Hot Deformation Behavior Of AZ70 Magnesium Alloy

Magnesium alloys, as light structural materials, have many special advantages, such as low density, high specific strength, high specific stiffness, good casting capability, electromagnetic shielding performance and easy reclamation etc. So magnesium alloys are extensively used in automobiles, airplanes, computers and communication equipments etc. They are known as 'the green project materials having development potential and best future in the 21st century'. Magnesium has hcp crystal structure and little slip system, so its plastic deformation is difficult. The most common deformation mode of magnesium alloys is die-casting and plasticity processing is limitedly used in the magnesium processing. However, plasticity processed magnesium alloys are highly improved in the microstructure and properties compared with die-casting products. So the development of plasticity processing is significant for the magnesium alloys.In this paper, AZ70 alloy was chosen to investigate the processing characteristic of magnesium alloys. Firstly the method of semi-continuous casting was used to prepare the qualified AZ70 cast ingots and then the research on the craft of its homogeneous annealing proceeded. It is discovered that coarse and reticular Mg₁₇Al₁₂ phase distribute on theα-Mg matrix before the homogeneous annealing of AZ70 ingot. With the temperature and time of homogeneous annealing increasing, Mg₁₇Al₁₂ phase is dissolved, grain becomes symmetrical and the hardness reduces; but the grains grow up when the temperature is too high. So it is determined that the optimum parameter of homogeneous annealing of AZ70 is 410℃/15-20h.The results indicate that the tensile strength and elongation rate is separately increased by 23.1% and 23.6% after the homogeneous annealing at 410°C for 15 hours.Hot compression tests of AZ70 magnesium alloy after homogeneous annealing were performed on Gleeble 1500 at deformation temperature ranged in 3 00-420℃and strain rates 0.001-1s^-1. The results show that firstly the flow stress rapidly ascends with increase of the true strain and gradually descends when the peak value appears and keeps steadily when certain value of true strain is achieved. The flow stress is controlled by strain rate and deformation temperature. The flow stress increases with strain rate increasing and decreases with deformation temperature increasing. The deformation activation energy 165.833kJ/mol and the corresponding stress exponent 5.963 were determined respectively. The constitutive equation was established at the high temperature. The microstructure is changed in the hot compression tests. The grain is refined and dynamic recrystallization is fully completed with increase of deformation. Dynamic recrystallization is accelerated with increase of temperature, but at the same time the grain is easy to grow coarse. The grain is refined with increase of the deformation rate, but dynamic recrystallization can't be fully completed when the deformation rate is too high. The average dynamically recrystallized grain size (drec) is decreased with reduction of deformation temperature and increase of strain rate.Hot tensile tests of AZ70 magnesium alloy after homogeneous annealing were performed on Sintech 20/G at deformation temperature ranged in 260-420℃and strain rate 0.05mm/s. The results show that resistance of deformation descends and elongation percentage ascends with increase of the deformation temperature when the samples are drawn at the same strain rate. But the oxidization is heavy when the deformation temperature is 420℃. Taking the factors such as resistance of compression, microstructure evolvement and the performance changes of tensile testing at high temperature into account, it is considered that in the present work the good deformation condition is temperature of 340-400℃with stain rate of 0.001-0.1 s^-1 and the lower strain rate is preferred.On the basis of the research on the hot deformation behavior of AZ70 magnesium alloy, extrusion tests was carried on and bars, pipes and sectional bars were trial-produced. Not only to establish certain foundation for the further research on the magnesium alloys, but also the results of the experiment provide certain basis for the establishment of craft parameter during producing.