Study On DC Casting Billet Preparation And Extrusion Process Of The Magnesium Alloys With High Rare Earth

Magnesium alloy is the lightest metal structural materials, with high specific strength, high stiffness, good damping capacity and electromagnetic shielding, etc. But its strength is not high and high temperature performance is poor. With high-strength, heat-resistant, corrosion resistance and other properties, the rare earth magnesium alloy has been widely used in the aerospace and military industry. The Mg-Y-Nd-Zr, Mg-Gd-Y-Zr and other new high-strength and heat-resistant magnesium alloys are the typical examples. But the preparation of large size billets is difficult and the plastic deformation ability is poor, that’s the main reason for its application is limited. In this study, we prepared the Mg-9Gd-3Y-1.5Zn-0.6Zr alloy through different casting process, and then we study its extrusion process and heat treatment process. Research has achieved conclusion as follows:(1) The main constituents in as-cast Mg-9Gd-3Y-1.5Zn-0.6Zr alloys are a-Mg and Mg24(GdYZn)5; The suitable homogenization technological condition is 530℃×5h, then water-cooled; The precipitate phases are Mg12Zn(YGd) after homogenization.(2) Experiments using the Low-Frequency Electromagnetic Casting technique by the different Moulds (hot-top, graphite and aluminum sleeve) to prepare the Mg-9Gd-3Y-1.5Zn-0.6Zr alloy. The results show that the grain size of hot-top billets is the smallest; the average grain size can be 39μm. When using the hot-top mould, it will obtain the high quality billet by increasing the current strength and reduce casting velocity appropriatly.(3) Casting process has a significant influence on the rheological behavior of the Mg-9Gd-3Y-1.5Zn-0.6Zr alloys, the hot-top billets applied by a lower current intensity and casted by a lower casting velocity are more suitable for the high strain rates at a low temperature.(4) Casting process and extrusion velocity has a significant influence on the microstructure and mechanical properties of as-extruded alloy. Low-velocity extrusion can improve alloy deformation uniformity, refine the microstructure, and improve the mechanical properties. The tensile strength and the yield strength of the hot-top alloy (v=55mm/min, f=30Hz, I=68A) which extrusion velocity is 2.5cm/s are 390 MPa and 330 MPa, respectively, the elongation is 10.6%.(5) The hot-top alloy (v=65mm/min, f=30Hz, I=68A) which extrusion speed is 2.5cm/s gets the optimal tensile property after 30h aging treatment at 200℃; the tensile strength and the yield strength are 465MPa and 364MPa, respectively, the elongation is 3.5%. When the tensile temperature is 250℃, the tensile strength is 315MPa and the elongation is 44.7%.