X-ray Computed Tomography Measurements Research Articles

Computed tomography as distortion mitigation method for selective laser sintering mass production

Most additive manufacturing (AM) technologies use heat to fuse materials together to create the manufactured part. The heat used in the AM process distorts the parts. Powder bed–based 3D printers can print multiple parts in their build chamber. The distortion is not uniform across the different locations of the build volume. Parts printed in different locations will have different thermal histories and therefore different distortions. In some cases, the achievable accuracy of the parts is insufficient due to the distortion. Subtractive processes such as milling, turning, and grinding make it difficult or impossible to improve part accuracy. For AM to produce more accurate parts, a distortion reduction method must be implemented. To take advantage of the ability to print multiple parts in a powder-based polymer 3D printing process in one build unit, a distortion mitigation technique must be applied to all the parts being printed simultaneously in the build chamber. The performance of the distortion mitigation method can be evaluated by measuring the dimensional accuracy of the uncompensated and compensated parts. Uncompensated 3D printing uses the nominal 3D model, which is the normal use of the 3D printers. Compensated 3D printing uses a distorted 3D model that is used for the printing. The 3D model is compensated with the reversed distortion data obtained from uncompensated manufacturing. X-ray computed tomography (XCT) is the chosen measurement method to extract the point cloud for the dimensional measurements. Unlike optical 3D scanners and coordinate measuring machines (CMM), the XCT is able to measure undercut and internal surfaces. The nominal difference % is improved by 18% by using compensation for the 3D models in the case of distances between two parallel planes. The standard deviation of the measured values was also improved. The distortion reduction method studied can significantly reduce the calibration errors of the 3D printer build chamber. When the tolerances of the parts are close to the limit of the 3D printer, this method can reduce the number of rejected parts. The XCT measurement of the parts is costly, so this method can be cost effective for high value parts or large production volumes.

Lithium Dendrite Formation inside Li3PS4 Solid Electrolyte Observed Via Multimodal/Multiscale Operando X-Ray Computed Tomography

Sulfide-based all-solid-state batteries using a lithium metal anode are expected to be next-generation batteries due to their extremely high energy density. In order to use the lithium metal as the anode, suppressing dendrite of lithium metal during charge/discharge processes is essentially important. It has been reported that lithium dendrite formation occurs not from the lithium/sulfide solid electrolyte interface, but in the sulfide solid electrolyte, isolated from the interface1, 2. The formation of lithium dendrite within the sulfide solid electrolyte is caused by electron conduction in the sulfide solid electrolyte and at the sulfide solid electrolyte/void interface3. However, fundamental information on the mechanism of lithium dendrite formation in a sulfide solid electrolyte caused by its electron conduction is lacking. In this study, the three-dimensional morphological changes of the lithium dendrite in Li3PS4, which is a typical sulfide solid electrolyte, were observed directly using multimodal/multiscale operando computed tomography (CT) under an applied pressure.Li/Li3PS4/Li cells were constructed in a diameter of 10 mm and 1 mm for the critical current density measurements and X-ray CT measurements by using SPring-8 BL20XU respectively. X-ray CT images for behavior change with the electrochemical operation were collected in micro and nano scales at 25 °C every 30 mins. After a series of data processing steps, these images were converted to cross-sectional slices that were then stacked together to render a 3D reconstruction of the cell. The 3D imaging data coupled with precise species segmentation show that the lithium metal deposition start point is spatially separated from the lithium metal anode. The gradient in thickness of a lithium filament with repeated charging, widening the plating-susceptible region horizontally in the process and eventually led to cell failure. The lithium nucleation initiates along the pre-existing voids where local electronic conductivities are high during the plating. The deposition then widens from the nucleation across the electrolyte horizontally. Accompanied with streak fracture widening through the Li3PS4, does a Li/Li3PS4/Li cell finally short circuit. By combining the multimodal/multiscale operando X-ray computed tomography with X-ray absorption spectroscopy and electrochemical impedance spectroscopy measurements, we revealed that the electronic conduction of reductive decomposition products and the solid electrolyte/void interface cause the lithium deposition within the Li3PS4. These results suggests that the suppression of reductive decomposition and sulfide solid-state electrolytes with low electronic conductivity plays significant roles in suppressing the growth of lithium dendrites in the solid-state electrolyte layer.

In-situ detection of stochastic spatter-driven lack of fusion: Application of optical tomography and validation via ex-situ X-ray computed tomography

The presence of random defects in laser powder bed fusion (LPBF) parts is an issue that challenges the reliability of this manufacturing process and hinders its employment in structural, defect-sensitive components. A potential solution to increase the reliability of LPBF is employing in-process monitoring targeting defect detection. This study aims to detect stochastic defects driven by spatter particles via in-situ monitoring and validate the detection method ex-situ via X-ray computed tomography (XCT). By means of in-situ optical tomography (OT), monitoring images were registered layerwise during the manufacturing of Hastelloy X specimens. The images were analyzed to detect spatters landing within specimen boundaries, and the spatial coordinates of the detections were obtained. The specimens were also measured ex-situ by means of XCT, from which key features and coordinates of internal defects were obtained. The in-situ spatter detection method was then compared to the XCT measurements. It was found that 79 % of lack of fusion defects were detected in OT images. The detection was particularly successful for large defects. Spatter-induced lack of fusion defects were present in the specimens manufactured with optimized processing parameters in different degrees, depending on the robustness of the processing conditions to spatters. This study demonstrates the applicability of optical tomography in-situ monitoring for indirect detection of stochastic lack of fusion, whose presence is inferred from spatter redeposits on the powder bed.

Resonant ultrasound spectroscopy measurement and modeling of additively manufactured octet truss lattice cubes

In this work, the elastic characteristics of additively manufactured Ti-5553 octet truss lattice cubes were examined using resonant ultrasound spectroscopy (RUS) techniques and correlated to X-ray computed tomography measurements of strut continuity. RUS models and measurements identified an outlier lattice possessing lower effective modulus despite larger effective density. Experimental measurements of density and effective Young's modulus were treated as input parameters in a homogenous anisotropic continuum model. The continuum model exhibited good agreement with experimental RUS measurements, thus confirming the potential for a simplified approximation of the octet truss lattice.

Geometrical Influence on Material Properties for Ti6Al4V Parts in Powder Bed Fusion

One major advantage of additive manufacturing is the high freedom of design, which supports the fabrication of complex structures. However, geometrical features such as combined massive volumes and cellular structures in such parts can lead to an uneven heat distribution during processing, resulting in different material properties throughout the part. In this study, we demonstrate these effects, using a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux. We manufacture the parts via powder bed fusion of Ti6Al4V by applying a laser beam (PBF-LB/M) as well as an electron beam (PBF-EB). We investigate the impact of the different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements. Both simulations and thermography show an increase in inter-layer temperature with increasing part radius, subsequently leading to heat accumulation along the build direction. While the geometry and thermal history have a minor influence on the relative density of the parts, the microstructure is greatly affected by the thermal history in PBF-LB/M. The acicular martensitic structure in the narrow parts is decomposed into a mix of tempered lath-like martensite and an ultrafine α + β microstructure with increasing part radius. The EBM part exhibits a lamellar α + β microstructure for both the cylindric and conic structures. The different microstructures directly influence the hardness of the parts. For the PBF-LB part, the hardness ranges between 400 HV0.5 in the narrow sections and a maximum hardness of 450 HV0.5 in the broader sections, while the PBF-EB part exhibits hardness values between 280 and 380 HV0.5.

The realisation of fast X-ray computed tomography using a limited number of projection images for dimensional metrology

Due to the merit of establishing volumetric data, X-ray computed tomography (XCT) is increasingly used as a non-destructive evaluation technique in the quality control of advanced manufactured parts with complex non-line-of-sight features. However, the cost of measurement time and data storage hampers the adoption of the technique in production lines. Commercial fast XCT utilises X-ray detectors with fast detection capability, which can be expensive and result in a large amount of data. This paper discussed a different approach, where fast XCT was realised via the acquisition of a small number of projection images instead of full projection images. An established total variation (TV) algorithm was used to handle the reconstruction.The paper investigates the feasibility of using the TV algorithm in handling a significantly reduced number of projection images for reconstruction. This allows a reduction of measurement time from 52 min to 1 min for a typical industrial XCT system with an exposure time of 1 s per projection. It also enables a reduction of data size proportionally.A test strategy including both quantitative and qualitative test metrics was considered to evaluate the effectiveness of the reconstruction algorithm. The qualitative evaluation includes both the signal to noise ratio and the contrast to noise ratio. The quantitative evaluation was established using reference samples with different internal and external geometries. Simulation data were used in the assessment considering various influence factors, such as X-ray source property and instrument noise.The results demonstrated the possibility of using advanced reconstruction algorithms in handling XCT measurements with a significantly limited number of projection images for dimensional measurements. This work lays down the foundation for conducting fast XCT measurements without the need for instrument alteration or enhancement. Although the reconstruction time required is still considerable, various possibilities to improve this have been discussed.

Quantitative analysis of calcium oxalate monohydrate and dihydrate for elucidating the formation mechanism of calcium oxalate kidney stones.

We sought to identify and quantitatively analyze calcium oxalate (CaOx) kidney stones on the order of micrometers, with a focus on the quantitative identification of calcium oxalate monohydrate (COM) and dihydrate (COD). We performed Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), and microfocus X-ray computed tomography measurements (microfocus X-ray CT) and compared their results. An extended analysis of the FTIR spectrum focusing on the 780 cm-1 peak made it possible to achieve a reliable analysis of the COM/COD ratio. We succeeded in the quantitative analysis of COM/COD in 50-μm2 areas by applying microscopic FTIR for thin sections of kidney stones, and by applying microfocus X-ray CT system for bulk samples. The analysis results based on the PXRD measurements with micro-sampling, the microscopic FTIR analysis of thin sections, and the microfocus X-ray CT system observation of a bulk kidney stone sample showed roughly consistent results, indicating that all three methods can be used complementarily. This quantitative analysis method evaluates the detailed CaOx composition on the preserved stone surface and provides information on the stone formation processes. This information clarifies where and which crystal phase nucleates, how the crystals grow, and how the transition from the metastable phase to the stable phase proceeds. The phase transition affects the growth rate and hardness of kidney stones and thus provides crucial clues to the kidney stone formation process.

Evaluating the standard uncertainty due to the voxel size in dimensional computed tomography

X-ray computed tomography (XCT) is a promising tool for making dimensional measurements of complex engineering components. The adoption of XCT as a measurement tool is hindered by the inability to evaluate the uncertainty of XCT-based dimensional measurements; simply put, XCT users cannot specify how good (or bad) their measurements are. In this work, equations and a method are given to evaluate the standard uncertainty due to the voxel size; this being one of several sources of uncertainty in XCT-based dimensional measurements. It is envisioned that this standard uncertainty component will be combined with other standard uncertainties in a task-specific uncertainty budget, thus providing end users with a statement of XCT measurement uncertainty. It is claimed here that evaluating the standard uncertainty of the voxel size by means of a calibrated length leads to a traceable voxel size. For the example considered in this work, the voxel size is evaluated to be 80.005 μm ± 0.001 μm, with the voxel size uncertainty expressed as one standard deviation. When the standard uncertainty of the voxel size is propagated through to the final measurement result of a machined aluminium length bar, the standard uncertainty due to the voxel size is evaluated as ± 1.01 μm for a bi-directional length of nominally 55 mm.

Experimental and numerical study on SMA modified with an encapsulated polymeric healing agent

The present study aims at experimentally and numerically investigating the effect an encapsulated healing agent on the mechanical characteristics of a stone mastic asphalt (SMA). As a healing agent a thiol-containing urethane AR-polymer is used in this study. In order to gain a numerical insight into mechanical behavior of the capsules in SMA, a micromechanical finite element modeling is employed. The developed model allows capturing the stresses induced in the capsules at different load cases applied to the SMA on macro-scale. Particular attention is paid presently to the numerical evaluation of the local stress state that arises around capsules during compaction, operation, and also during crack initiation. SMA mixtures with various volumetric contents of healing capsules were manufactured and the capsules survival during mixture production was evaluated based on X-Ray Computed Tomography measurements. The effect of capsules on the self-healing properties of asphalt mixtures has furthermore been examined with repeated compressive strength tests. The obtained experimental results indicate that the absolute majority of capsules survive mixture production, and that their addition increases the SMA strength recovery during the healing period. The experimental and numerical results concerning capsules breakage are found to be in reasonable agreement. The developed micromechanical model may thus potentially provide a useful tool for optimization of capsules mechanical properties in order to improve their survival during mixture production as well as their timely activation.

XCT and DLW: Synergies of Two Techniques at Sub-Micrometer Resolution

Direct Laser Writing (DLW) and X-ray computed tomography (XCT) both offer unique possibilities in their respective fields. DLW produces full three-dimensional (3D) polymer structures on the microscale with resolutions below 100 nm. The fabricated structures can be analysed by XCT or X-ray microscopy (XRM), which incorporates additional X-ray lenses, in three dimensions down to a minimal basic spatial resolution of about 500 nm or 50 nm, respectively. In this work, two different DLW structures are analysed via XCT. Internal defects are detected and analysed for the purpose of quality control. Defects and structures with sizes down to 1.5 µm are successfully analysed. A 3D reconstruction and internal, hidden features of the fabricated structures are shown and discussed. In a first-of-its-kind study, we demonstrate the detectability of a single-voxel line inside a fabricated structure that would not be detectable with SEM or light microscopy. Furthermore, the direct fabrication on a PET substrate is shown to overcome the high X-ray absorbance of commonly used glass substrates. Attenuation spectra of SZ2080 and glass substrates are compared to a fabrication route direct on a 170 µm PET foil. The practical aspects of XCT measurements for DLW structures on different substrates will be discussed.

A floc structure perspective on sediment consolidation in thickened tailings

Based on X-ray computed tomography measurements of aggregate macro-structure and compressive dewatering tests, this paper demonstrates that aggregate breakage and structural re-organization during mixing in the thickening process can account for some of the experimentally measured changes in solids volume fraction that have been until now attributed to aggregate densification. This paper highlights the lack of information about the micro-scale mechanisms of flocculated sediment consolidation and suggests several experimental and theoretical experiments that could be conducted to improve the situation.

Section Area Estimation Methods for Determining the Mechanical Properties of Laser Powder Bed Fusion Thin Wall Structures

A critical aspect of generating mechanical property data is the accurate estimation of the cross-sectional area of the specimen being tested for computing stress. This ordinarily trivial matter assumes greater significance in the context of thin wall structures (defined here as less than 2 mm thick) fabricated using the laser powder bed fusion process commonly used in metal additive manufacturing. This is primarily on account of the significant surface roughness associated with additively manufactured parts as well as due to variations in cross-sectional area across the gauge. In this work, the thin-wall measurement capabilities of four different metrologies are examined – standard micrometer, point micrometer, blue light 3D scanning, and x-ray computed tomography (XCT) scanning. Seven specimens covering a thickness range of 0.3 to 2 mm are measured by each of the metrologies and tested mechanically, with stress-strain curves derived using the different section area estimates from these metrologies. XCT scanning is chosen as the baseline on account of having the highest resolution of the metrologies under study. Deviations from XCT scanning are established, showing that the standard micrometer is a poor method of choice, particularly at low thicknesses, but that point micrometer measurements are within 5% of XCT measurements across all thicknesses. Blue light scanning is also shown to be a reliable measurement method at all but the lowest thickness, and is also used to estimate the coefficient of variation (CV) in section area within each specimen, showing that the CV rises sharply as specimens get thinner. This paper thus demonstrates, for the first time, the validity of using a point-micrometer for section area estimation, while also showing how variations in section area for thin specimens can contribute to initiation of failure at lower strengths, and thus at least in part explain mechanical debits observed with reductions in thickness.

Towards efficient application-dependent dimensional measurements with computed tomography: optimized reduction of measurement duration using continuous scan mode: experimental investigations

Abstract. Due to the complex physical and technical relationships in dimensional X-ray computed tomography measurement technology, the precise influences on the measurement result are still not fully understood. A deeper understanding of these influences is essential in order to improve the measurement accuracy and economy of computed tomographic measurement systems. In addition, this enables better numerical determination of measurement uncertainties, which is also a topic of current research. Two factors that have a significant influence on the lead times are the scan mode and the number of projections per measurement. In this article, these factors are investigated experimentally using a calibrated workpiece, and the influence of the workpiece position on the measurement result is also examined. It can be shown that the workpiece position has a considerable influence on the quality of the measurement, especially in the case of a continuous scan. The results show a vast potential for application-dependent reduction of lead times.

Effects of fast x-ray cone-beam tomographic measurement on dimensional metrology

X-ray computed tomography (XCT) is increasingly used for dimensional metrology, where it can offer accurate measurements of internal features that are not accessible with other techniques. However, XCT scanning can be relatively slow, which often prevents routine uptake for many applications. This paper explores the feasibility of improving the speed of XCT measurements while maintaining the quality of the dimensional measurements derived from reconstructed volumes. In particular, we compare two approaches to fast XCT acquisition, the use of fewer XCT projections as well as the use of shortened x-ray exposure times for each projection. The study shows that the additional Poisson noise produced by reducing the exposure for each projection has significantly less impact on dimensional measurements compared to the artefacts associated with strategies that take fewer projection images, leading to about half the measurement error variability. Advanced reconstruction algorithms such as the conjugate gradient least squares method or total variation constrained approaches, are shown to allow further improvements in measurement speed, though this can come at the cost of increased measurement bias (e.g. 2.8% increase in relative error in one example) and variance (e.g. 25% in the same example).

Methodologies for model parameterization of virtual CTs for measurement uncertainty estimation

X-ray computed tomography (XCT) is a fast-growing technology for dimensional measurements in industrial applications. However, traceable and efficient methods to determine measurement uncertainties are not available. Guidelines like the VDI/VDE 2630 Part 2.1 suggest at least 20 repetitions of a specific measurement task, which is not feasible for industrial standards. Simulation-based approaches to determine task specific measurement uncertainties are promising, but require closely adjusted model parameters and an integration of error sources like geometrical deviations during a measurement. Unfortunately, the development of an automated process to parameterize and integrate geometrical deviations into XCT models is still an open issue. In this work, the whole processing chain of dimensional XCT measurements is taken into account with focus on the issues and requirements to determine suitable parameters of geometrical deviations. Starting off with baseline simulations of different XCT systems, two approaches are investigated to determine and integrate geometrical deviations of reference measurements. The first approach tries to iteratively estimate geometric deviation parameter values to match the characteristics of the missing error sources. The second approach estimates those values based on radiographs of a known calibrated reference object. In contrast to prior work both approaches only use a condensed set of parameters to map geometric deviations. In case of the iterative approach, some major issues regarding unhandled directional dependencies have been identified and discussed. Whereas the radiographic method resulted in task specific expanded measurements uncertainties below one micrometre even for bi-directional features, which is a step closer towards a true digital twin for uncertainty estimations in dimensional XCT.

Kinetic process of upward gas hydrate growth and water migration on the solid surface

Gas hydrates have gained great interest in the energy and environmental field as a medium for gas storage and transport, gas separation, and carbon dioxide sequestration. The presence of small doses of surfactants in the aqueous phase has been reported to enhance hydrate formation; however, the underlying mechanisms remain poorly understood. Thus, in situ high-resolution X-ray computed tomography measurements were performed to monitor the upward water migration and the resulting hydrate nucleation and growth. It was found that the presence of hydrate crystals at the gas–liquid–solid contact line triggered the enhanced growth of hydrates on the reactor wall. A time delay was observed between the disappearance of the bulk water reservoir and its transformation into hydrate. The lower interfacial tension between the hydrate surface and the solution facilitated its adsorption onto the reactor wall once a thin film of hydrate nucleated on the solid wall surface. These hydrate layers present on the reactor wall were found to be porous, wherein the porosity decreased with increased subcooling. These fundamental results will be of value in understanding the mechanism of hydrate growth in the presence of surfactants and its potential application in hydrate-based technologies.

Characterization of a small-scale crystallizer using CFD simulations and X-ray CT measurements

• Single and multiphase CFD of a small-scale crystallizer with two stirrer types. • Indications when each stirrer is preferably used based on hydrodynamics. • X-ray CT applied in mixing studies used for the validation of the CFD simulations. Although small-scale crystallizers have gained increasing importance in recent years, their hydrodynamic properties have not yet been thoroughly studied. The purpose of this work is to characterize an intensively stirred small-scale crystallizer and to analyze whether its fluid mechanics behavior deviates from being homogeneously well-mixed. To this aim, single and multiphase computational fluid dynamics (CFD) simulations have been conducted. Two types of stirrers, including one overhead stirrer (OS) and one magnetic stirrer (MS), were compared. The single-phase CFD simulations showed that the mixing characteristics of the two stirrers differ significantly, thereby leading to variations in the distribution of solids. The MS is an axial flow type impeller, which is better at handling solid suspensions than the OS. The simulations were complemented by X-ray computed tomography measurements, which confirmed that spatial variations in solid distribution occur and that assuming a homogeneous suspension might lead to errors when modeling crystallization processes.

Toward traceable XCT measurement of AM lattice structures: Uncertainty in calibrated reference object measurement

Lattice structures created via additive manufacturing are of interest in several demanding industries, however the qualification of these components via X-ray computed tomography (XCT) is limited due to traceability requirements. In this work, a novel measurement reference object is presented to study task specific uncertainty in the XCT measurement of a simplified lattice structure. The design of this reference object, the calibration routines, and XCT acquisition are described. The description of uncertainty calculation is presented. The methodology for data sampling, registration, and measurement is detailed. The measurement results are tested for normality and show variations in the underlying distribution for different measurands. A discussion on uncertainty variation and normality of measurements is then presented. Variations in measurement bias and uncertainty are analyzed across various features of the reference object. It is shown that the measurement uncertainty and bias are not consistent across pin diameter measurement, indicating the need for similarity between the reference object and subsequent component measurements.

Reference object for traceability establishment in X-ray computed tomography measurements of fiber length in fiber-reinforced polymeric materials

X-ray computed tomography (CT) is an advanced non-destructive measuring technique that enables holistic three-dimensional measurements of outer as well as inner geometries and micro-scale features. In the field of fiber-reinforced polymeric (FRP) materials, CT can overcome the limitations of conventional methods typically based on time-consuming and destructive operations used for the measurement of fiber orientation and length, which are two characteristics that have a great impact on physical and mechanical properties of FRP parts. This work specifically addresses the CT measurement of fiber length, which is particularly complex because – differently from the fiber orientation analysis – it requires the individual fibers to be properly identified and segmented. Moreover, the traceability of such measurement has not been achieved yet. This work investigates an experimental methodology for traceability establishment and measurement uncertainty determination, based on the substitution approach. In particular, a task-specific reference object, including a selection of fibers with different lengths and configurations, was designed, manufactured and calibrated for the purpose.

Using x-ray computed tomography to estimate hydrate saturation in sediment cores from Green Canyon 955, northern Gulf of Mexico

Quantifying the amount of gas hydrate in a reservoir provides valuable insight on gas hydrates’ potential as a resource and their contribution to the global carbon cycle. We develop a new method to estimate hydrate saturation from density-sensitive X-ray computed tomography (XCT) scans of pressure cores from northern Gulf of Mexico (Green Canyon Block 955). While several studies have used XCT to quantify gas hydrate in the fractures of fine-grained sediments, our study is the one of the few to use XCT for coarse-grained (sand and silt) reservoirs. We calibrate our new method with grain density and quantitative degassing measurements from the same intervals. This paper uses the density difference between gas hydrate (0.924 g/cm3) and porewater brine (1.035 g/cm3) and assume these changes linearly affect the XCT measurements allowing for estimates of hydrate saturation. Overall, the XCT calculations agree with the hydrate saturation from quantitative degassing. For example, in core H005-3FB-3, quantitative degassing indicated a hydrate saturation of 88 ±33.5%, while the XCT calculations yielded a 90 ±4 4.8% saturation. These results are encouraging as they suggest that XCT analysis has the potential to nondestructively estimate hydrate saturation.