Ultrasonic metal refinement

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This post was last edited by cy1125915 on 2015-9-29 at 11:17. To refine the microstructure of aluminum alloy ingots, using ultrasonic vibration to treat the molten metal is an extremely effective method. Ultrasonic waves possess unique acoustic effects; when applied during the solidification of metals or alloys, they cause the solidification structure of the ingot to change from coarse columnar crystals to uniform, fine equiaxed crystals. At the same time, both macroscopic and microscopic segregation in the ingot are reduced. There are several theories regarding the mechanism by which ultrasound refines the microstructure of ingots; the more widely accepted view is that the cavity effect promotes the formation of nuclei in the liquid metal, that is, the nucleus formation theory ； Another is the dendrite fracture theory. Ultrasonic treatment is a new method for purifying aluminum alloy melts. It utilizes the cavitation effect of ultrasonic waves in the melt to disrupt the continuity of the liquid phase and create voids; these voids cause the gases dissolved in the aluminum melt to gather together. The elastic oscillations of the ultrasonic waves facilitate the formation of crystal cores within these bubbles, as well as their aggregation to a certain size, thereby ensuring the release of those gases. Based on the current research status of the application of ultrasound in steel metallurgy, it can be concluded that ultrasound holds broad application prospects in this field; it can be used to remove gases and inclusions from molten steel, improve the surface quality and internal microstructure of cast billets, and is also applicable to the development of alloys and new materials with specific directional requirements. 　　However, its applications in the metallurgical industry are limited to flaw detection aside from that. Nowadays, thanks to the successful development of new materials, high-performance ultrasonic transducers have been created, overcoming the limitations of traditional transducers such as low power and poor energy efficiency. This has made it possible to utilize ultrasound in the metallurgical industry, particularly in refining processes. At the same time, China’s steel industry is at a favorable stage where it is shifting focus from increasing production volume to improving quality and reducing energy consumption across the whole society, which gives ultrasound excellent prospects for research and development as well as practical value. Ultrasonic removal of inclusions: It is very difficult for tiny inclusions in molten steel to rise to the surface; they can only float more easily when they aggregate. Research has been conducted on the effect of ultrasonic waves on inclusions in solutions, and it was concluded that ultrasonic waves can successfully cause the particles to separate and aggregate. Vibration-based impurity removal: As the vibration frequency increases, the number of layers formed by the aggregation of small particles in the solution increases, and the time it takes for these particles to reach positions where suspension polymerization can occur decreases. This makes it possible to remove impurities from molten metal by controlling the frequency and using ultrasonic waves in combination. Ultrasonic degassing: When elastic vibrations are introduced into molten metal, cavitation occurs. This is due to the formation of voids as the continuity of the liquid phase is disrupted, and as a result, the gases dissolved in the molten metal gather in those voids. The elastic vibrations of sound waves, particularly those of ultrasonic waves, induce the formation of bubble cores, causing them to grow continuously until they reach a size that allows them to be easily expelled from the molten metal. The effect of ultrasound on billet quality Ultrasound can be applied to the mold to improve the surface quality of billets; the vibration generated by ultrasound in the mold can be utilized for small square billets, large square billets, and slab billets. Negative sliding is not required when using ultrasonic vibration. Applying ultrasonic vibration to the mold during the casting of small and large billets results in a very smooth surface on the cast billet. After applying ultrasonic vibration during slab casting, the consumption of flux increases, and the depth of the vibration marks also increases. Refining grains: Since initial results showing grain refinement through the application of simple mechanical vibrations during the solidification of steel ingots were obtained, research on vibration solidification has attracted widespread attention. In the early days, vibration methods involved simple mechanical vibrations with low frequencies and low energy densities, such as using cams to cause the mold to vibrate up and down. When producing castings using the ultrasonic vibration solidification method, ultrasonic waves generate alternating positive and negative sound pressures as they propagate, thereby forming jets. Meanwhile, due to nonlinear effects, acoustic flows and micro-acoustic flows are produced. Ultrasonic cavitation creates high-speed micro-jets at the solid-liquid interface. All of these effects help to break apart dendrites, impact the solidification front, and enhance mixing and diffusion processes. It can be seen that it has an effect wherever a liquid can reach and where a sound field exists. By utilizing the cavitation effect in this process, it is possible to purify the tissue, refine the grains, and homogenize the structure. In addition to the mechanical action caused by vibration, which damages the dendrites, another important effect of ultrasonic vibration solidification is that it increases the effective supercooling degree of the molten metal, reduces the critical nucleus radius, thereby increasing the nucleation rate and refining the grain structure. Ultrasonic treatment during the continuous casting of aluminum-silicon alloys can refine the grain structure of the cast billets, thereby improving the plasticity and ductility of these alloys and enabling their better use in construction materials and automotive engine pistons.