Study On Microstructures And Properties Of Casting Mg-RE Alloys

Several typical sand cast Mg-RE alloys were choosed to investigate. The influence of cast process, the kinds and contents of alloy element to the microstructure, age hardening response, ambient and elevated mechanical properties were systematically studied. The main research results are listed as follows:Medium content of rare earth elements of Mg-5Gd-4Y-0.5Zr(mass %) magnesium alloy were adopted to study the effect of addition of Zn or Nd on the microstructure and mechanical properties by sand-casting tensile test bar. The results indicate that addition of Zn or Nd in Mg-5Gd-4Y-0.5Zr alloy can obviously refine grain size and modify the kinds, morphology,and quantities of second phases in the as-cast condition, and improve its tensile mechanical properties at ambient temperature. The solution treatments of Mg-5Gd-4Y-0.5Zr,Mg-5Gd-4Y-0.5Zn-0.5Zr and Mg-5Gd-4Y-0.8Nd-0.5Zr magnesium alloys were optimized.The grain size of Mg-5Gd-4Y-0.8Nd-0.5Zr alloy near the casting surface grew large unnormal after two stage solution condition, which can be avoided by adjusting one stage solution treatment. The optimized solution treatment condition of the three alloys are500℃×6h, 500℃×6h, 500℃×2h, respectively. Moreover, microalloying of Nd and Zn can effectively improve the aging hardening response of the Mg-5Gd-4Y-0.5Zr alloy as well as its strength at ambient and elevated temperature. The ultimate tensile strength, yield strength, and elongation of the Mg-5Gd-4Y-0.8Nd-Zr alloy are 303 MPa, 215 MPa, and 1.2% and 283 MPa,200MPa, and 7.2%, at room and elevated temperature, respectively.Effect of Gd and Y elements contents on microstructure and mechanical properties were adopted by Mg-5Gd-4Y-0.5Zr, Mg-6Gd-3Y-0.5Zr and Mg-7Gd-4Y-0.5Zr magnesium alloys with sand-casting tensile test bar. The results showe that the second phase of these alloys are Mg24(Gd, Y)5, which is island discontinuous distribution along grain boundary, and white bright cuboid-shaped Mg5(Gd, Y) phase, meanwhile, the quantities of the second phases in these alloys increases and grain size decreases with increasing Gd, Y contents. After solution treated at 500℃ for 6h, the Mg24(Gd, Y)5 phase is completely dissolved, while the Mg5(Gd, Y)phase still exists. Mg-5Gd-4Y-0.5Zr alloy has relatively high elongation, but its strength is lowest. Mg-7Gd-4Y-0.5Zr alloy has very high strength, but its elongation is low. The Mg-6Gd-3Y-0.5Zr alloy owns the better comprehensive mechanical properties. The ultimate tensile strength, yield strength, and elongation of Mg-6Gd-3Y-0.5Zr alloy in T6 condition are237 Mpa, 162 Mpa and 12.1% and 208 Mpa, 146 Mpa and 13.5% at room and elevated temperature, respectively.Two casting methods of gravity casting and centrifugal casting were adopted with ZM6 alloy to produce two circle workpieces. In the gravity casting of ZM6 alloy, uniform equiaxed grains distribute along the radial direction. Meanwhile, mechanical properties are consistent along the radial direction. However, in the centrifugal casting of ZM6 alloy, grain size issmaller than that of gravity casting and is different along the radial direction. There is a fine grain size zone about 5mm distance to the outside surface of centrifugal casted workpiece and mechanical properties are unconsistent along the radial direction. It is coincidence with the fine grain size circle, which owns optimum mechanical properties, the fine grain size cirle is3-6mm distance from the outside surface of centrifugal casted circle. The optimized centrifugal condition of ZM6 alloy is confirmed by 30 s after pouring melting alloy into mould.