基于分子动力学的单晶硅纳米振动切削过程

为了研究脆性材料单晶硅原子级的纳米振动切削加工去除机制，应用分子动力学，通过改变刀具椭圆振动切削的模式，进行单晶硅纳米振动切削仿真．仿真结果表明，主切削力和法向力的变化趋势总体上呈现正弦曲线变化，并且已加工表面不同深度的亚表层存在残余应力；同时在不同切削模式下，由于刀具法向的振幅增加，使得法向力和残余应力增大；当刀具通过已加工表面时，残余应力随着亚表层深度降低而减弱；刀具法向振幅高于主切削力方向振幅时，法向力峰值高于主切削力峰值，已加工表面亚表层没有明显驰豫现象．     

刀具磨损对于单点金刚石切削单晶硅的影响（英文）

单点金刚石切削(SPDT)是加工单晶硅最常用的方法,刀具磨损是影响加工表面或工件表面质量的重要因素,但是其中的磨损机制尚不清楚。为了研究刀具磨损对于切削机制的影响,本研究建立了单点金刚石切削单晶硅的分子动力学(MD)仿真模型。仿真结果表明随着刀具磨损程度的增加,切削力、表面损伤层厚度、位错分布面积、剪切变形和相变程度均增加。当使用已经磨损的刀具切削单晶硅时,挤压起主要作用,当使用未磨损刀具时,剪切变形起主要作用,工件表面损伤层主要是由硅的非晶相组成,使用磨损的刀具时产生的轴向力F_t约是未磨损刀具的四倍。模拟结果同时表明使用未磨损金刚石刀具时会导致工件发生塑性变形,当刀具发生磨损后切削过程中会伴随有脆性断裂。     

膜厚对四面体非晶碳膜机械性能的影响

采用过滤阴极真空电弧技术以相同的工艺条件在p(100)单晶硅衬底上制备了不同厚度的四面体非晶碳薄膜,并利用表面轮廓仪测试薄膜的厚度和应力,利用纳米压入仪测试薄膜的硬度、杨氏模量和临界刮擦载荷.试验表明,在一定的扫描波形条件下,薄膜大约以0.7 nm/s的沉积速率稳定生长.随着膜厚的增加,薄膜的应力持续降低,当膜厚超过30nm时,应力将低于5GPa;当膜厚超过300nm时,硬度和杨氏模量分别将近70GPa和750GPa,已经十分接近体金刚石的性能指标.另外,随着膜厚增加所产生的应力变化,也导致了可见光拉曼光谱非对称宽峰的峰位逐渐向低频偏移.     

切削深度对单晶γ-TiAl合金亚表面缺陷及残余应力的影响

本工作采用分子动力学方法模拟了单晶γ-TiAl合金在不同切削深度下的切削过程,分析了不同切削深度下微观缺陷演化及稳定切削后的内应力演变,研究了切削后残余应力和von Mises应力等在不同切削深度下的分布规律,讨论了不同切削深度下位错和层错等微观缺陷演化及内应力演变之间的关系.结果表明:位错反应及层错演化随着切削深度的增加越来越剧烈,位错反应对Lomer-Cottrell位错的形成影响较大;刀具挤压工件表面形成的残余压应力受层错演化及位错反应的影响,位错反应的剧烈程度影响内应力的大小,且残余压应力存在于亚表面下的一定深度内;同时发现切削深度的变化对von Mises应力的影响较小.     

微观状态SiCp/Al复合材料铣削残余应力模拟分析

为了获得更高精度的高体积分数SiCp/Al复合材料铣削加工表面残余应力数据,采用有限元分析软件ABAQUS建立了基于正交切削的微观平面应变模型。复合材料两相材料属性被分别定义,改善了在宏观状态下整体定义材料属性的精度缺陷,并对其微观状态切削残余应力产生过程进行模拟分析。分析并得出了不同切削参数对工件加工表面残余应力的影响规律以及沿工件层深方向的残余应力分布情况。结果表明切削速度和每齿进给量对SiCp/Al复合材料已加工表面残余应力有着相似的影响规律,但每齿进给量的影响作用较为显著。     

不同厚度四面体非晶碳薄膜的拉曼表征和内应力

为了探讨膜厚对四面体非晶碳薄膜拉曼结构表征和内应力的影响规律，进而确定应力与拉曼光谱之间的关系，采用过滤阴极真空电弧技术以相同的工艺条件在P（100）单晶抛光硅衬底上制备了从3nm～350nm不同厚度的四面体非晶碳薄膜。利用表面轮廓仪和原子力显微镜测试膜厚，表面轮廓仪确定曲率半径并计算薄膜应力，共聚焦拉曼光谱表征薄膜的结构细节。实验发现，随着膜厚的增加，四面体非晶碳薄膜的应力持续下降，当膜厚超过30nlll时，应力的下降趋势变得平缓，并保持在小于5GPa的较低水平。随着膜厚的增加，可见光拉曼光谱中衬底硅的一阶和二阶谱峰强度逐渐降低，在50nm～80nm膜厚范围，半高宽最窄，峰强最高，能够最有效地获得拉曼结构信息。随着膜厚的增加和应力的下降，非晶碳一阶谱峰的峰位表现为逐渐向低频偏移。     

纳米磨削过程中加工表面形成与材料去除机理的分子动力学仿真

目前，以线性断裂力学为基础的加工理论对解释微切削加工机理还存在不足．分子动力学分析的方法在研究纳米尺度或原子尺度下的固体变形方面具有独特的优势；辅以压痕挤压机理分析，解释纳米磨真削过程中加工表面形成和材料去除机理．研究表明：静水压力对非结晶变形程度影响很大；晶格重构原子与一部分非晶层原子堆积在磨粒的前上方。由于磨粒不断前移，最终形成磨屑而实现材料去除，处在磨粒前下方的非晶层原子在压应力的作用下与已加工表层断裂的原子键结合重构形成已加工表面变质层；变质层由内外两层组成，外层是非晶层，内层是晶格变形层。     

氧化锆层状复合陶瓷表面压应力与相变增韧的关系

利用维氏硬度仪和X射线应力分析仪、X射线衍射仪等手段分别对单层和层状氧化锆陶瓷进行了力学性能测试和分析,研究结果表明,在ZrO2层状复合陶瓷中,压痕裂纹的形成除了因塑性区体积变化产生的残余应力外,还与相变应力和表面压应力有关,表面压应力对表面裂纹具有较大的抑制作用.层状陶瓷断裂韧性提高,主要是通过表面压应力对压痕裂纹区应力强度因子的贡献、提高断裂相变量,强化相变增韧效果、细化晶粒等几个方面来实现的.     

铣削和喷丸工艺对TC4钛合金残余应力和半高宽的影响

采用X射线衍射法研究了TC4钛合金铣削及铣削加喷丸状态的残余应力场和半高宽分布。结果显示:铣削加工的TC4钛合金残余应力场和半高宽分布受到刀具的切削作用和接触塑性形变作用的共同影响;切削线速率越大,切削作用越强,则压缩残余应力和深度越小,线速率对表面半高宽影响不大;切削深度越大,塑性形变作用越强,使得残余应力分布越复杂,表面半高宽越大;喷丸强化引入了倒钩型残余应力场,最大残余压应力接近屈服强度,喷丸强化引起的塑性形变层深度大于铣削加工的,使喷丸强化前不同铣削加工的残余应力状态近一致化。     

预弯曲变形对CP800复相钢力学性能的影响

对CP800复相钢进行冲压成型,制备预弯曲试样,并利用EBSD、X射线残余应力分析仪、拉伸试验机、DIC技术等研究预弯曲变形对钢的微观组织、残余应力和力学性能的影响.结果表明:预弯曲后残余应力分布情况呈现为拉-压-拉-压交替分布,即内表面(压缩层)呈现拉应力而外表面(拉伸层)呈现压应力,这种特殊分布情况会导致预弯曲后材料的屈服应力降低16％.同时,由于冷变形导致的材料硬化和位错强化效果,预弯曲后材料伸长率降低25％而抗拉强度增大24％.此外,预弯曲后内表面由于存在拉伸残余应力而导致更大的塑性应变和损伤,并早于外表面发生断裂.     

A fundamental study on the impact of surface integrity by hard turning on rolling contact fatigue

Hard turning has the potential to produce favorable surface integrity that would improve component life in rolling contact. However, the effects of the process-induced residual stress profile and the white layer on rolling contact fatigue (RCF) are poorly understood. This study aims to answer the long-standing question of how residual stress and the white layer affect RCF. Based on the developed real-time RCF testing system, a series of RCF tests were conducted for hard turned AISI 52100 steel components. The test results have shown that the acoustic emission amplitude is most consistent and sensitive to fatigue damage than other AE parameters. A white layer induced by hard turning is very detrimental to RCF. A component free of a white layer can have a life six times that of a white layer component. As the white layer increases in thickness, the fatigue life decreases. Surface residual stresses and near-surface residual stress profiles are significant factors for RCF, while the depth of maximum compressive residual stress in subsurface is not critical. Surface integrity affects RCF through the mechanism of near surface damage rather than subsurface damage.     

Process parameter effects on residual stress and phase purity after microlaser-assisted machining of silicon

Machining brittle materials, such as silicon, is expensive and often causes detrimental damage, but a relatively new technique, termed microlaser-assisted machining (micro-LAM), improves the efficacy of the machining process using traditional single point diamond turning with simultaneous laser assistance. Prior work shows it provides a smooth surface finish and reduces the likelihood of fracture by increasing the ductility of the material during the machining process. However, the quality of the finish and the utility of the machined silicon depends on having good phase purity and low residual stresses. Using Raman microspectroscopy and a wide range of micro-LAM machining parameters the current work has shown that the technique can give excellent results in terms of low residual stress, high phase purity, and good relative crystallinity. However, poor choice of process parameters can be very detrimental leading to high residual stresses (over 400?MPa) and multiple silicon phases being present.     

Fixed Abrasive Diamond Wire Saw Slicing of Single-Crystal Silicon Carbide Wafers

This article investigates the slicing of single-crystal silicon carbide (SiC) with a fixed abrasive diamond wire. A spool-to-spool rocking motion diamond wire saw machine using a 0.22 mm nominal diameter diamond wire with 20 µm average size diamond grit was used. The effect of wire downfeed speed on wafer surface roughness and subsurface damage was first investigated. The surface marks generated by loose diamond grit and stagnation of the wire during the change of the wire-cutting direction were studied. The use of scanning acoustic microscopy (SAcM) as a nondestructive evaluation method to identify the subsurface damage was explored. Effects of using a new diamond wire on cutting forces and surface roughness were also investigated. Scanning electron microscopy has been used to examine the machined surfaces and wire wear. This study demonstrated the feasibility of fixed abrasive diamond wire cutting of SiC wafers and the usage of a SAcM to examine the subsurface damage.     

brittle-ductile TRANSITION IN DIAMOND CUTTING OF SILICON SINGLE CRYSTALS

Silicon single crystals are not amenable to conventional machining operations because of their inherent low fracture toughness. This paper deals with an investigation of brittle-ductile transition in diamond cutting of silicon from the viewpoint of material response and tool geometry. Micro indentation and scribing tests were conducted in order to investigate the influence of applied loads on the deformation characteristics. The transition of material removal from brittle to ductile was observed by continuously changing the cutting depth. The effect of tool rake angle on the machined surface quality was studied by actual diamond turning. A mirror surface, with a roughness of 5 nm R_a, was produced using a tool with a -25° rake angle. The reason for the difference in the machined surface quality is discussed based on the analysis of stress distribution in the microcutting process.     

Einfluß des Festwalzens auf die Oberflächen‐ und Randzoneneigenschaften der Magnesiumlegierungen AZ31 und AZ91

Influence of burnishing operations on surface‐ and subsurface properties of AZ31 and AZ91 A mechanical surface treatment on magnesium‐alloys can be used to improve surface and subsurface characteristics after the cutting process. Investigations show, that the grooves, resulting from the turning process, are planed. The hardness of the surface rises. Compared to turned and polished samples, the increased strength and the induced residual stress of burnished samples cause an increased fatigue strength under rotating bending stresses. The corrosion resistance of AZ31 is enhanced by roller burnishing.     

Factors affecting surface quality in diamond turning of oxygen-free high-conductance copper

In identical cutting conditions, it has been shown that the surface quality of ultraprecisely machined oxygen-free high-conductance copper samples, for which a single-point diamond tool is used, depends on the microstructure and properties of the materials, as well as the processing sequence. It has been found that samples subjected to colddeformation-recrystallization annealing have the best surface quality after being diamond turned. In addition, crystal uniformity and the processing sequence affect the roughness and residual stress of the finished surface.     

Investigation on diamond tool wear in ultrasonic vibration-assisted turning die steels

This paper presents comprehensive theoretical analyses and experimental investigations for evaluating the ultrasonic vibration-assisted turning (UVAT) of die steels with single-crystal diamond tools. The diamond tool wear was found to rely heavily on the feed rate and the cutting speed while being insensitive to the depth of cut and the tool relief angle under the cutting conditions used in the tests. The tool wear characteristics were further studied based on the observation of wear zone using Raman spectral analysis and energy-dispersive X-ray (EDX) analysis. The detection results of the tool worn topography, the phase transformation and the carbon diffusion of diamond crystals revealed that tool wear mainly occurred on the tool flank face due to the graphitization and the diffusion of the diamond tool. Analytical results of the function mechanisms of the ultrasonic turning indicated that the friction force between the tool flank face and the machined surface, which depended mainly on the ratio of the cutting speed and the vibration speed, could be effectively reduced in ultrasonic turning process. The analytical and experimental results indicated that compared with conventional turning (CT), the cutting performance, in terms of the tool life, was markedly improved by applying ultrasonic vibration to the cutting tool.     

EFFECT OF TOOL WEAR AND TOOL SETTING ON PROFILE ACCURACY OF DIAMOND-TURNED NONFERROUS COMPONENTS

Diamond turning technology has gained great importance in high-precision optical component fabrication. The quality of machined optical surfaces is mainly affected by the machine tool's accuracy, cutting tool's quality, and dynamic machining effects. This study investigated the effects of cutting tool conditions and tool set-up error on the surface distortion. Controlled cutting tests were performed on a two-axis diamond turning machine. Spherical mirrors with preset tool offset values and tool height values were turned. The relationship among machined form accuracy, tool offset, and tool height was investigated based on experimental and analytical results. The influence of tool wear on machined surface quality was studied. Factors governing uneven wear along the cutting edge in contour machining were analyzed. A spherical surface with a form accuracy better than λ/10 was produced. Methods for minimizing the effect of tool wear are also discussed.     

Oberflächen‐ und Randschichtcharakterisierung von hochgeschwindigkeitsgefrästem Stahl

Surface and subsurface characterization of high speed milled steel Surfaces und subsurface regions of workpieces generated by high speed milling were studied to support the understanding of micro‐processes of deformation and cutting. Microstructure, crystal structure, residual stress, and topography of the workpiece, and microstructure and shape of the chips were characterized. With low carbon and low alloyed steel the materials state is significantly influenced by the cutting parameters. The surface topography is related to the microtopography of the cutting edge of the tool. A deformation of the microstructure happens down to 15 μm under the surface. Residual stress has been measured within a 60 μm thick surface layer. The curvature of the ships increases with the cutting speed.     

Effect of cutting speed and tool rake angle on residual stress distribution in machining 2024-T351 aluminium alloy — unlubricated conditions

The residual stress distribution in the machining of 2024-T351 aluminium alloy was measured using an electrolytic etching technique. Ring-shape specimens were machined under unlubricated orthogonal conditions with high-speed steel tools having rake angles of 10, 15, 20 and 25° at cutting speeds ranging between 0.5 and 1.25 m sec^–1. The results of the investigation show that the residual stresses are compressive at the machined surface and decrease with depth beneath the machined surface. The maximum (near-surface) residual stress and the depth of the severely stressed region increase with an increase in the cutting speed. There seems to be little change in the residual stress distribution due to a change in the rake angle. The results are interpreted in terms of the variations in the amount of surface-region deformation produced by changes in cutting conditions.