Focus Variation Microscopy Research Articles

A Novel 3D Topography Stitching Algorithm Based on Reflectance and Multimap

Surface topography is an efficient tool for the understanding of physical phenomena, especially if multiscale roughness analysis is performed. However, the observable scale range in a topography measured with 3D optical profilometers is quite limited. Therefore, all scales linked to a physical phenomenon might not be measured, which impedes the correct analysis of the surface. Stitching of 3D topographies, a technique combining elementary topographic maps into a larger one, can be used to increase the scale range for an objective lens. A high resolution over a large field of measurement topography is then generated. A literature review of 3D topography stitching algorithm highlights the stitching procedure, and detailed explanations on in-plane registration algorithms are provided. However, some existing 3D topography stitching algorithms are not sufficiently accurate for the registration of surface, especially at smaller scales. This paper proposes a new reflectance-based multimap 3D stitching algorithm and three of its variants. These algorithm variants are compared to three existing 3D stitching algorithms (geometric, cross-correlation and global optimization of differences) on four test cases, containing measured elementary topographic maps obtained on four surfaces and with four 3D optical profilometers (two focus variation microscopes and two interferometers). Five qualitative and quantitative criteria and indicators are proposed for the comparison of 3D topography stitching algorithms: visual inspection, run time, memory usage, mean repositioning error and stitching error estimator. Lastly, two quantitative indicators and criteria are new indicators proposed in this article. Overall, the new 3D stitching algorithms based on reflectance and multimaps have a lower mean repositioning error and stitching error estimator compared to other existing algorithms. This highlights the relevance of multimap stitching algorithms in the case of 3D topographies. A new decision-helping tool, the stitching gain lift plot (SGL plot), is described for the selection of the best stitching algorithm for a given test case. The SGL plot especially highlights the higher performance of two of the variants of the novel algorithm compared to the three existing 3D stitching algorithms.

CONCEPT FOR A RESEARCH METHODOLOGY OF SURFACE TOPOGRAPHY – TESTING AND ANALYSIS OF TRIBOLOGICAL WEAR TRACES

The methodology of surface topography research plays a key role in identifying and describing the wear mechanism, including the tribological wear traces. In the published literature, wear traces are most often described based on the surface morphology created in the operation process (microscopic image – optical microscope OM, scanning electron microscope SEM, atomic force microscope AFM); less frequently used are 2D profile description (contact or non-contact profilometer) or 3D topography (profilometer, confocal microscope, interference microscope, focus variation microscope). Each method of testing the worn surface has its own characteristics and purposefulness of use. The combination of different methods enables a comprehensive assessment of the surface topography created in the operation process and the characterisation of tribological wear traces. This work presents the main issues in the range of research methodology of the surface topography created in the operation process (operated/worn surface topography) – the purpose of research, research programme, research methods and tools, analysis of research results (quantitative – parametric and qualitative – nonparametric). The research methodology of the surface topography and tribological wear traces is presented on selected examples. The method of conducting a comprehensive analysis, including all elements – surface morphology, 2D profile analysis and 3D topographic analysis – is discussed.

Design of tool grinding processes for indexable inserts made of rocks

Using natural rocks as alternative cutting tool material poses a possibility to meet actual environmental, economic and geopolitical challenges. The present state of knowledge, however, is not sufficient to allow a knowledge-based design of the tool grinding process of cutting tools made of rock. For this reason, this study presents an investigation of the significance of the grinding process parameters and grinding tool specifications for the flank face and cutting edge roughness as well as for the cutting edge microgeometry besides an analysis of the scatter of the grinding results in tool grinding of rock inserts. Thus, the study contributes to a knowledge-based design of tool grinding processes of rock tools. In this context, confocal and focus variation microscopes are used besides SEM images to investigate the above mentioned factors in the plunge face grinding of rock inserts from five different rocks. The results identify the axial feed velocity of the plunge face grinding process as a highly significant influence factor for cutting edge roughness and microgeometry, while cutting speed only shows a significant influence on cutting edge microgeometry. Besides that, highly significant influences of the used rock type and the abrasive grain size are identified for all three mentioned factors. Grinding result analyses show a scatter between 0.04 and 25.00 µm depending on the parameter and rock investigated. Additionally, recommendations for the design of the tool grinding process of rock tools are presented deduced from the obtained results.

Understanding the effects of temporal waveform modulation of laser emission power in laser powder bed fusion: Part II - Experimental investigation

The laser powder bed fusion (LPBF) process has historically been operated with high-brilliance fibre laser sources with continuous wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high-frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This paper is the second part of an investigation on this topic, which aims to validate the analytical model proposed in the first part of the work. The LPBF system that was developed enabled the power emission profiles to be programmed during single-track depositions. Four different waveform shapes were tested (namely square wave, ramp up, ramp down and triangle wave) at different levels of waveform amplitude (ΔP= 200–400 W) and different frequencies (fw = 2–4–6–8 kHz) during the single-track deposition of stainless steel AISI316L. High-speed imaging acquisitions allowed the melt dynamics to be disclosed and the melt-oscillation frequency to be identified. Larger waveform amplitudes and waveforms with sudden variations of emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing ramp up and triangle waveforms with ΔP = 200. Melt-surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power, thus validating the analytical model of Part I, which correlated the melt-surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high-speed video observations. Three-dimensional reconstructions of the depositions via focus variation microscopy allowed the build rates and roughness of the single tracks to be determined. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission, and beneficial effects over the roughness were reported.

An integrated image processing approach for 3D scanning and micro-defect detection

Restoration of high-value components via additive manufacturing requires autonomous surface scanning and defect identification. The 3D free-form surface can be reconstructed with a point cloud obtained from the scanning. Laser line triangulation-based surface scanning is a promising method for generating a 3D point cloud of the component surface. In this article, a robotic defect scanning system developed using py_openshowvar, an open-source cross-platform communication interface is presented. For effective scanning of micro-scale features with minimal noise, it is crucial to optimize the scanning parameters. The scanner parameters such as exposure time and stand-off distance have been optimized for accurate feature detection. After selecting optimal scanning parameters, a generic algorithm is presented for generating a scanning path for automatic scanning of the 3D parts. Surfaces with pre-fabricated micro-defects are automatically scanned using this algorithm, and an integrated image-processing-based defect identification technique is presented. The geometries obtained from the presented technique were validated using focus variation microscopy, and the results are in good agreement with actual defect geometry, and the measurement error is below 9%.

Comparison of the contact and focus variation measurement methods in the process of surface topography evaluation of additively manufactured models with different geometry complexity

One of the key measurement parameters of the surface topography is the measurement area. It influences the possibility of using filters separating components of surface texture and thus determines the reliability of the obtained measurement results. The currently applicable standard does not define the size of the measuring area. To determine its size, fractal analysis was carried out in the article. The paper presents research on two types of geometry: simple geometry in the form of cylindrical and spherical surfaces and more complex geometry represented by free surfaces such as crowns and molars of teeth. In the process of making the research models, four 3D printing techniques were used: Fused Deposition Modeling (FDM), Melted and Extruded Modeling (MEM) Fused Filament Fabrication (FFF) and Material Jetting (MJ). 3D measurements of surface texture were made using a contact profilometer and a focus variation microscope. The analysis of topography images and selected parameters of the surface topography showed that the optical method gave better measurement results than the contact method. In the case of models made with the FDM and MEM techniques, similar values of the Sa parameter were obtained. Slightly smaller values of Sa were recorded for FFF models, while the highest for MJ models. Models made using the FFF method were also characterized by the lowest variability of results. Models made using the MJ method were characterized by relatively deep valleys in comparison with the other models, which was reflected in the Ssk and Svk parameters. The valleys counting from the top surface of the specimen were the shallowest for models made with the FFF method. Surfaces with simpler geometry were characterized by smaller variability of parameters values.

(Invited) 3-D Visualization of Li Metal Plating and Stripping with Operando Video Microscopy

In recent years, there has been an explosion of research into Li metal anodes for high-energy-density batteries. However, despite tremendous progress in the field, the reversible plating and stripping of Li has been hindered by the complex interplay between electrode morphology, surface chemistry, and mechanics. This has led to many empirical observations of improved performance, but our ability to rationally design solutions to solve the challenges of reversibility remain limited by our fundamental understanding of the complex electrodeposition and dissolution processes involved [1]. In an analogous example, Li plating on graphite anodes during fast-charging also plagues the Li-ion community [2], but fundamental studies of the complex nature of Li plating on intercalation electrodes is equally lacking.To address these challenges, in situ/operando analyses are of paramount importance to the community. However, given the dynamic nature of Li plating and stripping, challenges arise with respect to tradeoffs in spatial and temporal resolution. To address these challenges, we have recently integrated our optical visualization cells with a digital microscope capable of focus variation microscopy. This enables 3-D visualization of the electrode morphology with high temporal resolution, allowing for video capture of Li plating and stripping [2-3].In this talk, I will discuss the application of operando 3-D microscopy for visualization of both Li metal anodes and Li plating on graphite surfaces during fast-charging of Li-ion batteries. In the case of Li metal anodes, we observe significant anisotropy in the geometric shape of individual pits during stripping [3]. The nucleation density and anisotropy are shown to be strongly influenced by the surface microstructure and underlying crystallographic texture of the Li metal surface. As a results, pits can exhibit strong faceting, which influences the nature of nucleation at pit edges in subsequent cycles [4]. As a parallel example, we demonstrate 3-D visualization of Li plating on graphite electrodes during fast-charge, which illustrates the coupling between nucleation of Li plating and local heterogeneity in state-of-charge [2]. Overall, these results highlight the importance of both spatial and temporal heterogeneity in the nature of nucleation, growth, dissolution, pitting, and dead Li formationReferences[1] K. N. Wood, M. Noked, N. P. Dasgupta, ACS Energy Lett. 2, 664 (2017).[2] Y. Chen, K.-H. Chen, A. J. Sanchez, E. Kazyak, V. Goel, Y. Gorlin, J. Christensen, K. Thornton, N. P. Dasgupta, J. Mater Chem. A 9, 23522 (2021).[3] A. J. Sanchez, E. Kazyak, Y. Chen, J. Lasso, N. P. Dasgupta, J. Mater Chem. A. 9, 21013 (2021).[4] A. J. Sanchez, E. Kazyak, Y. Chen, K.-H. Chen, E. R. Pattison, N. P. Dasgupta, ACS Energy Lett. 5, 994 (2020).

Surface topography data fusion of additive manufacturing based on confocal and focus variation microscopy.

In this paper, two innovative data fusion methods are proposed for reconstructing the surfaces produced by directed energy deposition (DED) additive manufacturing. The surface topographic data were obtained from confocal laser scanning microscopy (CLSM) and focus variation microscopy (FV). The first method (competitive data fusion) aims to improve the data quality by combining both the advantages of the CLSM and FV techniques, while the second method (cooperative data integration) is designed for generating a single representation that contains not only global information but also local details. The results show that both fusion methods achieved satisfactory results: in the competitive fusion, the fused data preserved the characteristics of FV data while its vertical resolution is also improved by integrating the short waves from the CLSM data; the cooperative data fusion achieved one pixel precision of the surface registration which adopted the feature-based registration method with the help of color image information. The computational complexity is reduced from O((m×n)2) to O(m×n + k). Both proposed data fusion methods provided innovative solutions for the microscopic surface reconstruction and surface representation in multiscales in the field of additive manufacturing.

Outlier Elimination in Rough Surface Profilometry with Focus Variation Microscopy

Rough surfaces such as metal additive manufactured surfaces are quite challenging for measurement. Artifacts caused by irregular and difficult-to-measure geometries are inevitable. Removing all the artifacts would cause a portion of surface information to be missing. Different from previous works, the postprocessing in this paper includes an additional step to eliminate artifacts based on autocorrelation functions of particular subimages instead of simply removing them. This increases the accuracy with respect to surface roughness and provides a more comprehensive view on the topography. In addition, a dome shape LED array ring light is proposed to provide all-round lighting due to the high degree of irregularity of workpiece surfaces. The experimental results obtained from FVM are validated and compared with the given roughness values of a Rubert Microsurf 329 comparator test panel as well as measurement results of a metal additive workpiece by a confocal microscope.

A dimensional assessment of small features and lattice structures manufactured by laser powder bed fusion

The understanding of dimensional variations produced by laser powder bed fusion is critical in components with small features and with dimensions close to the inherent limits of the process. In this context, two reference geometries were used: (a) straight walls to quantify dimensional relative error for small features and (b) a latticed neck region of a fatigued specimen (cubic and hexagonal cell design, with design strut sizes of 250 µm, 500 µm, and 1000 µm in cell size). Samples were fabricated out of AISI 316L stainless steel powder with different building orientations. The metrology techniques used were the following: focus variation microscopy, optical microscopy and micro-computed tomography. The straight wall characterization shows that built orientation does not influence dimensional relative error for walls with less than 750 µm. Acceptable dimensional relative errors (~ 2% to ~ 15%) are achieved only in walls with 750 µm in width of more. For lattice structures, the fine struts (250 µm) show a significant level of dimensional relative error (~ 5% to ~ 25%). This additive manufacturing process delivers more consistent dimensions for coarse struts (500 µm), with relative errors between ~ 2% and ~ 4%. All metrology techniques showed the same trends in terms of capturing the dimensional variations for fine and coarse struts.

Effect of Surface Texture on the Structural Adhesive Joining Properties of Aluminum 7075 and TEPEX®.

In the process of continuous improvement of manufacturing processes, this study was developed within the framework of the Ecovoss project, based on the inclusion of lightweight and new materials parts in the automotive sector. The objective was based on the replacement of aluminum welding operations with the option of adhesive operations with other types of materials such as polyamides or, in this case, a TEPEX® composite material (Dynalite 202-c200/50% TYP 13). The aim of this work is to test the best texturing of substrate made in 7075 aluminum specimens manufactured by robotic polishing with an ABB 6640 robot. Another substrate is TEPEX composite. A structural adhesive film AF-163-2 from the 3M company (St Paul, MN, USA) is used, which must be applied according to the manufacturing procedure. The tests carried out are based on the topographic measurement of the surfaces to be joined with an Alicona focus variation microscope, and the uniaxial shear tests of adhesive samples have been analyzed. The texture of the surface failure has been analyzed, and the results confirm a significant correlation between the texture parameters of initial surfaces and maximum shear stress. The expected results should provide a better understanding of the surfaces aimed to optimize the adhesion of the studied materials.

Discharge Energy as a Key Contributing Factor Determining Microgeometry of Aluminum Samples Created by Electrical Discharge Machining

The aim of this study is first to determine the effect of the discharge energy on the surface microgeometry of aluminum samples created by electrical discharge machining (EDM). Secondly, an additional purpose is to demonstrate the differences between the geometric multiscale methods: length-, area-scale, and curvature. Eleven samples were manufactured using discharge energies ranging from 0.486 mJ to 1389.18 mJ and, subsequently, measured with focus variation microscopy. Standard ISO and multiscale parameters were calculated and used for surface discrimination and regression analysis. The results of linear, logarithmic, and exponential regression analyses revealed a strong correlation (R2 > 0.9) between the geometrical features of the surface topography and the discharge energy. The approach presented in this paper shows that it is possible to shape surface microgeometry by changing the energy of electrical discharges, and these dependencies are visible in various scales of observation. The similarities of the results produced by curvature and length-scale methods were observed, despite the significant differences in the essence of those methods.

Non-Contact Multiscale Analysis of a DPP 3D-Printed Injection Die for Investment Casting

The investment casting method supported with 3D-printing technology, allows the production of unit castings or prototypes with properties most similar to those of final products. Due to the complexity of the process, it is very important to control the dimensions in the initial stages of the process. This paper presents a comparison of non-contact measurement systems applied for testing of photopolymer 3D-printed injection die used in investment casting. Due to the required high quality of the surface parameters, the authors decided to use the DPP (Daylight Polymer Printing) 3D-printing technology to produce an analyzed injection die. The X-ray CT, Structured blue-light scanner and focus variation microscope measurement techniques were used to avoid any additional damages to the injection die that may arise during the measurement. The main objective of the research was to analyze the possibility of using non-contact measurement systems as a tool for analyzing the quality of the surface of a 3D-printed injection die. Dimensional accuracy analysis, form and position deviations, defect detection, and comparison with a CAD model were carried out.

Development of a compact focus variation microscopy sensor for on-machine surface topography measurement

On-machine areal surface topography measuring instruments are required for fast and accurate measurement of parts inside production machines without reducing production rates. This paper presents the design and development of a compact focus variation microscopy sensor that can be integrated into various types of machine tools. The paper focuses on the development of the linear stage of the sensor, which was the major engineering challenge. The overall developed sensor has dimensions of 78 mm diameter and 200 mm length, with a 20 mm travel range. Simulations of tolerance stack-ups for the sensor assembly were performed before the manufacturing of the sensor’s linear motion components to assure they can be appropriately assembled. The linear motion accuracy of the sensor is 2 μm, calibrated using laser interferometry. From measurement in a controlled laboratory, the measurement noise of the sensor is 0.4 μm. Finally, demonstrations of calibrated artefact measurements with the sensor are presented. A single image field measurement with the sensor requires less than 20 s.

Internal surface roughness measurement of metal additively manufactured samples via x-ray CT: the influence of surrounding material thickness

X-ray computed tomography (XCT) can be used to measure the internal and external surfaces of an object non-destructively and with micron-level spatial resolution. XCT is therefore an appealing method for measuring and characterising the internal surface roughness of additively manufactured parts that cannot be accessed by traditional tactile and optical surface roughness instruments. In this work, an additively manufactured aluminium spherical surface roughness sample is designed, fabricated and its surface roughness measured via a focus variation microscope, the sample is then XCT scanned when embedded in varying thicknesses of surrounding material. A quantitative and qualitative comparison between the optical and XCT surface roughness measurements is made; the results show that the Sa of the XCT-based surface roughness measurements increases as a function of surrounding material thickness.

Three-dimensional transfer function of optical microscopes in reflection mode.

Three-dimensional (3D) transfer functions build the basis for a comprehensive characterization of optical imaging systems in the spatial frequency domain. Utilizing the projection-slice theorem, the 2D modulation transfer function of an incoherent imaging system can be derived from a 3D transfer function by integration with respect to the axial spatial frequency. For a diffraction limited microscope with homogeneous incoherent pupil illumination, the modulation transfer function equals the 2D autocorrelation function of a circular disc. However, until now to the best of our knowledge no 3D transfer function has been published, which exactly leads to the 2D modulation transfer function of a diffraction limited microscope in reflection mode. In this article, we derive a formula, which after integration with respect to the axial spatial frequency coordinate perfectly fits to the diffraction limited 2D modulation transfer function. The inverse three-dimensional Fourier transform of the 3D transfer function results in a complex-valued 3D point spread function, from which the depth of field, the lateral resolution and, in addition, the corresponding 3D point spread function of both, a conventional and an interference microscope, can beobtained.

Contact angle analysis of surface topographies created by electric discharge machining

The objective of this work is to investigate the relationship between wettability and surface microgeometry of 6060 aluminum alloy created by electric discharge machining (EDM). The analyzes are enriched with the calculation of the correlation strengths between contact angles and geometric characteristics of surface topographies changing with the scale of observations. Surface topographies are measured by focus variation microscope and characterized with ISO parameters, geometric motifs and multiscale curvature tensor analysis. The wettability of the finished structures is investigated indirectly by contact angle measurements. The analysis were carried out using aqueous solutions of the semi-synthetic polymer, with different concentration and viscosity. The study showed that wetting behavior of aluminum alloy can be easily controlled through appropriate EDM finishing.

Tissue characterization of the medical fungus Hericium coralloides by focus-variation microscopy

ABSTRACT In this study, the potential of focus-variation microscopic imaging was evaluated in a study of morphological patterns of the potential medicinal fungus Hericium coralloides (Basidiomycota). We created three-dimensional reconstructions and visualizations using the imaging technique on a fresh H. coralloides basidioma. The aim was to approximate the spore dispersal efficiency of this basidiomata type regarding the investment of tissue biomass and its reproductive output (production of basidiospores). Results were correlated with published data gained from magnetic resonance imaging and micro-computed tomography. It is demonstrated that focus-variation microscopic imaging results in a more distinct picture of the morphology of the edible and potentially medicinal H. coralloides basidiomata. However, a direct measurement of spore production was not possible. Spore production could only be estimated in combination with a mathematical model because the surface was not directly measurable due to the cellular heterogeneity. However, focus-variation microscopic imaging allows a better and faster estimation of spore production compared with the published methods. Furthermore, it was found that a scanning resolution of 5× is sufficient for determining the fungal surface precisely because at a higher resolution artifacts occur, resulting in adulteration of the image.

Prediction of Extreme Value Areal Parameters in Laser Powder Bed Fusion of Nickel Superalloy 625

Important to the success of additive manufacturing (AM) is the ability to inspect and qualify parts. The research community is pushing to identify correlations between part function and surface topography, yet little guidance specific to AM surface measurement exists. Thus, development of inspection methods for surface finish are Required. In laser powder bed fusion (LPBF) AM, parts are built through a complex process with many variables, and the length scales of interest on the surface cover a wide range. Full characterization of the surface is time consuming and costly as high resolution in surface measurements decreases field-of-view (FoV), requiring stitching multiple FoVs to cover large areas. Statistical methods exist to estimate the maximum value based on a sample of FoVs, but are not yet commonplace in AM surface measurement. The goal of this work is to understand the use of these statistical methods in the estimation of maximum area valley depth (Sv) of a surface, an extreme value parameter, for which researchers have already found relationship to fatigue life. This work also investigates the effect of microscope objective, measurement region size, and nesting index of areal filters on Sv. A large (i.e., greater than 40 mm × 40 mm) planar LPBF surface is fabricated in nickel superalloy 625 and measured using a focus variation microscope with a 10x objective and again with a 20x objective. Results show that there is little difference in the maximum value of Sv between the two objectives, but the nesting index does have some effect. Results also show that a Type 1 Generalized Extreme Value, or Gumbel, distribution can be used to accurately estimate the maximum value of Sv for a surface from a small set of measurements, providing a framework for users to develop inspection routines that balance measurement time and accuracy of estimation.

Wear performance and mechanisms of PCBN tools in boring of powder metallurgy steels

Wear performance and mechanisms of PCBN tools with different CBN content (80% vol. and 95% vol.) and cutting edge geometry (honed edge radius, 0.05 mm chamfered edge width, and 0.10 mm chamfered edge width) in boring powder metallurgy (PM) steels had been evaluated. Post-cutting examination of the worn cutting inserts was performed using focus variation microscopy (FVM), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The advanced three-dimensional wear characterization parameters WRM, WMD, WAM, WMH, and WAA are used to measure the wear of PCBN tools. A novel characterization method of removed material (η1 = WMD/WRM) and adhered material (η2 = WMH/WAM) was used to measure the trend of tool wear in this paper. With the increasing of CBN content, the PCBN tool wear parameters WRM, WMD, and WAA were decreased; adhesion was increased by WAM and WMH parameter characterization. For the variation of cutting edge geometry, chamfered edge showed a higher tool life and better wear performance than honed edge radius. With the increasing of chamfered edge width from 0.05 to 0.10 mm, the three-dimensional wear WRM, WMD, WAM, and WMH were increased. WAA showed an opposite trend and parameters η1 and η2 showed a not convergence trend. Besides typical wear mechanisms such as diffusion, adhesion, abrasive, microchipping was observed on PCBN tool; the tool protection layer (TPL) formation and peeling and impact load could be the reasons to explain the microchipping that occurred on cutting edge. Tool life is measured by surface roughness and calculated according to the number of workpieces processed. The results showed that PCBN tool with 95% vol. CBN content and 0.10 mm chamfered edge width achieves higher tool life.