Physical Fields In Ultrasonic Melt Treatment Of Magnesium Alloys

China is a country with plentiful magnesium resources, but the yield and quality of fabrication products of magnesium alloys lag far behind the developed countries. High-quality ingot is one of the key points in the development of Mg alloys with high performance. This study is a part of the projects,"The key fundamentals on the processing of high-performance magnesium alloys" in National Key Fundamental Research Program (973) and "The research on solidification behaviors of magnesium alloys under high-intensity ultrasonic" in National Natural Science Foundations. In the present study, the propagation of ultrasonic in the magnesium alloy melt was investigated. Ultrasonic grain refinement and ultrasonic purification for magnesium alloy were also studied by both numerical simulation and experiments. Based on the results of the effects of ultrasonic field and electromagnetic field on the fluid flow, heat transfer and solidification behavior of magnesium alloy, a new technology of semi-continuous casting of magnesium alloy under both the ultrasonic field and electromagnetic field was developed, which provided a highly effective economic ingot process method to China’s rapid growth of magnesium alloy industry.The propagation of the ultrasonic in the magnesium alloy melt was numerically simulated by finite element method. The effect of the ultrasonic power and frequency on the acoustic field distribution was studied. The results showed that compared with ultrasonic power, ultrasonic frequency had great effect on the spatial distribution of acoustic field and with a certain frequency a nearly ideal standing wave field could be obtained in the cell of ultrasonic melt treatment. Besides, the acoustic field distribution was slightly influenced by ultrasonic power, but the average acoustic pressure amplitude increased with the increase of ultrasonic power. To further explore the mechanism of the role of ultrasound in the magnesium alloy melt, ultrasonic cavitation behavior was studied by numerical simulation and theoretical analysis. The results indicated that the lower frequency, higher acoustic pressure amplitude and the initial radius of the cavitation bubble which was close to the resonance size were in favor of ultrasonic cavitation. In order to obtain effective cavitation effect, the ultrasonic frequency, acoustic pressure amplitude and initial radius of cavitation neclei should be within the range of20-22kHz,0.5-2MPa and0.3-12μm, respectively.Ultrasonic melt treatment of AZ80alloy for grain refinement was studied by numerical simulation of acoustic field in the melt and experiment. The results showed that the average grain size of solidification structure was in close relation with acoustic pressure amplitude. With the increase of acoustic pressure amplitude, the average grain size was accordingly decreased. Ultrasonic input intensity influenced the effect of grain refinement of solidification structure by changing acoustic pressure amplitude in the melt. This was because increasing acoustic pressure amplitude resulted in intensifying cavitation effect, which in turn promoted the effect of ultrasonic grain refinement and finally reduced the average grain size.Ultrasonic melt treatment of AZ31and AZ80Mg alloys for purification were studied by experiments and numerical simulations of ultrasonic field and ultrasonic aggregation of the inclusions in a standing wave field. The results showed that the depth of Mg alloy melt greatly influenced the formation of ultrasonic standing wave field. When the ultrasonic was introduced vertically from the upside, the depth of the melt should be adjusted as (3/4)λ to obtain a similar standing wave field. In this study, increasing ultrasonic power would increase acoustic radiation force and reduced the aggregation time, which in turn promoted effect of ultrasonic purification. However, excessive ultrasonic power might result into enhancement of ultrasonic cavitation and acoustic streaming, and finally weakened the effect of ultrasonic purification. The optimal ultrasonic conditions for ultrasonic purification of AZ31and AZ80Mg alloys were35Wx30s and80Wx60s, respectively.The interaction of multi-physical fields in the processes of conventional DC casting (DC), ultrasonic casting (UC), low frequency electromagnetic casting (LFEC) and ultrasonic-electromagnetic compound field casting (UC+LFEC) of Φ160mm AZ80Mg alloy billet were discussed and the effects of ultrasonic field, electromagnetic field and ultrasonic-electromagnetic compound field on fluid field, temperature field, solidification structure and mechanical properties of the alloy were investigated. Results showed:In the UC process, ultrasonic influenced both flow field and temperature field. With the effect of acoustic streaming the velocity of the melt flow was slightly increased and this trend was more obviously when high ultrasonic power applied; the sump depth was decreased by compared with DC casting, but with the increase of ultrasonic power the sump depth was increased. Increasing ultrasonic power could enlarge the effective region of ultrasonic cavitation, which ensured the effect of ultrasound treatment. In the LFEC process, with the effect of electromagnetic field the flow field was dramatically changed and this was different from UC process. A vigorous forced convection of the melt occurred, the direction of the melt flow was entirely changed and the velocity of the melt flow was remarkably increased; the sump depth was obviously decreased.When the ultrasonic emitter was inserted into the melt in the center of the billet during LFEC, the material types of the emitter (different relative magnetic permeability) greatly influenced electromagnetic field. A strong magnetic flux density and Lorentz force appeared in the region near the emitter when the emitter with high relative magnetic permeability applied. This resulted into a strong "fountain" whirlpool near the emitter and the "drain" whirlpool near the mould was weakened. The depth of the sump was increased. When the inserted emitter was made of carbon steel (relative magnetic permeability was about100), with the decrease of the electromagnetic frequency, the sump depth decreased; with the increase of the electromagnetic intensity, the sump depth firstly increased and then reduced, which was different from LFEC process during absence of ultrasonic emitter. In the process of UC+LFEC, the effect of acoustic streaming on flow field and temperature field was slightly and could not be comparable with electromagnetic field. It indicated that the effect of ultrasonic field on flow field and temperature field during LFEC was caused by ultrasonic emitter.Compared with conventional DC casting, the solidification structure and mechanical properties of the AZ80alloy billets cast by LFEC were evidently improved. However, because of the skin effect of electromagnetic field the solidification structures in the center part of the billets were much coarser than the others and they were not as uniform as their counterparts in other parts of the billets. With the application of UC, the solidification structure and mechanical properties of the billets in the whole cross-section were also considerably improved while comparing to the billets produced by conventional DC casting. But the structure in the edge part was obviously coarser than that in the center part. It was because the active ultrasonic treatment areas were limited due to the effect of attenuation during the propagation of ultrasound in the melt. In order to take advantages of both power ultrasonic field and electromagnetic field, a novel method UC+LFEC, which simultaneously applied power ultrasonic vibration and low frequency electromagnetic to the melt during semi-continuous casting process, was developed for AZ80Mg alloy billets. The microstructures and mechanical properties of the billets from edge to center were altered considerably and evenly with the application of UC+LFEC. The dendritic structure was broken into somewhat globular one and the average grain size of the three different parts from edge to center decreases to262μm,267μm and259μm, respectively. The ultimate tensile strengths in the three parts of the billets became much higher than others and they were188MPa in the edge part,187MPa in the R/2part and189MPa in the center part, respectively. By comparison with individual application of ultrasonic field or electromagnetic field, the imposing of the compound field during semi-continuous casting refined the microstructures of the billets more evenly and effectively in the whole cross-section of the billets. Also, the mechanical performances of the billets from the center to the edge were equally improved to a great extent.