Papers by Author: Bo Zhao

Abstract: The experimental research on CBN grinding wheel mechanical dressing assisted by ultrasonic vibration was carried out. Experimental results showed that ultrasonic vibration dressing is capable to improve surface topography and roundness of CBN wheel as other untraditional dressing methods. Compared with traditionally mechanical dressing, the counts of static effective grains of CBN grinding wheel dressed assisted by ultrasonic vibration increased clearly and it had clear relationship with the acoustic parameters and the dressing lead. The grains had a large protrusion height and kept good integrity after ultrasonic dressing. The technology of vibration dressing is able to prepare precisely the CBN grinding wheel.

Abstract: Because of the high hardness and high brittleness of ceramics, crack damage would be easy left in the finished surface. This will lower the reliability of ceramics parts. Studies show that Ultrasonic vibration Added Grinding (UAG) can greatly improve the surface quality, but how UAG to reduce the surface/subsurface damage isn’t very clear. To this purpose, the surface/subsurface crack mechanism of the ultrasonic vibration added grinding on ceramics was mainly analyzed in this article. The study shows that ultrasonic vibration can change the crack propagation direction to free surface, rather than propagate to the deep material which will happen in conventional grinding. And this will reduce the surface/subsurface crack defects.

Abstract: The surface roughness of machined surface is one of the important indicators to evaluate the workpiece machining quality. In order to clarify the change law of surface roughness in ultrasonic aided high-speed lapping with solid diamond abrasive, a series of ultrasonic and conventional lapping experiments of different engineering ceramics, such as Al2O3, ZrO2 and ZTA, were carried out on precision lathe with self-made high-speed ultrasonic lapping device. The research results show that the value of surface roughness in ultrasonic aided lapping is lower than that in conventional lapping. With the assistance of ultrasonic vibration, the surface quality can be improved obviously. In ultrasonic aided lapping, there is an optimum lapping pressure from which a better surface roughness value can be obtained. Because of different physical and mechanical properties, material removal mechanisms of the three kinds of engineering ceramics present different characteristics, which lead to the different surface roughness even though under the same lapping conditions. The research of the paper is helpful to optimize lapping technique and improve the surface quality.

Abstract: For the precision engineering ceramics parts, semi-ductile regime machining technologies are usually adopted to obtain the final surface. It is very important to research the brittle-ductile material removal characteristics and corresponding monitoring technology in ultrasonic aided lapping of engineering ceramics to obtain better surface quality. A series of lapping tests were carried out in the paper and the influence law of lapping parameters on the ductile percentage of ultrasonic lapped surface was achieved, which enables to off-line identify the material removal characteristics. Though the material removal characteristics can be online judged through monitoring the ratio of lapping component forces qualitatively, the monitoring effect is worse just because of lower response frequency of dynamometer and the relativity of lapping force. In this paper, a novel online monitoring technology of material removal characteristics is put forward based on AE technology. Through the wavelet package analysis of AE signals, the discrimination index and standard of material removal characteristics can be obtained. It is proved that this technology can realize online identification of the brittle-ductile material removal characteristics in ultrasonic aided lapping of engineering ceramics effectively.

Abstract: Based on good processing property of two-dimentional ultrasonic vibration assisted grinding (TUVAG), the precision finishing of nano-zirconia toughened alumina ceramics (nano-ZTA) is carried out. According to theoretical analysis, TUVAG may obtain higher machining efficiency and better surface quality. Especially, experimental results show it may obtain the compressive stress in the finished surface of nano-ZTA that may restrain the expansion of surface microcrack, and surface residual stress of nano-ZTA under TUVAG differs from that under diamond grinding, and TUVAG may obtain the better surface quality of nano-ZTA than diamond grinding, as is characterized by scanning electronic microscope (SEM). As a result, it is good for TUVAG as a kind of processing method for nano-ZTA.

Abstract: The effects of ultrasonic frequency on the grinding force and surface quality were analyzed from the grinding experiment on ZTA nano-composite ceramics. The results indicate that, in the same parameters, ultrasonic normal grinding force is about 65 to 85 percents of the ordinary grinding condition, and the surface quality is better than that of ordinary grinding condition, as the frequency increasing, this trend will be more evident. The reasons for it were also discussed from microscopic and the nonlocal theory in this paper.

Abstract: This paper presents a research on the effects of ultrasonic frequencies of 19.9 kHz, 23.6 kHz and 29.3 kHz on both of cyclic amplitude and fatigue life of Al2O3-ZrO2 nano-composite ceramics. According to the high-cyclic-life tests at low load level, some prohibition influence on the process of crack initiation and propagation under ultrasonic vibration is revealed as frequencies are raised from 19.9 kHz to 29.3 kHz. During the low load level tests, the toughness increment induced by the transcrystalline fracture is found in the scanning electronic micrographs of the fractures, which shows the existence of the inhibition effect on the crack initiation and propagation. At the same time, the fracture XRD indicates that t-m phase transition of ZrO2 under 29.3 kHz is higher than that under 19.9 kHz, which means that both the required cyclic amplitude and the fatigue life can be improved if the ultrasonic frequency is increased to 23.6 kHz. All the test results show that high frequencies exhibit stronger toughening improvements than the lower ones do. This paper is a pre-research for the ultrasonic ceramic grinding and the conclusions obtained are meaningful for the study of the crack propagation mechanism during the grinding.

Abstract: A new dressing device for dense type diamond grinding wheel, whose elliptic vibration was prompted by single longitudinal vibration, was designed using a local resonance method. It was employed to dress the metallic bond super-abrasive grinding wheel and the dressing force characteristics were analyzed. The results indicate that the thrust force decreased to 1/7-1/10 of that of conventional method, and the main cutting force decreased by 70%. Compared to other dressing parameters, the dressing force in elliptic ultrasonic vibration was more sensitive to grinding speed. Thrust force and main cutting force increased rapidly when rotation speed beyond 72rpm. But when the ultrasonic power was higher than 50W, dressing force decreased very slowly. It was observed that the new dressing method is contribute to better surface finish such as more even surface, less crush abrasives, more uniform abrasive abscission and so on than the conventional method.

Abstract: ZTA ceramics is a kind of high performance structural ceramics, and its surface/subsurface damages after machining have great effect on reliability and long-service life of ceramic parts, so it introduces two-dimensional ultrasonic vibration assisted grinding (TUVAG) which is applied in the precision finishing of ZTA ceramic, mathematic motion model of abrasive grain during TUVAG is established, and the special machining path of abrasive grain during TUVAG is analyzed, and the surface/subsurface damages of ZTA ceramics after TUVAG are further discussed. Experimental results prove that TUVAG may make materials removed in a plastic state mostly, the probability of cracks formation and propagation becomes less, and obviously reduce surface damages of ZTA ceramics, and the grinding badness layer of ceramics after TUVAG is shallower than that after diamond grinding at the same grinding parameters.

Abstract: Experimental research on grinding characteristics of nano-composites ceramics were carried out using diamond wheel in two-dimensional ultrasonic vibration grinding (TDUVG) and the topography of the scratched grooves were observed with Transmission Electron Microscope. The surface crack, plastic deformation and brittle-ductile transition during chip formation were analyzed. Subsequently, the grinding model of nano-composites ceramics machining in TDUVG was established based on fracture theory of micro-indentation in fracture region and plastic information mechanism of hard-brittle materials. The metamorphic layer can be divided into three regions: brittle-fracture, non-elastic deformation and residual damage region. Ultrasonic grinding process can be divided into three phase: elastic friction and extrusion, crack growth and crack propagation and chip formation.