Study On The Ultrasonic-assisted Casting Of Al-Si Alloy By Mould Vibration

Acoustic-assisted casting is a new casting method to refine grain size and improve performance by ultrasonic vibration treatment of melt in traditional casting process.Generally,the application method of ultrasonic wave is insert the horn directly into the alloy melt.This method is suitable for continuous casting,but it is difficult to apply to the mould casting with a certain structural shape.In this case,mould-based ultrasonic vibration has the potential to apply ultrasonic waves to the desired parts of the casting through the mould cavity during the filling/solidification process.However,the ultrasonic vibration of the mould usually requires a resonant design to generate good vibration,which has not received in-depth attention at present.In addition,the vibration of the mould often has the characteristics of uneven distribution,which will inevitably affect the flow field of the casting process and then affect the fluidity,filling performance and feeding performance of the casting.However,due to the lack of ultrasonic research on shape casting,The relevant theories are immature.To this end,this project specially built a vibration system composed of an ultrasonic vibration device and a metal mould.Based on the COMSOL Multiphysics software,the modal mode shape of the mould was simulated,and the influence of the uneven ultrasonic vibration of the mould on the AlSi₉Cu₃ alloy melt flow was studied.Furthermore,a mould with uniform mode shape was designed,and the effect mechanism of uniform ultrasonic vibration of the mould on the melt flow and solidification of Al-Si alloy was studied.Specifically:（1）Based on the simulation design of a helical die with uneven vibration dynamics,and by setting the number of vibration sources,two modal shapes with large vibration gradient and small vibration gradient are obtained.The experimental results of fluidity under the condition of uneven vibration of the mould show that the large vibration gradient will reduce the fluidity of the AlSi₉Cu₃ alloy melt,while the small vibration gradient can improve the fluidity of the AlSi₉Cu₃ alloy melt.The study found that ultrasonic mould vibration has the positive aspect of promoting melt flow,and also has the negative effect of mould vibration gradient reducing flow.When the positive aspects dominate,the flow of the melt will be improved,whereas when the negative effects（ie,large die vibration gradients）dominate,the flow will be reduced.Among them,the positive effect of promoting melt flow is mainly reflected in three aspects:（1）The cavitation effect of ultrasonic vibration can break up dendrites in the mushy area during the solidification process,thereby reducing the internal resistance of melt flow;（2）The ultrasonic wave of the mould The vibration promotes the wetting between the melt and the mould interface,reduces the surface tension,and then reduces the additional pressure on the flow front;（3）The inertial force generated by the mould vibration does work on the melt and promotes the flow of the melt.The negative effect of reducing the flow is mainly attributed to the non-uniform vibration of the mould resulting in a large pressure gradient in the melt,which in turn induces acoustic flow and intensifies turbulence,which consumes the energy of the melt flow.（2）A rectangular test mould with uniform vibration mode was designed based on simulation,and the effect of uniform ultrasonic vibration of the mould on the melt flow and solidification process of Al-Si alloy was studied.The results show that the uniform ultrasonic vibration based on the mould can improve the fluidity of the hypoeutectic AlSi₉Cu₃ alloy and AlSi₉Cu₃ alloy,but too strong vibration will aggravate the turbulent flow in the melt,thereby reducing the melt fluidity.However,this method fails to have a significant effect on the fluidity of the hypereutectic AlSi₉Cu₃ alloy melt,because the solidification mode of the hypereutectic AlSi₉Cu₃ alloy is different.The dendrites grow so that it is difficult for the ultrasonic vibration of the mould to refine the primary silicon.In addition,the hypereutectic AlSi₉Cu₃ alloy cannot form a dendrite skeleton during solidification,which further weakens the effect of ultrasonic vibration on reducing the internal resistance of the melt.（3）The effect of mould ultrasonic vibration on the solidification structure of Al-Si alloys with different crystallization temperature ranges was studied.The study found that for hypoeutectic Al-Si alloys,under the same cooling rate,the wider the crystallization temperature range,the more developed the dendrite growth,the longer the ultrasonic vibration of the mould,and the better the corresponding solidification structure refinement effect.For hypereutectic Al-Si alloys,although ultrasonic vibration cannot effectively refine the primary crystalline silicon phase,the acoustic flow effect induced by ultrasonic vibration in the melt can promote the solute and thermal convection and tend to be uniformly distributed,reducing the amount of primary crystalline silicon.The precipitation leads to the growth of coarse primary Alaround the Si phase caused by the local lack of Si element in the melt.