| Magnesium alloy, as a kind of metal structural material with high specific strength and stiffness, good mechanical properties, and many other advantages, has a wide range in traffic and aerospace field. Compared with commercial magnesium alloy, magnesium alloys reinforced by long period ordered structure show excellent room temperature and high temperature performance. Because of the uneven distribution of long period structure and coarse grain, the as-cast Mg-Y-Cu alloy reinforced by long period ordered structure has poor mechanical properties. Solidified structure has important influence on mechanical properties of magnesium alloy.Alloying treatment or magnetic field treatment during solidification of alloys, with the advantages of convenient operation and significant effect, has attracted a wide spread attention. In order to improve mechanical properties of magnesium alloy reinforced by long period ordered structure, Mg97Y2Cu1 alloy were employed as raw materials for this study. Solidified structure and mechanical properties of Mg97Y2Cu1 alloy with Sr and Zr alloying treatment, DC magnetic field treatment, AC magnetic field treatment and compound treatment of alloying and magnetic field were studied, and the related mechanism was also discussed. The major conclusions are as follows.1) Sr or Zr can improve solidification structure and mechanical properties of Mg97Y2Cu1 alloy. The effect of compound addition of Zr and Sr is better than the effect of single addition of Zr or Sr. When the amount of Sr added in the range of0-0.2% or Zr added in the range of 0-1.2%, with the increase of Sr or Zr, grain size of Mg-Y-Cu alloy decreases gradually, the distribution of the second phase becomes more uniform, and its volume fraction also improves. When single addition or compound addition of Sr is over 0.2% in the alloy, the alloy is over-modified. A spot of granular CuZr2 phase forms in the alloy after Zr addition. Mg17Sr2 phase forms in the alloy when the addition of Sr is more than 0.2%. With the combined addition of0.12%Sr and 0.8%Zr, CuZr2 phase and Mg17Sr2 phase form in the alloy at same time.The change law of Sr and Zr on mechanical properties of the alloy is similar to the change law of Sr and Zr on grain size of the alloy. With the combined addition of0.12%Sr and 0.8%Zr, the tensile strength and elongation of the alloy reach 222.5MPa and 10.7%, which are increased by up to 29% and 84%, respectively, compared withthe alloy without treatment.2) DC or AC magnetic field can refine the primary grains of Mg-Y-Cu alloy,improve the uniformity of distribution of second phase and increase the solute content in grain interior. Solidification structure of Mg-Y-Cu alloy can be further refined with compound treatment of Zr, Sr alloying and DC or AC magnetic field. The change law of mechanical properties of Mg-Y-Cu-(Zr-Sr) alloy is similar to the change law of its solidification structure. When the DC magnetic field strength is 0.15 T, the tensile strength and elongation of Mg-Y-Cu alloy are increased by up to 20% and 40%,respectively, compared with the alloy in traditional condition. When the voltage of AC magnetic field is 200 V, tensile strength and elongation of Mg-Y-Cu alloy are increased by up to 16% and 47%, respectively, compared with the alloy without treatment. The mechanical properties of Mg-Y-Cu alloy can be further improved by compound treatment of Zr, Sr alloying and DC or AC magnetic field.3) When strength of DC magnetic field is at 0-0.15 T, with the increase of magnetic field strength, grain size of Mg-Y-Cu alloy decreases grossly. When pouring temperature is at 660-750℃, with the increase of the pouring temperature, grain size of Mg-Y-Cu alloy increases gradually, but grain size of Mg-Y-Cu-Zr-Sr alloy decreases gradually. When the mold temperature is at 20-600℃, with the increase of mold temperature, grain size of Mg-Y-Cu alloy or Mg-Y-Cu-Zr-Sr alloy decreases firstly, and then increases, and the turning point is 200 ℃. DC magnetic field can change the crystal orientation of Mg-Y-Cu alloy significantly. The(101) diffraction peak replaces the(002) to be the most strongest peak in the XRD patterns. Compared with magnetic field strength, the effect of pouring temperature or mold temperature on crystallization orientation of Mg-Y-Cu-Zr-Sr alloy is smaller.4) When excitation voltage of AC magnetic field is at 0-250 V, with the increase of excitation voltage, grain size of Mg-Y-Cu alloy decreases at first, and then increases. The turning point of excitation voltage is 200 V. When pouring temperature is at 660-750℃, with the increase of the pouring temperature, grain size of Mg-Y-Cu alloy increases grossly, but grain size of Mg-Y-Cu-Zr-Sr alloy decreases at first, then increases, decreases at last. When mold temperature is at 20-600℃, with the increase of mold temperature, grain size of Mg-Y-Cu alloy or Mg-Y-Cu-Zr-Sr alloy increases gradually. AC magnetic field can improve solute content in grain interior. With the lower pouring temperature or mold temperature, the influence of AC magnetic field on the solute content in grain interior is more obvious. |
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