工程陶瓷预压应力下超声振动辅助划痕实验研究

目的结合超声振动加工方法,探究工程陶瓷预压应力加工过程的工件表面损伤特性。方法建立预压应力下工程陶瓷超声振动辅助加工过程的工程学模型,结合Al_2O_3陶瓷划痕过程的离散元仿真结果和实验结果进行分析,采用扫描电镜对加工表面进行观察,使用三向动态压电测力仪测量划痕力。结果预压应力下超声振动辅助划痕过程能够去除沟槽边缘处的材料堆积,并且划痕沟槽边缘破碎呈现周期性。当预压应力为200 MPa、理论划痕深度为10μm时,普通划痕深度为7.58μm,宽度107.5μm,超声振动辅助划痕深度为8.55μm,宽度为143.5μm。结合仿真结果,超声振动辅助划痕过程可减小划痕沟槽的径向裂纹数量,增大径向裂纹深度。同时,两种划痕过程动态切向力出现明显差异,超声振动辅助划痕过程动态切向力较小,变化相对平稳。结论超声振动辅助加工过程可以减小工程陶瓷预压应力加工过程的切削力,提高材料加工效率。     

高体积分数SiCp/Al复合材料超声辅助划切微观去除机理

目的 揭示高体积分数SiC_p/Al复合材料在超声辅助加工条件下的材料去除机理。方法 采用SiC_p/Al复合材料的超声辅助划切试验,探究划切参数变化对超声振幅、划切力及摩擦因数的影响规律,并通过扫描电子显微镜和激光共聚焦显微镜对划痕表面微观形貌进行观察,分析单点金刚石磨粒工具超声辅助划切材料去除的特点。结果 随着划切深度从0.01 mm增加到0.05 mm,电流值逐渐降低,电流值变化量从12 mA增加到25m A,超声振幅逐渐衰减,金刚石压头的轴向冲击作用减弱。划切深度和划切速度的增加使切向挤压切削作用增强,划切力和摩擦因数增大。在材料去除过程中,碳化硅颗粒存在破碎成小颗粒、剪切断裂破碎和拔出等多种去除形式,铝基体出现明显的塑性流动和涂覆现象,并形成切削沟槽外侧堆积。结论 当切削深度和进给速度较小时,材料去除主要是在轴向的高频振动冲击作用下完成,材料表面加工质量较好;当切削深度和进给速度逐渐增大时,材料去除是在轴向冲击破碎和切向挤压切削共同作用下完成,材料表面加工质量逐渐降低。     

纵-扭超声磨削氧化锆陶瓷磨削力仿真分析

为揭示纵-扭超声振动对磨削氧化锆陶瓷磨削力的影响,分析纵-扭超声磨削（L-TUG）运动特性,利用ABAQUS开展单颗金刚石磨粒划擦氧化锆仿真模拟,并进行单磨粒磨削氧化锆试验验证。在此基础上,分析仿真条件下普通磨削（OG）与L-TUG材料表面应力的差异性;探讨磨削表征参数对L-TUG磨削力的影响规律。结果表明：OG与L-TUG磨削力仿真结果和试验结果吻合度较高,验证了仿真模拟的可行性;与OG相比,超声振动能够有效降低材料表面磨削应力;在相同表征参数下,整体上L-TUG轴向力始终大于法向力,且对轴向力与法向力的影响程度和变化趋势呈现不同的规律性。     

氧化锆陶瓷旋转超声加工脆塑转变特性研究

目的揭示旋转超声振动对硬脆材料脆塑转变特性及工艺参数对材料脆塑转变临界切削深度的影响规律。方法以氧化锆陶瓷为研究对象,在硬脆材料压痕断裂试验基础上,从理论上分析了超声振动加工硬脆材料脆塑转变的临界条件,并进行了纵向振动及纵扭共振形式的旋转超声振动划痕与普通划痕对比试验。结果在相同试验条件下,超声振动划痕较普通划痕有较高的材料脆塑转变临界切削深度。适当地增大超声能量,纵扭共振比纵向振动具有更大的脆塑转变临界切削深度值;而随着进给速度的增大,纵向振动比纵扭共振具有更大的材料脆塑转变临界切削深度。结论通过不同划痕条件的对比,超声振动能有效提高氧化锆陶瓷的脆塑转变临界切削深度,增大塑性域加工范围,提高材料表面加工质量,验证了理论分析的正确性。     

纵扭超声辅助磨削氮化硅亚表面损伤及其试验研究

目的 探究纵扭超声辅助磨削工艺参数对氮化硅陶瓷亚表面损伤的影响规律。方法 首先,建立纵扭超声振动下单颗磨粒的切削轨迹及其切削弧长模型,分析纵扭超声辅助磨削独特的加工机理。其次,考虑砂轮表面磨粒的随机分布特性,并基于硬脆材料脆塑转变特性及其临界转角界定,给出纵扭超声辅助磨削单颗磨粒未变形切屑厚度的概率学模型,进而建立纵扭超声辅助磨削过程中单颗磨粒的平均法向磨削力模型。最后,建立纵扭超声辅助磨削氮化硅亚表面损伤深度模型,并进行试验验证。结果 纵扭超声振动的引入增大了纵扭超声辅助磨削过程中单颗磨粒的切削弧长,减小了单颗磨粒平均未变形切屑厚度,降低了单颗磨粒的法向磨削力,最终降低了氮化硅陶瓷亚表面损伤的深度,获得了较好的氮化硅陶瓷表面加工质量。氮化硅亚表面损伤深度随着超声振幅的增大而降低,当超声振幅为6μm时,亚表面损伤深度为5.65μm,相较于普通磨削亚表面损伤深度降低了33.6%。理论模型预测结果与试验结果趋势一致,预测结果与试验结果的最大误差为13.38%,平均误差为8.34%,因此该模型能够为氮化硅实际加工中亚表面损伤深度的预测提供一定参考。结论 纵扭超声辅助磨削能够有效降低氮化硅陶瓷...     

基于改进本构模型的超声辅助渐进成形仿真研究

在渐进成形加工中施加高频振动会引起被加工材料的塑性性能改变,并产生软化现象,从而降低加工过程中所需的成形力。首先,基于晶体塑性理论建立了描述金属塑性变形过程中应力-应变关系的理论模型,通过调整位错密度演化和热激活过程体现了超声施加后的声软化效应,构建了超声辅助成形加工时材料的本构模型。基于改进后的本构模型,通过ANSYS/LS-DYNA软件,对超声辅助渐进成形过程进行仿真模拟,获得了成形过程中成形力随成形深度的演变规律。通过超声辅助点成形实验,识别了本构模型参数并验证了仿真模拟结果,获得在一定频率下施加不同振幅时成形力的变化曲线。结果表明,在仿真时使用考虑软化效应的本构模型能获得更好的成形力预测效果。     

单颗金刚石划擦玻璃的实验研究

本文利用自行制备的单颗粒金刚石工具划擦普通玻璃,观察不同划擦深度的划痕形貌并进行统计分析,同时采集和分析了划擦过程中的磨削力。实验结果表明,划痕的实际宽度与理论宽度有相同的变化趋势。随着切深的增加,实际宽度与理论宽度趋于比较恒定的比值。当划擦磨粒不变时,单颗磨粒的法向磨削力和切向磨削力都随着划痕长度的增加而线性增加。     

基于DEFORM的单颗磨粒对零件表面冲击仿真与实验

磨削加工过程中,工件表面材料的去除是磨粒冲击、切削、划擦等综合作用的结果。为了研究磨削过程中的冲击现象,针对45~#钢工件,运用DEFORM-3D软件对单颗磨粒对工件表面的冲击强化过程进行有限元仿真,获得了工件内部应力场的分布情况,对磨削过程中工件表面等效应力与应变率进行了分析,得到了磨削参数对磨削过程的影响规律。对等效应力的分析表明:磨削过程中,工件受力是由很多脉冲力合成的。对应变率的分析表明:磨粒对工件表面的冲击强化效应显著。实验研究结果验证了理论与仿真分析结果的正确性,采用的仿真方法可有效分析与预测工件表面强化层指标。     

钨合金超声椭圆振动切削表面完整性研究

目的 研究钨合金超声椭圆振动切削表面完整性的变化规律,为实现钨合金高表面完整性加工提供理论基础.方法 设计单因素试验,采用单晶金刚石刀具开展钨合金超声椭圆振动切削试验,并与普通切削进行对比,研究不同切削深度下超声椭圆振动对工件表面形貌、表面粗糙度、微观组织、位错密度、显微硬度以及表面残余应力的影响.结果 普通切削或超声椭圆振动切削后,工件表面的位错密度、显微硬度以及残余应力均随着切削深度的增加而增大,且亚表面的晶粒都发生了一定程度的塑性变形,并出现了晶粒细化.与普通切削相比,超声椭圆振动切削可以有效抑制加工过程中鳞刺和犁沟的产生,改善表面粗糙度;工件表面会产生更高的硬化程度、残余压应力和位错密度,位错密度的量级在108 mm–2;亚表面的变质层厚度更小.结论 相比于普通切削,超声椭圆振动切削可以降低表面粗糙度,增大表面残余压应力,提高工件表面完整性.     

二维超声磨削纳米复相陶瓷的磨削特性研究(1)

本文通过对二维超声磨削纳米复相陶瓷的磨削特性进行理论分析及试验研究，尤其是磨削力的特性及其影响因素，从而探索磨削加工表面质量的影响因素并提出改善磨削效果的措施。研究结果表明，磨削力随着切深的增大而增大，随着磨削深度的进一步增加，超声振动在磨削加工中所起的作用减弱；二维超声振动磨削大大扩大了复相陶瓷磨削的塑性加工区域，二维超声振动磨削过程的塑性域是磨削深度小于5μm，而普通磨削塑性域是在磨削深度小于2μm；适当增大磨削速度，既可以增强磨削砂轮的自锐能力，获得较高的去除率，又可以增加塑性变形，改善工件的表面质量；砂轮线速度的变化对二维超声振动磨削过程中的磨削力影响比对普通磨削过程中的磨削力的影响小，故二维超声振动磨削可以选用较大的砂轮线速度；工件速度对二维超声振动磨削表面粗糙度影响很大，其值随着工件速度的增加而增大。     

Material removal behavior in ultrasonic-assisted scratching of SiC ceramics with a single diamond tool

Ultrasonic-assisted grinding (UAG) has been extensively employed in manufacturing industries for processing hard and brittle materials. However, its potential has not been sufficiently developed because the material removal mechanism in UAG has not been elucidated. This paper focuses on the material removal mechanism in the UAG of silicon carbide (SiC) ceramics by investigating the material removal behaviors in ultrasonic-assisted scratching (UAS) of SiC ceramics. The UAS test was conducted on an NC (Numerical Control) surface grinder connected to a self-designed ultrasonic unit; a single diamond tool was fixed onto the end-face of the ultrasonic unit to achieve an ultrasonic vibration (UV). Conventional scratching (CS) test was also carried out on the same experimental rig but without UV for comparison. During testing, the material deformation/fracture behavior and the scratching forces were investigated. The experimental results show that (1) there are two scratching modes in the UAS process depending on actual depth of cut: an intermittent mode and a continuous mode; (2) the mean groove depth in UAS is much bigger than that in CS, indicating that the cutting ability of the tool was significantly improved by the assistance of the UV; (3) the critical depth of cut is increased by 56.25% once UV is applied; the stiffness of the experiment setup is improved in the UAS compared to CS; the UV in y-direction strongly contributes to the material removal, whereas the UV in z-direction only results in variation of the cutting trace and hardly contributes to the material removal in the UAS process; (4) in UAS, the scratching forces sinusoidally fluctuate with the same period as that of the UV of the tool when the material removal is in ductile mode, when the material removal is in brittle mode, the forces heavily vary but the period is different from that of the UV of the tool; (5) the cutting efficiency of the tool is improved by the assistance of the UV. The impact and cutting action at the tool tip on the machining surface are the main factors contributing to the material removal. The observed features were rationalized by analyzing kinematic characteristics of the tool and material removal mechanism in UAS. This study confirms that UAG is a highly effective processing method for machining hard and brittle materials.     

WC涂层纳米划痕过程中摩擦学性能及演化特征的分子动力学研究

为研究纳米划擦过程中WC涂层纳米级摩擦演化特征,利用大型原子/分子大规模并行模拟器建立了不同条件（载荷、划痕深度、划痕速度）下的分子动力学模拟模型。结果表明：摩擦力和摩擦系数随着划痕深度的增加而增大;当压头划擦试样时,沿划痕方向在压头前方及凹槽两侧的原子被挤压、剪切、堆积。瞬时摩擦曲线在初始阶段和稳定阶段表现出明显的摩擦学特征,摩擦过程中压头下方区域晶体出现错位、滑移、间隙或空位。随着划痕速度的增加,体系应变能超过原子间相互约束,原子突破约束,在划痕沟槽两侧堆积,堆积的表面形貌和外缘变得粗糙,亚表面晶体结构产生缺陷。本研究有助于在纳米尺度了解WC涂层摩擦过程的微观磨损机理。     

Improvement effects of vibration on cutting force in rotary ultrasonic machining of BK7 glass

Rotary ultrasonic machining (RUM) exhibits a high potential for a significant reduction in the cutting force, which directly associates with tool wear, machining accuracy, machining temperature, and surface integrity. However, the improvement mechanisms of the ultrasonic vibration on the cutting force are still not fully recognized, restricting the currently optimization methods for further reducing the cutting force occurred during the RUM process. In this research, by incorporating the kinematics principles of the abrasive, the evolution features of the material strain rate in the loading phase were first discussed with respect to the indentation mechanics theory. Taking these features into account, the RUM scratching tests were carried out on the polished specimen surfaces under various process parameters to capture the integrated damage patterns evoked in the high strain rate stage. Following, the comparative indentation tests were respectively conducted on the RUM scratches and the gentle polished surfaces. The indentation-induced damage structures and the load–displacement curves were characterized and assessed to investigate the improvement mechanisms of the superimposed ultrasonic on the cutting force in formal RUM process. It was found that superimposing an ultrasonic vibration led to the incipient cracks nucleated in the abrasive loading phase, and their propagations would increase the material removal rate (MMR) obtained in formal RUM process. Furthermore, the incipient cracks provided a shielding effect to the indentation force, which was a dominant factor in diminishing the cutting force of the diamond tool. The nucleation of the incipient cracks resulted in more energy dissipation after the abrasives penetrating into the hard substrate of the material, which would lead to a higher residual stress on final RUM surface. In addition, a failure pattern (plastic deformation or brittle fracture) evolution model involved in abrasive loading phase was developed with respect to the strain rate effects of the material.     

Scratching of Al2O3 under pre-stressing

Pre-stressing scratching tests have been preformed on polished surfaces of Al₂O₃ ceramic under a Rockwell diamond indenter which moved with uniform speed and constant normal load to investigate how the pre-stress contributes to the material removal mechanism. With the measurement of acoustic emission signals as well as indenter tangential forces, surface damages and cross-section of grooves of Al₂O₃ ceramic were evaluated under the action of different values of pre-stress. It was found that the scratched groove width was increased with the increasing of pre-stress when same normal loads were applied. The existence of pre-stress tends to restrain the crack propagation along the direction of pre-stress, and obvious plastic deformation at the bottom of scratched groove has been observed. Moreover, the fluctuation of tangential force was obviously enhanced, and the magnitude of tangential force in the test of pre-stress was higher than that of without pre-stress. The acoustic emission signals showed that fewer damages were produced in the process of scratching with an appropriate pre-stress. However, the continuing increase of pre-stress would aggravate the machining process.     

钨合金的磨削加工去除机理

目的 解决钨合金磨削加工去除机理不明晰的问题。方法 基于单磨粒刻划有限元仿真、单磨粒刻划和磨削加工实验，探究钨合金的磨削加工去除机理。结果 在刻划过程中，划痕的不同位置材料的去除特性存在显著差异。在单颗粒刻划切入端，材料依次发生了塑性变形、隆起、微裂纹，再到钨相与黏结相的混杂交融。在划痕中段以材料去除为主，出现了材料微卷起和材料卷起现象，沿着刻划方向卷起现象越来越严重。在划痕切出端，划痕边缘和尾部均出现了“飞边”现象，且相较于切入端，切出端的形貌较差，实验与仿真吻合。此外，在不同相位处，材料的去除特性也存在一定不同。在钨相区域，同时存在脆性特征和塑性特征。在黏结相区域，刻划深度较浅时主要呈现塑性变形、塑性流动等特征，刻划中端深度较大时主要呈现与钨相的混杂和交融。在钨相与黏结相的相界处，相邻钨颗粒呈现不同的损伤或去除特征，且相界会阻断特征形貌的传递。最后，磨削后的钨合金表面存在单颗粒刻划痕上出现的所有去除特征，与单颗粒划痕的去除特征吻合。不同的是，磨削后划痕底部出现了区域性和放射状的裂纹。结论 钨合金的两相特性使得磨削表面的去除特征较复杂，存在塑性变形、微裂纹、微卷起、卷起、裂纹和两相交...     

基于不同纳米划痕顺序的6H-SiC单晶片材料去除机理研究

通过微纳米力学测试系统对6H-SiC单晶片(0001)晶面进行不同间距和不同顺序的纳米刻划试验,并用摩擦力传感器、超景深显微系统和三维形貌仪对产生的划痕的划痕横截面轮廓曲线、划痕深度、摩擦力以及表面形貌进行分析,研究单晶片刻划过程中不同划痕间距和划痕顺序下的材料去除过程.结果表明:当静载荷为100 mN时,不同划痕间距...     

激光选区熔化钛合金超声辅助铣削性能研究

目的 针对激光选区熔化钛合金开展超声振动辅助铣削性能和作用机理研究,提高增材制造钛合金表面加工质量、加工精度及加工效率,推动增材制造钛合金构件在高端装备业领域的广泛应用。方法 在传统铣削和超声振动辅助铣削下,采用聚晶金刚石刀具开展激光选区熔化钛合金铣削试验研究,分析不同条件下的表面硬度、切削力、表面形貌、表面粗糙度和切屑黏结的差异性。结果 激光选区熔化钛合金硬度单次测量及其平均值均高于传统钛合金。常规干铣削激光选区熔化钛合金时,切削力随着转速的增大而呈现下降趋势,随着进给速度和切削深度的增加表现出逐渐增大的趋势。在传统铣削下,传统钛合金表面形貌存在明显的刀具划痕,而超声振动铣削时,激光选区熔化钛合金表面形貌总体表现出更加的光滑和平整。激光选区熔化钛合金在常规铣削和超声辅助铣削过程中,刀具前后刀面都出现了严重的钛合金切屑黏结现象。结论 激光选区熔化钛合金常规干铣削时,增大转速或降低进给速度和切削深度能够降低切削力。在相同切削参数下,激光选区熔化钛合金超声铣削质量优于传统钛合金常规铣削表面质量。激光选区熔化钛合金表面质量改善的作用机理主要归因于激光选区熔化钛合金的金相组织特性及超声振动时断...     

Subsurface damage analysis by TEM and 3D FIB crack mapping in alumina and alumina/5vol.%SiC nanocomposites

TEM and 3D crack analysis by focused ion beam (FIB) cross sectioning have been used to quantify the subsurface damage beneath scratches made by a 120° cone indenter loaded to 1 N in monolithic polycrystalline alumina and alumina/5vol.%SiC nanocomposites. In the nanocomposite, an extensive plastic deformation zone was found under the scratch grooves, extending beyond the first layer of grains to a maximum plastic deformation depth of ~7 μm below the surface of the track. In the alumina, however, the plastically deformed region only extends to a maximum depth of ~4 μm and is contained within the first layer of grains adjacent to the groove surface. The 3D morphologies of the cracks under the scratches have been determined by FIB sectioning, showing that a high density of microcracks exists under the scratches in both ceramics. Differences between the plastic deformation and subsurface facture modes of the alumina and the nanocomposite are discussed.