We present a pixel-specific, measurement-driven correction that effectively reduces errors in detector response that give rise to the ring artifacts commonly seen in X-ray computed tomography (CT) scans. This correction is easy to implement, suppresses CT artifacts significantly, and is effective enough for use with both absorption and phase contrast imaging. It can be used as a standalone correction or in conjunction with existing ring artifact removal algorithms to further improve image quality. We validate this method using two X-ray CT data sets acquired using monochromatic sources, showing post-correction signal-to-noise increases of up to 55%, and we define an image quality metric to use specifically for the assessment of ring artifact suppression.

Ring artifact suppression in X-ray computed tomography using a simple, pixel-wise response correction

Quantification and prediction of lack-of-fusion porosity in the high porosity regime during laser powder bed fusion of Ti-6Al-4V

Evaporative cooling has been widely used due to its environment-friendly characteristic of cooling air by water evaporation. Nowadays, there are two common forms of evaporative cooling: indirect evaporative cooling (IEC) and dew point evaporative cooling (DPEC). While, the heat-moisture migration in the wet surface composed of porous materials is an important factor that affecting the heat and mass transfer of IEC. Therefore, the application of porous materials in IEC has been reviewed in this paper. Firstly, a comprehensive classification of the porous materials used in the IEC was introduced, which could be divided into porous ceramics and fibers. Porous ceramics had the function of storing water, and the fiber in the form of composite membranes was the current research hotspot. Then, by referring theoretical analysis and experimental research, the structural parameters (Porosity, Specific surface, Tortuosity) and characteristic parameters (Wicking height, Diffusivity, Evaporation rate) of porous materials were introduced in detail, which could be used as important indicators for selecting suitable porous materials. Since the heat and mass transfer process brought by the unique porous structure was quite complicated, the heat and mass transfer model of porous materials in IEC could be simplified. Based on the spray water flow, the theoretical research of the IEC mathematical model can be summarized into three situations. Among them, moderate water spray had the best cooling capacity for IEC, and could predict the temperature change of the water film surface well. Finally, four future research directions based on the characteristics of porous materials were also proposed in this paper. The aim of this paper is to provide a reference for the field of IEC.

The performance and model of porous materials in the indirect evaporative cooling system: A review

http://manuscript.elsevier.com/S0007850622001275/pdf/S0007850622001275.pdf

Advances in the metrological traceability and performance of X-ray computed tomography

Advanced manufacturing technologies, led by additive manufacturing, have undergone significant growth in recent years. These technologies enable engineers to design parts with reduced weight while maintaining structural and functional integrity. In particular, metal additive manufacturing parts are increasingly used in application areas such as aerospace, where a failure of a mission-critical part can have dire safety consequences. Therefore, the quality of these components is extremely important. A critical aspect of quality control is dimensional evaluation, where measurements provide quantitative results that are traceable to the standard unit of length, the metre. Dimensional measurements allow designers, manufacturers and users to check product conformity against engineering drawings and enable the same quality standard to be used across the supply chain nationally and internationally. However, there is a lack of development of measurement techniques that provide non-destructive dimensional measurements beyond common non-destructive evaluation focused on defect detection. X-ray computed tomography (XCT) technology has great potential to be used as a non-destructive dimensional evaluation technology. However, technology development is behind the demand and growth for advanced manufactured parts. Both the size and the value of advanced manufactured parts have grown significantly in recent years, leading to new requirements of dimensional measurement technologies. This paper is a cross-disciplinary review of state-of-the-art non-destructive dimensional measuring techniques relevant to advanced manufacturing of metallic parts at larger length scales, especially the use of high energy XCT with source energy of greater than 400 kV to address the need in measuring large advanced manufactured parts. Technologies considered as potential high energy x-ray generators include both conventional x-ray tubes, linear accelerators, and alternative technologies such as inverse Compton scattering sources, synchrotron sources and laser-driven plasma sources. Their technology advances and challenges are elaborated on. The paper also outlines the development of XCT for dimensional metrology and future needs.

Review of high energy x-ray computed tomography for non-destructive dimensional metrology of large metallic advanced manufactured components

If additively manufactured metal parts are to be used as performance critical components in the aerospace and other high-value industries, then non-destructive methods for measuring the porosity of these parts are required; X-ray computed tomography (XCT) is a promising instrument for this measurement task. One disadvantage of using XCT for this purpose is the presence of artefacts in XCT data of high-density materials (aluminium, titanium, steel and Inconel). These artefacts are problematic for segmentation algorithms that are required for porosity measurement. In this work we propose an adaptive thresholding algorithm for segmenting XCT data. The algorithm is specifically designed to overcome the influence of beam hardening (cupping) artefacts that are typically observed when XCT scanning high-density metal parts. The algorithm is also designed to have as few as possible user-defined settings in order to minimise the influence the user has on the segmentation result. The results show that the algorithm is able to segment small voids in low contrast-to-noise ratio XCT data and that the algorithm is robust to cupping artefacts. The accuracy of the method is evaluated for a set of Inconel cubes approximately 10 mm × 10 mm × 10 mm in size (H × W × D), reference porosity measurements are made using Archimedes’ principle. The mean error and standard deviation of the XCT percentage porosity measurements evaluated by four different users are found to be: −0.38 ± 0.09%, 0.09 ± 0.09% and −0.23 ± 0.22% for samples with bulk porosities of 0.23 ± 0.28%, 1.42 ± 0.28% and 3.00 ± 0.28%, respectively.

An adaptive thresholding algorithm for porosity measurement of additively manufactured metal test samples via X-ray computed tomography

X-ray computed tomography (XCT) is a well-established method for measuring and inspecting an object's internal structure in a non-destructive manner. Ring artefacts are unwanted high- or low-intensity rings that appear in CT images that influence CT-based measurements. The cause of the subtle ring artefacts found in high-resolution XCT data is found to be due to inadequate flat field detector correction. Therefore, in this work, the usual two-point flat field correction is replaced by a multi-point, piecewise linear flat field correction. The proposed method is shown to increase the signal-to-noise ratio of exemplary CT data by up to 12.1%. Based on the results presented, it is recommended that a minimum of seven open field images should be used for flat field correction.

Ring artefact reduction via multi-point piecewise linear flat field correction for X-ray computed tomography.

Algorithms for linear and non-linear least squares fitting of Bezier surfaces to unstructured point clouds are derived from first principles. The presented derivation includes the analytical form of the partial derivatives that are required for minimising the objective functions, these have been computed numerically in previous work concerning Bezier curve fitting, not surface fitting. Results of fitting fourth degree Bezier surfaces to complex simulated and measured surfaces are presented, a quantitative comparison is made between fitting Bezier surfaces and fitting polynomial surfaces. The developed fitting algorithm is used to remove the geometric form of a complex engineered surface such that the surface roughness can be evaluated.

Non-linear least squares fitting of Bézier surfaces to unstructured point clouds

X-ray sources with acceleration voltages of 450 kV typically have large focal spot sizes that lead to low spatial resolution computed tomography (CT) data. In this work a method to improve the spatial resolution of 450 kV CT is developed. The proposed method relies on using vertical subpixel detector shifting and deconvolution to generate a projection-set that has double the number of pixels in the direction parallel to the axis of rotation. The results show that the proposed method is able to increase the 10% modulation transfer function of the considered system from 3 lp/mm to 4 lp/mm. In addition, a scan of a metal additively manufactured component with periodic features of approximately 200 μm is used to demonstrate the method; small features that were not fully resolved in a conventional scan become resolvable when the proposed method is used.

Tong Liu

Papers

An adaptive thresholding algorithm for porosity measurement of additively manufactured metal test samples via X-ray computed tomography

Ring artefact reduction via multi-point piecewise linear flat field correction for X-ray computed tomography.

Non-linear least squares fitting of Bézier surfaces to unstructured point clouds

Spatial resolution improvement of 450 kV computed tomography via vertical subpixel detector shifting