Research On Microstructure And Properties During The Fabrication Of Fine-grained Mg-5Zn-1Mn Magnesium Alloy Sheets

Mg-Zn-Mn alloy system with high zinc content as an important wrought magnesium alloy has the advantages of age hardening, excellent corrosion resistance and low cost due to no noble metal elements. Thus, the research on the preparation technology of this kind of alloy thin sheet with fine grains has great significance. The traditional rolling process of multi-pass and small deformation commonly exists a lots of problems, such as long production cycle, low finished product rate, poor plastic etc. In this study, the effects of high strain rate on deformation behaviour and microstructure evolution of ZM51 alloy were studied by hot compression simulation experiment. On the basis of these results, the ZM51 magnesium alloy sheets with high strength and high plasticity were preparated by high strain rate rolling(HSRR) technology. During high strain rate rolling, dynamic recrystallization induced by twin nucleation can be enhanced to achieve grain refinement and nanometer precipitates can be dynamicly induced through large plastic deformation.In this paper, the hot compression tests were performed on Mg-5Zn-lMn alloy with the large deformation of 80% by the Gleeble-3500 material simulation machine. Microstructure evolution and flow behaviour were studied with the deformation temperature ranged from 250 ? to 400 ? and the strain rate ranged from 1s-1 to 40s-1. The results show that dynamic recrystallization (DRX) occurs obviously during hot compression and the DRX microstructures formed at high strain rate are more uniform and the size of the grains is smaller than the microstructures formed at low strain rate. With increase of strain rate and deformation temperature, the deformation activation energy decreases gradually. The optimal process parameters of large plastic deformation for ZM51 alloy are that the strain rate ranges from 10s-1 to 20s-1 and the deformation temperature ranges from 250 ? to 400 ?.Based on the thermal simulation experiment, the effects of rolling temperature on the microstructure evolution, dynamic precipitation behavior and mechanical properties of ZM51 alloy sheets were studied by HSRR. The results show that the microstructures of surface and center in the ZM51 sheets rolled at low rolling temperature are inhomogeneous. With increase of rolling temperature, the overall microstructures of the sheets tend to be uniform. Dynamic precipitation behavior occurred obviously in ZM51 alloy sheets during HSRR. The nanometer Mg-Zn phases were dynamically precipitated by heterogeneous nucleation on high density of dislocations induced during HSRR and a large amount of ?-Mn particles formed during homogenization treatment. On the one hand, the typical bimodal microstructure can be obtained in the HSRRed sheets due to the dynamic precipitates strongly pinning the boundaries of the DRX grains. On the other hand, the Mg-Zn phases can be rapidly precipitated accompanying the formation of dislocations and then the precipitates will inhibit DRX behaviour by strongly pinning the dislocation and the boundaries of subgrain. The sheet fabricated by HSRR at 300? exhibits high strength and excellent ductility, with the ultimate tensile strength (UTS), yield tensile strength (YS) and elongation (EL) to failure of 359MPa,258MPa and 20.1%, respectively. The superior mechanical properties can be attributed to the ultrafine DRX grains featured with the bimodal microstructure and the nanometer precipitates with high density.