Laser cutting of thick sheet metals: Residual stress analysis

Laser cutting of tailored blanks from a thick mild steel sheet is considered. Temperature and stress field in the cutting sections are modeled using the finite element method. The residual stress developed in the cutting section is determined using the X-ray diffraction (XRD) technique and is compared with the predictions. The structural and morphological changes in the cut section are examined using the optical microscopy and scanning electron microscopy (SEM). It is found that temperature and von Mises stress increase sharply in the cutting section, particularly in the direction normal to the cutting direction. The residual stress remains high in the region close to the cutting section.     

Laser cutting of rectangular geometry into alumina tiles

Laser cutting of rectangular geometry into the 5 mm thick alumina tiles is carried out. Temperature and stress fields, which are developed during the cutting process, are simulated in line with the experimental conditions. The morphological changes in the cutting sections are examined using optical and electron scanning microscopes, energy dispersive spectroscopy, and X-ray diffraction technique. The predictions of surface temperature and the residual stress are validated through the experimental data. It is found that von Mises stress attains high values in the region of the mid-thickness of the workpiece. The laser cut sections are free from major cracks and large scale sideways burning. The predictions of surface temperature and residual stress agree well with their counterparts obtained from the experiment.     

Laser cutting of Kevlar laminates and thermal stress formed at cutting sections

Laser cutting of Kevlar laminates is carried out and thermal stress field developed in the cutting region is predicted using the finite element code. Temperature predictions are validated through the thermocouple data. The morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that temperature predictions agree well with the thermocouple data. High values of von Mises stress are observed at the cutting edges and at the mid-thickness of the Kevlar laminate due to thermal compression formed in this region. The laser cut edges are free from whiskers; however, striation formation and some small sideways burning is observed at the kerf edges.     

Laser cutting of holes in thick sheet metals: Development of stress field

Laser cutting of hole in a mild steel thick sheet metal is investigated. Temperature and stress fields developed around the cutting section are simulated using the finite element method. An experimental is carried out accommodating the simulation parameters. The residual stress developed in the cutting section is measured using the XRD technique and findings are compared with the predictions. Optical microscopy and SEM are carried out to examine the morphological changes in the cutting sections. It is found that temperature decays sharply in the region of the laser heat source, which results in high temperature gradient in this region. This causes the development of high stress levels around the cut edges. The residual stresses predicted are in agreement with the measured results.     

Laser cutting of rectangular geometry into aluminum alloy: Effect of cut sizes on thermal stress field

Laser cutting of a rectangular geometry into aluminum alloy 2024 is carried out. Temperature and stress fields are predicted in the cutting section using the ABAQUS finite element code in line with the experimental conditions. Effect of the size of the rectangular geometry on the thermal stress fields is examined in the cutting section. Temperature predictions are validated through the thermocouple data. To identify the morphological changes in the cutting section, an experiment is carried out and the resulting cutting sections are examined under optical and scanning electron microscopes. It is found that temperature and stress fields are affected by the size of the rectangular cut geometry. Temperature and von Mises stress attains higher values for small size rectangular geometry as compared to its counterpart corresponding to the large size geometry. Laser cut sections are free from large size asperities including sideways burning and out-off flatness at the cut edges. Locally scattered some small dross attachments are observed at the kerf exit.     

Laser welding of low carbon steel and thermal stress analysis

Laser welding of mild steel sheets is carried out under nitrogen assisting gas ambient. Temperature and stress fields are computed in the welding region through the finite element method. The residual stress developed in the welding region is measured using the XRD technique and the results are compared with the predictions. Optical microscopy and the SEM are used for the metallurgical examination of the welding sites. It is found that von Mises stress attains high values in the cooling cycle after the solidification of the molten regions. The residual stress predicted agreed well with the XRD results.     

Laser control melting of alumina surfaces and thermal stress analysis

Laser gas assisted melting of alumina surface is carried out and temperature as well as stress fields developed in the irradiated region are predicted using the finite element method (FEM). An experiment is conducted resembling the simulation conditions. Optical and scanning electron microscope (SEM) are used to examine the morphological and the metallurgical changes in the laser treated region. The X-ray diffraction (XRD) technique is used to determine the residual stress developed in the irradiated region. It is found that the residual stress predicted agreed with the measurement result. High heating and cooling rates result in high von Mises stress levels in the surface region.     

Laser hole cutting into bronze: Thermal stress analysis

Laser hole cutting in bronze is carried out and the thermal stress formed in the cutting section is examined using a finite element code. The cut geometry and microstructural changes in the cutting section are examined using optical microscope, scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). It is found that the high conductivity of bronze increases the cooling rates within the cutting section, which influences the thermal stress field in the cutting region. The residual stress predicted is in the order of 200 MPa within the vicinity of the hole circumference. The striation pattern at the kerf surface changes towards the hole exit, which is associated with the drag forces developed in this region.     

管道连接装配型ITER增强热负荷第一壁的热工水力分析

对管道连接装配型ITER增强热负荷第一壁(EHF FW)的标准手指对、边缘手指对及中心梁(CB)开展了热工水力分析.分析结果显示流速分布合理,低于1m·s-1或高于10m·s-1的区域非常小;手指对间的流量分配较合理;整个第一壁的压降为0.351MPa,小于0.4MPa的限定值;在8个循环周期内CB最高温度为409.2...     

Dangling bond modulating the electronic and magnetic properties of zigzag SiGe nanoribbon

First-principles nonmagnetic calculations reveal a metallic character in zigzag SiGe nanoribbons (ZSiGeNRs) regardless of their width. The partial DOS projected onto the Si and Ge atoms of ZSiGeNR shows that a sharp peak at the Fermi level is derived from the edge Si and Ge atoms. The charge density contours show the Si–Ge bond is covalent bond, while for the Si–H bond and Ge–H bond, the valence charges are strongly accumulated around H atoms due to their stronger 1 s potential and the higher electronegativity of 2.20 than that of 1.90 for Si atom and 2.01 for Ge atom, so that a significant charge transformation from Si or Ge atoms to H atoms and thus an ionic binding feature. Spin–polarization calculations show that the band structures of ZSiGeNR are modified by the dangling bonds. Compared with perfect ZSiGeNR which is a ferrimagnetic semiconductor, the bands of the ZSiGeNRs with bare Si edge, bare Ge edge, and bare Si and Ge edges shift up and nearly flat extra bands appear at the Fermi level. The ZSiGeNR with bare Si edge or bare Ge edge is a ferrimagnetic metal, while ZSiGeNR with bare Si and Ge edges is a nonmagnetic metal.     

Influence of boundary stress singularities on the vibration of clamped and simply-supported sectorial plates with arbitrary radial edge conditions

This paper extends previous studies made for sectorial plates having re-entrant (i.e., interior) corners causing stress singularities, to provide accurate frequencies when the circular edge is either clamped or simply-supported. An extensive review of the literature is also given herein spanning nearly the past two decades explaining the free vibration characteristics of sectorial plates. In this work, the classical Ritz method is employed with two sets of admissible functions assumed for the transverse vibratory displacements. These sets include: (1) mathematically complete algebraic-trigonometric polynomials which guarantee convergence to exact frequencies as sufficient terms are retained and (2) corner functions which account for the bending moment singularities at the re-entrant vertex corner of the radial edges having arbitrary edge conditions. Extensive convergence studies summarized herein confirm that the corner functions substantially enhance the convergence and accuracy of non-dimensional frequencies for sectorial plates having either a clamped or hinged circumferential edge and various combinations of clamped, hinged, and free conditions on the radial edges. Accurate (to at least four significant figure) frequencies and normalized contours of the transverse vibratory displacement are presented for the spectra of sector angles [90°, 180° (semi-circular), 270°, 300°, 330°, 350°, 355°, 360° (complete circular)] causing a re-entrant vertex corner of the radial edges. For sector angles of 360°, a clamped-clamped, clamped-hinged, clamped-free, hinged-free or free-free radial crack ensues. One general observation is the substantial reduction in the first six frequencies as the sector angle increases for all plates, except in the first two modes of plates having free-free radial edges.     

Laser cutting of steel and thermal stress development

In laser cutting of sheet metals, thermal stresses are developed in the region of the cutting section. Depending on the cutting conditions and substrate material properties, the thermal stress levels can attain high values. In the present study, thermal stress developed in the region of the laser cut edges is modeled and temperature as well as stress fields are predicted. Temperature predictions are validated through the experimental results. It was found that the temporal variation of the maximum temperature along y-axis follows the laser heating source. However, temporal variation of von-Mises stress deviates slightly from the temporal variation of temperature along the cutting direction. Increase in scanning speed enhances the von-Mises stress levels due to the attainment of high temperature gradients in the substrate material.     

Coulomb attraction during the carpet growth mode of NaCl

The submonolayer growth of NaCl bilayer high-rectangular shaped islands on Ag(111) is investigated at around room temperature by using low temperature scanning tunneling microscopy. The growth at the step edges is preferred. Two kinds of islands are observed. They either grow with their non-polar edge at the step edge of Ag(111) or the islands overgrow in a carpet-like mode with the polar direction parallel to the edge. In the latter case, the Ag step is rearranged and considerable, while the NaCl layer is bent. This study clarifies the nature of the interaction of an alkali halide nanostructure with a metal step edge.     

气相爆轰波在分叉管中传播现象的数值研究

数值研究气相爆轰波在分叉管中的传播现象.用二阶附加半隐龙格-库塔法和5阶WENO格式求解二维欧拉方程,用基元反应描述爆轰化学反应过程,得到了密度、压力、温度、典型组元质量分数场及数值胞格结构和爆轰波平均速度.结果表明:气相爆轰波在分叉管中传播,分叉口左尖点的稀疏波导致诱导激波后压力、温度急剧下降,诱导激波和化学反应区分离,爆轰波衰减为爆燃波(即爆轰熄灭).分离后的诱导激波在垂直支管右壁面反射,并导致二次起爆.畸变的诱导激波在水平和垂直支管中均发生马赫反射.分叉口上游均匀胞格区和分叉口附近大胞格区的边界不是直线,其起点通常位于分叉口左尖点上游或恰在左尖点.水平支管中马赫反射三波点迹线始于右尖点下游.分叉口左尖点附近的流场中出现了复杂的旋涡结构、未反应区及激波与旋涡作用.旋涡加速了未反应区的化学反应速率.反射激波与旋涡作用并使旋涡破碎.反射激波与未反应区作用,加速其反应消耗,并形成一个内嵌的射流.数值计算得到的波系演变和胞格结构与实验定性一致.     

Carbon nanosheets by microwave plasma enhanced chemical vapor deposition in CH4-Ar system

We employ a new gas mixture of CH₄-Ar to fabricate carbon nanosheets by microwave plasma enhanced chemical vapor deposition at the growth temperature of less than 500 °C. The catalyst-free nanosheets possess flower-like structures with a large amount of sharp edges, which consist of a few layers of graphene sheets according to the observation by transmission electron microscopy. These high-quality carbon nanosheets demonstrated a faster electron transfer between the electrolyte and the nanosheet surface, due to their edge defects and graphene structures.     

Atomic-scale friction on stepped surfaces of ionic crystals

We report on high-resolution friction force microscopy on a stepped NaCl(001) surface in ultrahigh vacuum. The measurements were performed on single cleavage step edges. When blunt tips are used, friction is found to increase while scanning both up and down a step edge. With atomically sharp tips, friction still increases upwards, but it decreases and even changes sign downwards. Our observations extend previous results obtained without resolving atomic features and are associated with the competition between the Schw?bel barrier and the asymmetric potential well accompanying the step edges.     

STM/STS investigation of edge structure in epitaxial graphene

In this paper, we have used low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study zigzag or armchair edges of epitaxial graphene on 6H-SiC (0001). The monolayer carbon structures exhibit occasionally one-dimensional ridge (1D) in close vicinity to step edge. This ridge exhibits different edges orientations in armchair–zigzag transition which give rise to different local density of states (LDOS) along this 1D structure. This ridge formation is likely explained by residual compressive in-plane stresses.     

Beam damage by the induced electric field in transmission electron microscopy

Electric fields can be induced by electron irradiation of insulating thin film materials. In this work, the electric fields under a broad beam illumination in transmission electron microscopy (TEM) are analyzed for insulating samples. Some damage phenomena observed can be interpreted by the mechanism of damage by the induced electric field (DIEF). For broad-beam illumination in an ultra-thin specimen, the electric field near the center of the illumination may not be strong, but at the periphery of the illumination the electric field can be significant. Therefore, damage may be easily observed in these regions rather than at the center of the illumination. For a beam which is broad compared to the specimen thickness, e.g. 100 ∼ 1000 nm, a strong electric field pointing inward into the specimen near the surface region may result in cation diffusion into the specimen and/or anion diffusion out to the surface region. Meanwhile, a strong electric field perpendicular to the beam direction near the edge of the illumination may attract anions into the illuminated region, but eject cations to the periphery. For a wedge-shaped specimen, the electric field points inward into thicker region, driving cations toward the thicker region, while attracting anions to the edge region. On the sharp edge, a strong electric field pointing outward may be responsible for the edge-smoothing effect observed in insulating materials.     

Edge-and strain-induced band bending in bilayer-monolayer Pb_2Se_3 heterostructures

By using scanning tunneling microscope/microscopy(STM/STS), we reveal the detailed electronic structures around the sharp edges and strained terraces of lateral monolayer-bilayer Pd₂Se₃ heterostructures. We find that the edges of such heterostructures are well-defined zigzag type. Band bending and alignment are observed across the zigzag edge, forming a monolayer-bilayer heterojunction. In addition, an n-type band bending is induced by strain on a confined bilayer Pd₂Se₃ terrace. These results provide effective toolsets to tune the band structures in Pd₂Se₃-based heterostructures and devices.     

Microstructure, hardness and stress in melted zone of 42CrMo steel by wide-band laser surface melting

Using laser surface melting (LSM) of a roller, to obtain the desired distribution of the microstructure, hardness and residual stresses with minimum distortion, is essential in order to improve machining efficiency and to achieve reliable service performance. In this study, a 3D finite element model has been developed to simulate the wide-band LSM process and predict the thermal and mechanical properties in the melted zone. The microstructure evolution, hardness distribution and stress field in the melted zone with different laser power were simulated. With the increase of the laser power from 3000 to 3800 W, the width and the depth of the laser melted layer increase, while the laser power has a little effect on the martensite contents, which exceed 90% in the melt-hardened zone. It greatly affects the mechanical properties in the melt-hardened zone with its volumetric expansion effect and the hardness increases by 2-3 times. The residual stress distributed within the melt-hardened zone is always of the compressive type. The amplitude of compressive stress exists in the transition region, and the amplitude of von Mises stress within the heat affected-zone (HAZ) decreases with the increase in laser power. The accuracy of the developed finite element simulation strategy is validated for phase proportion and hardness distributions through the wide-band LSM on roller steel with proper instrumentation for data measurement. This agreement is encouraging.