Papers by Author: Bo Zhao

Abstract: Ultrasonic lapping can effectively improve machining efficiency and obtain higher machining precision, which can be as the final process. This paper discusses the ultrasonic lapping mechanism through simulating the lapping tracks. Based on the contrast experiments between common and ultrasonic lapping, the influence of different lapping parameters on material removal rate and surface roughness were studied and the surface microstructure of workpiece was analyzed. The experimental results show that material removal rate of ultrasonic lapping is nearly two times that of the common lapping, especially in ultrasonic axial lapping, and the better surface quality can be obtained during ultrasonic lapping. Meanwhile, the reasons for material removal rate increasing and surface quality improving were analyzed.

Abstract: This paper researched the crack propagation and the variation of microstructure for ZTA ceramics under ultrasonic vibration grinding. To interpret the phenomenon, the nonlocal theory was introduced to establish the constitutive model under the ultrasonic vibration. Grinding experiments on surface quality of nanoZrO2 ceramic are carried out using diamond grinding in different condition, both with and without ultrasonic vibration. Experimental results show the surface quality after two-dimensional ultrasonic assisted grinding is superior to that after common grinding. Meanwhile, the effects of amplitudes and frequencies on ZrO2 ceramics crack propagation were analyzed by SEM. The results show that the crack propagation rate decrease with the amplitudes and frequencies increasing, this is because that the nonlocal attenuation ratio reduced and the nonlocal effects also the strongest. The results of experiments are in accordance with the theoretical conclusions drawn from the nonlocal theory, which verifies that the act ion of ultrasonic vibration can actually postpone the ceramic crack propagation rate.

Abstract: Carbon/carbon composite is composite material using carbon fiber as reinforcement and carbon as matrix. However, it is the difficult-to-machine material due to some characteristics of the material. The thesis uses ultrasonic milling cutting in order to improve the surface quality. Surface quality becomes better with the improvement of milling speed; surface quality becomes poor with the increases of per tooth feed and cutting depth; S_ku increases after adding ultrasonic, and which explains that the mechanism of ultrasonic milling is different from that of ordinary milling; the increase of S_al indicates that surface defects of all kinds reduces in ultrasonic milling and the surface quality is better than that of ordinary milling.

Abstract: In order to better reveal the theoretical nature of internal ultrasonic grinding which is an efficient grinding technology, the characteristics of grits motion is analyzed under axial vibration and the grits kinematic equation is established. By using Matlab the single particles trajectory is simulated, then the displacement of a grit is given at any time they contact. Through the theoretical analysis and Matlab simulation, we can achieve that for axial ultrasonic vibration the influences of axial feed on the grits trajectory is too litter to be ignored. So when calculate the contact length of the grinding wheel and the work piece, the grinding wheels axial feed can be neglected. Then the contact length of the grinding wheel and the work piece is achieved. It has a significant value for simplifying the calculation of the grinding force and the analysis of internal ultrasonic grinding.

Abstract: Based on similarity principle, cutting modes of turning, milling, grinding etc are equivalent to three load forms of dot, line and face. And three different loads of dot, line and face are applied on the nano-ZrO2 workpiece by three different indenter models of dot, line and face. To one-dimensional ultrasonic vibration system, the influence is studied which is of different load forms and different load magnitudes on acoustic system characteristics of ultrasonic machining. The experimental results show that, the change of loads has little influence on the acoustic system parameters of ultrasonic machining when the load modes on the nano-ZrO2 workpiece are dot and line. And when the load mode is surface contact, the change of loads has significant influence on the acoustic system parameters of ultrasonic machining.

Abstract: The nonlocal theory was introduced to establish the nonlocal constitutive model and analyze the effect of ultrasonic on the tensile strength and fracture surface microstructure. ZTA ceramics tensile fracture experimental results show that the tensile stress obviously reduces and the fracture changes under ultrasonic vibration. The experimental results are coincided with theoretical analyses.

Abstract: Through considering the influence factors of ultrasonic excitation parameters, the nonlocal kernel parameter formula under ultrasonic excitation is established and the nonlocal elastic kernel function is also deduced. The attenuation characteristics and effects of the elastic kernel function are also studied in this paper. The nonlocal stress under ultrasonic vibration is calculated based on the nonlocal elastic kernel formula. It is concluded that stress decay phenomenon can explain through nonlocal kernel function attenuation in frequencies.

Abstract: Hard brittle materials have been popular used in manufacture experience because of the excellent performances. However the low plasticity, frangibility and non-conduction make the machining very difficult. Therefore, ultrasonic vibration was introduced to common lapping for the purpose of higher removal rate and fine surface quality. The acoustic model of ultrasonic lapping was proposed, the track equations of ultrasonic lapping with different vibration modes were deduced, the influencing factors of lapping tracks were obtained, and the lapping tracks with different lapping parameters had been emulated. It can be concluded that ultrasonic lapping is a high efficient processing method adapting to ceramic materials.

Abstract: The surface residual stress of nano-composite ceramics prepared by mixed coherence system and machined both in ordinary grinding(OG) and ultrasonic vibration grinding(TDUVG) respectively were described. Researches showed that the characteristics of residual stress on ceramic surface is affected by grinding load, ultrasonic vibration frequency and released stress induced by generation of cracks whose magnitude is affected by material removal mechanism. Whether in ordinary grinding or ultrasonic grinding, tangential residual stress (TRS) in X direction is tensile stress, and in Y direction is compressive stress. The surface residual tensile stress in ordinary grinding is larger than that in ultrasonic grinding with the same grinding conditions. The influences of multi-frequency on the residual stress are studied.

Abstract: The surface roughness of nano-composite ceramics machined by means of multi-frequency ultrasonic vibration grinding was investigated with the diamond grinding wheel dressed by elliptical vibration. The results indicate that the surface roughness is influenced by ultrasonic vibration frequency, feeding speed, grinding depth. The ultrasonic vibration frequency and the grinding depth are not the higher the better and there are suitable values. The surface roughness gets worse with the increase of feeding speed.