【作者】 刘国东；

【导师】 祝锡晶；

【作者基本信息】 中北大学 ， 机械设计及理论， 2013， 博士

【摘要】 功率超声珩磨磨削过程中常采用煤油或乳化液作为切削液,在超声波作用下磨削区会出现超声空化。空化泡通常处于非稳定状态,不断地膨胀、压缩甚至溃灭,当空化泡溃灭时会在磨削区产生冲击波和微射流等一系列非线性声学现象,这些现象已经在功率超声振动加工界引起可足够重视。基于空化效应来研究功率超声珩磨技术,对于完善功率超声珩磨磨削机理具有重要的现实意义。本文从磨削区空化产发生机理、空化泡动力学、空化效应及空化声场等方面来研究功率超声珩磨磨削区的空化。主要研究内容及结论如下：(1)揭示了磨削区空化发生机理。在珩磨头体的扰动下,连续的冷却液被离散成大量微小的液滴,出现少量游移的空化泡,进而在超声振动下发生超声空化。通过求解势流动伯努利方程和拉普拉斯方程,得出油石表面超声振动声压幅值远超过切削液的空化阈值,理论上验证了油石表面必然会发生超声振动空化效应,并进行了实验验证。(2)通过建立磨削区单个球形空化泡的模型,计算出空化泡完全闭合所需时间、气泡表面流体径向速度以及气泡闭合释放的冲击压力。在此基础上,对油石表面半球状空化泡群动力学模型进行计算,得出在接近泡群中心无量纲半径r’=70-80处,溃灭会产生明显的局部压力脉冲现象。(3)研究了空化产生的噪声、空蚀和颤振对功率超声珩磨的影响。利用声波的反射、折射和镜像原理建立了油石壁面附近单、双空化泡辐射声压模型,并对溃灭产生的辐射声压进行了数值模拟。通过计算空化泡群在壁面上溃灭时压力脉冲发生几率,得出空化泡群内只有少数大尺寸空化泡溃灭释放的压力脉冲才能导致固壁空蚀,并带来材料塑性和脆性破坏。建立了功率超声珩磨再生型颤振动力学模型,得出当考虑空化泡溃灭作用时,会加剧系统颤振的频率,而对功率超声珩磨颤振振幅的基本没有影响。(4)利用有限元分析方法,建立了谐振系统——流体介质——刚性圆柱形吸收边界的耦合声场模型,得出油石表面声压幅值与声场分布的关系：当谐振系统处于谐振动状态时,产生的声场均匀分布在油石座及油石外轮廓线上,出现了声压幅值的集中,油石表面振动空化强烈且分布均匀,主要集中在油石中部区域。这表明谐振系统声场的分布直接影响空化效应的强弱,而且通过声场的分布还能判定谐振系统是否处于最优振动状态,为谐振系统的优化设计以及振动控制提供了一种重要的技术支持。(5)通过弱酸试纸法定性测量了功率超声珩磨磨削区空化声场,得到了不同超声频率下声场分布,并定量测量出油石表面最大声压幅值为1.3MPa,验证了谐振系统声场有限元分析的科学性和可靠性。更多还原

【Abstract】 Power ultrasonic honing during grinding often uses the kerosene or emulsified liquid as the cutting fluid. When the ultrasonic wave passes the grinding zone, the cavitation effect can happen. The cavitation bubbles in the grinding area usually are unstable, constantly grow, expend, compress and even collapse. When the cavitation bubble collapsed, shock wave can be generated and a fluid microjet can be released, which attract enough attention in the power ultrasonic vibration industry. The study on the technology of power ultrasonic honing from the perspective of cavitation effect is very important for the grinding mechanism of the power ultrasonic honing. The cavitation mechanism in the grinding zone can be explained. The dynamics of cavitation bubble and the effects of cavitaiton can be also discussed.The work of this paper and main results are as follow:(1) The mechanism of cavitation in the grinding region of power ultrasonic honing was explained. The process of cavitation occurrence was explained. The continuous cooling liquid can be dispersed into a large number of tiny liquid for the perturbation of the ultrasonic honing head body, and then the cavitation can occur with the ultrasonic vibration. The cavitaion threshold value in the grinding area was calculated. The pressure of the oilstone surface was greater than the cavitaion threshold value of cutting fluid with Flow and Bernoulli equation and the Laplace’s equation, which verified the cavitation effect occured on the oilstone surface, and this theoretical result fits the experiments well.(2) The dynamic model of an individual cavitation bubble in the grinding zone was established. The desired closed time range of the spherical cavitation bubble completely and its collapse distance were obtained. And then, the dynamics model of the hemispherical cavitation bubbles cluster on the oilstone surface can also be established. The numerical results show that the distance between the center of the bubbles cluster and its boundary of cavitation bubbles cluster was about the dimensionless radius r=70～80, which can generate significant local pressure pulse phenomenon.(3) The influence of the cavitation noise, cavitation erosion and chatter caused by cavitation on the power ultrasonic honing can be discussed. Taking the reflection, refraction and mirroring principle of ultrasonic in the solid surface into consideration, the radiated acoustic pressure model of single and double cavitation bubbles near the oilstone wall was studied. The occurrence probability of the pressure pulse generated by collapsing cavitation bubbles on the oilstone was calculated. The collapsed pressure released by a few large cavitation bubbles can generate cavitation erosion on a rigid wall, and even bring ductile deformation and brittle fracture.Comparing the model of power ultrasonic honing chatter with considerations of power ultrasonic cavitation noise, it is observed that the chatter time domain should be short when considering cavitation noise, but there is not obvious effect to the ultrasonic vibration amplitude.(4) The fluid-coupled acoustic model of resonant system, fluid medium and rigid cylindrical absorbing boundary were established. The acoustic field distribution of the renonant system and its relationship between the resonant frequency and the vibrational state can be obtained. The results show that the sound field of resonant system is uniformly distributed in the outer line of the oilstone and oilstone seat, and that the amplitude values focus on the surface of the oilstone, when the resonant system is located in the optimal vibration status. It presents that the strength of the cavitations can be determined by the distribution of the resonant system directly, and the optimal resonant state can also be evaluated. This method provides a crucial support for the designing and optimization of resonant system.(5) The cavitation acoustic field in the grinding area was measured qualitatively by a weak acid paper method. The acoustic distribution by different ultrasonic frequency can be obtained, and the maximum sound pressure on the oilstone surface amplitude is1.3Mpa. Compared with the experiments, the method used for acoustic field analysis of resonant system is verified reasonable and feasible.更多还原