Straight-in Ultrasonic Vibration Casting Test Its Solidification Kinetics Study

Introducing ultrasonic field into materials processing is the front technology in materials science. Melt processing by use of ultrasonic vibration is not only a very complicated physical process, but also a multi-field coupling process. Ultrasonic vibration could refine solidification structure and improve temperature field as well as stress field, which has conducive to aluminum alloy forming process and facilitates to obtain high-performance casting products. Taking the 7050 aluminum alloy ultrasonic casting as research object, the research work was done as follows:1. The grain-refining effect on large 7050 aluminum alloy ingots of different diameters was studied by doing semi-continuous ultrasonic casting experiments. The average grain size across the cross-section and at special points was analyzed. The results showed that the grains across the ingot cross-section were refined under ultrasonic vibration.2. By doing ultrasonic crucible foundry experiments, the effects on cooling time and solidification structure were studied, the grain-refining mechanism was analyzed. The results showed that, with ultrasonic vibration applied, the cooling time was longer when the temperature was between 655℃and 670℃or between 625℃and 640℃, and was obviously shorter when the temperature was between 590℃and 625℃; the solidification structure was much better when ultrasonic was applied between 625℃and 640℃than between 590℃and 625℃, and was good when ultrasonic was applied as the temperature was below liquidus 10℃. The simulation results showed that acoustic streaming and mechanical effect caused by ultrasonic could not break dendrite arm. Combining with crucible foundry experiments, the grain-refining mechanism that the root primary dendrite secondary arm fused was presented.3. The mechanism of effect on the solidification process from ultrasonic resonance was analyzed mainly from the following aspects. First, the natural frequencies of ultrafine particles and macroscopic solid balls were calculated and analyzed; the results showed that the particles with radius of 1μm to 10μm caused resonance in melts. Second, the results of particle oscillation model showed that ultrasonic vibration had an exciting effect on the particles, whose natural frequency was around 2×104Hz, on the interface of solidification, which caused resonance. Third, the hermodynamic calculation results showed that the undercooling increased under ultrasonic vibration, which made nucleation rate increase quickly, so as to refine the solidification structure. Forth, resonance could promote the scouring of solidification interface by melts, which refined the grains across the cross-section.