Study On Fabricating And Rheoforming Of In-Situ Synthesized Magnesium Matrix Composites By Ultrasonic Vibration

With the enhancement of people’s consciousness of energy saving andenvironmental protection, magnesium alloy as one of the lightest metal structuralmaterials has attracted more and more people’s attention. Particle reinforcedmagnesium matrix composite is a new material developed in recent years, whichpreparation methods are classified into the external addition method and in-situmethod according to the sources of particles. Compared with the external additionmethod, the in-situ method has many advantages such as small particle size, nopollution at the interface, good thermal stability and low preparation cost. In recentyears, high intensity ultrasonic has been applied to treating metal melt. acousticcavitation and acoustic streaming effect induced by ultrasonic in the melt can changesome physical, chemical and biological properties of the material, improve thewettability between particles and melt, and enforce the particles dispersing uniformlyin the melt. The technique has been widely applied to the preparation of composites.As a new process for metal forming in21st century, semi-solid forming technologyincludes thixoforming and rheoforming. In contrast with thixoforming, rheoforminghas many advantages such as simple process, simple equipment, energy preservation,little environmental contamination and low production cost. In this dissertation, theAl2Y/AZ91magnesium matrix composites were fabricated by in-situ synthesiscombined with ultrasonic, and its rheoforming process were investigated. The mainwork and results are as follows:The forming mechanism of Al2Y in AZ91melt was discussed fromthermodynamics and kinetics, the effect of external factors on the reaction wasanalyzed as well as. The excess free-energy, enthalpies of formation, excess entropiesof Mg-Al-Y ternary alloy were calculated with computer programming. The dynamicmodel of the in situ synthesis of Al2Y was set up and the kinetic equation was educed.The Mg-Y master alloy was added into AZ91magnesium alloy, the Al2Yreinforced phase was formed. The effect of the amount of Y and ultrasonic parameterson the microstructure of Al2Y and composites were studied. The microstructure ofcomposites compared to the matrix was obviously refined, α-Mg phase were refinedand the quantity of which become more. The shape of β-Mg17Al12phase change fromdiscontinuous skeletal shape to discontinuous short chain or granular shape. With the increase of ultrasonic power and ultra ultrasonic time, the Al2Y phases are refinedand distributed uniformly in the melt, the amount of which are also increasedobviously. When the adding amount of Y are no more than3%, the Al2Y phases donot agglomerate.Semi-solid Al2Y/AZ91magnesium matrix composites slurry was prepared byultrasonic vibration, effect of different ultrasonic parameters on microstructure ofsemi-solid slurry was investigated. The formation mechanism of magnesium matrixcomposites semi-solid microstructure in the ultrasonic field was revealed. The bestsemi-solid slurry with average grain diameter of75μm and shape factor0f0.7wasgained after the melt was treated by ultrasonic vibration for about60s at neartemperature of the liquid phase(600℃).The rheological properties of Al2Y/AZ91magnesium matrix composite preparedby in-situ synthesis combined with ultrasonic were studied, based on the experimentaldata, the rheological model of semi-solid Al2Y/AZ91magnesium matrix compositesprepared by ultrasonic vibration was established. With the increase of volume fractionof Al2Y and solid fraction, the apparent viscosity of semi-solid Al2Y/AZ91magnesium matrix composites slurry increases. When the solid fraction is lower than0.23, the apparent viscosity increases slowly, meanwhile the solid fraction is greaterthan0.23, the apparent viscosity increases fast. Under the same experimentalconditions, the apparent viscosity decreases with the increase of ultrasonic power.Based on the second developing platform, the simulation of semi-solidrheoforming of Al2Y/AZ91magnesium matrix composites was done. The filling andsolidification process were analyzed, and the casting defects were predicted, whichcan provide theoretical guidance for rheological casting.Semi-solid slurry of was prepared by ultrasonic vibration, and the rheologicalforming of the composites has been implemented. The effect of different processparameters on the mechanical properties of composite has been studied, which canprovide the experimental and theoretical basis for the popularization and applicationof magnesium matrix composites.