Lack of assurance of quality with additively manufactured (AM) parts is a key technological barrier that prevents manufacturers from adopting AM technologies, especially for high-value applications ...where component failure cannot be tolerated. Developments in process control have allowed significant enhancement of AM techniques and marked improvements in surface roughness and material properties, along with a reduction in inter-build variation and the occurrence of embedded material discontinuities. As a result, the exploitation of AM processes continues to accelerate. Unlike established subtractive processes, where in-process monitoring is now commonplace, factory-ready AM processes have not yet incorporated monitoring technologies that allow discontinuities to be detected in process. Researchers have investigated new forms of instrumentation and adaptive approaches which, when integrated, will allow further enhancement to the assurance that can be offered when producing AM components. The state-of-the-art with respect to inspection methodologies compatible with AM processes is explored here. Their suitability for the inspection and identification of typical material discontinuities and failure modes is discussed with the intention of identifying new avenues for research and proposing approaches to integration into future generations of AM systems.
Example concepts for integration of a sensor unit (green) for in-situ monitoring of laser powder bed fusion. Display omitted
•An overview of state-of-the-art methods for assessing the quality of additive manufacturing processes is presented.•The need for new sensors and monitoring methods for emergent additive manufacturing processes is introduced.•Material discontinuities resulting from well understood processes are explored and the case for in-situ monitoring is made.•The industrial opportunities and potential benefits of using these advanced methods are explored.
The challenges of measuring the surface topography of metallic surfaces produced by additive manufacturing are investigated. The differences between measurements made using various optical and ...non-optical technologies, including confocal and focus-variation microscopy, coherence scanning interferometry and x-ray computed tomography, are examined. As opposed to concentrating on differences which may arise through computing surface texture parameters from measured topography datasets, a comparative analysis is performed focussing on investigation of the quality of the topographic reconstruction of a series of surface features. The investigation is carried out by considering the typical surface features of a metal powder-bed fusion process: weld tracks, weld ripples, attached particles and surface recesses. Results show that no single measurement technology provides a completely reliable rendition of the topographic features that characterise the metal powder-bed fusion process. However, through analysis of measurement discrepancies, light can be shed on where instruments are more susceptible to error, and why differences between measurements occur. The results presented in this work increase the understanding of the behaviour and performance of areal topography measurement, and thus promote the development of improved surface characterisation pipelines.
Industrial x-ray computed tomography (XCT) is seen as a potentially effective tool for the industrial inspection of complex parts. In particular, XCT is an attractive solution for the measurement of ...internal geometries, which are inaccessible by conventional coordinate measuring systems. While the technology is available and the benefits are recognized, methods to establish the measurement assurance of XCT systems are lacking. More specifically, the assessment of measurement uncertainty and the subsequent establishment of measurement traceability is a largely unknown process. This paper is a review of research that contributes to the development of a geometrical calibration procedure for XCT systems. A brief introduction to the geometry of cone-beam tomography systems is given, after which the geometrical influence factors are outlined. Mathematical measurement models play a significant role in understanding how geometrical offsets and misalignments contribute to error in measurements; therefore, the application of mathematical models in simulating geometrical errors is discussed and the corresponding literature is presented. Then, the various methods that have been developed to measure certain geometrical errors are reviewed. The findings from this review are discussed and suggestions are provided for future work towards the development of a comprehensive and practical geometrical calibration procedure.
Camera-based methods for optical coordinate metrology, such as digital fringe projection, rely on accurate calibration of the cameras in the system. Camera calibration is the process of determining ...the intrinsic and distortion parameters which define the camera model and relies on the localisation of targets (in this case, circular dots) within a set of calibration images. Localising these features with sub-pixel accuracy is key to providing high quality calibration results which in turn allows for high quality measurement results. A popular solution to the localisation of calibration features is provided in the OpenCV library. In this paper, we adopt a hybrid machine learning approach where an initial localisation is given by OpenCV which is then refined through a convolutional neural network based on the EfficientNet architecture. Our proposed localisation method is then compared with the OpenCV locations without refinement, and to an alternative refinement method based on traditional image processing. We show that under ideal imaging conditions, both refinement methods provide a reduction in the mean residual reprojection error of approximately 50%. However, in adverse imaging conditions, with high noise levels and specular reflection, we show that the traditional refinement degrades the results given by pure OpenCV, increasing the mean residual magnitude by 34%, which corresponds to 0.2 pixels. In contrast, the EfficientNet refinement is shown to be robust to the unideal conditions and is still able to reduce the mean residual magnitude by 50% compared to OpenCV. The EfficientNet feature localisation refinement, therefore, enables a greater range of viable imaging positions across the measurement volume. leading to more robust camera parameter estimations.
Despite the advancement of additive manufacturing (AM)/3-dimensional (3D) printing, single-step fabrication of multifunctional parts using AM is limited. With the view of enabling multifunctional AM ...(MFAM), in this study, sintering of metal nanoparticles was performed to obtain conductivity for continuous line inkjet printing of electronics. This was achieved using a bespoke three-dimensional (3D) inkjet-printing machine, JETx, capable of printing a range of materials and utilizing different post processing procedures to print multilayered 3D structures in a single manufacturing step. Multiple layers of silver were printed from an ink containing silver nanoparticles (AgNPs) and infrared sintered using a swathe-by-swathe (SS) and layer-by-layer sintering (LS) regime. The differences in the heat profile for the SS and LS was observed to influence the coalescence of the AgNPs. Void percentage of both SS and LS samples was higher toward the top layer than the bottom layer due to relatively less IR exposure in the top than the bottom. The results depicted a homogeneous microstructure for LS of AgNPs and showed less deformation compared to the SS. Electrical resistivity of the LS tracks (13.6 ± 1 μΩ cm) was lower than the SS tracks (22.5 ± 1 μΩ cm). This study recommends the use of LS method to sinter the AgNPs to obtain a conductive track in 25% less time than SS method for MFAM.
Calibration of the scales of areal surface topography measuring instruments requires testing of the resolution. Several designs of artefact that allow testing of the resolution of such instruments ...are currently available; however, analysis methods need to be developed to provide comparable results. A novel method for determining the lateral resolution of areal surface topography measuring instruments is presented. The method uses a type ASP (star-shaped) material measure. To demonstrate the validity of the method, the resolution of a phase shifting interferometer was determined based on the ISO definition of the lateral period limit. Using the proposed method, the type ASP material measure, which is often used to judge qualitatively an instrument's resolution, can be used to quantitatively estimate the resolution of instruments using the topography data.
•The paper presents a methodology to develop fast and accurate in-line optical surface topography measuring instruments at millimetre- to micrometre-scale.•An information-rich metrology (IRM) ...framework is presented to support the methodology.•A case study of an in-line measuring instrument development for additive surface finishing by using the methodology is presented.•The performance improvement of an optical metrology instrument utilising machine vision and machine learning is presented.
The productivity rate of a manufacturing process is limited by the speed of any measurement processes at the quality control stage. Fast and effective in-line measurements are required to overcome this limitation. Optical instruments are the most promising methods for in-line measurement because they are faster than tactile measurements, able to collect high-density data, can be highly flexible to access complex features and are free from the risk of surface damage. In this paper, a methodology for the development of fast and effective in-line optical measuring instruments for the surfaces of parts with millimetre- to micrometre-size is presented and its implementation demonstrated on an industrial case study in additive manufacturing. Definitions related to in-line measurement and barriers to implementing in-line optical measuring instruments are discussed.
A predistortion-based 3-D phase mapping method with linear epipolar constraints for fringe projection system calibration and measurement is reported. Hybrid system calibration methods have been ...developed recently to provide accurate measurement, high speed, and flexibility for fringe projection profilometry (FPP). However, these methods still have significant limitations. With projector distortion, for example, rigorous post-undistortion often results in low efficiency in computation. Conversely, simple, approximated undistortion is only effective for low levels of distortion. As a result, projector undistortion is usually not prioritized in practice. Aiming to achieve high-accuracy 3-D reconstruction with FPP, an easily implementable projector undistortion method is proposed in this article. Theoretical analyses and simulations confirm that the proposed method is rigorous. Experiments validate that the pixelwise root-mean-square reconstruction error is 0.042 mm, a 24% reduction from that of a simple post-undistortion method. The proposed method is also efficient computationally. Finally, a 3-D phase mapping lookup table is built. The results show that the proposed method is effective for achieving high accuracy and speed in 3-D surface measurement.
Methods for determining the amplification coefficient, linearity and squareness of the axes of areal surface topography measuring instruments are presented. The methods are compliant with draft ...international specification standards on areal surface texture. A method of calibrating the z-axis scale according to the guidelines given in surface profile specification standards, which is applied to areal measurements, is first presented. Then a method of calibrating the scales of the x and y axes using cross grating artefacts, and which is not based on pitch measurement, is introduced. A method for extending the calibrated range of the z-axis scale, which uses multiple overlapped measurements of a step height artefact, is also discussed.
This paper describes the development and initial testing of a novel three-axis vibrating micro-scale co-ordinate measuring machine (micro-CMM) probe. The vibrating micro-CMM probe is designed to ...address the needs of micro-manufacturing industry, in particular the requirement to measure high aspect ratio micrometre sized features to nanometre accuracy. The vibrating micro-CMM probe was also designed to address the problems inherent with micrometre and nanometre scale co-ordinate measurements caused by surface interaction forces. The initial concepts were first developed using extensive computational modelling and materials analysis. Production techniques were also investigated. The result was a micro-CMM probe consisting of three flexures, instrumented with piezoelectric actuators and sensors. The micro-CMM probe is capable of controlled vibrations in three axes; an essential feature of the design that directly addresses the problems inherent with tactile CMM probe interactions with measurement surfaces on the micrometre and nanometre scale. The ability of this micro-CMM probe to accurately measure high aspect ratio features will be dependant on the aspect ratio of the stylus. Investigations have been conducted to determine the optimum dimensions of the stylus.
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