Article

# X-ray CT and image analysis methodology for local roughness characterization in cooling channels made by metal additive manufacturing

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## Abstract

The increasingly complex shapes and geometries being produced using additive manufacturing necessitate new characterization techniques that can address the corresponding challenges. Standard techniques for roughness and texture measurements are inept at characterizing the internal surfaces in freeform geometries. Hence, this work presents a new methodology for extracting and quantitatively characterizing the roughness on internal surfaces. The methodology links X-ray CT with complete roughness characterization of channels manufactured by laser powder bed fusion through a novel image analysis approach of X-ray CT data. Global and local orientation parameters are defined to enable a full 360° description of the roughness inside additively manufactured channels. X-ray CT data is analyzed to generate 3D deviation data – based on which multiple local roughness profiles are extracted and analyzed in accordance with the ISO 4287:1997 standard. To demonstrate the proposed methodology, seven circular 17-4 PH stainless steel channels produced at different inclinations and with a diameter of 2 mm are investigated as a case study. Qualitative and quantitative characterization of the roughness is obtained through the use of the proposed methodology. A strong dependence of the local roughness on the corresponding α and β orientations is found. A simple regression model is subsequently extracted from the calculated roughness values and allows prediction of Ra-values in the channels for the ranges between 0° ≤ α ≤ 90° and 80° ≤ β ≤ 280°. In addition to decreasing the effective hydraulic diameter of a cooling channel, the surface roughness also influences the local Nusselt number, which is quantified using the extracted regression model.

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... These fundamental contributions characterized additively manufactured channels at first, establishing this particular field. More recently, KLINGAA ET AL. [166] developed and demonstrated a 360° surface characterization methodology for additively manufactured channels based on X-ray CT and image analysis. ...
... In the second image process chain, a methodology developed by KLINGAA ET AL. [166] is employed for further characterization. The methodology includes the use of the software Avizo 9.2. ...
... the surface texture in circular channels is many-faceted due to: 1) the inaccessible nature of internal features, which necessitate non-destructive testing methods (NDT) and 2) the concurrent existence of the different build directions' full spectrum. Particularly for additively manufactured channels, the roughness, for instance, varies depending on the channel's direction with regard to its build direction (global orientation, denoted with α = [0° -90°]) and the local position around the channel's circumference (local orientation, denoted with ß = [0° -360°]) [166]. Thus, the surface roughness values Ra are individually assessed depending on their α-and ß-orientation in the following. ...
Thesis
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... We use high-resolution X-ray micro-computed tomography (CT) scanning to visualize scale deposition. The surface texture parameters of the scaled deposition cells are quantified using a custom-written Python code [41]. ...
... The surface texture characterization performed in this study was based on the methodology proposed by Klingaa et al. [41], which uses an in-house Python code to extract surface texture information from 3D point cloud data, generated from the exported stack of images. The surface profile parameters used in this work are defined as: ...
... If = 3 the profile has an equal distribution of soft and sharp peaks and valleys. If > 3 the profile is characterized as being spiked [41]. These can therefore be used to quantify what kind of height distribution deposited material exhibit on a surface. ...
Article
Crystallization fouling presents a significant challenge in a wide range of industries. Accurate understanding of crystal formation is crucial for planning preventative measures and maximizing the effectiveness of maintenance interventions. In this study, we demonstrate that understanding net deposition rates depends on the knowledge of the detachment mechanisms and deposition distribution characteristics. We quantify deposition in a once-through flow set-up and visualize crystal formation through high-resolution X-ray micro-computed tomography scanning. Additionally, we quantify the height distribution of deposited crystals through computed surface texture parameters. Finally, we used computational fluid dynamics, implementing large-eddy simulations turbulence modeling and Eulerian transport of chemical species, to describe bulk and wall reactions and quantify energy and mass transport in turbulent eddies. Results show that attachment and detachment processes depend on fluid hydrodynamics; the influx of material determines the overall deposition to the surface, while the deposition pattern is governed by the surface morphology of the initial surface morphology. Our findings provide a foundation for understanding fouling mechanisms and present a template for developing more accurate prediction models.
... For these reasons, conventionally manufactured IN718 is widely used for high temperature applications in aerospace industry, gas turbines, turbocharger rotors, nuclear reactors, highly-loaded rotating parts, and a variety of other applications [11][12][13]. For example, IN718 is of interest for use in aircraft engine components such as combustion chambers with a modern cooling system consisting of multiple thin internal channels [14][15][16][17][18]. For the latter applications, the surface roughness and dimensional tolerance of internal channels are critical. ...
... After analysis of internal surface roughness and dimensionality from CT, the heat flux was tested on an experimental facility, allowing a correlation between surface roughness, friction factor and heat flux. Moreover, Klingaa et al. [18] investigated channels with a diameter of 2 mm produced at different inclinations by LPBF in 17-4 PH stainless steel. They extracted local and radial roughness profiles from CT data and calculated the roughness values in the channels by means of a simple regression model. ...
... CT has already been used to examine internal canals of different configurations, e.g. [15][16][17][18], but the approaches for analysis and comparison differ widely. Snyder et al. [17], for example, used the maximum cylindrical concentricity and maximum cylindrical run-out for analysis. ...
Article
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A laser powder bed fusion (LPBF) strategy was developed for manufacturing small channels with high dimensional accuracy in Inconel 718 structures. Particular attention was paid to surface characteristics such as equivalent diameter and shape factor. The inherent surface quality of external surfaces was optimized by a systematic variation of the LPBF contour parameters as well as the channel cross-section. The mean arithmetic roughness Sa was analysed for upskin, vertical and downskin surfaces with respect to the build platform. Simultaneously, the effect of the build direction on the quality of internal free-shaped surfaces was investigated on channels with diameters from 500 to 1000 µm and build orientations from the horizontal (0°) to vertical (90°). A significant improvement in dimensional accuracy was achieved by using an optimized droplet-shaped cross-section that is scaled as a function with the build inclination. An angular analysis of the surface roughness in different regions of the channels confirms that this modified cross-section reduces the fraction of channel regions that show a particularly high surface roughness due to inward melting. In combination with an optimized contour processing strategy, the modified channel resulted in the best properties for inclinations below 45°. The shape factor increased from 0.4 to almost 0.9, i.e., close to the ideally round shape.
... The surface topography characterization inside channels performed in this study was based on the methodology proposed by Klingaa et al. [29], which uses 3D point cloud data generated from 3D voxel-based data to extract surface profiles along the length of channels and at 360 • around the channel periphery. The surface profiles are obtained similar to running a contact stylus across the surface of the channel. ...
... First, it can be seen how all channels, except the D nom = 1 mm channel, follow a general dross formation pattern with regions of high dross formation on both sides of β = 180 • . This is in contrast to the dross formation pattern observed in steel LPBF channels [29,38] where the dross formation was continuous from β = 90 • to β = 270 • and with maximum dross at β = 180 • . This discrepancy could be caused by the use of different process parameters at the down skin surfaces. ...
... First, it can be seen how all channels, except the Dnom = 1 mm channel, follow a general dross formation pattern with regions of high dross formation on both sides of β = 180°. This is in contrast to the dross formation pattern observed in steel LPBF channels [29,38] where the dross formation was continuous from β = 90° to β = 270° and with maximum dross at β = 180°. This discrepancy could be caused by the use of different process parameters at the down skin surfaces. ...
Article
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Channels manufactured by laser powder bed fusion have an inherent process-induced dross formation and surface texture that require proper characterization for design and process optimization. This work undertakes surface texture characterization of AlSi10Mg channels of nominal diameter sizes ranging from 1 mm to 9 mm using X-ray computed tomography. Profile parameters, including Pa, Pz, and Pq, were found to be interchangeable for qualitative characterization of surface texture variation. Psk, Pvv, and the fractal dimension could identify the presence of extreme dross and sintered particles on the measured profiles. A method for predicting the equivalent diameter of the unobstructed cross-sectional area (Deq) was presented and its reduction was found to follow a logarithmic trend, as a function of channel length. An empirical model Pa (β, D), as a function of local angular position (β) and channel diameter (D), was demonstrated on a perfect channel geometry, resulting in well-predicted roughness and internal geometry.
... As described by Klingaa et al. [45], the surface roughness of samples obtained by AM depends on the orientation respect to the build direction. In the present study, it was decided to manufacture all the different channel geometries keeping the same build direction and the vertical one was selected because, as stated by Klingaa et al. [45], it leads the lowest surface roughness. ...
... As described by Klingaa et al. [45], the surface roughness of samples obtained by AM depends on the orientation respect to the build direction. In the present study, it was decided to manufacture all the different channel geometries keeping the same build direction and the vertical one was selected because, as stated by Klingaa et al. [45], it leads the lowest surface roughness. This study is carried out in the framework of the fusion energy program and it aims at developing an efficient thermal management system embedded in the MITICA acceleration grid (Fig. 1). ...
... It can be seen that the calibrated model tends to underestimate the pressure drops measured for these channels and this can be related to both the different geometrical properties of the channels and to the AM process. The reasons for the revealed tendency to underestimation may be attributed to a different wall absolute roughness on the non-vertical walls, as suggested by Klingaa et al. [45]. This one can affect the wall-fluid interaction, especially at boundary layer level when the geometrical complexity increases as it clearly happens in the case of the DUNED and NICE configurations. ...
Article
Full-text available
Recently, Additive Manufacturing (AM) of metal components has opened new frontiers in heat transfer applications, going beyond the capabilities of conventional technologies. Despite the great design freedom offered by AM, when dealing with metals, there are a few issues that should be considered to exploit the great capabilities of this manufacturing technology. In fact, the surface roughness of the components is expected to affect the performance of the devices, which can be remarkably different from the ones simulated with computer codes. This paper presents a critical analysis of the accuracy of the numerical tools to simulate the fluid flow behaviour inside channels obtained via AM, showing the major limitations of the standard approaches to accurately predict the pressure drops in straight and complex channels. Three different copper channels of growing complexity were built via SLM (Selective Laser Melting) and then they were experimentally tested to evaluate the predictive abilities of the numerical model. The results revealed that the surface roughness deeply affects the fluid flow, thus the numerical models need to be calibrated to become reliable design tools. The proposed procedure can be considered the first attempt in this direction and allows for a proper integration of the AM with the numerical simulation tools to boost the design capabilities of SLM technology.
... In some cases, where the cooling channels and dimensions of the design features are small, the inherent roughness may even be high enough to fully cover the features. It has been shown in former work by the current authors, that the internal roughness in channels is largely dependent on the global and local orientation of a channel surface [15], but it has yet to be investigated how the position along the channel length influences the local roughness. This would be important knowledge when optimizing the placement of internal design features. ...
... Thus the characterization should be based on the classic approach for profile roughness according to ISO 4288:1997 [22]. This work seeks to utilize a two-step methodology introduced by the current authors [15,23], in order to characterize the through length variation of internal surface roughness of seven SLM manufactured straight channels made in 17-PH stainless steel. ...
... Each image stack was used as input for nominal/actual deviation analysis performed by utilizing a methodology and in-house Python code developed by the current authors [15,23]. The methodology was based on a two-step approach and will in the following be presented in short. ...
Article
Conformal cooling channels, made by the laser powder bed fusion process, are a promising design strategy for optimising cooling and process control of manufacturing tools and injections moulds. The addition of internal features such as corrugations and ribs are becoming an interesting addition to the cooling channel designs. Internal surfaces of laser powder bed fusion components have an inherent process-induced roughness. The roughness may affect the flow in the channels and may affect the addition of internal features by changing the actual geometry of the channels. This work seeks to characterise the variation of the internal surface roughness along the length of seven straight channels, manufactured using the laser powder bed fusion process, by utilising X-ray CT and image analysis. The characterisation showed that the surface roughness varied discontinuously along the length of the channels and that the roughness magnitudes and roughness variations along the channel lengths were dependent on the orientation of the channels with respect to the build direction. Therefore, the actual geometries of multiple, nominally equal, embedded internal features would vary differently from nominal design, dependent on the location of the feature along the channel length and orientation in the channel.
... These fundamental contributions characterized additively manufactured channels at first; establishing this particular field. More recently, Klingaa et al. developed and demonstrated a 360° surface characterization methodology for additively manufactured channels which is based on X-ray CT and image analysis [23]. ...
... Another research goal is to validate the hypothesis that channels made by BJ have the potential to show a high degree of path and form stability and low levels of 360° surface deviations when compared to LPBF with similar layer thickness. Hence, as a continuation of the work done in [23], the developed methodology is applied and paired with commercial inspection software to study and compare different industrial-grade channels made by BJ and LPBF in stainless steel 17−4PH for the first time. Form deviations and surface texture are quantitatively characterized depending on build directions according to geometric dimensioning and tolerancing (GD&T) and ISO standards. ...
... In the second image process chain, a methodology developed by the authors in [23] is employed for further characterization. The methodology includes the use of the software Avizo 9.2. ...
... Surface profile parameters were obtained similar to the methodology proposed by Klingaa et al. [33]. The exported stacks of segmented 2D orthoslices were used for extracting surface texture parameters through an in-house Python code. ...
... Deconstructing tomographic data allows for the characterization of surfaces and the energetic competition between various modes of surface growth. In this work, a method based on texture parameters, which gives a range of texture parameters describing the height distribution of crystals is adopted [33]. One such parameter is which is a measure of the average height of the surface protuberances and is often used to describe roughness. ...
Article
Full-text available
Fouling processes present significant challenges in many industries; understanding these processes is crucial for accurate prediction and planning effective mitigation protocols. Composite fouling is prevalent in many industrial applications, however, studies of these systems are scarce. In this work, we investigate crystallization fouling in composite systems to understand how substrate properties affect foulant growth behaviour. Crystal (BaSO4 and CaCO3) deposition studies were performed in a once-through flow setup. We used high-resolution X-ray micro-computed tomography scanning to visualize the formed crystals. We extracted the distribution of the shapes, frequencies of the protuberances, surface coverage and density of clusters through image analysis. Our results reveal that deposition behaviour depends on complex substrate-foulant interactions. Whereas the net deposition of CaCO3 on a BaSO4 substrate increased with time, significant detachment processes were observed for the reverse case. The increased detachment of BaSO4 on a CaCO3 substrate can be attributed to either the weak interaction within the CaCO3 crystal structure or the CaCO3/steel interface. We also observe that substrate properties govern crystal cluster sizes and distribution. The results from this work provide a basis for the development of more accurate prediction models.
... Texture parameters are the description of the surfaces height distribution. A surfaces height distribution can, through different standards, be converted into the texture parameters (Klingaa, 2020). These are used in many industries, and the surfaces are typically measured with probe techniques, see Figure 2. ...
... The method is based on the work of Klingaa et al. and more about the avaiable parameters can be found in Ref (Klingaa, 2020). In summary, XCHANTO isolates the internal geometry of each image in the stack and then generates a mesh of the internal surface. ...
Conference Paper
Without methods for surface characterization of tubing and pipeline, corrosion and scaling cannot be mitigated. One standardized characterization method would enable comparison between various surfaces, which would give new insight into the mechanism behind both corrosion and scaling. We aim to showcase a novel surface characterization software and how it can be used for industry and research purposes. We aim to highlight the capabilities of this tool through 2 analysis campaigns. Our tool is called XCHANTO (X-ray CHannel ANalysis TOol). XCHANTO is an in-house written Python code that can extract surface texture information from 3D point cloud data generated from a stack of images. XCHANTO is based on X-ray CT scanning and calculates standardized Metrologic parameters. In the first campaign we show how XCHANTO can aid the industry in characterizing decommissioned tubing. We performed a single in-depth analysis of a channel. The investigation includes global averages of texture parameters, cylindrical averages in spherical coordinates, and visualization of the height reduction. This investigation was concluded by benchmarking the obtained texture parameters to values obtained from international peer-reviewed journals. Secondly, we have shown how XCHANTO could be useful for researchers. This included using texture parameters to describe surface growth with a temporal resolution and compare in between larger datasets. The quality of XCHANTOs output is dependent on the input CT data. Therefore, for optimal usage of XCHANTO, it will require an experienced operator to acquire and segment high-quality data. When data is acquired, XCHANTO offers a simple way of sophisticated analysis.
... In fact, microCT has been found to be an especially well-suited evaluation method for complex parts produced using AM [29][30][31][32]. In addition to routine porosity and defect inspection, this method has been employed in assessing part geometric accuracy, surface characterization [33,34], and corrosion evaluation [35]. ...
Article
Full-text available
Laser powder bed fusion (LPBF) has become an established method for manufacturing end-use metal components. Exploiting the geometric freedom of additive manufacturing (AM) offers broad possibilities for part optimization and enables performance enhancements across industry sectors. However, part shape and feature size have been found to locally affect residual stresses, melt pool cooling rates, microstructure, and thus the mechanical properties of components. Even though the mesoscale structure can locally induce microstructural changes, there are no prior studies on how it influences corrosion. Using AM-produced, optimized parts in critical applications necessitates a better understanding of their long-term performance. In this study, lattice structures were used to probe the influence of feature size and shape on corrosion susceptibility and its spatial localization. The susceptibility of submillimeter LPBF-fabricated 316 L stainless steel lattice structures to corrosion was investigated by conducting a 21-day immersion corrosion test in an aqueous 3.5 wt% NaCl solution. Schoen gyroid and Schwarz diamond triply periodic minimal surface lattices were manufactured with three unit cell sizes and wall thicknesses (0.867, 0.515, and 0.323 mm). The nominal surface and cross-sectional areas were the same for the two geometries. X-ray microcomputed tomography (microCT) scans before and after the corrosion test were compared for volumetric losses. In addition, the mechanical properties and microstructure of the samples were evaluated. As part of the study, a workflow to register, index, and analyze volumetric changes of consecutive microCT image stacks was developed. The method is fully reported and applicable to time-lapse studies with microCT. Three out of five of the 0.323 mm wall thickness lattices displayed visually aggressive pitting. Based on the microcomputed tomography data, the mass losses were localized either in the entrapped powder particles or partially melted surface globules. Corrosion did not occur in the dense base material. The total mass losses ranged from 8 to 19 mg. Despite visual indications to support a higher corrosion susceptibility for the smallest lattice sizes, the mass loss values did not confirm this conclusion. The tensile test results did not provide any clear indications of latent corrosion effects on mechanical properties.
... In previous work, an X-ray Computed Tomography (X-CT) and image analysis approach was developed [1] and applied to study the effect of build direction on channels made by LPBF with a nominal diameter dN = 2mm [2]. Based on the same methodology, a state-of-theart benchmark study of differently inclined channels (α = 0° -90°) with a constant nominal diameter (dN = 2mm) made by different BJ and LPBF manufacturers was performed [3]. ...
Conference Paper
Full-text available
Additively manufactured channels enable conformal cooling for applications such as injection molds or fuel injectors. Understanding the scalability of these channels for applications of different sizes is essential to pave the way for a more widespread adoption. This work uses a non-destructive testing approach to characterize and compare channels of different sizes made by two popular Metal Additive Manufacturing (MAM) technologies: Binder Jetting (BJ) and Laser Powder Bed Fusion (LPBF). The results reveal systematic differences in deviations depending on the nominal diameter and the used AM process.
... In addition, straight channels may be manufactured by setting the optimal wall direction for AM. In fact, the direction of the wall with respect to that of the building influences the surface roughness of the wall itself [41,42], and the results reported in Table 5 are in accordance with this behaviour. ...
Article
The surface roughness can be considered one of the main drawbacks of the Laser Powder Bed Fusion (LPBF) technology applied to thermal applications. Favero et al. [1] experimentally and numerically demonstrated that the surface roughness of printed copper channels deeply affects their fluid dynamic performance and limits the predictive capabilities of the CFD tools. The authors proposed a methodology to calibrate the numerical tools in turbulent flow. However, the problem remains unsolved because the printed copper channels exhibit remarkably higher pressure drops when compared to conventional smooth channels, hindering the proper deployment of the technology in the thermal sector. This paper proves how it is possible to smooth the internal walls of channels obtained via LPBF to reduce the surface roughness. In fact, the chemical milling process is proposed and applied. The smoothed channels showed a fluid dynamic behaviour similar to the conventional ones. Finally, by coupling Computer Tomography to the CFD tool, the fluid dynamic behaviour inside the smoothed channels is fairly predicted, confirming that the surface roughness and the channel dimensions are the controlling parameters of the fluid dynamic performance of channels printed via LPBF.
... When regarding to the pore structure of 3D-pinted cementitious materials, a highly porous structure of the interlayer zone (IZ) between two neighbored filaments was extensively reported [33,35]. With the aid of advanced non-destructive test techniques like X-ray computed tomography (XCT), the pore structure differences between the matrix and the interlayer of filaments can be quantitatively characterized [36,37]. Kruger et al. [38] measured the interlayer and matrix pore structure of a 3D-printed high-performance mortar by XCT, and reported the more porous interlayer structure with the increased printing time interval. ...
Article
Ordinary 3D-printed neat cement paste generally possesses a porous interface between two neighbored filaments. Our work demonstrates that the addition of glass microfibers into cementitious slurries may lead to an anomalous pore structure, i.e., the increased porosity of the inner and outer matrices in filaments and the densified interlayer between filaments in fiber-reinforced cementitious composites (FRCC). Limited glass microfiber addition (0.6%) has no benefits to the bending strength of FRCC beams, the compressive strength of filament matrix, and the bonding behaviors between filaments. The coupled mechanisms of lubrication layer and pressurized bleeding on the surface of filaments, and air-void entrapments in the matrix of filaments were proposed to account for the formation of the anomalous pore structure and strengths. Our findings would deepen the understanding of matrix-interlayer structures in 3D-printed FRCC towards better tuning and design of 3D printing-based additive manufacturing.
... [53][54][55] Aloisi and Simone [56,57] identified significant inconsistencies in AM surface roughness measurements by comparing the use of (i) a coordinate-measuring machine, (ii) a 2D X-ray method and (iii) a 3D noncontact approach. 3D X-ray computer tomography (CT) [58] has distinct advantages. However, noticeable uncertainties can arise due to scan resolution, material density, artifacts in data analysis and lack of a standard measuring procedure. ...
Article
The design freedom offered by additive manufacturing (AM) enables the fabrication of components with internal surfaces that are challenging to access post-manufacture. This is of concern, as the surface condition can markedly deteriorate fatigue performance. Additionally, the adaptation of surface finishing methods for AM components with topologically optimised designs can be a costly practice. It is therefore desirable to consider deploying AM parts with no or minimal surface processing for targeted applications. This requires an in-depth understanding of the formation of various types of AM surfaces, including the variation in surface condition and controlling factors, and their influence on mechanical performance. The last few years have seen significant research advances in these aspects. Ti-6Al-4V is the most extensively studied alloy for AM. The research data available now allows an informative treatment of this topic for both practical applications and future research. Using laser powder bed fusion (LPBF) of Ti-6Al-4V as a model AM−alloy system, this article examines (i) the characteristics of various types of LPBF surfaces including horizontal, vertical, inclined, upward, downward, internal isolated, and slotted surfaces; (ii) the design features and LPBF variables that affect the surface topography; (iii) the capabilities of existing post-AM surface processing methods; and (iv) the influence of AM surface topography on mechanical properties by focusing on the fatigue performance. On this basis, design considerations are recommended for AM of consistent surfaces, and priority surface-related research issues are identified. The purpose is to establish an essential knowledge base for improved commercial designs for LPBF for suitable dynamically loaded applications, with no or minimal surface processing. While centring on LPBF of Ti-6Al-4V, the insights derived are expected to be applicable to other AM processes or metallic materials.
... It is essential to investigate the geometrical imperfections of actual printed parts through nondestructive testing (NDT). Recently, computed tomography (CT) technology has been widely employed as an NDT technique for detecting imperfections and identify- [8,15]. Melancon et al. integrated CT and statistical analysis of geometric imperfections to generate statistical data based on numerical models [21]. ...
Article
Full-text available
There are inherent geometric imperfections in the additive manufacturing process, which affect the performance and lifespan of a lattice structure and are not easy to observe directly. In this study, a novel intelligent defect detection method based on the YOLOv3 network architecture is proposed for additively manufactured diamond lattice structures. A strategy based on the K-medoids clustering algorithm is proposed to improve the performance of YOLOv3 for detecting small defects. Moreover, to improve the resolution of defects in computer tomography slices, a super-resolution convolutional neural network model is used. Thus, a modified YOLOv3 network model with super-resolution convolutional neural network is constructed, trained, and tested. Experimental results demonstrate that the typical internal defects of lattice structure samples are identified effectively, and the mean average precision is 94.55%.
... X-ray computed tomography (XCT) is widely used to characterize material properties of fibre reinforced polymers, as well as for materials like concrete and metals. [7][8][9] XCT has the advantage of non-destructive testing and a high resolution, that is, micron level voxel size. [10][11] However, the resolution obtained by XCT cannot compete with that obtained by scanning electron microscopy (SEM), reaching sub-micron resolutions. ...
Article
This study presents a holistic segmentation procedure, which can be used to obtain individual fibre inclination angles from X-ray computed tomography. The segmentation approach is based on principal component analysis and was successfully applied for a unidirectional and an air-textured glass fibre–reinforced composite profile. The inclination results show a weighted mean fibre inclination of 2.1° and 8.0° for the unidirectional and air-textured profile, respectively. For the air-textured composite, fibre inclinations of up to 55° were successfully segmented. The results were verified by comparative analysis with equivalent results obtained from structure tensor analysis – showing no notable deviation. The comparable characteristics in combination with the distinct differences of the two material systems make this case study ideal for verification and validation of idealized models. It is shown how this approach can provide fast, accurate and repeatable inclination estimates with a high degree of automation.
... The friction factor decreases with the increased wavelength. Klingaa et al. [37,38] and Dahmen et al. [11] built seven cuboids with internal cooling channel inclined at particular building angles (0 • , 15 • , 30 • , 45 • , 60 • , 75 • , and 90 • ) using 17-4 PH SS. The authors used X-ray CT and image analysis to measure the profile and surface roughness of the channels. ...
Article
Laser powder bed fusion (L-PBF), or selective laser melting, can be used to significantly reduce the size and weight of hydraulic components. However, the current design approach still greatly depends on the technician’s experience. The friction loss of the L-PBF fabricated fluid channels is greatly affected by the fabrication quality but remains unknown, which is essential for the design approach. In this study, circular 316 L SS fluid channels with various diameters (4-10 mm) and build angles (0°-90°) were fabricated using L-PBF. Fabrication quality was characterized, and the friction factors were measured and analyzed. The results indicate that the geometric tolerance and surface roughness are closely related to the build angle. The friction factor of the L-PBF fabricated channels, which is higher than the classical theory in the turbulent flow, is influenced by both channel diameters and build angles. A comprehensive model is developed to predict friction factors of the L-PBF fabricated fluid channels. Design guidelines of fluid channels are proposed, for the first time, by considering both fabrication quality and friction factors.
... In addition, the staircase structure of the sample surface may also cause overestimation [2], which leads to unexpected error. Conversely, the representative models do not reveal every feature but highlight certain kinds of defects (e.g., varying cross-section [24] or local roughness [17]) and weaken other features with minimal influence on accuracy. In this way, guaranteed prediction accuracy can be achieved while keeping the cost of FEA acceptable. ...
Article
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Currently, the potential use of selective laser melting (SLM) technology to fabricate and optimise the performance of human bone implants with functionally graded scaffold (FGS) structures has drawn an increasing amount of attention. Magnesium alloy is a high-quality light metal that shows impressive potential in bioengineering; thus, it is chosen in this study. However, the defects that occur in magnesium during manufacturing may generate unexpected mechanical differences between the products and the ideal products, so there is a need to determine an efficient way to model actual products. In this article, samples were manufactured with different laser energy densities and subjected to compression experiments. The formation of spherical defects was analysed according to the principle of additive manufacturing. Hence, based on the observed characteristics, finite element (FE) models with different geometries were built, and changes in the stress distribution during compression were discussed. By comparing the mechanical properties obtained from compression tests and FE modelling, the simulation accuracy of each model was also estimated, from which the model that highlighted the stacking relationship was notable. Moreover, the expanded Hertzian contact stress theorem verified the influence of spherical defects on the compressive behaviour of the sample. The accuracy of the FE results was obviously improved by rectifying the equivalent bearing area.
... X-ray Computed Tomography is a non-destructive inspection method that allows for the 3D spatial reconstruction of pores, inclusions, cracks or discontinuities at a resolution level from the nano-, micro-, to macroscales [11,12]. An additional advantage of X-ray CT is the ability to perform coordinate measurements of components that are difficult to access (holes [13], connections [14]) or which are completely inaccessible (conformal channels [15], lattice structure [16]). Due to the examination of a sample with high resolution, XCT is widely used in the area of additive manufacturing [17] for investigating the number of defects, as well as their size, shape and spatial distribution in three dimension. ...
Article
Full-text available
The aim of this work is to present the quantitative evaluation of the Additive Manufacturing (AM) laser powder bed fusion (L-PBF) process steps and validation of the FE (Finite Elements) simulation results using the X-ray Computed Tomography (XCT) based approach. The characterization of the production process was made using standard methods, including: laser light diffraction, optical and scanning electron microscopy, hardness examinations, and the investigation of mechanical properties. The XCT method was used to conduct the quality of powder feedstock, the influence of the L-PBF build strategy on the samples’ porosity, and also to evaluate a quantitative analysis of damage behavior before and after ex-situ mechanical testing. The porosity recorded for the powder feedstock was 0.2% and for test samples did not exceed 0.02%. The majority of the observed pores are aligned in patterns connected with the scanning strategy and were arranged in lines appearing along the borders between adjacent stripes. It was observed that increasing the resolution of the XCT scanning significantly influenced the mapping of the shape of pores and – to a lesser extent – the recorded porosity. The comparison of geometry deviations after numerical simulation and tensile test for FE models obtained with different XCT resolutions allowed to show differences in the geometry of experimentally and numerically damaged test samples.
... Measurements of Pa and D eq for channels were described in recent work by Klingaa et al. [23]. An in-house Python code, developed by Klingaa et al. for the quantitative analysis of X-ray CT scanned channels, was used for the quality metric calculations [24,25]. The tool uses cross-sectional images of channels to perform automated quantitative evaluations. ...
Article
Metal additive manufacturing is increasingly used as a complementary manufacturing technique in industrial settings and slowly moving from pure prototyping applications toward full production. In parallel, there is an emergence of Industry 4.0, where the applicability of concepts such as digital twins of manufacturing machines and components are being investigated. Compared to conventionally manufactured parts, typical quality metrics of metal additively manufactured components such as dimensions, roughness, porosity, and hardness are underperforming in an as-built state. As a mitigation strategy, the build chamber variables are often measured and logged by the metal additive manufacturing system to maintain a stable production environment. Thus, proper insight into the expected responses in part quality from changes in those build chamber variables is important in the pursuit of digital twins and process improvement. This sheds more light on the influence of the gas flow variables, namely gas flow speed, relative pressure, and oxygen content on the metal additive manufacturing quality metrics, specifically channel roughness, bulk porosity, average diameter, the equivalent diameter of the unobstructed cross-sectional area, and hardness of the bulk. A Design of Experiments was implemented on two laser powder bed fusion systems, namely an SLM 280 processing 316L stainless steel and an SLM 500 processing Ti6Al4V. The current work found that surface oxidation of 316L and Ti6Al4V components may be classified based on simple red, green, and blue (RGB) color constituent analysis. The influence of gas flow variables was found to be different in the two investigated SLM systems, suggesting a high dependency on the processed material. Oxygen content in the build chamber had the highest standalone effect on the selected quality metrics, while the gas flow speed had the lowest standalone effect. The second-order effects were found to be, in general, more significant than the main effects. The findings of the current work is a step towards an improved understanding of the interaction effects of gas flow conditions on typical quality metrics of metal additive manufactured components. By the creation of simple but computationally fast response surface models, in-line assessments may be carried out and the effect of process variability on component quality may be evaluated in-situ while being one step away from the full feedback control implementation in the digital twin. Following the methodology of the current work for other laser powder bed fusion systems enables the generation of 3D point cloud visualizations for decision making under uncertainty.
... However, their studies were performed on external AM surfaces. More recently, Klingaa et al. [30,31] extended the CT analysis to internal surfaces of LPBF channels, by modelling the roughness value depending on the defined global build orientation (α) and local orientation (β). This approach allows establishing novel concepts, like the presented rough belt concept with an angular width, which showed how the angular width directly correlates to the α-orientation. ...
Article
Channels and bores in metal components produced by laser powder bed fusion (LPBF) are internal features that are typically affected by defects such as dross and sag formation, dimensional errors and global deformations in different proportions. Such deviations from the ideal geometry may strongly limit the functionality of the channels, but are difficult to prevent, due to complex multi-physical production aspects. Different destructive and non-destructive approaches are available to investigate the geometry of the internal features and possibly correlate their results to the LPBF process parameters; however, such approaches do not offer a systematic method to derive key characteristics of the main contributors for channel deviations. Hence, this work proposes a novel tomographic non-destructive analysis of LPBF channels and bores, focusing on the derivation of sag and dross key parameters. The methodology works on polar-transformed profiles obtained from image stacks which are extracted perpendicularly to the channel axis from the X-ray computed tomography (CT) reconstructed volume. The method allows for the clear determination of surface characteristics and includes the quantitative evaluation of descriptors through an algorithm specifically developed for the purpose. In particular, general form deviations are addressed by fitting sinusoidals on the unwrapped mean surface profile, to tackle deviations induced by thermal residual stresses. Proposed descriptors of sag and dross are the onset angle of protrusions, separation criteria between sag and dross effects, and the peak analysis of the mean profile after approximation with a least squares spline. The developed algorithm is tested in the case study of a LPBF AlSi7Mg0.6 benchmark part comprising hollow cylinders and inter-connecting frusta with different diameters. The resulting evaluation of the benchmark part also corroborates how the proposed methodology can help to obtain more precise information regarding the correlation of LPBF fabrication conditions and obtained channels geometrical deviations. Furthermore, the results show possible routes to enable an a-priori compensation of the nominal channel design for first-time right LPBF manufacturing.
... The defects in AM-built product can trap undesirable materials (e.g. voids, uncured polymers, and impurities), which influence the local density of structure, scatter the local stress fields and serve as a crack initiator causing mechanical propagation until failure of physical products [12,13]. NDE has emerged as prominent and promising tool for the evaluation of internal damages and inclusions in three-dimensional (3D) products via AM [14][15][16]. ...
Article
Poor surface quality necessitates polishing of the parts manufactured by laser powder bed fusion (LPBF) to meet the requirements of use and service. Electrochemical polishing is one of the effective post-treatment methods to reduce the surface roughness. In this manuscript, polishing behavior of 316L stainless steel fabricated by LPBF in the eco-friendly NaNO3 solution was investigated. Different electrochemical tests including current efficiency, open circuit potential, and polarization behavior were carried out in this solution to investigate the influence of section orientation on the electrochemical polishing behavior. Test results indicated that electrochemical dissolution of the transverse section is more difficult than the longitudinal section because of the near-equiaxed dendrites and smaller grains diameter. The electrochemical polishing experiments showed that the polished surface presents hollowed-out retiform structure which is not conducive for the reduction of surface roughness value at low current density and the roughness value significantly decreases with the increase of total quantity of electricity at high current density. The weight loss of the transverse section is smaller than that of the longitudinal section at all current densities and electric charges, but the difference decreased with increasing current density. The electrochemical polishing at the current density of 40 A·cm⁻² resulted in maximum reduction of the surface roughness with minimum weight loss. Finally, interior surfaces with longitudinal section and transverse section were polished successfully. The roughness values decreased from 17.3 and 18.3 μm to 2.4 and 2.6 μm, respectively.
Article
Additive manufacturing is an emerging technique for manufacturing complex shapes rather than traditional manufacturing procedures due to the tool free manufacturing method. The complex components manufactured by this technique have a high surface roughness, which reduces fatigue strength, influences the wear of mating components, reduces the cooling efficiency of complex cooling channels, and so on. The surface roughness of complex cooling channels, for example, affects the formation of boundary layers, partial liquid flow, and heat transfer coefficients. As a result, the finishing processes are required for additively manufactured components to reduce the surface roughness. Chemical based finishing processes for additively manufactured parts have been developed by the researchers because it is a noncontact and automatic finishing process. However, a lack of understanding of the fundamental mechanisms governing these finishing processes may limit their practical uses in areas like aerospace, automobiles, and defense. In this review study, the mechanisms of various chemical and electrochemical based finishing processes are explained first, followed by the current state of the finishing processes of additively manufactured components. Surface integrity, for example, is controlled by experimental factors that are revealed first, followed by various researchers choosing acceptable input parameters to achieve the low surface roughness. In addition, this article provides future perspectives on finishing processes and a view into the process selection based on the component complexity and cost.
Article
Laser ultrasonic (LU) testing is a promising technique that meets the constraints of additive manufacturing (AM) online monitoring, which is vital for the quality control of AM products and the practical application of AM. However, the scattering noise caused by surface roughness of AM parts and the ambient noise can generally result in defect imaging with low resolution. This is not conducive to defect detection. In terms of this problem, a multi-feature fusion imaging methodology is proposed instead of the traditional method containing a single feature, i.e., maximum amplitude within a set window of all A-scans. The most discriminative features in each A-scan are automatically extracted by principal component analysis (PCA). These features are subsequently fused by a trained artificial neural network (ANN) to obtain an evaluated value that determines whether a A-scan is the normal case or defective case (denoted by 0 and 1, respectively). Finally, a C-scan image with high signal to noise ratio (SNR) can be produced by plotting the binary dataset of 0 and 1. LU inspection on a selective laser melting (SLM) component with the pre-induced micro hole defects on the rough surface is performed to validate the effectiveness of the methodology. The defects are distributed in 2 rows and 6 columns where the diameter of one row is 100 μm and the other is 50 μm. It is demonstrated that the proposed methodology can detect and size all micro defects by the obtained high SNR C-scan.
Article
In this study, three manifold-microchannel heat exchangers were additively manufactured by direct metal laser sintering. The heat exchangers were manufactured from stainless steel (SS17-4) with an overall dimension of $64.2\times 46.0\times27.1$ mm. The thermal and hydrodynamic performance of the heat exchangers was experimentally evaluated at different air and water flow rates. The microchannel fins had a thickness of 0.48 mm which featured the smallest length scale used in these heat exchangers. While the overall dimensions of the heat exchangers were identical, their interior designs were slightly different. Heat exchanger A was based on the original manifold-microchannel heat exchanger design concept while heat exchangers B and C had pin fins on their air manifolds. Heat exchangers B and C were different in their microchannel orientations as the microchannels in heat exchanger C were along the streamwise direction of the air inlet manifolds. Despite the interior design modifications between the three heat exchangers, results suggested comparable thermal and hydrodynamic performances. For a water inlet temperature of 60 °C, a normalized rate of heat transfer as high as around 4.76 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> was achieved for an air-side Reynolds number of around 4000. The air-side convection heat transfer coefficient was obtained as high as around 800 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> K. The range of air-side pressure drop was between around 2000 and 4000 Pa, depending on the airflow rate. Heat exchanger A demonstrated the lowest pressure drop and heat exchangers B and C, which incorporated pin fins on their air manifold, caused higher air-side pressure drops.
Article
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Featured Application: This paper presents the first investigations towards closed loop feedback control of the selective laser melting (SLM) process. Insight gained from this work can be applied to facilitate in-process optimization of the SLM process for maximizing part quality and minimizing surface roughness. Abstract: Additive manufacturing provides a number of benefits in terms of infinite freedom to design complex parts and reduced lead-times while globally reducing the size of supply chains as it brings all production processes under one roof. However, additive manufacturing (AM) lags far behind conventional manufacturing in terms of surface quality. This proves a hindrance for many companies considering investment in AM. The aim of this work is to investigate the effect of varying process parameters on the resultant roughness of the down-facing surfaces in selective laser melting (SLM). A systematic experimental study was carried out and the effects of the interaction of the different parameters and their effect on the surface roughness (Sa) were analyzed. It was found that the interaction and interdependency between parameters were of greatest significance to the obtainable surface roughness, though their effects vary greatly depending on the applied levels. This behavior was mainly attributed to the difference in energy absorbed by the powder. Predictive process models for optimization of process parameters for minimizing the obtained Sa in 45 • and 35 • down-facing surface, individually, were achieved with average error percentages of 5% and 6.3%, respectively, however further investigation is still warranted.
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Surface topographies of metal additively manufactured components are inherently characterized by the presence of complex surface characteristics that are not accessible by contact or optical measuring techniques. Micro X-ray computed tomography is capable of measuring non-accessible surfaces and micro-scale surface features, including undercuts. In this work, an innovative approach for evaluating the accuracy and establishing the traceability of surface topography measurements obtained by X-ray computed tomography is presented. Reference samples produced by selective laser melting of Ti6Al4V were specifically designed in order to acquire reference cross-sectional surface profiles representing the actual morphology (including re-entrant features) using an imaging probing system. Surface topographies were measured on these samples by using three different techniques: X-ray computed tomography, confocal microscopy and focus variation. Moreover, the effect of different voxel dimensions on the accuracy of surface topography measurements performed by X-ray computed tomography was investigated. Results showed that X-ray computed tomography (at the highest tested resolution) can acquire surfaces and re-entrant features with small deviations with respect to the reference profiles; the deviations were quantified. On the contrary, focus variation and confocal microscopy can measure surfaces obtaining results that are close to the reference profiles only if such surfaces have no undercuts or inaccessible features.
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The use of microCT of 10 mm coupon samples produced by AM has the potential to provide useful information of mean density and detailed porosity information of the interior of the samples. In addition, the same scan data can be used to provide surface roughness analysis of the as-built surfaces of the same coupon samples. This can be used to compare process parameters or new materials. While surface roughness is traditionally done using tactile probes or with non-contact interferometric techniques, the complex surfaces in AM are sometimes difficult to access and may be very rough, with undercuts and may be difficult to accurately measure using traditional techniques which are meant for smoother surfaces. This standard workflow demonstrates on a coupon sample how to acquire surface roughness results, and compares the results from roughly the same area of the same sample with tactile probe results. The same principle can be applied to more complex parts, keeping in mind the resolution limit vs sample size of microCT.
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X-ray microcomputed tomography (microCT) has become an established method of testing and analyzing additively manufactured parts in recent years, being especially useful and accurate for dimensional measurement and porosity analysis. While this nondestructive analysis method is gaining traction among additive manufacturing (AM) researchers and engineers, the capabilities of the method are not yet fully appreciated and are still being developed. This review aims to summarize the many diverse ways this technique has been applied to AM, including new and specialized applications. Examples are shown of many of these newly developed methods, while also discussing the practicality and limitations of each. The review ends with perspectives on the most time- and cost-effective ways to make use of microCT for various AM applications from R&D up to industrial production, with suggestions for scan strategies for different types of analyses.
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X-ray computed tomography (CT) is increasingly used in dimensional metrology. However, several influencing factors affect CT dimensional measurements. In particular, significant deviations can be observed between CT and tactile measurements especially when measuring parts with rough surfaces. The dependence of such deviations from surface morphology has not been thoroughly studied yet. In this work, the influence of surface roughness on CT dimensional measurements is investigated considering the combined effect of surface morphology and CT measurement characteristics. Experimental investigations and numerical simulations are used to determine the systematic effect on CT dimensional measurements for roughness profiles with different material distribution.
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This article aims to provide an overview of applications of rapid prototyping–assisted conformal cooling channel and shows the potential of this technology in different manufacturing processes. This review article also reports one case study from open literature where rapid prototyping–assisted conformal cooling channel has been successfully used in the manufacturing process. This study concludes that rapid prototyping technique can replace conventional manufacturing for complicated structure conformal cooling channel which improves quality and productivity. The outcome based on literature review and case study strongly suggested that rapid prototyping–assisted conformal cooling channel might become standard procedure in manufacturing process in near future. Advanced technologies such as computer-aided manufacturing, computer-aided engineering, computer fluid dynamics, and rapid tooling made it possible to fabricate the conformal cooling channel. Rapid prototyping–assisted conformal cooling channel can easily transfer the simulation into actual fabrication. This article is beneficial to study the development and application of rapid prototyping–assisted conformal cooling channel in different manufacturing processes.
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This paper presents a regression analysis of the relationship between values of profile and areal surface texture parameters extracted from the same surfaces. The problem of tolerancing for areal surface texture evaluation is briefly introduced. Basic concepts of and differences between areal and profile evaluation are discussed. Measurements of 21 samples of diverse origin are used as data for linear regression of dependencies of areal surface texture parameters Sa, Sq and Sz on their roughness profile equivalents Ra, Rq and Rz. Recommendations for calculation of areal surface texture parameter’s tolerance values are discussed based on the results of the analysis. The resulting regression equations provide a fast method for selection of areal surface texture tolerance values based on previous experience with profile parameters.
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The development of additive manufacturing has allowed for increased flexibility and complexity of designs over formative and subtractive manufacturing. However, a limiting factor of additive manufacturing is the as-built surface quality as well as the difficulty in maintaining an acceptable surface roughness in overhanging structures. In order to optimize surface roughness in these structures, samples covering a range of overhang angles and process parameters were built in a laser powder bed fusion system. Analysis of the surface roughness was then performed to determine a relationship between process parameters, angle of the overhanging surface, and surface roughness. It was found that the analysis of surface roughness metrics, such as Rpc, Rsm, and Rc, can indicate a shift between surfaces dominated by partially melted powder particles and surfaces dominated by material from the re-solidified melt track.
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The often used Nusselt number is critically questioned with respect to its physical meaning. Based on a rigorous dimensional analysis, alternative assessment numbers are found that in a systematic way separately account for the quantitative and qualitative aspect of a heat transfer process. The qualitative aspect is related to the entropy generated in the temperature field of a real, irreversible heat transfer. The irreversibility can be quantified by referring it to the so-called entropic potential of the energy involved in the transfer process.
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The efficiency of fabricating an overhanging structure by selective laser melting (SLM) is an important indicator of the performance of metallic parts. This is due to the fact that defects such as warpage and adherent dross may occur during fabrication of the curved surfaces of overhanging structures. In order to investigate the optimum conditions for fabrication of the curved surfaces of the overhanging structures, experiments were carried out using 316-L stainless steel powder. Initially, the almost 100 % dense parts were fabricated. Then, a model that has a circular curved surface along the Z axis was designed. For a given fabrication depth of 25 μm, several overhanging structures were produced when the laser scanning energy input ranges from 0.15 to 0.6 J/mm. Results show that the upper surface of the almost 100 % dense cube fluctuates like ripples and that the fabrication quality of the curved surface of the overhanging structure varies greatly depending on the energy input and the obliquity angle. For a given energy input of 0.2 J/mm, the obliquity angle for fabricating a totally overhanging surface is as low as 30°. The warpage and adherent dross grow with an increase in the energy input and a decrease in the obliquity angle. Warpage may accumulate, and the accumulated warpage of many layers significantly exceeds the predetermined thickness of the layer. All the four overhanging structures fabricated using varying energy inputs have the following four zones: no dross surface, dense-sinking transition surface, totally sinking surface, and forming failure surface. In the overhanging structures, fabricated with varying laser energy parameters, the angle corresponding to each region was different. The quality of the overhanging surface can be improved by reducing the laser energy. Additionally, a better overhanging surface can be obtained by increasing the obliquity angle. The variation trend of the roughness Rz was almost the same as that of Ra, but the variation range of Rz was much larger than that of Ra. Finally, a foldable abacus with several curved-surface overhanging structures was fabricated to verify the research results. Fundamental methods for controlling and optimizing the SLM-based direct fabrication of curved surfaces of overhanging structures are proposed in this paper, from the perspectives of crafting and design.
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Recent technological advances in the field of additive manufacturing (AM), particularly with direct metal laser sintering (DMLS), have increased the potential for building gas turbine components with AM. Using the DMLS for turbine components broadens the design space and allows for increasingly small and complex geometries to be fabricated with little increase in time or cost. Challenges arise when attempting to evaluate the advantages of the DMLS for specific applications, particularly because of how little is known regarding the effects of surface roughness. This paper presents pressure drop and heat transfer results of flow through small, as produced channels that have been manufactured using the DMLS in an effort to better understand roughness. Ten different coupons made with the DMLS all having multiple rectangular channels were evaluated in this study. Measurements were collected at various flow conditions and reduced to a friction factor and a Nusselt number. Results showed significant augmentation of these parameters compared to smooth channels, particularly with the friction factor for minichannels with small hydraulic diameters. However, augmentation of Nusselt number did not increase proportionally with the augmentation of the friction factor.
Chapter
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The vast majority of surface texture parameters are the field parameters. The term field refers to the use of every data point measured in the evaluation area, as opposed to feature parameters that only take into account specific points, lines or areas. Field parameters allow the characterisation of surface heights, slopes, complexity, wavelength content, etc. They are defined in the specification standard ISO 25178 part 2. In this chapter the ISO areal field parameters will be presented along with limited guidance on their use.
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Marching Cubes' methods first offered visual access to experimental and theoretical data. The implementation of this method usually relies on a small lookup table. Many enhancements and optimizations of March-ing Cubes still use it. However, this lookup table can lead to cracks and inconsistent topology. This paper introduces a full implementation of Chernyaev's technique to ensure a topologically correct result, i.e. a manifold mesh for any input data. It completes the original paper for the ambiguity resolution and for the feasibility of the implementation. More-over, the cube interpolation provided here can be used in a wider range of methods. The source code is available online. Figure 1: Implicit surface of linked tori generated by the classical Marching Cubes algorithm, and ours.
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The main advantage of laser powder bed fusion (LPBF) is its use for directly manufacturing metal components with highly complex geometries. But the LPBF manufacture of overhang structures, also known as downward-facing surfaces, is a challenge because of the possibility of incurring distortion and dross defects. This paper presents a systematic examination of the manufacturability and structural integrity of AlSi10Mg overhang structures fabricated by LPBF using computational and experimental techniques. The experimental and simulation results indicate that the use of support structures facilitates the manufacturability and structural integrity of both full-circle and half-circle overhang structures. The influence of supports on circularity was found to be more beneficial as the diameter increased above 15 mm. The experiments also suggest that the use of supports plays a significant role in maintaining mechanical performance by successful fabrication of downward-facing surfaces free of dross defects. From a design perspective, small overhang features are preferable to large overhangs, especially when support removal is impractical. This study significantly contributes to design for metal additive manufacturing by providing an improved understanding of the manufacturability of overhang structures in applications intended for lightweight structural performance.
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Agglomerates are often composed of amorphous and irregular primary particles, especially in the food industry. The spatial morphology of this kind of soft agglomerate, here maltodextrin, can be quantified by fractal dimension. Previous research in this regard was focused on simulated agglomerates or studied 2D projected images of real agglomerates. In this work 3D volume images of agglomerates are generated with the help of X-ray computed tomography. Primary particles are distinguished and separated by means of a sequence of image processing steps. Thus center coordinates and volume of each particle are extracted. Based on this information, the radius of gyration is calculated and compared for either monodisperse or polydisperse primary particles. The primary particles comprising the maltodextrin agglomerates follow a broad size distribution, hence considering the polydispersity is highly recommended. Next, radii of primary particles are determined in order to calculate 3D fractal dimension and prefactor from power law equation. Due to the irregular shape of primary particles, two different ways of calculating primary particle radius are investigated. It is observed that differences in primary particle radius affect the partial overlapping of particles which mostly influences the prefactor value, while only slight changes are noticed in the fractal dimension. Further, the gyration radius and fractal dimension are obtained directly from voxel data. Though voxel based method is more accurate, it requires more effort and time. Therefore, by considering some error in the values of fractal dimension and gyration radius, the separated polydisperse primary particle model is suggested as a proper option. Finally, fractal dimension is also calculated by the box counting method. The proper implementation of this method for 3D structures is discussed and the results are compared with the classical power law function.
Book
This book acts as a one-stop-shop resource for students and users of X-ray computed tomography in both academia and industry. It presents the fundamental principles of the technique, detailed descriptions of the various components (hardware and software), current developments in calibration and performance verification and a wealth of example applications. The book also highlights where there is still work to do, in the perspective that X-ray computed tomography will be an essential part of Industry 4.0.
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In this paper, a selective laser melting (SLM) physical model describing the melt pool dynamics and the response of downward-facing surface morphology evolution of overhanging structure under different laser processing conditions was proposed, in which an enormous difference in thermal conductivity and laser absorption capacity between the as-fabricated part and powder material was taken into consideration. The underlying thermal physical mechanism of the dross formation phenomenon during SLM preparing overhanging surface was revealed by numerical simulation analysis and experimental studies. It was found that both high and low laser volume energy density (ω) resulted in an inferior downward-facing surface quality. As an optimal processing parameter (60–80 J/mm³) was settled, the overhanging structure obtained a relatively smooth downward-facing surface due to the sound melt pool dimension and steady melt flow behavior. The experimental studies were compared with the simulated results, showing a good agreement with the predictions obtained in the simulations. It was interesting to find that the variation rules of surface quality and densification level of overhanging structure with different ω were exactly converse. As the ω decreased from 80 J/mm³ to 60 J/mm³, the surface roughness could be reduced from 59 μm to 33 μm while, contrarily, the porosity was elevated from 3.2% to 8.4%. In order to fabricate complicated metal parts with lower risk, four solutions for improving the processability of hard-to-process overhanging structure were provided.
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The past few decades have seen substantial growth in Additive Manufacturing (AM) technologies. However, this growth has mainly been process-driven. The evolution of engineering design to take advantage of the possibilities afforded by AM and to manage the constraints associated with the technology has lagged behind. This paper presents the major opportunities, constraints, and economic considerations for Design for Additive Manufacturing. It explores issues related to design and redesign for direct and indirect AM production. It also highlights key industrial applications, outlines future challenges, and identifies promising directions for research and the exploitation of AM's full potential in industry.
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A comprehensive analysis of literature pertaining to surface texture metrology for metal additive manufacturing has been performed. This review paper structures the results of this analysis into sections that address specific areas of interest: industrial domain, additive manufacturing processes and materials; types of surface investigated; surface measurement technology and surface texture characterisation. Each section reports on how frequently specific techniques, processes or materials have been utilised and discusses how and why they are employed. Based on these results, possible optimisation of methods and reporting is suggested and the areas that may have significant potential for future research are highlighted.
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The Additive Manufacturing (AM) process is a computer-controlled process that uses CAD representations to build physical parts layer by layer. Almost all computer-aided design (CAD) packages allow the creation of stereolithography (STL) files, which are translated into machine commands to drive the AM process. Unfortunately, the STL file has many weaknesses and is prone to error. Moreover, there is a need to investigate and understand the issues and errors associated with the software formats and how to control, eliminate, or minimize these errors. Failure to deal with these issues will lead to building poor parts and delayed lead time, which will result in a bad physical model. This chapter summarizes the CAD model of the AM process and explains the various software issues related to the selected CAD representation.
Article
The surface texture of additively manufactured metallic surfaces made by powder bed methods is affected by a number of factors, including the powder's particle size distribution, the effect of the heat source, the thickness of the printed layers, the angle of the surface relative to the horizontal build bed and the effect of any post processing/finishing. The aim of the research reported here is to understand the way these surfaces should be measured in order to characterise them. In published research to date, the surface texture is generally reported as an Ra value, measured across the lay. The appropriateness of this method for such surfaces is investigated here. A preliminary investigation was carried out on two additive manufacturing processes—selective laser melting (SLM) and electron beam melting (EBM)—focusing on the effect of build angle and post processing. The surfaces were measured using both tactile and optical methods and a range of profile and areal parameters were reported. Test coupons were manufactured at four angles relative to the horizontal plane of the powder bed using both SLM and EBM. The effect of lay—caused by the layered nature of the manufacturing process—was investigated, as was the required sample area for optical measurements. The surfaces were also measured before and after grit blasting.
Article
The number of industrial applications of Computed Tomography (CT) is large and rapidly increasing. After a brief market overview, the paper gives a survey of state of the art and upcoming CT technologies, covering types of CT systems, scanning capabilities, and technological advances. The paper contains a survey of application examples from the manufacturing industry as well as from other industries, e.g., electrical and electronic devices, inhomogeneous materials, and from the food industry. Challenges as well as major national and international coordinated activities in the field of industrial CT are also presented.
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This paper gives an overview of the progress which has been made in surface metrology over the past ten years. It updates the surface classification system, and discusses the practical and theoretical reasons for the technological shifts which have occurred. This includes the use of surfaces with predetermined features as an alternative to traditional machined surfaces, and the move from simple to freeform shapes. The paper discusses technological shifts in association, filtration, numeric parametric techniques, fractals associated with function and standardisation. Many examples are given in order to contextualise the significance of these technological changes. This paper should help to predict the direction of future developments in surface metrology, and therefore emphasise its importance in functional applications in advanced manufacture.
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The paper gives a survey of the upcoming use of X-ray computed tomography (CT) for dimensional quality control purposes: i.e. for traceable measurement of dimensions of technical (mechanical) components and for tolerance verification of such components. It describes the basic principles of CT metrology, putting emphasis on issues as accuracy, traceability to the unit of length (the meter) and measurement uncertainty. It provides a state of the art (anno 2011) and application examples, showing the aptitude of CT metrology to: (i) check internal dimensions that cannot be measured using traditional coordinate measuring machines and (ii) combine dimensional quality control with material quality control in one single quality inspection run.
Article
The object of this paper is to furnish the engineer with a simple means of estimating the friction factors to be used in computing the loss of head in clean new pipes and in closed conduits running full with steady flow. The modern developments in the application of theoretical hydrodynamics to the fluid-friction problem are impressive and scattered through an extensive literature. This paper is not intended as a critical survey of this wide field. For a concise review, Professor Bakhmeteff’s (1) small book on the mechanics of fluid flow is an excellent reference. Prandtl and Tietjens (2) and Rouse (3) have also made notable contributions to the subject. The author does not claim to offer anything particularly new or original, his aim merely being to embody the now accepted conclusions in convenient form for engineering use.
Article
The effects of surface roughness on gas turbine performance are reviewed based on publications in the open literature over the past 60 years. Empirical roughness correlations routinely employed for drag and heat transfer estimates are summarized and found wanting. No single correlation appears to capture all of the relevant physics for both engineered and service-related (e.g., wear or environmentally induced) roughness. Roughness influences engine performance by causing earlier boundary layer transition, increased boundary layer momentum loss (i.e., thickness), and/or flow separation. Roughness effects in the compressor and turbine are dependent on Reynolds number, roughness size, and to a lesser extent Mach number. At low Re, roughness can eliminate laminar separation bubbles (thus reducing loss) while at high Re (when the boundary layer is already turbulent), roughness can thicken the boundary layer to the point of separation (thus increasing loss). In the turbine, roughness has the added effect of augmenting convective heat transfer. While this is desirable in an internal turbine coolant channel, it is clearly undesirable on the external turbine surface. Recent advances in roughness modeling for computational fluid dynamics are also reviewed. The conclusion remains that considerable research is yet necessary to fully understand the role of roughness in gas turbines.
Article
The convective heat transfer and pressure drop characteristics of flow in corrugated channels have been experimentally investigated. Experiments were performed on channels of uniform wall temperature and of fixed corrugation ratio over a range of Reynolds number, 3220≤Re≤9420. The effects of channel spacing and phase shift variations on heat transfer and pressure drop are discussed. Results of corrugated channels flow showed a significant heat transfer enhancement accompanied by increased pressure drop penalty. The average heat transfer coefficient and pressure drop enhanced by a factor of 2.6 up to 3.2 and 1.9 to 2.6 relative to those for parallel plate channel, respectively, depending upon the spacing and phase shift. The friction factor increased with increasing channel spacing and its phase shift. The effect of spacing variations on heat transfer and friction factor was more pronounced than that of phase shift variation, especially at high Reynolds number. Comparing results of the tested channels by considering the flow area goodness factor (j/f), it was better for corrugated channel with spacing ratio, ɛ≤3.0 and of phase shift, Ø≤90°. Comparisons of the present data with those available in literature are presented and discussed.
Article
In modern industry, mass production has migrated to third world countries. To be competitive, European companies are forced to rapidly switch towards manufacturing of short series of customised products with added value. In European industry, a great effort has been made in order to customise products and give them an added value by developing new fabrication technologies. Additive layered manufacturing (ALM), also known as rapid manufacturing (RM), is a powerful tool that offers the necessary competitiveness to European companies. ALM comprises the use of layer-by-layer manufacturing in order to build a part by addition of material. Fabrication is performed directly from the 3D CAD model, which is sliced into layers that are printed one upon the other. Also known as free form fabrication, additive fabrication ‘unlocks’ design potential since part design obeys functionality, pushing the limits of manufacturability. In this paper, the authors review ALM technologies and the state-of-the-art of ALM applications in tooling, biomedicine and lightweight structures for the automotive and aerospace sectors. The authors present their experience in industrial application of additive fabrication through various industrial technology transfer projects made to transfer ALM technology to SMEs. Various case studies are presented and the achieved benefits of ALM are shown.
Build direction effects on additively manufactured channels
• J C Snyder
• C K Stimpson
• K A Thole
• D Mongillo
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