Effect Rules And Function Mechanism Of Ultrasonic Field On The Solidification Of High Strength Aluminum Alloy

Abstract:The formation of the qualified structure and performance is difficult in the casting of large size aluminum alloy ingot, which is a technical bottleneck in the development of large-scale integrated parts required for aerospace and weapon equipment in our country. Exerting ultrasonic disturbance to the solidification of aluminum alloy is a novel process approach for improving casting defects, enhancing ingot quality and promoting material performace. A comprehensive understanding and systematic mastering about the effect rules and function mechanism of ultrasonic field on the solidification of aluminum alloy is the key to realize this process. However, most of the comtemporary study had been empirical and phenomenological rather than quantitative expression and essential revealment. Based on this, the7xxx series aluminum alloy with a high strength and high tenacity was taken as the research object in this dissertation. Through the experimental testing, theoretical modeling, numerical simulation and other methods, the solidification beheavior of the aluminum alloy treated with ultrasound and its generation mechanism were discussed throughly. The main research contents and conclusions are as follows:(1)A series of experiments were carried to analyze the heat transfer during the solidification of aluminum alloy. The results show that ultrasonic treatment (UT) can not only improve the uniformity of temperature field distribution, but also speed up the solidification process of the melt obviously, in especial, significantly increase the cooling rate of solid-liquid phase zone melt so that its corresponding cooling time was shortened by about2/3. But on the other side, the ultrasonic radiator itself has little efffect on the heat transfer of the aluminum alloy melt so it can be be negligible.(2)The aluminum alloy melt was treated with different process conditions to study the rules of solidification structure formation of the aluminum alloy. The experiments confirm that the grain-refinement effect of the ingot obtained with UT is neither resulted from nucleation caused by the chilling effect of the radiating face nor the titanium element separated from the radiator, but owes to the nucleation proliferation induced by ultrasonic cavitation and the growth inhibition of crystals caused by resonance effect.(3)Based on the summarization of experimental phenomena, the models for pressure-suppercooling, capillary infiltration and the primary crystal resonance were established respectively. The dominant function mechanism of ultrasonic field on the solidification of Al alloy melt at different solidification stages was revealed comprehensively through the mathematical calculations and theoretical analysis. Researches show that with UT in the temperature range of crystallization, cavitation effect plays a leading role in grain refinement by promoting the nucleation of Al alloy melt in the ways of pressure-supercooling and heterogeneity activation; While with UT in the solid-liquid phase coexistence temperature range, the resonance effect plays a main role in grain refinement by inhibiting the grain growth in the ways that the phase transformation driving force is reduced as the free energy level of crystal interface increases, which is due to the increase of crystal vibration amplitude. Moreover, through further calculation, it is found that primary crystals with a size1?10?m are prone to resonate among the grains in the solidification sytem of aluminum alloy..(4)The industrial experiments of semi-continuous casting of large size7050aluminum alloy ingot with UT were conducted successfully. The size of the ingot reached ?550mm and1320mm×500mm. Results show that the cooperative treatment of multi-source ultrasonic field can not only implement a homogeneous grain refinement effect of the whole cross section in ingots of which the crystal degree was level1(billet) and level2(slab) repectively, but also can significantly improved the grain boundary segregation and macro-segregation of solute elements. The solution rate of the main alloy elements was increased by more than20%. Meanwhile, the yield of the ingots and the uniformity of mechanical properties are able to be improved.(5)For the technology of ultrasonic semi-continuous casting of aluminum alloy slab with a size of1320mm×500mm, a three-dimensional numerical model based on the coupling of acoustic field, flow field and temperature field was established through adding an extra source term. It was realized by Fluent soft and user subroutine that the coupling process of the multi-field in semi-continuous casting with multi-source ultrasonic field was simulated. The matching rules among the ultrasonic parameters were preliminarily set down from the simulation results of distribution characteristics of macroscopical acoustic fied, flow field and temperature field that obtained with UT of difference frequency and amplitude.These studies will provide technical base and theoretical reference for promoting the development of the casting technology of aluminum alloy with outer ultarsoncid fied and its industrial applications.