Design and fabrication of a scanning electron microscope using a finite element analysis for electron optical system

In this paper, we describe the design and fabrication of a thermionic scanning electron microscope (SEM) and examine its characteristics analytically. In the design process, the dimensions and capacity of the SEM components, such as the electron column, lenses, and apertures, were determined using finite element analysis. All components were integrated systematically during fabrication in order to achieve the maximum performance by adjusting the lens parameters, high voltage source, and image calibration methods. As a result, a thermionic SEM image with high resolution was achieved. We discuss the primary considerations required to achieve a high-performance image.     

Analysis of digitizing errors of a laser scanning system

The digitizing errors of a high-speed 3D laser scanning system are analyzed and characterized in this paper. As the laser scanner is an electro-optical device and based on the principle of optical triangulation, the measurement accuracy is affected by the measured part geometry and its position within the scanning window. Commercial laser scanners are often calibrated in the scanning plane to account for variation of the incident angle of the laser beam. The effects of the scan depth and the projected angle, characterizing the surface normal of the measured part external to the scanning plane, on the measurement accuracy are not considered in the standard calibration process and have been identified by experiments in the present work. Experimental results indicate that the random error of the scanned data is close to the nominal value provided by the manufacturer. The systematic error shows a bilinear relationship with the scan depth and the projected angle and has a maximum value of about 160 μm. The developed empirical model correctly predicts the systematic error with a maximum deviation of only 25 μm.     

Bundle adjustment of multi-position measurements using the Mahalanobis distance

Precision three-dimensional metrology frequently involves the measurement of common points by several three-dimensional measurement systems. These can be laser tracking interferometers, electronic theodolites, etc. A new analysis technique has been developed that fully takes into account the uncertainty ellipsoid for each measurement point. This technique solves for the positions and orientations of the measurement systems while determining the optimal fit to each physical point. No a priori knowledge of the location and orientation of the measurement systems is required. An initial estimate for the location and orientation of the measurement systems is derived from the measurement data by assuming equal uncertainties for each data point. Then a merit function is minimized to determine the optimized location and orientation of the measurement systems and the weighted mean for the position estimates of the physical points. This merit function is based on the Mahalanobis distance from multivariate statistics and takes into account the particular shape and orientation of the three-dimensional uncertainty ellipsoid of each data point from each measurement system. This technique can utilize data from differing types of three-dimensional measurement systems including distance only and angle only measurements, evaluate the “strength” of a measurement configuration and accommodate missing data points from some of the measurement systems.     

Emissivity estimation for high temperature radiation pyrometry on Ti–6Al–4V

The paper demonstrates a versatile procedure suitable for industrial implementation of temperature measurement on a hot titanium alloy. The driving force has been the need for an accurate temperature measurement during additive manufacturing using laser welding technology where Ti–6Al–4V-wire is melted. The challenges consider both industrial constraints and the varying emissivity of the surface. Measurements makes use of a narrow bandwidth spot radiation pyrometer and a calibration procedure for estimation of the surface temperature through spectral emissivity estimation. The theoretical results are validated through experiments. A number of difficulties in radiation temperature measurements for metals with varying surface properties are discussed; especially the case of surface oxidation. The uncertainty in temperature reading due to the uncertainty in the emissivity estimate is established along with a model that qualitatively describes surface oxidation. The procedure is expected to be useful for several manufacturing applications where it is important to control high temperatures.     

Erosion Debris Particle Observations and the Micromachining Mechanism of Erosion

Erosion debris particles produced by particle impact erosion of pure Ni and a stainless steel have been examined in the scanning electron microscope for the purpose of determining whether micro-machining is an operative mechanism of erosion by alumina particles. Macroscopic machining chips generally exhibit well-defined lamellae on the side of the chip away from the tool face, and such lamellae are also observed in micromachining chips produced by abrasion or scratch testing. The aspect ratio of such chips is generally large. In the present work, the aspect ratios and shapes of erosion debris particles formed at angles of incidence below the peak erosion angle (α_c) were generally consistent with the dimensions of the impact craters formed on the eroded surface and with the hypothesis that they were formed by micromachining. However, most of the debris particles did not exhibit characteristic lamellae. This may be explained by the fact that the surface from which they are formed is very rough even on a scale similar to the size of the debris particles. This is not true in abrasion: Micromachining chips formed from such a surface would be expected to have surfaces which would obscure the existence of lamellae. However, some chips would be expected to come from the few relatively smooth areas of the surface, and these should show lamellae. Examples of such chips were, indeed, found, and micrographs of these chips are nearly indistinguishable from micrographs of micromachining chips formed by abrasion or scratch tests. It is concluded that micromachining is an operative mechanism of erosion which is of greatest importance at low angles of incidence. Debris particles formed at higher angles of incidence are generally more platelike.     

An Experimental Study and Statistical Analysis of the Effect of Laser Pulse Energy on the Geometric Quality During Laser Precision Machining

In laser precision machining, process parameters have critical effects on the geometric quality of the machined parts. Due to the nature of the interrelated process parameters involved, an operator has to make a host of complex decisions, based on trial‐and‐error methods, to set the process control parameters related to the laser, workpiece material, and motion system. The objective of this work is to investigate experimentally the effect of laser pulse energy on the geometric quality of the machined parts in terms of accuracy, precision, and surface quality. Experimental study of formation of machined craters on thin copper foil with variation in laser pulse energy, the geometry and the surface topology of craters is presented. The geometrical parameters were measured and statistically analyzed with respect to incident pulse energy. Statistical analysis including pattern recognition was employed to analyze the experimental data systematically and to serve proper selection of the process parameters to achieve the desired geometric quality of the machined parts. Plausible trends in the crater geometry with respect to the laser pulse energy are discussed. The technique has been verified experimentally on simple geometrical features such as circles and grooves, and the geometric quality is evaluated.     

Microstructure and failure modes during scratch testing of laser cladded WC–NiCrBSi coatings with spherical and angular carbides

Abstract

Laser cladded coatings have been used extensively to extend the service life of components exposed to severe abrasive wear. One of the main wear resistant materials used in laser cladding is ceramic–metallic composite. Despite extensive use of this class of material, there is very limited knowledge regarding mechanical degradation mechanisms, such as cracking and plastic deformation, under different wear conditions. In this investigation a mixture of nickel alloy and tungsten carbide powders were used to deposit the coating. Two types of tungsten carbide powders with spherical and angular carbides were employed. The microstructures of the coatings were analysed thoroughly by optical microscopy, electron probe microanalysis and wavelength dispersive spectrometry. Failure and cracking mechanisms of laser cladded coatings under normal and tangential loading were systematically investigated using scratch testing. In the nickel alloy matrix, fine mixed secondary carbides were formed due to partial dissolution and formation of the secondary tungsten carbide during laser cladding. These secondary carbides were rich in chromium, tungsten and nickel and had a blocky and/or bar-like shape. Failure mechanisms associated with scratch testing were dependent on the microstructure and carbide morphology, applied stress and location of carbide particles with regard to the scratch groove. Owing to the high binder mean free path between the carbide particles, plastic deformation of the binder was the dominant failure mechanism. Additionally, partial or whole fragmentation of carbides, carbide/binder interface cracking and limited binder fracture were observed.     

316L粉末间接激光烧结的工艺参数对温度场的影响

提出了一种新的热物性参数计算方法,以有限元分析软件ANSYS为平台,利用参数化设计语言APDL语言编写程序,对316L不锈钢粉末与PA6粉末的混合粉末激光烧结的温度场进行了数值模拟.通过工艺参数的对比分析,找到了合适的工艺参数.研究了激光功率和扫描速度这两个工艺参数.对研究金属粉末间接法激光烧结的温度场具有参考价值.     

Water mapping in hydrated soft materials

We present a method based on spatially resolved electron energy-loss spectroscopy in the cryo-STEM to map the spatial distribution of water in frozen-hydrated polymers. The spatial resolution is limited by the dose constraints imposed by radiation damage, and to stay within these constraints, the use of fine electron-probe sizes comes at the cost of reduced counts in the energy-loss spectra. Thus, at the resolution limit, the detection of isolated water-rich pixels or the identification of minor variations in water content across the specimen is complicated because one must distinguish significant fluctuations from noise. Here we develop a criterion with which to guide such a distinction. We characterize the intrinsic noise associated with spectral measurements under given illumination and acquisition conditions. We then use that noise in combination with scatter diagrams to threshold spectrum images and objectively identify statistically significant compositional fluctuations. We illustrate these ideas using a simulated spectrum dataset for a hypothetical blend of hydrophilic and hydrophobic homopolymers. We show that while a direct inspection of the water map may not allow any meaningful conclusions to be drawn, after applying the thresholding approach we can clearly identify the regions of the specimen that are rich in water. We also experimentally study a model blend system comprised of hydrophilic poly(vinyl pyrrolidone) (PVP) dispersed in a hydrophobic matrix of poly(styrene) (PS). By MLS fitting using damaged and undamaged PVP reference spectra, we determine that the critical dose characteristic of dry PVP is approximately 8000 e/nm2 using 200 keV incident electrons. Irradiating frozen-hydrated PVP gives rise to noticeable hydrogen evolution at doses of approximately 1500 e/nm2. To stay within this constraint we use doses of 400 e/nm2 and a pixel spacing in the spectrum imaging of 100 nm. In order to quantitatively map the water, PVP, and PS compositions, we measure their total inelastic scattering cross-sections. Direct inspection of the composition maps reveals the presence of large water-rich domains of the order of approximately 1 microm and the scatter-diagram thresholding approach identifies small water-rich domains one pixel in size.     

Thermal stress developed during the laser cutting process: consideration of different materials

The laser cutting of metallic substrates results in the development of thermal stresses around the cut edges. Depending on the cutting speed, laser power intensity, and material properties, stress levels reaching and exceeding the yielding limit of the substrate material can result. In the present study, the laser cutting situation is simulated and temperature as well as thermal stress fields are computed for steel, Inconel 625, and Ti-6Al-4V alloy. The cutting speed of the laser is considered to be constant and a constant temperature heat source with a focused spot diameter is assumed along the kerf surface at the cut edge, resembling the laser heat source. The equations for energy and thermal stresses are solved numerically using the finite element method (FEM). It is found that the temperature decays sharply in the vicinity of the cut edges and that the equivalent stress attains high values in this region. Inconel 625 results in the highest thermal stress levels in the vicinity of the cut edges and is then followed by steel and titanium alloy.