【作者】 赵明利；

【导师】 王裕清； 赵波；

【作者基本信息】 河南理工大学 ， 机械制造及其自动化， 2010， 博士

【摘要】 随着硬脆材料在国防等尖端领域的需求日趋增多,对其零件加工质量的要求也越来越苛刻,但加工过程产生的表面/亚表面损伤成为遏制其广泛应用的瓶颈。因此,探求硬脆材料高质高效的精密加工方法是一项十分有意义的工作。针对超声振动在硬脆材料加工方面独特的优越性以及ZrO2陶瓷特殊的相变性能,本文将超声振动与磨削过程相结合对纳米ZrO2陶瓷进行加工,着重研究多频率超声磨削过程中纳米ZrO2陶瓷材料的损伤机理及损伤特征,揭示超声振动塑性域磨削硬脆材料精密高效的本质。本文主要研究内容包括:基于波动理论对声学系统进行设计计算,通过有限元方法对其优化,研制了不同频率的超声振动磨削装置,给出了结构参数对系统振动特性的影响规律,并通过试验获取声学系统的振动特性。从分析振动频率、振幅、振动方式等参数对磨粒轨迹的影响出发,研究了超声磨削表面的创成过程,结果表明:1)超声振动磨削改变了磨粒运动的直线型轨迹,增强了磨粒之间的干涉,使磨粒在试件表面产生往复熨压、研磨作用;2)二维超声振动能量的叠加使得超声振动对磨削过程的干预能力增强;超声振幅越大,磨粒对材料的干涉宽度越宽;频率越高,磨粒轨迹越密,磨粒对材料干涉的程度增大;3)超声磨削时磨粒前磨刃面对材料产生周期性变化的挤压力,该挤压力的作用相当于压痕过程中压头在材料作用面上不断的加载卸载过程,从而引发细小横向裂纹产生并扩展至表面使材料以细碎和粉末方式去除。建立了超声磨削过程中单颗磨粒的通用磨削力模型以及应力场模型,拓宽了以往研究中单一的声学系统装夹角度、以及两个方向上要求超声波频率相同等苛刻条件,使系统在工作台上不同装夹角度、不同振动频率时的单颗磨粒磨削力模型及应力场模型达到了统一。在压痕断裂力学基础上,建立中位裂纹成核的临界磨削深度模型,并通过试验定性分析了超声振动对裂纹成核临界磨削深度的影响,研究表明高频振动冲击下材料的等效硬度降低,断裂韧性增加,超声振动可以拓宽裂纹成核的临界磨削深度,有助于实现材料的塑性域磨削。研制了能够有效模拟金刚石砂轮磨削过程的超声辅助刻划试验装置,对多频率条件下一维和二维超声辅助刻划时的陶瓷材料损伤情况进行研究,得出材料损伤情况随超声维数和频率变化的规律:二维刻划优于一维超声刻划,一维超声刻划又优于普通刻划;材料损伤与频率之间基本呈上开口抛物线形变化趋势,即在超声频率不高于35kHz时,材料损伤程度随频率增大而降低,频率达到35kHz左右,损伤程度最小,之后随着频率增大损伤程度又逐渐增大。这一规律在磨削试验中得到了验证。通过多频率超声磨削试验得出了表面残余应力随超声维数和频率变化的规律:二维超声磨削表面的残余压应力大于一维超声磨削,普通磨削时最小;残余应力与频率之间基本呈下开口抛物线形变化趋势,即在超声频率不高于35kHz时,残余压应力随频率增大而增大,在35kHz频率磨削时残余压应力达到最大值,之后随着频率增大,残余压应力又逐渐减小。研究表明超声磨削对硬脆材料而言是一种有效的加工方法,尤其是35kHz二维超声振动磨削时,超声振动对材料的干预能力大大增强,扩大了材料塑性域去除的范围,材料表面亚表面损伤情况显著改善,磨削表面产生较大的残余压应力,在一定范围内可有效闭合磨削表面微裂纹,提高材料的强度。更多还原

【Abstract】 With the increasing demand of hard brittle materials in national defense and other cutting-edge fields, requirements for part quality become increasingly harsh, but the surface and subsurface damage generated in machining process is the bottleneck hindering its widespread applications. Therefore, it is very significant to seek the high quality and efficiency precision machining method for hard brittle materials.According to the unique superiority of ultrasonic vibration for hard brittle materials and special phase transition property of ZrO2 ceramics, nano-ZrO2 ceramics was machined by combination of ultrasonic vibration and grinding in this paper, and the damage mechanism and characteristics of nano-ZrO2 ceramics in multi-frequency ultrasonic vibration grinding were mainly studied, so as to reveal the precision and high efficiency essence of ultrasonic vibration ductile grinding hard brittle materials. The main contents of this study are as follows:The acoustic systems were calculated and designed on the basis of the wave theory,and optimized by finite element method. The ultrasonic vibration grinding devices with different frequencies were developed, influence rules of structural parameters on system vibration characteristics were got, and vibration performances of devices were obtained by experiments.From the analysis of effects of vibration frequency, amplitude and mode on abrasive trace, the ultrasonic ground surface generation process was discussed. The results show that: 1) Ultrasonic vibration can change the straight line trace of abrasive grains, and enhance the interference between abrasive grains, thus, abrasive grains can play a reciprocating extrusion and lapping role on the ground surface. 2) The energy superposition of two-dimensional ultrasonic vibration reinforces the ability of ultrasonic vibration intervening to grinding process. The bigger of ultrasonic vibration amplitude, the wider interference width of abrasive grains to material, and the higher of frequency, the denser of abrasive trace, so the interference degree of abrasive grain to material is increased. 3) In ultrasonic grinding, periodic vibration extrusion force is generated by rake faces of abrasive grains, and the effect of this extrusion force is equivalent to the indenters’loading and unloading process on action surface of material during indentation, therefore fine transverse cracks generated and propagated to surface and make the material removal in fine crushing and pulverizing.In the present paper, the general grinding force model and stress field model of single-grit in ultrasonic grinding were built, which broaden harsh conditions of previous studies, such as single clamping angle of acoustic system and the same ultrasonic frequency in two directions, in addition, unify the grinding force model and stress field model of single grit with diverse clamping angles and different vibration frequencies.Based on indentation fracture mechanics, the critical grinding depth model of radial crack nucleation was built, and the effect of ultrasonic vibration on the critical grinding depth was qualitatively analyzed through the experiment. The results show that under high frequency ultrasonic vibration, the material equivalent hardness is decreased, and its fracture toughness is increased. Ultrasonic vibration can widen the critical grinding depth of crack nucleation, and contribute to the ductile grinding of material.The experimental devices of ultrasonic assisted single-grit scratching were developed, which can effectively simulate the grinding process of diamond wheel. From the study of ceramics damage on the condition of one dimensional and two dimensional ultrasonic vibration assisted scratching with multi-frequency, the rules of material damage with ultrasonic dimension and frequency variation were obtained. The research shows the damage in two dimensional ultrasonic assisted scratching is better than that in one dimensional and the scratch in one dimensional ultrasonic is better than that without ultrasonic; the material damage shows parabola with open side up changing along with frequency, in other words, the damage decreases with the growth of frequency under 35KHz, and the damage degree is the lowest at 35KHz, later the damage increase with frequency increasing. The above rules were validated in grinding experiment.From multi-frequency ultrasonic grinding test, the rules of the surface compressive residual stress changing with ultrasonic dimension and frequency can be drawn, that is, the surface residual stress in two dimensional ultrasonic grinding is bigger than that in one dimensional with the same frequency, and it is minimum in common grinding; the surface residual compressive stress shows parabola with open side down changing along with frequency, that is, the residual stress increases with the growth of frequency under 35KHz, and the residual compressive stress is the biggest at 35KHz, later the residual stress decreases with frequency increasing.The results show that ultrasonic vibration assisted grinding is an effective method for hard brittle materials. Especially in two-dimensional ultrasonic vibration grinding with 35kHz frequency, the interference ability of the ultrasonic vibration to machined material was greatly enhanced, and the scope of material ductile regime removal was expanded, so that the surface and subsurface damage was significantly alleviated. The relatively large compressive residual stress which generated on the ground surface can close surface micro-cracks effectively and improve the material strength in a certain range.更多还原