一(yī)、Rotary ultrasonic machining of brittle materials

Rotary ultrasonic machining (RUM) is widely used in the processing of brittle materials. It is a special processing technology aiming at the technical problems of hard and brittle materials, which is composed of common grinding and ultrasonic vibration. Most rotary ultrasonic machining tools are electroplated with a layer of diamond or CBN and other high-hardness materials at the bit position. During the machining process, the tool will rotate at high speed and vibrate at micron level along the tool axis, which can effectively reduce cutting force, residual stress and surface damage, improve machining accuracy and efficiency, and prolong tool life. It can be clearly seen from the effect diagram of rotary ultrasonic sawing sapphire shown in Figure 3 that after the introduction of ultrasonic vibration, the plastic removal ratio is reduced, and no large peeling occurs.

Ultrasonic machining includes three machining forms: drilling, face milling and side milling, and its material removal mechanism is shown in Figure 4. Among them, in drilling and face grinding, the abrasive grains on the tool face and workpiece materials are periodically cut and separated under the action of ultrasonic vibration, and hammering effect is added. This process characteristic has a good effect on the high hardness and low fracture toughness of hard and brittle materials, which has attracted wide attention of scholars at home and abroad. A lot of research has been done, and many advantages of rotary ultrasonic machining have been summarized.  

In terms of reducing cutting force, abrasive particles are periodically separated from the workpiece at high frequency during rotary ultrasonic machining, the cutting area is opened, and cutting fluid can enter, which improves lubrication and cooling. Moreover, high-frequency hammering during machining makes the machined surface powder and reduces cutting force. In order to analyze the mechanism of cutting force reduction in rotary ultrasonic machining, researchers have done a lot of research by theoretical modeling and adjusting process parameters. Zhu Xu of Huaqiao University found that the radial cutting force decreased by 50%~80% and the axial force decreased by 35%~50% compared with ordinary sawing after cutting sapphire by rotary ultrasonic machining.

At present, domestic and foreign scholars have relatively little research on the surface roughness of rotary ultrasonic machining, and there is a big controversy, that is, whether rotary ultrasonic machining can reduce the surface roughness is inconclusive. The research results obtained by scholars through establishing surface roughness prediction model or experimental verification more support that rotary ultrasonic machining can significantly reduce surface roughness in side grinding or V-groove machining, while drilling and end grinding can increase surface roughness more often, but it can be reduced by increasing spindle speed, reducing cutting depth and feed rate and matching appropriate ultrasonic parameters.

Improving machining efficiency is an important advantage of rotary ultrasonic machining, which is mainly reflected in the increase of material removal rate. Especially under the condition of constant force feed, the material removal rate of rotary ultrasonic machining is significantly higher than that of ordinary grinding. At present, rotary ultrasonic machining has obvious advantages over ordinary grinding in the field of hard and brittle materials processing, but the research on material removal mechanism under ultrasonic vibration is not deep at present. Further study on the removal mechanism can further reveal the surface roughness law and the formation mechanism of subsurface damage in rotary ultrasonic machining, which is of great significance to the popularization of rotary ultrasonic machining technology. 

二、Ultrasonic polishing

The principle of ultrasonic polishing is to apply ultrasonic vibration to the workpiece or horn, so that the abrasive particles in the abrasive suspension will move relative to the workpiece to achieve the effect of impact and polishing, which can improve the polishing speed and uniformity and reduce the surface roughness. Kala, Indian Institute of Technology, etc. used magnetic field assisted ultrasonic polishing to improve the surface smoothness of brass. Jeffrey et al. applied transverse ultrasonic vibration to the horn and polishing plate respectively, and found that the transverse ultrasonic vibration acted on the silicon wafer in the form of stirring suspension, which improved the polishing uniformity. Xu Wenhu et al. conducted a comparative experiment with or without ultrasonic polishing sapphire, and found that the removal rate of ultrasonic vibration assisted polishing was twice that of non-ultrasonic polishing, and the surface roughness decreased.

Ultrasonic-assisted polishing has been applied in mechanical polishing, chemical polishing, magnetorheological polishing, ion beam polishing and other polishing categories, and has achieved certain improvement results. However, at present, the research on the mechanism of material removal in the process of ultrasonic polishing is not deep enough, especially the influence mechanism of ultrasonic cavitation on the surface integrity of the machined surface has not been revealed, so it is necessary to further strengthen the research of basic theory.