刀具几何参数对金属钛纳米切削影响的分子动力学研究（英文）

基于分子动力学的基本原理,构建了钛的纳米切削分子动力学仿真模型。工件原子间采用嵌入原子势EAM(Embedded atom method),工件原子与刀具原子间采用Morse势函数,研究了在不同刃口半径和刀具前角条件下,钛纳米切削过程中工件形态、系统势能、切削力以及工件温度等的变化规律。结果表明:随着刀具刃口半径增大,加工表面粗糙度增加,切削力和工件温度降低,切屑变薄;当刀具前角由负值增加到正值,钛工件承受的压应力逐渐变为剪应力,正前角刀具更有利于切削,同时在不同的刀具前角下,切向力和法向力的大小也有显著变化。     

刀具几何参数对金属钛纳米切削影响的分子动力学研究

基于分子动力学的基本原理,构建了钛的纳米切削分子动力学仿真模型。工件原子间采用嵌入原子势EAM（Embedded atom method）,工件原子与刀具原子间采用Morse势函数,研究了在不同刃口半径和刀具前角条件下,钛纳米切削过程中工件形态、系统势能、切削力以及工件温度等的变化规律。结果表明：随着刀具刃口半径增大,加工表面粗糙度增加,切削力和工件温度降低,切屑变薄;当刀具前角由负值增加到正值,钛工件承受的压应力逐渐变为剪应力,正前角刀具更有利于切削,同时在不同的刀具前角下,切向力和法向力的大小也有显著变化。     

单晶锗纳米切削温度场分布及各向异性对切削温度的影响研究（英文）

为深入理解单晶锗纳米切削特性,提高纳米锗器件光学表面质量,首次采用三维分子动力学(MD)的方法研究了单晶锗纳米切削过程中工件原子的温度分布情况,研究了晶体的各向异性(100),(110),(111)晶面对切削温度的影响及切削温度对切削力的影响。结果表明,在切削过程中最高切削温度分布在切屑当中,达到了460 K。刀具的后刀面与已加工表面之间的区域也有较高的温度,在400 K以上。在3个不同的晶面中,(111)晶面的切削温度最高,(111)晶面的原子密度最大,即为单晶锗的密排面,释放出的能量最多。切削温度对切削力也有影响,切削温度越高,工件中原子受到的切削力越小。     

单晶锗纳米切削温度场分布及各项异性对切削温度的影响研究

为深入理解单晶锗纳米切削特性,提高纳米锗器件光学表面质量,首次采用三维分子动力学(MD)的方法研究了单晶锗纳米切削过程中工件原子的温度分布情况,研究了晶体的各向异性(100), (110), (111)晶面对切削温度的影响及切削温度对切削力的影响。结果表明,在切削过程中最高切削温度分布在切屑当中,达到了460K。刀具的后刀面与已加工表面之间的区域也有较高的温度,在400K以上。在三个不同的晶面中,(111)晶面的切削温度最高,其根本原因是由于不同晶面间的原子空间结构不同,(111)晶面的原子密度最大即为单晶锗的密排面,释放出的能量最多。切削温度对切削力也有影响,切削温度越高,工件中原子受到的切削力越小。     

单晶铜纳米加工过程中热效应及缺陷结构的原子尺度模拟(英文)

基于建立的新型三维仿真模型,采用分子动力学方法模拟单晶铜(100)表面纳米加工过程,研究材料的去除机理和纳米加工过程中系统的温度分布与演化规律。仿真结果表明:系统的温度分布呈同心型,切屑温度最高,并且在金刚石刀具中存在较大的温度梯度。采用中心对称参数法区分工件中材料缺陷结构的形成与扩展。位错和点缺陷是纳米加工过程中工件内部的主要缺陷结构。工件中的残余缺陷结构对于工件材料的物理属性和已加工表面质量具有重要影响。位错的成核与扩展、缺陷结构的类型均与纳米加工过程中系统的温度有关。加工区域温度升高有利于位错从工件表面释放,使工件内部位错结构进一步分解为点缺陷。采用相对高的加工速度时,工件中残留缺陷结构较少,有利于获得高质量的加工表面。     

单晶γ-TiAl合金纳米切削过程的分子动力学模拟

采用分子动力学方法研究单晶γ-TiAl合金纳米切削过程,通过对单晶γ-TiAl合金的建模、计算和分析,讨论了不同切削深度和切削速度对切削过程的影响,结果发现:在切削过程中,随着切削深度的增大,切屑体积逐渐增大,切屑中原子排列越来越紧密,位错密度也会随之增大;但随着切削速度的增大,位错密度反而会随之降低。在一定的切削深度和切削速度范围内,切削过程中刀具前方都会产生"V"型位错环,工件的温度和势能也都会相应的增大。特别是,当切削速度为400 m/s时,刀具前方的切削表面上未出现原子错排。     

加工参数对单晶γ-TiAl表面质量和亚表层损伤的影响机理

为研究加工工艺参数对纳米切削单晶γ-TiAl合金表面质量和亚表层损伤的影响机理,以分子动力学（molecular dynamics, MD）为基础理论,采用非刚性金刚石刀具建立三维纳米切削模型,通过研究切屑体积、表面粗糙度、静水压分布、位错密度、位错演化、相变原子数,详细分析不同切削速度和切削深度对表面和亚表面结构的影响。结果发现:随着切削速度的增加,切屑体积增大,加工效率提升,且存在切削速度为100 m/s的临界值。表面粗糙度先减小后增大,同样存在切削速度为100 m/s的临界值。位错的复杂程度降低,位错密度减小,塑性变形程度增加;随着切削深度的增加,切屑体积增大,加工效率提升,表面粗糙度、位错密度以及塑性变形程度显著增加。在切削过程中,发现位错主要分布在刀具前方和下方,在刀具前方45°方向存在V形位错和梯杆位错以及位错间的相互反应,且切削完成后残留下空位和原子团簇等稳定缺陷。      

切削过程中刀/屑的切削温度预报

本文以有限差分法为基础建立了连续切削和铣削的数值模型,该数值模型用于预报切削过程中刀具和切屑的温度场.连续或稳态切削(如正交切削),可用刀具-前刀面接触区刀具切屑导热(热传导)模型加以研究.该模型考虑了第一变形区的剪切能、前刀面-切屑接触区的摩擦能、运动刀屑和固定刀具之间的热平衡.用有限差分法求解温度分布,可将该模型延用到断续切削和切削厚度随时间而变化的铣削加工中.根据刀具转角,将切屑划分为微元.刀具转角是由工件主轴速度和离散时间所决定.每一个微元的温度场可看成是一阶动态系统,它的时间常数由刀具和工件材料的导热性能和前一个切屑段的初始温度所决定.瞬态温度变化的估算是依次求解连续切屑单元的一阶热传递问题.模型对连续切削稳态温度和切屑、加工过程不连续变化的断续切削的瞬态进行预报.数值模型和仿真结果与文献报告的实验温度相符.     

单晶铜在不同温度下的纳米切削机理研究

目的 虽然纳米切削是21世纪超精密加工技术的重要发展方向之一,但现有的纳米切削机理仍不完善。因此,采用数值模拟方法,从晶体结构、力学和粒子运动等方面对纳米切削机理进行补全。方法 首先,基于分子动力学方法对纳米尺度下的单晶铜进行了拉伸模拟,总结其在不同温度下的韧脆性特征;其次,对纳米尺度下的单晶铜进行了切削模拟,系统性地研究了切削过程中晶体结构、切削力、应力应变分布,以及原子运动特征在不同材料韧脆性下的变化规律。结果 拉伸模拟结果表明,低温下单晶铜脆性特征显著,但仍具有一定的韧性。随着温度的升高,单晶铜脆性减弱,韧性增强。切削模拟结果表明,靠近工件自由面的材料沿主剪切方向发生持续的剪切滑移和周期性的长距离错动,形成多种晶体结构有序分布的块状切屑。靠近刀具的材料在推挤作用下由晶体结构变为非晶结构,之后持续流动形成切屑。随着切削温度的升高,块状切屑中的长距离错动频率提高,通过剪切形成的块状切屑尺寸减小,而通过推挤形成的流动状切屑厚度增加。结论 切屑的形成方式包括剪切和推挤2种类型。低温下,剪切切屑形成过程占据主导地位,切屑呈现明显的块状。随着温度升高,切屑形成机理从剪切向推挤转变。     

石墨烯纳米流体加工Ti-6Al-4V合金的黏着磨损、表面粗糙度和切削力

提出了一种基于石墨烯纳米颗粒分散在菜籽油中的切削液为加工区域提供润滑/冷却的新加工方式,确定了该纳米流体对刀具切屑粘附层的影响。与干切削相比,使用菜籽油+石墨烯加工后的刀面和前刀面的切屑粘附层厚度分别降低了38.8%和28.8%,切削力降低51.4%,工件表面粗糙度降低50.1%。石墨烯较高的导热系数可以降低切割区域的温度。此外,石墨烯可以渗透到刀具与工件之间的接触区域,有效地保护了刀具的涂层材料,减少了粘附在工件表面的切屑,并且填充了工件表面形成的凹坑,从而提升了表面质量。     

Investigation on AFM-based micro/nano-CNC machining system

To solve the problems in the atomic force microscope (AFM)-based nanomachining process such as nonlinearity and low repeatability positioning accuracy of AFM scanner in a larger scale, a novel micro/nanomachining system similar to conventional CNC machine tools was presented. The system integrated AFM with a precision stage. AFM optical lever detection method was employed to apply a very light normal load on the sample surface. An AFM diamond tip was utilized as a cutting tool to scratch the sample. The precision stage was used as a worktable to move the sample. This process was different from machining with a planer. Based on the system, effects of tip geometry, the scratching direction, the normal load, the machining velocity and the feed on the machining depth were discussed. Fabricating techniques of two-dimensional and three-dimensional complex micro/nano-structures using AFM-based mechanical scratching method based on this system were presented. Complex regular structures were fabricated. Moreover, AFM-tip-induced local anodic oxidation was also carried out based on this system, which displayed higher repeatability positioning accuracy with a larger machining dimensions and a significant machining ability of this system.     

Analytical solution of flexural vibration responses on nanoscale processing using atomic force microscopy

An analytical solution is developed to deal with the flexural vibration problem during a nanomachining process which involves an atomic force microscope (AFM) cantilever. The modal superposition method is employed to analyze the response of an AFM subjected to a cutting force with an excitation force of an arbitrarily chosen frequency. The cutting forces were transformed into distributed transversal and bending loading, and were applied to the end region of the AFM by means of the tip holder. The effects of transverse stress and bending stress were adopted to solve the dynamic model. Based on the result, applying a cutting force with an excitation force near the high-order modal frequencies and using a wide tip holder are recommended when nanoscale processing using AFM is performed.     

Possibility of high-pressure transformation during nanoindentation of SiC

High-pressure transformation to the rocksalt structure has been proposed as a mechanism that underlies the ductile wear observed during nanomachining of SiC. However, in contrast to other brittle materials (e.g. Si), no such transformation has been directly observed either during machining or during nanoindentation of SiC. Here, we performed large-scale molecular dynamics simulations of nanoindentation with spherical indenters of various sizes and surface roughness to determine whether SiC can undergo a nanoindentation-induced transformation from the zincblende to the rocksalt structure. The calculations of possible states of stresses under the indenter have been combined with a thermodynamic analysis to estimate the effects of dislocation density, shear stresses and temperature on the phase transformation pressure in SiC. Our analysis shows that the high-pressure transformation is highly unlikely under the conditions of nanomachining. We conclude that the primary response of SiC to nanoindentation is dislocation nucleation and propagation in the low-pressure (zincblende) phase.     

连续升温、降温过程中纯Fe的微观结构分析

通过分子动力学模拟,采用较先进的键型指数法HA及原子团类型指数法CTIM-2,对Fe连续升温、降温过程中微观结构进行模拟研究.结果表明:连续升温过程,Fe的微观结构变化是bcc→fcc\hcp→bcc→液体；连续降温过程,Fe的微观结构变化是液体→fcc\hcp.Fe凝固结束没有形成大量的高温bcc晶体,原因是在高温液态中bcc结构原子稳定性较差,fcc和hcp结构原子更易稳定存在.此外,温度变化速率过快,可诱导晶体生长过程中发生层错,促使Fe在升温、降温过程出现fcc和hcp晶体的交替分层分布,这与fcc和hcp晶体的原子能量相近、晶体的致密度相同、原子空间堆垛方式局部相同有关.     

FEM–MD combined method for evaluation of microscopic stress distribution by considering microscopic heterogeneity and deformation in close vicinity to grain boundary of steels

Finite element method (FEM)–molecular dynamics (MD) combined method is proposed for the microscopic stress analysis of steels. In this numerical method, FEM is applied to the stress analysis inside grains, and MD is applied to the calculation of the atomic configuration near the grain boundary in order to consider the microscopic heterogeneity and the deformation near the grain boundary that influences the stress distribution. Slip length between two grains caused by the mismatch of the displacement near the grain boundary is calculated by FEM. Slip resistance, which is necessary to calculate slip length, is obtained by calculating the atomic configuration near the grain boundary by MD. The combination of FEM and MD is realized by using slip resistance in FEM and slip length in MD. The validity of modelling of the deformation near the grain boundary is investigated by comparing the deformation near the grain boundary calculated by FEM–MD combined method to that observed in the experiment in the case of a load applied to the specimen. Calculated slip length coincides with measured slip length. FEM–MD combined method is applied to the investigation of the influence of change in the grain shape caused by the thermal history such as the weld zone upon the strength characteristic. The high stress region tends to increase the incidence of larger grain diameter and it is indicated that grain coarsening due to the weld thermal history increases the possibility of the crack initiation. FEM–MD combined method is expected to be helpful in investigating the mechanism of fracture or the strength characteristic of the complicated microstructure such as the weld zone by evaluating the microscopic stress distribution.     

分子动力学法对Cu-Ag合金熔化及凝固过程的模拟

借助镶嵌原子势(EAM)利用等压等温的分子动力学方法(NPT-MD)模拟共晶Cu40Ag60合金的熔化,凝固过程.利用径向分布函数和对分析技术研究Cu40Ag60台金在不同的冷却速率(1×1011,1×1012,5×1012,1×1013。及1×1014 K/s)下的非晶形成能力和微观结构演化.研究发现Cu40Ag60合金形成非晶态需要极大的冷却速率.模拟中的结构分析揭示了合金在冷却过程中微观结构的演化规律     

弹性波在纳米单晶铜杆中的传播

采用预弛豫和对试件一个端部进行速度加载（另一端固定）的方法在分子动力学模拟中实现了纳米杆真实的动态单向加载过程,并提出了与宏观一致的应变率定义;对长脉冲和短脉冲在纳米铜杆中的传播过程进行了分子动力学模拟,同时也对短脉冲在纳米铜杆中的传播进行了连续介质力学三维有限元模拟．长脉冲的模拟结果表明,纳米铜杆的弹性模量会随着试件截面的增大而增大．短脉冲的模拟结果显示,分子动力学模拟结果相当于在连续介质力学三维有限元模拟结果上叠加了一个抖动,且两种方法模拟得到的波速和Rayleigh波速基本吻合,这表明边界扰动在纳米尺度试件中的传播仍然具有一维弹性应力波和横向惯性效应引起的几何弥散的特征;而叠加的抖动则体现了表面原子对弹性波传播的影响,且与加载前未能将试件完全弛豫到自然状态有关．     

End milling tool breakage detection using lifting scheme and Mahalanobis distance

In this paper, a novel method based on lifting scheme and Mahalanobis distance (MD) is proposed for detection of tool breakage via acoustic emission (AE) signals generated in end milling process. The method consists of three stages. First, by investigating the specialty of AE signals, a biorthogonal wavelet with impact property is constructed using lifting scheme, and wavelet transform is carried out to separate AE components from the original signals. Second, Hilbert transform is adopted to demodulate signal envelope on wavelet coefficients and salient features indicating the tool state (i.e., normal conditions, slight breakage, and serious breakage) are extracted. Finally, tool conditions are identified directly through the recognition of these features by means of MD. Practical application results on a CNC vertical milling machine tool show that the proposed method is accurate for feature extraction and efficient for condition monitoring of cutting tools in end milling process.     

高腐蚀性介质精馏塔塔内件的选用

通过对氯化苄（C7H7Cl）及类似体系物料的分离 过程中精馏塔设备及内件材质的分析与选择,论述了设备及内件本身的制造工艺、成本、安 装及维修等方面的问题,提出了在高腐蚀性介质条件下精馏塔,特别是塔内件的选用方法.