| Numerous studies have shown that the alloys of Mg, Al, and other low melting point metal Pb-Sn which were treated by ultrasound could refine the solidification structure, but the possible pollution of ultrasound may restrict the industrial applications of ultrasonic melt treatment. However, the ultrosound iron rod has excellent durability in Mg alloy melt because of its low solubility of Fe, so ultrasonic treatment of Mg alloy melt has bright pospects in industrial application. Many scholars in domestic and foreign have achieved many achievements in metal solidification and theoretical study about ultrasound. But the current research mainly focused on the impact of solidification, which couldn’t response to the answer that how the ultrasonic cavitation effect the nucleation and growth of melt. The ultrasound on the metal melt could reflect the structural changes of melt resistance, which is an important physical parameters, so we can understand the effects of ultrasound on the solidification behavior of the mechanism, and which also can provid more specific and reliable theoretical basis in the control of solidification.This article as a part of National Natural Science Foundation project "The correlational research between ultrasound on solidification behavior of Mg alloy melt and melt resistivity changes", which adopt four-electrode method to measure the influence orderliness of resistance of ultrasonic treatment on Pb-20%Sn, Pb-10%Sn and Al-12%Si, and study the continuous ultrasonic treatment power, time, temprature and holding time after ultrasound treatment; the influence of continuous ultrasound on resistance in alloy melt space heating and cooling process; pulse width and pulse frequency on the influence orderliness of melt resistance. We get the following results:(1) The metal melt electrical resistance measurement unit we designed has good stability, which could accurate measuring the small changes of metal melt electrical resistance and could synchronous sampling for electrical resistance and melt temperature, measuring accuracy was0.001mΩ. We adopt this device could realize the accurate measurement of metal melt electrical resistance in the process of ultrasound treating;(2) Studies show that the four-electrode resistivity measurements could accurately reflect the ultrasonic treatment on the resistance of melt, indicating that ultrasound treatment on the melt stucture can be characterized by melt electrical resistance measurement. The changes of ultrasound on metal melt resistance exists instantaneous change and lasting changes, and pointing out instantaneous electrical resistance change is connected with ultrasonic cavitation on the ordered structure of periodic vibration. While the durable variation of resistance reflect the orderlineness incidence of ultrasonic cavitation on microstructure, and which is directly related to important physical quantities of solidification behavior.The accurate measurement of this physical quantity for in-depth understanding the influence of metal melt solidification behavior and provideing an important basic data for optimizing ultrasonic treatment process;(3) The results show that ultrasonic treatment make the resistance of Al-based alloy melt increased significantly,but the resistance of Pb-Sn-based alloy decreased, indicating that ultrasonic treatment on Al-based alloys and Pb-Sn-based alloy melt could have different directions for ordered microstructure influence. And which also shows that the content of Sn in Pb-Sn will impact the changes of resistance significantly, indicating that the ultrasonic treatment could made the resistance change more significantly when content of Sn is high;(4) The laws of ultrasound treating conditions effecting on trancsient electriccal resistance variable quantity△R and persistent electrical resistance variable quantity△r was accurate measured, indicated that AR and△r increased significantly after raising the ultrasound treating power and temperature, and properly prolonging ultrasound treating time could increased△r value significantly, but Ar had no appreciable change if ultrasound treating time was too long.That indicated that raising the ultrasound treating power, temperature and properly prolonging ultrasound treating time could increase the rate of ordered change and it is same as the fact that ultrasonic treatment influenced on the solidification structure, and further explained the△r relating to the solidification behaviors,but vibration of microstructure is influenced by the time and temperature of ultrasonic treatment and has nothing to do with ultrasonic treating time;(5) After studied on the aging of ultrasonic treatment we found that the△t and△t1are relative to the ultrasound technology conditions of treating power,treating temperature and treating time.△t and△t1increased with the treating power increasing.△t showed a tendency to increasing first and reducing later and the optimum temperature:400W and600W at350℃,800W at400℃;△t, increased with the treating time raising. The regular pattern of△t at300℃and450℃was same as△t1at450℃,△t1increased with treating time expand.when the treating time less than7min,△t1had a downward tendency when the the treating time more than7min.There is a maximum when the treating time is5min at300℃;(6) The studies of Pb-20%on heating and cooling process shows that the ultrasound on the change in resistance lasting influence is zero in the two-phase zone, while the transient change of resistance decreases with the solid fraction increased and then decreased, indicating that ultrasonic treatment can’t change the ordering level of the two-phase region. Which also shows that when the solid fraction is high,the reduce of solid fraction will increase the disturbance of primary solid particles, thereby increasing the collision opportunities of particles which will made the resistance instantaneous variation increased.But the solid phase collision decreases and the instantaneous change of resistance increases then gradually decreases when the solid fraction is lower. |
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