摘要:
建立了高能超声制备碳纳米管增强AZ91D复合材料的声场计算模型，并采用有限元方法计算了20 kHz超声直接作用下AZ91D熔体的声场分布，熔体声场呈辐射状分布，距离声源越远，声压幅值越低。采用超声作用下单一气泡变化模型描述超声作用下AZ91D 熔体中的空化效应，通过对Rayleigh-Plesset方程的求解，得到了不同声压作用下气泡的变化规律，获得了声压幅值与熔体空化效应的关系，声压幅值越大，气泡溃灭半径阈值越小，熔体发生空化效应越容易。计算了固定坩埚尺寸、不同超声探头没入熔体深度情况下的声场，得到了超声探头最优没入深度为30 mm左右。将声场计算结果以及AZ91D熔体中空化效应的发生规律进行综合分析，得到了超声功率对有效空化区域的影响规律，超声功率较大时，有效空化区域体积随超声功率近似成线性增大。最后，通过甘油水溶液超声处理实验，验证了模拟计算的准确性。
关键词:
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超声处理
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声场
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空化效应
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模拟计算
Abstract:
The sound field in the melt processed by 20 kHz ultrasonic to fabricate CNT-AZ91D is investigated by numerical simulation. Firstly, the distribution of sound pressure in the AZ91D melt is calculated by the finite element method after the model of the sound filed of the ultrasonic processing has been built. The simulation results show that a radial sound field forms under the ultrasonic probe, which means that the sound pressure decreases with increasing distance from the sound source. After the sound field is revealed, we study the ultrasonic cavitation in the AZ91D melt with a single-bubbly-change model and examine the bubble change rule under different sound pressures by solving the Rayleigh-Plesset equation. The relationship between sound pressure amplitude and the ultrasonic cavitation in the melt is also discovered. The higher the sound pressure amplitude, the smaller the threshold radius for the bubble collapse is, and thus the ultrasonic cavitation in AZ91 melt happens more easily. Secondly, the sound fields of the melt with different immersed depths of the ultrasonic probe are calculated. The results show the optimal immersed depth is about 30 mm for the same crucible size used in the present study. Furthermore, the corresponding optimal immersed depth can be calculated for different crucible sizes or different melts by the present numerical method, which is important for the practical ultrasonic processing. After analyzing the calculated results for sound field and the cavitation rule of in the AZ91D synthetically, we find that the volume of the effective cavitation zone rapidly increases with the smaller ultrasonic power and then rises almost linearly with the ultrasonic power larger than 500 W. Finally, to verify the simulation method of the sound field, the contrast study between the simulation and experiment of ultrasonic processing using glycerol-water solution is performed. The simulation result of the sound field in glycerol-water solution is similar to that in AZ91D melt. The highest sound pressure occurs near the end face of ultrasonic probe, while the experimental observation shows that the strongest cavitation also happens near the end face of ultrasonic probe, which indicates that the highest sound pressure occurs in the zone.
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