Grain Refinement Of Sip/ZA27 In-situ Composites And Structural Evolution During Partial Remelting

In this thesis, the effects of refiners(Ti, B-Ti and Zr) and RE elements (La,Ce and Ce-rich mixture) on the as-cast microstructure of in-situ Si_P/ZA27 composites were investigated. The microstructural evolution of the Si_P/ZA27 composite during partial remelting, were also investigated. The results are shown as followings:(1) The efficiencies of modification of Zr,B and Ti were investigated, and it was found that the Zr had the best modification effect. After being modified by Zr, the primary a became fine.(2) The primaryαdendrites of the in-situ Si_P/ZA27 composites became finer after the additions of Zr and rare metals. The addition of Ce-rich mixture had the best refine effect ,while the addition of La had the worstest effect. With the increase of RE content, the size of the primary a dendrites decreased, the size and shape of Si particles had no change, while the sizes of rare-earth metals contained compound increased and their shape became un-regular.(3) The primary phase of the Si_P/ZA27 composites modified by 0.2%Zr and 0.05% Ce-rich RE mixture and ZA27 alloy modified by 0.2% Zr became into spherical particles after being heated for 20min and 30min respectively, at semisolid temperature 475℃. However, The Si_P/ZA27 composite with developed dendrites must be heated over 120 min in order to achieve the globular grains, and the resulting grains were quite large.(4) During partial remelting at semisolid temperature 475℃, The microstructures of both the Si_P/ZA27 composites and ZA27 alloy with fine equiaxed dendrites experienced four stages: the initial coarsening stage, structure separation stage, spheroidization stage and the final coarsening stage. The differences between them are : the separation speed of Si_P/ZA27 composites is slower than ZA27 alloy at heating stage, the increase rates of the primary a particle size and shaper factor of Si_P/ZA27 composites is lower.(5) The heating temperature is one of the important parametrs during partial remelting. The size and the shape factor of the primary phase of the Si_P/ZA27 composites with fine equiaxed dendrites are lowest,after heated at 475℃for 30min. With the increase of heating temperature, the size of the primary phase decresed and the roundness incresed while the temperature was lower than 475℃,however,the trend changed while the temperature higher than 475℃.