads to different thermal histories and microstructures. This variability can significantly impact the performance of AM components, particularly those with internal structures. Ultrasonic nondestructive evaluation (NDE) methods are sensitive to microstructure and material properties. In this study, we assess the ability of ultrasonic NDE to resolve internal structures in AM components and evaluate the influence of microstructural heterogeneity. A systematic protocol was developed to map the viable parameter space for laser powder bed fusion (LPBF) of SS316L using a Lumex Avance 25 system. Process parameters were varied by adjusting scanning speed and laser power while maintaining a constant linear energy density to induce distinct microstructures. Samples were fabricated in two groups: solid specimens and specimens with internal structures, with each internal structure sample having a corresponding solid counterpart. Ultrasonic measurements were compared to intended geometries and X-ray CT scans to evaluate the effects of process-induced microstructural heterogeneity on feature resolution. This work highlights the potential and limitations of using ultrasound to characterize internal features in AM components, providing insights into the impact of microstructure heterogeneity on ultrasonic wave propagation.

Diffuse ultrasonic backscatter, which results from the interaction of elastic waves with material heterogeneity, can be used to characterize microstructural information. The scattering that occurs within the material can be complex and accurate models are needed to interpret measurements quantitatively, particularly for higher-scattering materials. Recently, a double scattering model was derived in which it was assumed that the energy scatters at most twice within the microstructure prior to detection. The significance of this model, with a focus on valid ranges of applicability, has not been thoroughly explored. In this article, theoretical and experimental results are examined to compare the single and double scattering models. The theoretical results show that single scattering models are appropriate for weakly scattering materials for a wide range of experimental setups while stronger scattering materials predict significant second-order scattering for certain measurement parameters, including transducer frequency and material path. Experimental results for steel, a strongly scattering material, are presented for frequencies of 5, 7.5, 10, and 15 MHz and show the domain for which the doubly scattered response becomes significant, as well as model limitations, due to selected experimental parameters. Finally, guidelines are provided for experiment design in order for measurements to remain within the desired scattering regime.

Objective To determine the sensitivity of vascular endothelial cells to long durations of low-intensity pulsed ultrasound (LIPUS) compared to normal flow and identify the duration that maximizes expression of two mechanosensitive genes related to healthy endothelial function, endothelial nitric oxide synthase (eNOS) and Krüppel-like factor 2 (KLF2). Methods Custom ultrasound exposure tanks were developed and the acoustic field was characterized. Human umbilical vein endothelial cells were seeded into culture plates and exposed to LIPUS at a frequency of 1 MHz and acoustic pressure of 217 kPa for 20 min, 1 h, 6 h, 9 h, or 24 h. As a comparator, other cells were exposed to normal flow. RT-qPCR was used to assess mRNA expression of eNOS and KLF2. Results Maximum eNOS and KLF2 expression occurred at 6 h and was localized to the beam path. Both genes exhibited qualitatively similar patterns of expression under LIPUS compared to normal flow. LIPUS induced a more rapid beneficial response compared to normal flow, but flow induced higher expression of both genes. eNOS expression after 6 h of LIPUS was dependent on RNA yield and culture duration prior to experiments. Conclusion Endothelial cells exposed to longer durations of LIPUS than typically employed exhibited greater expression of beneficial genes. The temporal gene expression patterns resulting from LIPUS and normal flow suggest activation of similar signaling pathways. However, LIPUS also caused increased RNA yield that may be linked to proliferation, which would suggest more of a wound healing than atheroprotective phenotype.

Fused Filament Fabrication (FFF) is one of the most extensively used additive manufacturing (AM) methods for the manufacturing of thermoplastic polymers. However, there are still many problems related to the quality of the printed parts. Most of the current research work for improving the part quality is limited to optimizing process parameters and using imaging techniques to analyze the part quality. Due to lack of consistency of quality across printers and printed parts, these efforts are insufficient to characterize and predict local material properties within the parts. Ultrasonic evaluation techniques are becoming increasingly recognized in assessing parts created by AM. In this work, Polylactic Acid (PLA) cubes were printed with varying fan activation layers. Ultrasonic and X-ray CT testing are employed to analyze and detect the controllable and observable deformation due to the change in in-situ cooling condition within the cubes. At the same time, mechanical properties and defect content were evaluated. It is important to understand the relationship between the fan cooling and mechanical properties so that effective fan cooling strategies can be deployed for consistent fabrication of higher quality parts across printers.

In recent years, the use of ultrasonic nondestructive evaluation (NDE) for in situ and ex situ assessment of Additive Manufacturing (AM) parts has garnered interest due to its proficiency in defect detection and material characterization. This study focuses on evaluating the ability to resolve internal structures created by AM and hybrid AM using ultrasonic NDE as well as assessing the impact of microstructure heterogeneity. For this purpose, first the parameter space for laser powder bed fusion (LPBF) of SS316L is mapped in a Lumex Avance 25 system. Based on this parameter mapping, optimal process parameters are identified and sets of process parameters are selected to vary the thermal history of the samples, while minimizing the influence of porosity. Two groups of samples are printed with these parameter sets: a solid group of samples, and a group with internal structures, such that each internal structure sample has an equivalent solid sample. Measurements of ultrasonic velocity, attenuation, and backscatter amplitude are collected and compared against the intended geometry to assess the impact of non-optimal processing on the ability to resolve internal structures with ultrasound. The viability and limitations of using ultrasound to assess internal features created with AM are discussed.

Ultrasonic evaluation enables nondestructive defect detection and material characterization in situ and ex situ. Contact ultrasound is used for field inspections due to its portability, although it lacks resolution. Immersion testing offers higher resolution: however, these systems are more expensive and tend to be bulky. The need to balance higher resolution with portability and accessibility presents a significant gap for field testing and academic research. Our project introduces an economical and practical immersion setup. We modified a commercial 3D printer kit by incorporating plexiglass into the aluminum rails and sealing it with silicone paste to ensure watertight integrity. The original servo motors from the printer were repurposed to maneuver a custom transducer holder for precise transducer movement control. This setup achieved a resolution of 500 µm in step size for pulse-echo data collection. Constructed for about $2000 in less than a couple of days, this system exemplifies frugal engineering by delivering ultrasonic evaluations at a fraction of the cost and complexity of conventional systems. This portable setup can serve as an additional step for on-field inspections, providing greater resolution. Beyond industrial applications, it holds potential for enhancing ultrasonic education and workforce development in NDE.

Metal additive manufacturing (AM) has brought about the need for nondestructive evaluation (NDE) methods to assess part quality. AM processes result in complex microstructures, porosity, material texture, and residual stresses all of which may vary spatially throughout the part. Thus, knowledge of ultrasonic NDE methods to characterize materials is relevant to the AM community. To address this challenge, a new graduate-level course entitled “Ultrasound for Metal Additive Manufacturing,” was created at the University of Nebraska-Lincoln. The course followed a cognitive-situative blended learning approach to give students practical ultrasonics training regardless of prior acoustics background. The theory introduced key topics including: the general wave equation, plane wave solutions, waves in isotropic solids, impact of material anisotropy, role of material interfaces, reflection and transmission, surface waves, scattering, dispersion, material dissipation, and scattering attenuation. A series of laboratory experiments allowed students to learn: experimental setups, signal processing, transducer properties and selection, beam mapping, wave speed and attenuation measurements, and diffuse-field measurements. Finally, each student developed a project incorporating ultrasound measurements into their individual AM research. Their findings were included in research articles, conference presentations, and PhD dissertations. This presentation will describe the course design, successes, and recommendations for future implementations.

Metal components with functionally organized microstructures for specific applications are emerging thanks to hybrid additive manufacturing (AM). The customization of these high value components accentuates the need for nondestructive methods to characterize their microstructural functional patterns. Nondestructive evaluation (NDE) methods that are economical, fast, energy efficient, and easy to integrate into routine component inspections are preferred. Most importantly, NDE methods must be sensitive to changes in the microstructure such that regions that do not satisfy the design requirements (i.e. out-of-spec regions) can be detected. In this work, ultrasonic NDE methods grounded in diffuse backscatter modeling were used to detect and quantify spatial property variations resulting from a hybrid AM process. The manufacturing process coupled directed energy deposition (DED) with milling in a cyclical manner. These methods were successfully implemented to evaluate the microstructural uniformity of Ti6Al4V samples as well as to make comparisons across an ensemble of samples manufactured with identical parameters. Out-of-spec regions were mapped with respect to the sample geometry on a layer-by-layer basis. The results of this work are expected to inform future NDE strategies for both research and practitioner contexts, and limitations are discussed.

Additive manufacturing (AM) often results in high strength but poor ductility in titanium alloys. Hybrid AM is a solution capable of improving both ductility and strength. In this study, hybrid AM of Ti-6Al-4 V was achieved by coupling directed energy deposition with interlayer machining. The microstructure, residual stress, and microhardness were examined to explain how interlayer machining caused a 63% improvement in ductility while retaining an equivalent strength to as-printed samples. Interlayer machining introduced recurrent interruptions in printing that allowed for slow cooling-induced coarsening of acicular alpha laths at the machined interfaces. The coarse alpha laths on the selectively machined layers increased dislocation motion under tensile loads and improved bulk ductility. The results highlighted in this publication demonstrate the feasibility of hybrid AM to enhance the toughness of titanium alloys.

This work pertains to the nondestructive evaluation of additively deposited coatings using ultrasound measurements. The specific objective was to evaluate using ultrasound surface wave measurements the quality of Stellite 21 coatings deposited on Inconel 718 substrates by the directed energy deposition (DED) additive manufacturing (AM) process. The surface wave speed and diffuse backscatter amplitude of the ultrasound waves were correlated with both cracking and warpage of the coating. This research is important, because, the integrity of DED-processed coatings currently requires a combination of destructive metallographic studies and X-ray computed tomography analysis, which are both expensive and time consuming. Instead of using a normal incidence configuration for the ultrasound measurements, the surface-wave approach allows the inspection of the near surface coating integrity. Three different frequencies were used to excite surface waves because each frequency has a different penetration depth. The signals obtained were used to quantify and compare the surface wave speed and surface wave-diffuse ultrasonic backscatter amplitude from five different DED-processed Stellite coated samples. Considering destructive metallurgical characterization as a reference, surface wave measurements were found to be effective for examining coating integrity. Further, the backscatter amplitude of the surface waves was correlated with the crack density and warpage. These flaws also change the stress state of the Stellite coating and consequently the surface wave propagation speed and its scattering behavior.

Hybrid additive manufacturing (AM) of metals includes synergistic secondary processes such as milling or peening that impart changes to the component for improved performance. Although a valuable concept, implementation is difficult to verify due to a current lack of in situ inspection methods. In this presentation, two different strategies are described with respect to ultrasonic inspections for hybrid metal AM. Both are used for inspections during laser-based directed energy deposition with milling as the hybrid process. The first uses a transducer mounted below the build plate. This configuration allows waves to interrogate the entire sample during all additive and subtractive manufacturing steps. Experiments with Ti6Al4V show the changes in ultrasonic wave speed and attenuation that track the manufacturing temporally. The second inspection approach uses ultrasonic surface waves at the top surface of the sample. These measurements are used on samples of 316L stainless steel at the end of additive steps as well as before and after milling to quantify localized information regarding the most recent layers. Wave speed and diffuse scattering information show the variability with respect to sample geometry, build height, and process parameters. Finally, advantages and limitations of both approaches and prospects for validated parts are discussed.

Advanced applications of polymer additive manufacturing (AM) require knowledge of the material acoustic and complex elastic properties. Recently, ultrasound nondestructive evaluation (NDE) methods have been applied to the understanding of AM polymers manufactured with a variety of methods. Nonetheless, the available information is still limited to a few materials and frequency ranges, and knowledge of shear acoustic properties and complex elastic properties of AM polymers is lacking. In this study, ultrasound measurements of compressional and shear phase velocity and attenuation are used to experimentally determine the complex elastic properties of 14 AM photopolymers manufactured using PolyJet systems. The results provided here are expected to aid in design of advanced polymer AM structures, and highlight the value of ultrasound NDE for the characterization of AM polymers.