【作者】 马辉；

【导师】 赵波；

【作者基本信息】 上海交通大学 ， 机械制造及自动化， 2011， 博士

【摘要】 针对陶瓷的脆性、不均匀性以及较低的韧性和强度,超声波加工作为最有效的方法之一被提出。为了更好的研究陶瓷材料的超声波加工技术,解释超声磨削中的高效延性的现象。本文基于非局部理论,着重于探索超声波与ZTA纳米复相陶瓷材料的作用机理,从磨削力特性、材料去除效率和磨削表面微观形貌等方面阐述了超声频率对陶瓷加工特性的影响,揭示了超声加工时所产生的延性高效现象的理论本质,为进一步完善超声加工的理论奠定了基础。本文主要研究内容包括:通过对ZTA纳米复相陶瓷的内禀长度和外部超声波长之间作用关系的研究,分析了材料在超声振动作用下的力学行为,引入非局部理论,建立了超声下的本构模型。通过ZTA纳米复相陶瓷试件波速实验,得出波速与频率的关系,解释了波速的衰减现象。根据波速的测量结果,通过计算获得了标准化非局部模量(非局部模量与经典模量的比值)随频率以及影响域的变化规律。针对标准化非局部模量进行积分计算,得出超声下的非局部应力衰减规律。通过超声磨削实验,研究了超声振动频率对磨削力的影响,进一步验证了超声下的本构模型。针对超声磨削过程中磨削表面裂纹的疲劳断裂过程在动态下难以测量的情况,设计了疲劳等效实验,从而模拟实际加工过程的受力状况进行磨削疲劳研究。本文着重于研究大进给量磨削(5μm-15μm)和小进给量磨削(2μm-5μm)两种情况,分别采用大载荷循环加载和小载荷循环加载的两种实验方式进行分析。针对大载荷循环加载,采用修正后的Morrow方程进行等效转换,并进行疲劳等效实验,得出超声磨削裂纹断裂寿命与应力振幅的关系。而对于小载荷循环加载,通过逐级加载的实验方式,获得了不同超声频率下的裂纹断裂时间与强度,从相似的角度揭示了在小进给磨削过程中,磨削表面裂纹随超声频率升高而受到较强抑制作用,表面质量得到提高。为了获得超声频率对ZTA纳米复相陶瓷磨削效率(超声下临界切削厚度、脆延转变以及材料去除率)的影响规律,通过超声振动下轴向拉伸实验,研究超声下的断裂强度和动态断裂韧性随频率的变化规律。对比不同频率下的拉伸力学行为及试件断口微观特征的变化,并基于非局部理论的应力衰减原理,解释了超声下断裂强度随频率升高而降低的原因。通过超声振动下的磨削表面硬度实验以及磨削率实验,得出超声下的等效硬度值及磨削率随超声频率的变化趋势。根据实验结果分析得出,超声振动作用增大了临界切削厚度,扩大了脆延转变范围,提高了材料磨削去除率,进一步阐述了超声磨削中的高效延性本质特征。通过超声刻划及超声磨削实验,针对0kHz、19.9 kHz、29.3 kHz和35.6 kHz多个频点下的超声振动磨削表面质量进行研究,借助扫描电镜、XRD、白光干涉仪等检测手段,对磨削表面粗糙度、相变以及磨削表面轮廓进行检测,发现超声振动频率能够对ZTA纳米复相陶瓷磨削表面微观特性以及材料去除方式产生影响,改变了超声振动下的单颗磨粒刻划轨迹,从本质上解释了表面质量提高的原因。通过超声磨削表面的XRD物相峰形图来分析超声频率对相变的影响规律,进一步从微观的角度说明了表面质量与磨削频率的关系。更多还原

【Abstract】 Due to the large brittleness, bad uniformity, low reliability, less malleability and intensity, ultrasonic machining is used as an effective supplementary method for finishing advanced ceramics. In order to better research the ultrasonic machining technology and explain the essential characteristic of high efficiency and ductility. Based on the nonlocal theory, the research pays more attention to the action mechanism of ZTA nano-composite ceramics and ultrasonic vibration to illustrate the effects of ultrasonic vibration on grinding characteristics that from aspects of grinding force, grinding frequency, material removal rate and SEM of grinding surface etc, which discloses the essential characteristic of high efficiency and ductility and establishes the theory base. The main work and achievement of the paper is as follows:The mechanics behavior under the action of ultrasonic vibration is analyzed according to the relationship between the internal character length and the external character length （i.e. ultrasonic wave length） for ZTA composite nano-ceramics. To explain this phenomenon, the nonlocal theory is introduced to establish the constitutive model under the ultrasonic condition. The wave velocity attenuation and the relationship between wave velocity and frequency are obtained through the wave velocity experiment. According to the results, the variation of normalized nonlocal moduli with different frequencies are calculated （the ratio of nonlocal moduli and classical moduli）. When the normalized nonlocal moduli are integrated, the nonlocal stress attenuation rule can be obtained. The influence of ultrasonic frequencies on grinding force is researched through the ultrasonic grinding experiment, which also verifies the constitutive model under the action of ultrasonic vibration.As the fatigue and fracture process of grinding crack is difficult to measure during the ultrasonic grinding, the equivalent experiment is designed to study the actual grinding process. Both large （5μm-15μm） and small （2μm-5μm） feeding process are researched with corresponding loading styles. As the large load, the equivalent experiment is designed according to the modified function of Morrow, and the relationship between the crack fracture life and stress amplitude can be obtained. Otherwise, as the small load style is utilized, the relationship between fracture strength and life can be also acquired by specified loading process with step by step. In this way, the conclusion can be acquired that the suppressed grinding cracks and enhanced grinding surface quality can be achieved with higher frequencies.In order to obtain the influence of ultrasonic frequencies on the grinding efficiency, i.e. critical cutting thickness, brittle-ductility transition and material removal rate, the variation of the fracture strength and dynamic fracture toughness with frequencies are researched through the axial tensile experiment. Based on the nonlocal theory, the mechanical behavior and microcosmic characteristic with different frequencies are compared to explain the reduction of fracture strength. Through the experiments of equivalent grinding surface rigidity and grinding material removal rate, the change trend of equivalent rigidity and material removal rate with frequencies are acquired. According to the results, the action of ultrasonic vibration enlarges the critical cutting thickness, increases the transition range and the grinding material removal rate, by which the high efficiency and ductility characteristic is illustrated.Through the ultrasonic single-point scratching test and ultrasonic grinding experiment, the surface quality of ultrasonic grinding using frequencies of 0 kHz, 19.9 kHz, 29.3kHz and 35.6 kHz are researched respectively. It is found that the microcosmic characteristic and material removing style can be influenced by ultrasonic frequencies according to the experiments of grinding surface roughness, phase transition and surface profile,which illustrates the reason of the improvement of surface quality essentially. Finally, through the XRD experiment, the phase transition of ZrO₂ with the action of ultrasonic vibration can be observed, which further shows the relationship between surface quality and grinding frequencies in microscopic views .更多还原