Study On The Microstructure Refinement And Feeding Enhancement Of Aluminum Alloy By Ultrasonic-assisted Casting

Ultrasonic-assisted casting has been proven to significantly refine the grain size of castings,and related technologies and theories have also made significant progress in recent decades,especially the direct method of inserting ultrasonic tool heads directly into metal melts and refining alloy grains through intervention solidification,which has been applied industrially in the continuous casting of aluminum alloys.However,due to the attenuation and reflection characteristics of sound waves,the direct insertion method is difficult to adapt to the molding of complex structural castings.In this case,mold-based ultrasonic vibration has the potential to apply ultrasonic waves to the desired parts of the casting during the filling and solidification process through the mold cavity.However,due to the modal characteristics of mold vibration,resonance design and vibration distribution issues need to be taken into account,which is often overlooked at present.As a result,mold based ultrasonic vibration casting technology has not yet achieved satisfactory results.Therefore,to explore the effect of ultrasonic resonance mode characteristics of molds on castings,this study builds a vibration system consisting of ultrasonic vibration equipment and cores based on the metal mold gravity casting aluminum alloy end cover mold.The vibration mode of the core is simulated and designed using COMSOL Multiphysics software,The mechanism and law of the influence of ultrasonic vibration mode of the core on the solidification microstructure and feeding at the target hole wall position of Al Si₉Cu₃ castings were studied;Further study was conducted on the influencing factors of ultrasonic vibration of the mold on the operating distance of Al-Si alloy.The specific research is as follows:（1）Based on the vibration mode simulation of the hole walls at different positions in the end cover casting,the resonant mode of the core corresponding to the target hole position of the formed casting is obtained.The design principles of the core mode aimed at applying ultrasonic vibration to the target part of the casting and the influence of casting solidification on the resonant mode of the core ultrasonic wave are explored.The study has found that by adding a disk between the amplitude transformer and the core,the vibration system of the core undergoes a large cross-sectional mutation in the structure,to achieve a change in the direction of core vibration;Changing the size and structure of the non-formed part of the core can achieve strong vibration distribution in the formed part of the core.In addition,only when the pre-solidified part of the casting does not significantly change the structure of the vibration system,the resonance mode of the ultrasonic vibration system does not change much,thus achieving strong vibration of the post solidified part of the casting.Therefore,in the process of resonance design,it is necessary to fully consider the impact of the pre solidified part of the casting on the vibration of the subsequent solidified part.（2）Based on the metal mold of the end cover,two different vibration modes were obtained by applying ultrasonic vibration on two cores with different structures（"long"and"short"cores）.These two modes produced strong vibrations at the target hole positions（The walls ofφ24 mm andφ50 mm holes）of the castings,thereby refining the solidification microstructure at these locations.The grain size of theφ24 mm hole wall was refined from 623μm to 357μm,while the grain size of theφ50 mm hole wall was refined from 477μm to 258μm.The study investigated the influence mechanism of ultrasonic vibration on the solidification microstructure at the target locations of the castings.The study has found that the effect of ultrasound on refining microstructure depends on the vibration intensity,and only strong vibration can significantly affect the solidification microstructure of castings.Therefore,for castings with complex structures,elaborate ultrasonic modal design is required to refine the solidification microstructure at the target location of the casting.In addition,through the analysis of solidification microstructure with different wall thicknesses,it was found that the effect of ultrasonic vibration on the refinement of solidification microstructures largely depends on the cooling rate.For the high cooling rate parts of the casting,the nucleation rate of primaryα（Al）is high,so the effect of ultrasonic on promoting nucleation is not obvious,but the effect of breaking dendrite is more obvious.For the low cooling rate parts of the casting,both nucleation and breaking dendrites are promoted by ultrasonic vibration,and at low cooling rates,the effect of ultrasonic vibration on the secondary dendrite spacing is more significant.（3）Based on the metal mold end cover mold,the influence mechanism of ultrasonic vibration based on the core on the feeding performance of castings was studied.The strong vibration of the feeding channel of the end cover casting was achieved through the resonant mode design of the core,significantly improving the feeding performance of the casting.The study found that ultrasonic vibration breaking dendritic is the main reason for improving the feeding performance of castings.When the strong vibration position of the core is located at the lower end of the feeding channel of the casting,ultrasonic vibration can significantly improve the feeding performance of the casting.However,when the strong vibration position of the core is far from the feeding channel,the effect of ultrasound on feeding is not significant.Therefore,ultrasonic vibration aimed at improving the feeding capacity of the casting not only needs to consider the design of vibration modes,but also the positional relationship between the vibration modes and the feeding channel of the casting.（4）The cylindrical mold was designed based on simulation,and the influencing factors of ultrasonic vibration of the mold on the operating distance of Al-Si alloy were studied.The results show that the effect distance of ultrasonic vibration in the aluminum alloy melt mainly depends on the ultrasonic power and the crystallization temperature range of the alloy.Increasing ultrasonic power can extend the operating distance of ultrasonic vibration in the aluminum alloy melt.The crystallization temperature range of the alloy significantly affects the operating distance of ultrasonic vibration,with a wider range leading to a longer operating distance.Therefore,when high-power ultrasonic vibration act on alloys with a wide crystallization temperature range,their operating distance is longer.Additionally,the effect of ultrasonic vibration on the solidification microstructure of the alloy depends on the solidification characteristics of the alloy and the ultrasonic power.For the Al Si₉Cu₃ hypoeutectic alloy with obvious dendritic growth characteristics,the mechanism by which high-power ultrasonic vibration refine grains is mainly by breaking dendrites,while low-power ultrasonic vibration mainly promotes nucleation.For the Al Si₁₈ hypereutectic alloy,primary crystal silicon grows on small planes,and ultrasonic vibration mainly refine grains by promoting nucleation.