Study On Rheological Forming And Solidification Microstructure Of ZA27 Peritectic Alloy Prepared By Self-inoculation Method

ZA27 alloy is a traditional foundry alloy with excellent casting performance and wear resistance. Therefore, the alloy is extensively studied by the scholars at home and abroad. Although the alloy is very suitable for semi-solid processing, there are few researches of the alloy about rheological forming, Especially, the second solidification of the alloy in the peritectic reaction. In addition, the analysis of the peritectic reaction of the second solidification process has very important significance for other peritectic aloy.ZA27 peritectic alloy is investigated as the objective material in this thesis, the semi-solid slurry of the alloy is prepared by self-inoculation method. The mechanisms of grain refining are studied by self-inoculation method. The semi-solid slurry of the alloy is optimized by controlling the isothermal holding time and isothermal holding temperature, subsequently, the rheo-forming experiments are conducted and the characteristic of rheo-forming is studied. The results indicate that:The microstructucal evolution of slurry during the preparation process was researched by water-quenching method. The average size of primary grains increases with the prolonging of isothermal holding time, and the shape factor of grains increases firstly then decreases. Suitable isothermal holding temperature of ZA27 alloy for semi-solid rheo-forming is at 470-480℃and isothermal holding time is 3 min, while the corresponding average grain size is between 52.26 and 60.2. The solidification modes of remaining liquid in the alloy semi-solid slurry are: direct nucleation then growth; attaching to the primary grains then growth into dendrites or bulge shape; attaching to the primary grains then stable growth.The liquid phase segregation is found during rheo-diecast of ZA27 alloy, the secondary grains grow into “vermicular” in the area of the high stress. Remaining liquid has different solidification ways in the different positions of casting, but without coarse dendrites. In the positions of the low cooling rate, the peritectic phase(β) can be generated in coarse eutectic microstructures, where the attachment growth is the main way for secondary solidification. On the contrary, in the position of the high cooling rate, the main way is nucleation growing with a lot of secondary crystal nucleus and the fine eutectic microstructures. The grains of secondary solidification can either nucleate by α(Al), or grow directly by the nucleus of β phase. Cu element is enriched in the eutectic η phase.The die casting, permanent casting and sand casting are performed by prepared semi-solid slurry of ZA27 alloy, respectively. Then the solidification microstructures of die casting, permanent casting, sand casting are researched. It is found that the microstructures are obviously different under the different forming conditions. Remaining liquid solidification of the sand casting mainly attaches to primary grains and the sufficient peritectic reaction. The solidification of permanent casting not only attaches to the primary grains, but also has some new grains. Comparably, the microstructures of die-casting are fine grains, and the remaining liquid instantaneous nucleus under the high rate of the cooling, leading to the microstructures are very dense.