Dynamics And Nonlinear Vibrations Of Cavitation Bubbles Of Grinding Fluid Under Ultrasonic Vibration Honing

In the process of ultrasonic vibration honing(UVH), on the one hand, abrasives of honing stone are employed with mechanical vibration with constant high amplitude and high frequency. On the other hand, when the vibration of honing stone goes through the grinding fluid, it can cause cavitation effect, and generates a great number of cavitation bubbles. Cavitation bubbles with intensive vibration and they produced collapse effects such as shock wave and micro-jet impact continuously the grinding fluid and solid boundary around it, which can significantly improve the performance of grinding conditions exceedingly. It plays very important role in honing processing, in particular for cleaning honing conditions, enhancing machining precision and efficiency, suppressing chatter and noise and so on. At present, though many fundamental research attempts have been seen to deal with controlling and utilizing the cavitation bubbles of grinding fluid, the cavitation research is until now not fully understood, due to its numerous influencing parameters, and difficult real-time detection during processing, and complicated cavitation mechanism in different research environments, and especially in the UVH engineering field. Therefore, this paper prepares to study the cavitation of grinding fluid under UVH, from the point of view of cavitation mechanism, cavitation bubble dynamics under different boundary conditions, and nonlinear oscillation of cavitation bubbles etc. In this paper, the main work is described in the following aspects:1. The cavitation mechanism of grinding conditions under UVH is studied. Sound field analysis is carried out, by means of considering the coupling effect of structure-piezoelectric and structure-fluid for acoustic vibration system of UVH. Compared to the other locations, the middle zone of honing stone is easy to produce cavitation. Some concepts to help to account for cavitation inception such as “mobility” cavitation nuclei of grinding fluid, “instability”cavitation nuclei of honing stone, and “fixity” cavitation nuclei of honing cross hatch are provided. The cavitation number and cavitation threshold of grinding fluid are also revealed. Experimental observations on the grinding fluid of honing stone surface are carried out using the depth of field microscope. Based on the cavitation memory effect, the net distribution cavitation bubble groups of grinding fluid(water and kerosene) are found.2. The dynamical behaviors of single and double cavitation bubble of grinding fluid with free-surface condition under UVH are analysed. Firstly, the dynamic model of a single cavitation bubble with free-surface condition is established with consideration of ultrasonic honing velocity and honing pressure. The results show that the grinding fluid under UVH is easy to generate stable cavitation. The cavitation effect on the grinding fluid can be suppressed by honing pressure and fluid static pressure, and a critical point for the transient cavitation converted to the steady cavitation can be obtained, using control honing pressure reasonably. Secondly, based on the superposition principle of fluid velocity potential, the dynamic model of double cavitation bubbles in the grinding fluid with free-surface condition is established. A higher ultrasonic acoustic pressure and lower honing pressure obtained can also perform a better cavitation effect when the functional relationship between ultrasonic acoustic pressure, honing pressure and fluid static pressure need to stratify the following equation: α< pa –(p0+ph) <β(in this example, α=0.66 MPa,β=1.89 MPa). Thirdly, the dynamical behaviors of single and double cavitation bubble of grinding fluid are compared with each other. The results show that the application situations of bubble model can be determined by the critical values of pressure amplitude and fluid static pressure. The movement behaviors of the bubble under ultrasonic vibration honing and traditional honing are also compared. It can be found that the cavitation of grinding fluid in traditional honing is weaker than that in ultrasonic vibration honing.3. The dynamical behaviors of single and double cavitation bubble of grinding fluid near a plane rigid wall under UVH are analysed. Using the method of mirror image, the dynamic models of single and double cavitation bubbles near a plane rigid wall are derived. The results show that the effect of UVH on the bubble plays a significant role in suppressing the bubble oscillation with free and rigid boundary, and it can also limit the bubble interaction near a plane rigid wall. There exists an optimal acoustic pressure, at which the collapse of a bubble near a rigid wall can be the most violent, but the optimal acoustic pressure is not influenced by the bubble interaction. Moreover, the optimal acoustic pressure(almost 5.5 times bigger than ambient pressure) for UVH is higher than that(almost 3.5 times bigger than ambient pressure) for ultrasonic field. Furthermore, the relationship between the collapse velocity of a bubble near a rigid boundary and its micro-jet is declared. The results demonstrate that the velocity of micro-jet is dependent of that of bubble collapse, and it can be controlled by adjusting the velocity of bubble collapse indirectly.4. The nonlinear oscillation of single and double cavitation bubble of grinding fluid near a plane rigid wall under UVH is explored. Based on the bubble assumption of small amplitude oscillation, the resonant frequency and vibration displacement of single and double cavitation bubble with free and rigid boundary are derived, and then the velocity and pressure field distribution of that bubble near a plane rigid wall are obtained. The effects of initial bubble radius, ambient pressure and gas concentration inside a bubble on the resonant frequency of cavitation bubble are discussed. It can be seen that the analysis of resonant frequency of cavitation bubble can be used to come to the best match of ultrasonic energy and cavitation bubble energy. The resonant mode of the single and double cavitation near a plane rigid wall is sensitive to the external influence parameters. When the cavitation bubble with rigid boundary is in the resonance state, the most value of its velocity and pressure field focuses on a tiny space of the opposite side of the inner material from bubble. The amplitude of the velocity field for double cavitation bubbles is four orders of magnitude lower than that for single cavitation bubble, but its amplitude of the pressure field is two orders of magnitude higher than that for single cavitation bubble.5. Experiments of the cavitation intensity of grinding fluid are carried out. The cavitation intensity of grinding fluid near the different positions of honing stone surfaces is measured quantitatively by using acoustimeter. The results show that the cavitation of grinding fluid is occurred on the position near the external surface of the honing stone, and when the equipment of UVH is in the resonance state, the dramatic cavitation intensity of grinding fluid can be obtained. Nevertheless, when the ultrasonic frequency is deviated from the resonant frequency, the cavitation intensity of grinding fluid can also be explained by the theory of dynamics analysis of cavitation bubble. A surface roughness method of the material is provided to demonstrate indirectly the cavitation effect, which testifies the corresponding relationship between the surface roughness of the material and cavitation parameters. In addition, erosion pits generated by cavitation bubbles were observed by using the method of aluminum foil erosion, and the dimensions of a single erosion pit are larger than that of two adjacent pits, which fit the theoretical analysis well. What’s more, the impact strength produced by the cavitation bubble is much greater than that by abrasives.