| Multiple forging technology is a novel severe plastic deformation method to prepare structural materials with refined grain and high performance. Fever reports are available about the application of this technology in magnesium alloy. In the present dissertation, two typical types of magnesium alloys, i.e. AZ31 and AZ80 alloy of Mg-Al-Zn system, ZK60 alloy of Mg-Zn-Zr system are chosen as the samples to study the effects of homogenizing annealing process on the deformability and the deformation behaviors during uni-axial compression and multi-directional deformation at elevated temperature. The main conclusions are drawn as follows:(1) The effect of homogenizing annealing process on the microstructures and mechanical properties of AZ80 and ZK60 alloy are systematically investigated. The best homogenizing annealing regularities of the two alloys are annealing at 390℃for 16h and at 330℃for 16h, respectively. The initiation of the cracks in the as-cast AZ80 alloy or ZK60 alloy during upset test is related with the second phase or eutectic compound in the matrix. After solution treatment, the plastic forming ability of the alloys can be effectively improved and the maximum amplitude of increment reaches 20%~30%.(2) The deformation behaviors of Mg-Al-Zn alloys are studied by uni-axial compression (UC) testing under different deformation variables. The experimental results show that flow stress behavior, which is characteristic for hot working processes, strongly depends on the strain rate during high deformation temperatures(T≥250℃)except low temperature (T=200℃). The temperature rise is between 30~50℃and is a function of the strain when the specimens are compressed at high strain rate of 5s-1. Constitutive analysis suggests that the flow stress behaviors of the as-extruded AZ31 alloy strongly depend on the deformation temperature and the relationships between the flow stress and the deformation temperature as well as the strain rate can be represented by the exponential equation during deformation temperature below 350℃, and by the power equation when deformation temperature over 350℃, whereas the flow stress behaviors of as-cast AZ31 or AZ80 alloy can be represented by the exponent-type equation during the whole deformation temperatures (T≥250℃). Initial grain size and a distinctive basal texture lead to the great difference between as-extruded AZ31 alloy and the other two groups of cast alloys AZ31 and AZ80, including the variation in the flow stress equations and dynamic recrystallizaion behaviors. The increment of alloying element Al can decrease the stacking fault energy and enhance the process of dislocation climb, and thus reduce the tendency for dislocation pile-up to cross-slip. As a result, for cast AZ80 alloy, dynamic recrystallization (DRX) is delayed and the activation energy for the plastic deformation process sharply increases from 167 KJ/mol to 220 KJ/mol as compared with the cast AZ31 alloy under the same deformation condition.(3) Not only the effects of deformation variables, including deformation temperature, strain rate and strain, on the deformation behaviors of ZK60 alloy during UC are investigated, but also the static softening behaviors during multistage hot deformation of ZK60 alloy are studied by isothermal interrupted hot compression tests. The results show that the main static softening mechanism of the samples during the interrupted deformation is metadynamic recrystallization (MDRX). The MDRX model of ZK60 has also been established with the activation energy of approximately 50.12KJ/mol which is much lower than that of DRX, approximately 189 KJ/mol. In the present study, deformation temperature has a more important influence on the MDRX process than the strain rate.(4) The deformation behaviors of Mg-Al-Zn alloys under multi-compression (MC) at elevated temperature are systematically studied. A main characteristic of microstructure evolution is directly associated with grain splitting due to the formation of microbands that develop in various directions. Such microbands intersect with each other during hot MC processing, resulting in continuous subdivision of the coarse grains into misoriented fine domains. Further deformation leads to increase in the number and misorientation of these boundaries and finally development of fine equiaxed grains at high strain. During the deformation there exists a critical strainεc, above which a homogeneous microstructure with fine DRX grains can be attained, and after that the grains of the alloys can only be refined to a certain size. The second phase Mg17Al12 in the as-cast AZ80 alloy which remained after solution treatment experiences a series of change with the process of breaking up—dissolving—precipitating—re-dissolving. Fine precipitated phase locates at the DRX grain boundaries, restraining the slipping of the dislocations and the movement of the grain boundaries.(5) The effects of alloy composition on the mechanical properties of the Mg-Al-Zn alloys during hot MC process are systematically investigated. The results show that for AZ80 alloy and AZ31 alloy, both hardness and tensile strength firstly increase, then reduce gradually with increment of pass number. However, the elongation continuously increase with the pass number before 7-passes. Softening and the re-dissolving of the second phase Mg17Al12 during the re-heating process lead to the decrease of hardness of the AZ80 specimens. Decreasing deformation temperature in a certain range and deformation by multi-directional forging method can both refine the grains in the as-cast ingots and improve the mechanical properties. (6) The grain orientation of the as-cast AZ80 alloy during hot MC process is studied for the first time in the world. The results show that the preferred orientation during plastic flow is mainly resulted from grain rotation. For the samples deformed at small strain, slipping and twinning result in grain rotation and lead to the formation of relatively strong basal preferred orientation. Basal preferred orientation changes with the direction of the applied loading axis and is parallel to the compression direction during the first 9 passes except the 4th one. After 10 passes, no preferred orientation is observed by XRD. Forging temperature and deformation mode have an important influence on the evolution of grain orientation except for pass strain. Both decreasing deformation temperature in a certain range and multi-directional deformation mode can promote the formation of strong basal orientation.(7) Large sized AZ80 alloy samples with the dimensions of 110mm×70mm×60mm were produced by continuous multiple forging processing. The two fracture modes of the samples during forging are intensively investigated. Deformation temperature and pass strain are the key factors controlling the cracking of the samples during deformation.
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