| 其他摘要 | As structural materials magnesium alloys have a lot of advantages, such as low density, relatively high specific strength and specific elastic modules. They have been widely used in aeronautic, automotive, electronic industries in recent years. Applying pulsed magnetic field (abbreviated to PMF) during the solidification of metals is a new method developed in recent years. This dissertation investigates the effects of pulsed magnetic field on the solidified microstructures, average grain size, distribution of solute content, cooling curves and mechanical properties of pure Mg, AZ91D, AZ31 and Mg-Gd-Y-Zr magnesium alloys, and simulates the PMF processing, and discusses the microstructure refinement mechanism. The results are as follows:
A mathematical model is built to describe the interaction of the circuit-magnetic-flow fields during solidification by means of ANSYS software. The pulsed electric circuit is first solved and then it is substituted into the magnetic field model. The fluid flow model is solved with the acquired electromagnetic force. The effects of pulse voltage and frequency on the current wave and the distribution of magnetic and the flow fields are numerically studied.
The experimental results show that the remarkable microstructural refinement is achieved when the PMF is applied to the solidification of pure Mg, AZ91D, AZ31 and Mg-Gd-Y-Zr alloys. The average grain size of the 50mm ingots is refined to 260m, 104m, 107m and 37m respectively. Besides the grain refinement, the microstructures of pure Mg are considerably refined via columnar-to-equiaxed growth by the PMF; the morphology of the primary -Mg of AZ91D alloy is changed from dendritic to rosette, then to globular shape with changing the parameters of the PMF; the microstructures of AZ31 and Mg-Gd-Y-Zr alloys are also refined to whole fine equiaxed grains. The microstructure refinement effect is the same for the 100mm ingots, and the average grain size is refined to 110m, 104m, 109m and 47m for AZ91D, AZ31 and Mg-Gd-Y-Zr alloys respectively. The macrosegregation of solute elements of magnesium alloys is reduced by PMF treatment. The optimal processing parameters are 5Hz, 200V.
The pure Mg and magnesium alloys produced with a 5Hz, 200V PMF treatment exhibit improved mechanical properties, such as the ultimate compressive strength at room temperature is increased to 227MPa, 362MPa, 337MPa and fracture strain to 33.2%, 20.1%, 31.6% for pure Mg, AZ91D and AZ31 alloys respectively. The tensile strength and elongation are increased to 165.4 MPa, 181.6 MPa, 238.3MPa and 3.26%, 8.9%, 1.91% for AZ91D, AZ31 and Mg-Gd-Y-Zr alloys respectively.
The pulsed magnetic field causes melt convection during solidification, which makes the temperature of the whole melt homogenized, and produces an undercooling zone in front of the solid/liquid interface by the magnetic pressure, which increases the nucleation rate and prohibits dendrite growth. In addition, primary -Mg dendrites break into fine crystals, resulting in a refined solidification structure of the magnesium alloys. The Joule heat effect induced in the melt also strengthens the grain refinement effect and the spheroidization of dendrite arms. |
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