Study On The Effect Of Mold Ultrasonic Vibration On The Fluidity And Solidification Structure Of Al-Si Alloy

In recent decades,the use of ultrasonic-assisted aluminum alloy casting has received widespread attention,and related research has mainly focused on the treatment method of directly inserting the ultrasonic tool head into the alloy melt,which is used to intervene in the solidification process of the alloy melt in order to achieve the purpose of refining the grain size.However,this treatment method is difficult to apply to the mold casting of castings with certain structural shapes.Based on the ultrasonic vibration of the mold cavity wall,there is potential to apply ultrasonic waves to the desired part of the casting during the filling or solidification process.For casting forming,the fluidity of the alloy melt is an important indicator of its casting performance and a prerequisite for obtaining complete castings.The mold ultrasonic vibration will inevitably affect the fluidity of the alloy melt,and the fluidity of the alloy melt is not only affected by the pure fluid mechanics factors,but also closely related to its solidification behavior,which is currently not clear.Therefore,this study takes pure fluids（water and silicone oil）,pure metals（tin and bismuth）,and Al-Si series alloys(Al Si₉ and Al Si₁₈)as research objects,and studies the influence of mold ultrasonic vibration on the fluidity and solidification structure of cast aluminum-silicon alloy and its action mechanism from the aspects of hydrodynamics,solidification behavior,and solidification characteristics.Specifically:（1）In order to investigate the hydrodynamics factors of mold ultrasonic vibration affecting the fluidity of alloy melts（excluding solidification）,water and silicone oil were studied as pure fluids to investigate the effect of mold ultrasonic vibration on the fluidity of pure fluids.The results showed that the hydrodynamics of mold ultrasonic vibration affecting fluid flow mainly manifested in two aspects.On the one hand,the mold ultrasonic vibration can generate surface tension waves at the fluid-gas or fluid-liquid interface,disrupting the equilibrium of surface tension and reducing the fluid flow resistance.On the other hand,the boundary layer of the fluid and wall shear stress will undergo periodic changes under the mold ultrasonic vibration,thereby reducing the fluid flow resistance.When the fluid is in a laminar flow state,the fluid flow resistance decreases with increasing ultrasonic power.However,when the ultrasonic power reaches a certain intensity（>1080 W）,the disturbance of the wall vibration on the fluid is enhanced,and the fluid state changes from laminar to turbulent,which exacerbates the kinetic energy loss inside the fluid and leads to a significant increase in flow resistance.（2）In order to investigate the solidification factors of alloy melt flow affected by mold ultrasonic vibration,tin and bismuth,two low-melting pure metals（without crystallization temperature range）,were studied to examine the impact of mold ultrasonic vibration on the fluidity of pure metal melts.The results showed that the influence of mold ultrasonic vibration on the fluidity of pure metal melts depends not only on the hydrodynamics factors but also to a large extent on solidification effects.Specifically,the surface tension wave generated at the metal melt gas-liquid interface by mold ultrasonic vibration can destroy the oxide layer at the flow front and reduce the flow resistance of the melt.In addition,the melt absorbs some of the ultrasonic energy,leading to a temperature rise in the melt,which improves the thermal exchange efficiency within the melt and delays the solidification speed at the flow front.（3）Further investigation was conducted on the effects of mold ultrasonic vibration on the fluidity and solidification structure of two Al-Si series alloys(Al Si₉ and Al Si₁₈)with different crystallization temperature ranges.The study found that for Al Si₁₈ alloy without dendritic growth solidification characteristics,the solidification structure was almost unaffected by mold ultrasonic vibration,and the influence of mold ultrasonic vibration on its fluidity was mainly in the hydrodynamics aspect.That is to say,appropriate mold ultrasonic vibration can improve the fluidity of the melt by reducing the flow resistance,but when the vibration intensity is too high（>1080 W）,turbulence will be induced in the melt,greatly increasing the flow resistance and decreasing the fluidity.However,for Al Si₉ alloy with obvious dendritic growth solidification characteristics,mold ultrasonic vibration can affect solidification by disrupting theα（Al）dendrites during growth and refine the solidification structure.Therefore,mold ultrasonic vibration can not only improve the fluidity of Al Si₉ alloy from the hydrodynamics aspect,but also reduce the flow resistance by disrupting the dendritic network in the mushy zone,and the latter is more prominent.However,the fluid kinetic energy loss caused by turbulence induced by high-intensity mold ultrasonic vibration largely offsets the promotion of disrupting dendrites on fluid flow.