| High aluminum ZA alloy was widely applied in bearing and other fields for its high mechanical properties, excellent wear resistance and low production cost, etc.However, high aluminum ZA alloy was easily prone to form the defects, such as dendrite segregation and bottom shrinkage, in the normal casting. As the composition and microstructure were not controlled by the preparation technology at present, the semi-solid forming technology of ZA27alloy was researched in this paper. And the refining, heat treatment and semi-solid forming mechanism of ZA27alloy, which were base on the semi-solid forming technology was also researched. The main results were shown as follows:(1) The best extruding temperature of ZA27alloy was470℃at the semi-solid forming, and the size was refined obviously, the combinative degree was high, the distribution was uniform of the alloy grain at this temperature. The dendrite of casting alloy was broken; the microstructure was solidified to be spherical and distributed uniformly in ZA27alloy, and the grain size was25μm or less. Its density was improved to5.1931g/cm3, and the mechanical properties were also improved, the yield strength was214.51MPa, the tensile strength was415.62MPa, the elongation was15.85%and the hardness was139HB.(2) After Sc refining, the grain size of the casting alloy was20μm or less, the density was5.0789g/cm3, the yield strength was221.56MPa, the tensile strength was443.98MPa, the elongation was10.39%and the hardness was144HB. The microstructure was solidified to be the globular crystal and distributed uniformly in the alloy of Sc refining+semi-solid forming, the grain size was15μm or less, the density was5.2292g/cm3, the yield strength was236.73MPa, the tensile strength was462.51MPa, the elongation was16.25%and the hardness was152HB.(3) The strength and plasticity were greatly improved after heat treatment. The mechanical properties of alloy of Sc refining+semi-solid forming+heat treatment were highest, the yield strength was285.59MPa, the tensile strength was586.01MPa, the elongation was17.57%and the hardness was171HB.(4) It could be found that the field force which could prepare the semi-solid blank that microstructure was uniform, small and spherical, was formed by the mechanical agitation, and was the semi-solid forming mechanism. The method of extrusion ratio in11:1which could refine the grain size and improve the uniformity of the in the remelting of extrusion forming was presented.(5) It could be generated the gray heterogeneous nucleation Al2Sc phase particles when the Sc was added to the alloy. The Al2Sc phase particles were gathered in the grain boundary to prevent the grain from growing and the grain size was refined. As the primary α-Al phase was solidified to be spherical micro structure after preparing the semi-solid blank, it was inferred that the atomic diffusion of Zn and Al was transformed from anisotropy to isotropic by adding Sc at semi-solid forming, and the grain size was further refined.(6) It could be found after the heat treatment of350℃×8h+water cooling, the eutectoid structure of casting was inhibited, the non-equilibrium three-phase eutectic (α’+η+ε) which was brittle was greatly reduced, after aging treatment, the strengthening (α+η) two-phase microstructure could be obtained. And the micro phase of Mg32Al47Cu7was precipitated in the matrix, which could delay the (α’+η) eutectoid transforming, so that the dimension stability, strength and toughness of the alloy were improved. In the alloy of semi-solid forming+heat treatment, the part of the supersaturated solid solution was formed at semi-solid forming, and the (α+η) two-phase structure was further precipitated by the aging, and the mechanical properties of the alloy were improved. The growing up of the grain boundary and matrix of settling phase was inhibited in the aging treatment when the Sc was added to the alloy, and the mechanical property of ZA27alloy was further improved by the alloy of Sc refining+semi-solid forming+heat treatment. |
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