Focus Variation Microscopy Research Articles

Adaptive Threshold Algorithm for Outlier Elimination in 3D Topography Data of Metal Additive Manufactured Surfaces Obtained from Focus Variation Microscopy

The topography of surfaces produced by metal additive manufacturing is a challenge for optical measurement systems such as focus variation microscopes. These irregularities can lead to artifacts, such as incorrectly measured protrusions or spikes, hampering reliable topographic characterization. In order to eliminate this problem, we introduce a new algorithm based on dual convolving a vertical Sobel operator with cross sections of an image stack parallel to the scanning direction of the so-called depth scan. This has proven beneficial in order to distinguish the focus region from out-of-focus areas where outliers are frequently detected. This paper introduces a method for deriving self-adaptive thresholds from the convolution result and compares the effects of different operators in creating self-adaptive thresholds. Additionally, a simulation model of focus variation microscopy is introduced to validate both the measuring system and the proposed algorithm, thereby enhancing the overall performance of focus variation microscopy. Finally, comparisons of measurement results on rough metal additive manufacturing workpieces with and without self-adaptive thresholds are discussed to demonstrate the algorithm’s effectiveness.The utilization of self-adaptive thresholds demonstrably reduces the uncertainty range in roughness parameter calculations. For example, in the case of an additive manufactured metal sample due to outlier elimination, the Sz roughness value reduces from 543 µm to 413 µm.

Investigation into the swelling and dissolution behaviour of Polymer-Excipient blends of PEO Utilising dissolution imaging

The use of dissolution imaging in analysing the behaviourof hydrophilic matrices and various types of excipients is examined in this study.The main aim was to investigate how different ratios of excipients with different solubility properties, such as lactose, microcrystalline cellulose, and dicalcium phosphate impact on the swelling properties and propranolol hydrochloride (PPN) release characteristics of polyethylene oxide matrix compacts. The surface properties of the compacts were investigated using a focus variation microscope after which dissolution studies were conducted to determine compact swelling and drug release properties. Smr2, a surface parameter representing the percentage of deeper valley structures on the surface, was used to calculate the proportion of the compact surface available for retaining lubrication (dissolution media in this case). Smr2 values of 83 and 84 were measured for the 1:1 and 1:3 PEO lactose compacts, respectively. This parameter utilised in this experiment gives an indication of the compact surface available for the initial hydration process and suggests a higher rate of hydration for the 1:1 and 1:3 PEO lactose compacts. The swelling studies revealed that a higher PEO ratio (3:1) resulted in more extensive gel layer formation as compared to the 1:3 compacts. All PEO:excipient compacts exhibited faster drug release than the compacts comprising PEO as the sole excipient. The quantity of PEO present was thus crucial in influencing the capacity of the matrix to control the release of PPN. This study underscores the potential for modifying drug release by altering the quantity of the matrix gel-former (PEO in this case) as well as the type or ratio of excipient used. The study also highlights the novelty of using UV dissolution imaging to image and quantify swelling and drug dissolution processes as well as providing qualitative observations such as channel formation which can support formulation optimisation and mechanistic understanding.

Chromatic focus variation microscopy for surface metrology

Optical metrology plays a vital role in a wide range of research and inspection areas in the industry. At present, the market offers a variety of optical metrology instruments, among which the focus variation microscope stands out for its capability of measuring steep surfaces with high slopes. The traditional focus variation (FV) instrument mechanically scans the surface by sweeping the focal plane of the objective lens using linear motion stages and simultaneously capturing images at different scanning positions, forming a stack of images. The mechanical motion stages require regular maintenance and calibration to ensure accuracy over time. Another issue associated with the mechanical scanning methods is their physical size, which creates a limiting factor to compactness for in-situ measurement applications. This work proposes a chromatic focus variation (CFV) method that replaces mechanical scanning with a wavelength scanning mechanism to overcome the above limitations. Unlike traditional focus variation, the CFV system employs a dispersive objective lens (i.e. chromatically aberrated objective lens) to axially shift the focus along the optical axis to provide vertical/depth scanning. This approach brings significant enhancements in measurement speed and reduces the instrument size for on-machine metrology tasks. In this paper, a detailed analysis of the optical performance of the dispersive objective lens is conducted, and then the measurement performance of the proposed CFV system is validated using samples including a step height of 30 µm and a sine wave shape with a peak-to-valley amplitude of 19 µm. The experiment results were compared to those from the state-of-the-art commercial instrument (Alicona G5), which showed a good agreement between the two. Furthermore, a detailed analysis and discussions are provided to investigate the measurement’s accuracy.

Active illumination mode with checkerboard pattern in focus variation microscopy: Analysis and application

Optical measurement methods for surface topography offer the advantages of high accuracy, rapid measurement, and non-destructiveness. Each method has its own suitable application scenarios. Among them, focus variation microscopy is extensively employed in precision manufacturing, aerospace, and medical industries due to its ability to measure rough and large slopes surfaces. However, since the measurement depends on local grayscale differences between focused and blurred images, it cannot measure surfaces with low reflectivity or insufficient texture information. In this work, we propose an active illumination mode for focus variation method that utilizes a digital micromirror device (DMD) to generate a checkerboard pattern. This method introduces additional texture information, resulting in a usable local gradient of image grayscale. Additionally, we analyze the selection criteria for the checkerboard pattern parameters, including the period and light-dark ratio. Furthermore, measurements of two standard steps with different heights demonstrate that the measurement repeatability of the proposed method can reach the nanometer level, rendering it suitable for high-precision measurements. More importantly, the measurement noise results indicate significantly superior performance of active illumination mode compared to the uniform illumination mode. Finally, we reconstruct the surface topography of the microchannels in a microfluidic chip through the encapsulation layer, demonstrating the feasibility of the proposed method.

Roughness measurement results evaluation of 6082 aluminium alloy specimens after fatigue bending tests

In this paper, the topography of 6082 aluminium alloy specimens after fatigue bending tests was studied with a comprehensive evaluation of measurement noise caused by vibration. Roughness results were acquired by contactless Focus Variation Microscopy (FVM). Studied data were pre-processed, removing the non-measured points and outliers with regular methods, respectively, and high-frequency noise was considered. The variations in ISO 25178 roughness parameters were studied. Based on the previous studies, it was found that surfaces after fatigue bending tests can be difficult to consider when analyzing the measurement noise in a selected bandwidth. Some advantages of profile data extraction in selected directions, like horizontal, vertical or crack, were found deficient, even in studies by various functions, like autocorrelation, power spectral density, or texture direction ratio. When noise suppression methods depend on the details studied, boundary areas were extracted to compare and highlight the presence of high-frequency data characteristics. The proposed method was validated when contrasting standardised Gaussian or median filtering techniques with the spline filtering approach. A proper filter for the reduction of vibrational noise from the results of FVM topography measurements was suggested based on the proposed procedure. Finally, it was proposed how use the new method for reducing errors caused by high-frequency measurement noise in the surface topography of specimens after fatigue bending tests.

Surface roughness reduction in electron beam powder bed fusion additive manufacturing of super duplex stainless steel 2507: investigating optimisation techniques and face orientation-dependent irregularities

AbstractThe use of additive manufacturing in metals by powder bed fusion via electron beam (PBF-EB) is increasing for fabricating high-quality parts meeting industrial standards. However, high surface roughness poses a consistent challenge in PBF-EB. This study investigates two novel approaches to optimise surface roughness for a given machine and powder combination. Using machine control software’s recently introduced research mode functionality, we develop customised beam control code to effectively explore a vast parameter space. Additionally, we explored the impact of beam travel direction and spot morphology on surface roughness. Line-melt-based contours were explored by specimen manufacturing with layer-wise parameter change, whilst spot-melting-based samples were built using a full factorial design of experiments with four factors at three levels. Initial sample characterisation was done using a stylus-based contact profilometer, followed by detailed evaluation using focus variation microscopy. Results reveal that increasing beam power and spot energy exacerbate surface roughness. We also find that a well-defined energy distribution at the spot's edge contributes to smoother surfaces. Whilst the influence of beam travel direction on surface roughness remains uncertain, our findings underscore the importance of parameter selection in achieving optimal results. By adjusting contouring parameters, we achieve a vertical roughness of Ra17.7 ± 0.9 (Sa 21.6), significantly lower than in the current literature. These findings advance our understanding of surface roughness optimisation in PBF-EB and offer practical insights for improving part quality in industrial applications. By harnessing tailored beam control strategies, manufacturers can enhance the capabilities of additive manufacturing technologies in producing metal components.

Characterization of surface asperities to understand its effect on fatigue life of electron beam powder bed fusion manufactured Ti-6Al-4 V

Surface asperities play a leading role in determining the fatigue life of as-built Ti-6Al-4 V components manufactured by electron beam powder bed fusion (PBF-EB). Several roughness parameters are available to characterize the surface asperities This study focuses on identifying the surface roughness parameter that correlates best with fatigue life. To this end, several fatigue test specimens were manufactured using the PBF-EB process and utilizing different contour melting strategies, thus producing as-built surfaces with varying roughness. The focus variation microscopy technique was employed to obtain surface roughness parameters for the as-built surfaces. Selected specimens were characterized using x-ray computed tomography (XCT). Tomography can detect surface-connected features obscured by other parts of the surface that are not visible through optical microscopy. The fatigue life of all specimens was determined using four-point bend testing. Through regression model analysis, maximum pit height (Sv) was identified as the statistically significant roughness parameter with the best fit affecting fatigue life. The fracture zone was closely inspected based on the data collected through XCT prior to fatigue tests. This led to another estimate of the worst-case value for the statistically significant roughness parameter Sv. The Sv parameter values obtained from optical microscopy and XCT were used as the initial crack size in a crack growth model to predict fatigue life. It is observed that life estimates based solely on optical measurements of Sv can be overly optimistic, a situation that must be avoided in predictive design calculations.

Fracture surface topography measurements analysis of low-alloyed corrosion resistant steel after bending-torsion fatigue tests

In this paper, an assessment of a topography measurement method for fracture surfaces of 10HNAP steel after bending-torsion fatigue tests was performed. Surface roughness was measured by using a non-contact Focus Variation Microscopy (FVM) technique in which the non-measured points (NMPs) and outliers (spikes) were removed by the application of general methods. The results revealed, that the optical measurement method introduced variations in the high-frequency errors, considered as noise within the selected bandwidth. Therefore, the minimization of the high-frequency noise (HFN) was proposed based on an extensive examination of ISO 25178 roughness parameters. Additionally, a general S-filter was applied, as recommended by international standards and commercial software. It was used to identify and remove noise from the measured data after pre-processing. Consequently, levelling and eliminating of NMPs and spikes was successfully performed. Subsequently, the results obtained by using various filters were compared to further assess the impact of different filtration bandwidths. Finally, the proposed procedure was validated by implementing different general functions, such as autocorrelation (ACF), power spectral densities (PSD), and texture direction (TD). It was concluded, that coupled characteristics, including profile and areal measurements, should be studied simultaneously since they are necessary to analyze the fracture surfaces comprehensively.

Investigating demoulding characteristics of material jetted rapid mould inserts for micro­injection moulding using in­line monitoring and surface metrology

PurposeThis study aims to investigate the demoulding characteristics of material-jetted rapid mould inserts having different surface textures for micro-injection moulding using in-line measurements and surface metrology.Design/methodology/approachMaterial-jetted inserts with the negative cavity of a circular test product were fabricated using different surface finishes and printing configurations, including glossy, matte and vertical settings. In-line measurements included the recording of demoulding forces at 10 kHz, which was necessary to capture the highly-dynamic characteristics. A robust data processing algorithm was used to extract reliable demoulding energies per moulding run. Thermal imaging captured surface temperatures on the inserts after demoulding. Off-line measurements, including focus variation microscopy and scanning electron microscopy, compared surface textures after a total of 60 moulding runs.FindingsA framework for capturing demoulding energies from material-jetted rapid tools was demonstrated and compared to the literature. Glossy surfaces resulted in significantly reduced demoulding forces compared to the industry standard steel moulds in the literature and their material-jetted counterparts. Minimal changes in the surface textures of the material-jetted inserts were found, which could potentially permit their prolonged usage. Significant correlations between surface temperatures and demoulding energies were demonstrated.Originality/valueThe research presented here addresses the very topical issue of demoulding characteristics of soft, rapid tools, which affect the quality of prototyped products and tool durability. This was done using state-of-the-art, high-speed sensing technologies in conjunction with surface metrology and their durability for the first time in the literature.

Extracting focus variation data from coherence scanning interferometric measurements

Coherence scanning interferometry (CSI), based on the principle of interference, can achieve sub-nanometer precision for height measurements. On the other hand, focus variation microscopy (FVM), combining the small depth of field of the objective, is a widely used surface topography measurement method suited to surface topography that is mostly optically rough. In this paper, we propose a method to simultaneously obtain the interferometric fringe data and focus variation FVM image stack, from a single vertical scanning process, using a CSI instrument without any hardware modifications. Using a 3D Fourier transform, the FVM signal, looks takes the form of a “bowtie” and the CSI signal resembles two “umbrellas” that are separated in 3D K-space. The signal is recovered using a 3D inverse Fourier transform and the surface topography can be determined by fusing the CSI and FVM signals. Since both signals come from the same instrument and scanning process, there is no need for coordinate registration and data interpolation during the data fusion process. Our method combines the features of CSI and FVM measurement, thereby improving the robustness and data coverage of the measurement. An all-in-focus surface topography map can also be generated using this method. This focusing feature has the potential to significantly improve the defect detection and quality control ability of CSI instruments.

Preprocessing method for robust topography reconstruction of surfaces of metal additive manufactured parts based on focus variation microscopy

Abstract When using an areal measuring optical instrument to measure rough surfaces, especially surfaces generated by metal additive manufacturing (e.g. laser and electron beam powder bed fusion), topographical artifacts such as spikes on a reconstructed surface are nearly unavoidable. These artifacts may affect the determination of surface roughness parameters and lead to erroneous surface features. This paper proposes a new preprocessing method to eliminate most artifacts before extracting surface heights of rough surfaces measured by focus variation microscopy. In this method, the axial region where a surface height value is located with the highest probability is estimated, based on datasets of planes parallel to the axial scanning direction. Results regarding height measurements with and without the preprocessing method are compared by measuring a Rubert Microsurf 329 comparator test panel for reference and workpieces produced by metal additive manufacturing.

Electromagnetic modeling of interference, confocal, and focus variation microscopy

Electromagnetic modeling of interference, confocal, and focus variation microscopy

An investigation into the mechanisms that influence laser sintered polyamide-12 top surfaces

Purpose The purpose of this paper is to elucidate the extent to which the mechanisms of polymer melt viscous flow and finish layer powder particle adhesion influence the top surface topographies of laser sintered polyamide (PA12) components. Design/methodology/approach Laser sintered specimens were manufactured at varying laser parameters in accordance with a full factorial design of experiments. Focus variation microscopy was used to ascertain insight into their top surface heights and peak/valley distributions. Subsequently, regression expressions were generated to model the former with respect to applied laser parameters. Auxiliary experimental analysis was also performed to validate the proposed mechanisms and statistical models. Findings Within the parameter range tested, this work found the root mean square (Sq) and skewness (Ssk) roughness responses of laser sintered PA12 top surfaces to be inversely related to one another, and both also principally influenced by beam spacing. Furthermore, it was demonstrated that using optimised laser parameters (to promote polymer melt dispersion) and building without finish layers (to avert subsequent powder particle adhesion) reduced the mean Sq roughness of resultant topographies by 30.8% and 47.9% relative to standard laser sintered PA12 top surfaces, respectively. Practical implications The scope to which laser sintered PA12 top surfaces can be modified was highlighted. Originality/value This research demonstrated the impact the mechanisms of polymer melt viscous flow and finish layer powder particle adhesion have on laser sintered PA12 top surfaces.

Evaluation of different surface topography measurement methodologies for characterizing weld beads in shipbuilding

The weld bead is crucial in identifying defects during laser welding, aiding in the evaluation of weld strength and quality. However, accurately monitoring and identifying defects with cameras can be challenging in the preassembly phase. This paper compares surface topography of laser weld beads using in-process Optical Coherence Tomography (OCT) and post-process Focus Variation Microscopy (FVM). Both techniques were employed on welded A36 structural steel plates to detect the surface defects. The weld profile was evaluated according to EN ISO 13919-1, and the quality level B was given to the surface topography measurement from both techniques. The correspondence between these two methodologies is imperative for implementing profile data in advanced monitoring systems. Thus, by reducing the delay between defect formation and inspection, this paper supports a more intelligent welding process, particularly in the shipbuilding industry.

Topography measurement methods evaluation for entire bending-fatigued fracture surfaces of specimens obtained by explosive welding

In this paper, the methods of compensation of differences in the results of entire bending-fatigued fracture surface topographies were presented. The roughness evaluation was performed with a focus variation microscope and confocal surface topography measurement techniques. The differences in the ISO 25178 roughness parameters were investigated and procedures for their compensation were studied. It was found that various types of optical measurements can cause differences in the errors occurring in the measurement process, such as outliers, and noise. The reduction of differences in the various optical roughness measurements can be attained when measurement errors are compensated. For this study, the applications of general procedures available in commercial software can be suitable for improvements of the roughness measurement results, such as raw data thresholding technique, digital filtering (S-filter), power spectral density, and autocorrelation function analyses. The validation of measurement techniques was proposed for areal and profile studies, including analysis of differences in the calculation areal ISO 25178 roughness parameters.

Multiscale Evaluation of Jaw Geometry Reproduction Obtained Via the Use of Selected Orthodontic Materials in Dental Implants and Orthodontics—In Vitro Case Study

In this paper, the multiscale analysis of the reproduction accuracy of jaw geometry obtained via the use of selected orthodontic materials is discussed. Impressions were made from two types of impression material. An accuracy assessment of the model geometry mapping was performed using noncontact systems, including a fringe projection optical 3D scanner, computed tomography, and a focus variation microscope. Measurements were made in three modes for comparison, as were the silicone and polyether impression materials. These modes were a jaw model and impression, an impression and plaster model, and plaster and jaw models. The research results are presented as colorful maps of deviations. Data analysis showed that deviations were the smallest in the case of silicone and that the best fit occurred between the silicone impression and the plaster model. The conducted research confirmed the validity of the assumptions considering the use of multiscale analysis for geometric analysis. The use of modern multiscale measurement methods allows for shorter and more efficient prosthetic operations. At present, these devices are expensive and complicated to use, but developments in technology should simplify the process, and prosthetic professionals should be aware of the possibilities described in the paper.

The effect of aviation anti-wear additives on tribofilm formation and micropitting propensity

This article studies the effect of anti-wear additives on micropitting and tribofilm formation. In particular, anti-wear additives used in aviation applications have been compared against a commonly used automotive anti-wear additive (zinc dialkyldithiophosphate ((ZDDP)) that is known to promote micropitting. All tests were performed under test conditions representative of the Power GearBox within Rolls-Royce's new engine, the UltraFan©. Tests using a micropitting rig found that ZDDP formed micropits the fastest, which then propagated onto the largest amount of wear showing the catastrophic effects of micropitting. Whereas other aviation anti-wear additives, such as tricresyl phosphate and additive X, formed micropits more slowly but faster than the formulation without additives containing only base oil. Focus variation microscopy was used to characterise the micropits and found that ZDDP formed smaller and shallower micropits than the other anti-wear additives, and as a result, the wear track of the ZDDP roller had the lowest roughness. The unadditised base oil, whilst it generated micropits more slowly, propagated quickly to a more severe failure mode, showing the harmful effects of having no anti-wear additive present. The profilometer results showed the counterface ring roughness for ZDDP remained the highest, confirming that ZDDP forms a tribofilm quickly. This protects asperities, which consequently promotes micropitting, as high localised asperity contact pressures are maintained. This was further confirmed by evaluating the tribofilms using a mini traction machine with spacer layer imaging to form tribofilms under representative conditions. This demonstrated that ZDDP forms a thicker tribofilm than the aviation anti-additives and at a faster rate. Overall, the study showed that the aviation anti-wear additives promote micropitting less than ZDDP but more than base oil alone. The slower action of the tribofilm formation allows the initial running-in of surfaces, which slows down the initiation of micropitting.

Feasibility of using bio-mimicking fish scale textures in LPBF for water drag-reducing surfaces

In this work, bio-mimicking fish scale textures are produced by LPBF and AlSi7Mg0.6 powder to reduce drag forces on nautical components. For this purpose, a surface texture inspired by the European bass skin was modelled and parametrized. Textures were applied over the external surface of purpose-designed specimens. Additive manufacturing quality of textures was assessed using focus variation microscopy to examine surface roughness as well as geometrical errors. Once the feasibility of producing the desired bio-mimicking surfaces was confirmed, the designed surface patterns were analysed in the computation fluid dynamics modelling environment. The behaviour of the surfaces was characterized in terms of drag force generated over a fixed dimension plate model. The most promising configuration was further investigated in a sensitivity analysis where variations in main stream velocity and in surface roughness are applied. Drag reduction was related to the lowering of the viscous component and was found to be in the order of 1–2%, with respect to a smooth surface, for free stream velocity of 2.5–5 m s−1 and average roughness smaller than the as-built condition. The results confirm that the modelled surfaces can be reproduced with sufficient geometrical fidelity, showing great promise for drag-reducing metallic components produced by additive manufacturing.

The effect of friction stir processing on the hydrogen susceptibility of AA5083 specimens after hydrogen cathodic charging

The present study investigated the effect of hydrogen on the mechanical degradation of friction stir processed (FSPed) 5083 aluminum alloy by intense hydrogen cathodic charging (HCC). The effect of different numbers of FSP passes was investigated: 3 and 8 passes, respectively. Hydrogen-charged and uncharged specimens were subjected to tensile testing and microhardness evaluation analysis, and were examined through optical microscopy, focus variation microscopy (FVM), and scanning electron microscopy (SEM) both on the microstructure and fracture zone. The results showed that the FSP process introduced a refined microstructure with finer grains. This led to an improved mechanical response during tension tests of the uncharged specimens; the energy absorption increased from 85 MJ/m3 of the base material to 94 MJ/m3 and 97 MJ/m3 for the 3 and 8 FSP passes, respectively. The introduction of hydrogen through the HCC process led to a more brittle mechanical response with a decrease in the energy absorption capability for all the charged specimens. The more prone specimen was the 8 FSP passes specimen where the energy absorption dropped by 20% and 71% for the two different charging current densities. The 3 FSP passes specimen presented a reduction of energy absorption of 4% and 18%, respectively, where the base material presented a reduction of 8% and 14%, respectively. This brittle response is also evident from the microhardness testing where the hydrogen charging led to increased surface hardness values. The 3 FSP passes specimen presented a better mechanical response with respect to the base material specimen (and the 8 FSP passes specimen) for all the charging conditions, and this led to the conclusion that a small number FSP surface modification could be a beneficial surface modification process as it improves the mechanical response of the material and is not significantly affected by hydrogen charging environments.

A Multiscale Topographical Surface Analysis of High Entropy Alloys Coatings by Laser Melting.

High Entropy Alloys (HEAs) coatings obtained by laser melting (LM) technique were studied through a multiscale topographical surface analysis using a focus variation microscope. The laser melting creates a multiscale topography from under-powder size (incomplete or complete powder melting) to upper-powder size (process conditions). The surface topography must be optimized because of the significant influence on friction and material transfer during sliding wear. The analyses were shown that different pre-melting zone interactions were present. Statistical analysis based on covariance analyses is allowed to highlight the different process melting scales. The best LM parameter values to minimize Surface Heterogeneity were laser power (Pw) of 55 W, laser exposition time (te) of 1750 µs, and distance between two pulses (dp) of 100 µm.