Ultrasonic Treatment And Gd The Influence Of The Microstructure And Mechanical Properties Of Az31 Alloy

The purpose of this study is to improve the performances of AZ31magnesium alloy by introducing ultrasonic field and alloying. The smelting, casting, extruding processes, microstructures and properties of as-cast and as-extrusion alloys was systematically investigated in this paper. To find optimum smelting project by the orthogonal experiment of ultrasonic treatment process and alloying Gd content.The effect of ultrasonic treatment on the microstructure and performance of as-cast AZ31magnesium alloy and its mechanism was investigated, the results showed that the ultrasonic treatment can effectively refine the grains of AZ31magnesium alloy, make the a-Mg equiaxed and fine. Besides, the B-Mg17Al12dispersed particles were homogeneous and distributed rather than continuous on the grain boundary. The comprehensive mechanical properties of the alloy was improved by fine grain and dispersion strengthening mechanism. Ultrasonic treatment promoted grain refinement and the second phases became more and homogeneous by cavitation and acoustic streaming effect. The effect of acoustic cavitation played a dominant role in enhancing the nucleation rate and the effect of acoustic streaming through the acceleration of the melt flowing through stirring and promoting the heat and mass transfer to improve the microstructures of the alloys.The effect of Gd and its content on the microstructure of AZ31alloy was investigated. It was shown that Gd existed in as-cast AZ31alloy in the form of Al2Gd intermetallic compounds, so the amount of B-Mg17Al12became less and its distribution became more homogeneous. With increasing of Gd content in AZ31alloy, the grain size increased at first and then decreased. Without Gd, the grain size of AZ31was about270μm, with the Gd content weight of1%, the grain size was decreased to165μm.The orthogonal experimental results showed that the optimum smelting parameters for ultrasonic treatment and adding element Gd was690℃×120s×1%(Ultrasonic treatment temperaturexUltrasonic treatment timexGd content). Under these conditions, the grains of as-cast AZ31alloy became fine and round, and the comprehensive mechanical properties were the highest. Compared with simply applying ultrasonic field or adding Gd element, the grains of the alloy were refined greatly, from180μm for single factor effect to about60μm for composite effects.By the second phase strengthening and fine grain strengthening interaction, the mechanical properties of as-extrusion alloy were improved evidently. Compared with AZ31alloy, both applying ultrasonic field and adding Gd element were able to improve the hardness, yield strength and tensile strength of as-extrusion alloy obviously. From53HV,164MPa and270MPa to59HV,195MPa and295MPa respectively.