As a kind of important industrial raw material, the application of fine metal powder in the fields of electronics, information, metallurgy, energy and astronautics is expanding. With the metal injection molding, the development of thermal spraying, metal rapid prototyping, electronic surface mount and other technologies has gradually improved the performance requirements of fine powders, such as particle size, purity and morphology, and then promoted the development of powder preparation technology towards narrow grain size, low oxygen content, high efficiency and low cost. Although traditional powder preparation methods such as high energy fragmentation, water mist, gas atomization and centrifugal atomization technology have entered a large-scale industrial production stage, but the powder characteristics, such as particle size, size distribution, powder geometry and so on, are difficult to meet the requirements for the use of high performance metal powders in some fields. In order to meet the needs of this new situation, people are developing and perfecting the traditional metal powder preparation technology, and also developing new metal powder ultrasonic atomization technology.
The basic principle of ultrasonic atomization of metal is to use power source generator to convert frequency alternating current into high-frequency electromagnetic to provide ultrasonic transducer, the transducer transforms the high-frequency electromagnetic into weak mechanical vibrations by means of the scaling effect of the piezoelectric crystals, and the ultrasonic energy amplifier amplifies the particle displacement or velocity of the mechanical vibration and passes it to the ultrasonic tool head. When the molten metal flows from the liquid tube to the surface of the ultrasonic tool head, when the amplitude of the vibrating surface reaches a certain value, the thin liquid layer is crushed under the action of ultrasonic vibration, and the droplets from the vibrating plane form droplets.
In the actual atomization process, when the ultrasonic intensity exceeds the liquid's cavitation value, it often produces a strong cavitation effect in the liquid medium of the vibrating surface. The cavitation effect caused by the large number of bubbles in the process of continuous growth and collapse, on the periodic surface tension wave regularity caused by aperiodic disturbances, if the cavitation effect is considered, The atomization mechanism will be more complex. At present, there are two different viewpoints on the explanation of ultrasonic atomization mechanism, and the compromise view is that ultrasonic cavitation and tension wave effect play a role together. The theory holds that the ultrasonic vibration atomization is mainly caused by the droplets induced by the tension wave, and the acoustic cavitation, as a stochastic phenomenon, constitutes a disturbance to the periodic surface Zhang Lipo, which randomly produces the droplets with different particle size.
The metal ultrasonic atomization preparation equipment is generally by the smelting furnace, atomization tank, ultrasonic atomizer, powder collecting tank, vacuum air charging system, feed system, control system structure, installed in the middle of the atomization Room ultrasonic vibration system, from high-power piezoelectric transducer, luffing rod, tool head, ceramic reactor gas cooling hood composition, The ultrasonic generator signal is introduced from the atomizing outdoor part, and the bottom of the atomization chamber is provided with a powder collecting tank, and by changing the orifice aperture and adjusting the pressure difference between the melting furnace and the atomization chamber to control the atomized metal flow, the power output required by the normal atomization is the best critical state for the amplitude to The ultrasonic horn selects the alloy material with small acoustic impedance rate, good mechanical fatigue resistance and easy processing, and the length is selected as the integer times of Λ/2 and Λ/4, and λ is the longitudinal wave length of the corresponding frequency acoustic wave propagating in the material. In order to facilitate the formation of uniform thin liquid film on the vibrating surface, the top of the tool head is designed to facilitate the spreading of the melt. Through the melt temperature and flow control, the atomization state can be stabilized, and the powder particles are endowed with low pollution, good spherical size and narrow particle size distribution. Metal ultrasonic atomization technology involves the application of acoustics, materials, metallurgy, physics, electronics, automatic control and other disciplines, is the true meaning of multidisciplinary integration and development of new technologies. The key techniques to be controlled by ultrasonic atomization technology include the structural design of high performance ultrasonic atomizer, the theoretical mechanism of interaction between metal melt and ultrasonic wave, the acquisition of experimental law and the selection of key component materials. In particular, the main research contents include the establishment of the Power ultrasonic field (ultrasonic frequency, power, amplitude) and metal powder characteristics (morphology, particle average diameter, particle size distribution, etc.), the nature of the melt, mass flow, confining pressure, the type of media and other factors affecting the characteristics of the powder.