Study On The Hot Compression Deformation Behavior In AZ91D Magnesium Alloy

Magnesium alloys, as the lightest metal structural materials being used in industry application at present, have many good properties such as higher specific strength and specific stiffness, better electromagnetic shielding performance and easier manufacture and reclamation, etc.. So it is widely accepted that magnesium alloys are "the green metal engineering materials of the 21st century", and are suitable for many fields, especially in automobile, communication, electronics, aviation and aerospace industries, etc.. At present, most of the alloys are produced by the way of casting. Comparing with the above cast alloy, microstructures of magnesium alloys will be refined after plastic deformation, the cast defects will be eliminated and combined properties will be improved greatly. However, due to their hexagonal close-packed (h.c.p) crystal structure with limited number of operative slip systems, most magnesium alloys have low plastic deformability at room temperature, so the expansion of applications for the alloys is confined for the above reasons. Hence, studies on the plastic deformation of magnesium alloys are of great importance from both scientific and technical view.Of the as-cast alloys, AZ91D magnesium alloy has good cast property and is the most popular one for commercialized application, but there are few reports investigated about plastic deformation of the alloy. In the study, AZ91D magnesium alloy was chosen to investigate the deformation characteristic of casting magnesium alloys. In order to eliminate the arborescent crystal (Mg₁₇Al₁₂) and improve the imhomogeneity of the microstructure of the alloy, AZ91D ingot must be solution treated at 683K for 60h. For the purpose of studying the flow stress behavior of AZ91D magnesium alloy, hot compression tests of the alloy after solution treatment were performed on Gleeble-1500D at deformation temperatures ranging from 473K to 673K and strain rates from 0.005s^-1 to 5s^-1. Microstructure features and deformation mechanism of AZ91D magnesium alloy were investigated using optical microscope (OM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The proper processing parameters of AZ91D magnesium alloy are determined according to the processing map theory. The main conclusions are drawn as follows:1. The flow stress of AZ91D magnesium alloy is strongly influenced by deformation temperature and strain rate during hot compression, decreasing with the increase of deformation temperature and increasing with the increase of strain rate, and the curves exhibit obvious characters of dynamic recrystallization between 473K and 673K. The above hot deformation process is controlled by heat activation, and the relationship among flow stress (a),deformation temperature (T) and strain rate ((?)) is satisfied by hyperbolic sine equation, that is, the flow stress behavior of the alloy during hot deformation can be expressed by Zener-Hollomon parameter (Z parameter for short). The expressions of the parameter Z and the flow stress equation for the alloy are as follows, respectively, derived from the experiment data through regression analysis: 2.From the fabic analysis of AZ91D magnesium alloy after hot deformation through OM and TEM, it indicates that the effects of both deformation temperature and strain rate on the microstructure of the alloy are of great significance, and its deformation mechanism is of a little discrepance at various temperatures. The microstructure of the alloy basically consists of shear bands and twins at 473K. The interleaved extent of the above microstructure increases with the increase of the strain rate, and deformation modes of the alloy mainly comprise dislocation slipping and twinning at the above temperature. At 573K, the alloy entirely shows the typical characteristics of the necklace-shaped microstructure, that is the concurrent phenomenon of deformed grains and recystallized grains. When deformation temperature reaches 673K, the microstructure of the alloy is primarily composed of recrystallized grains, which exhibit obvious growing tendency. The higher strain rate, the smaller the size of the recrystallized grains, and so is the extent of the mischcrystals in the above microstructure. Deformation mode of the alloy principally includes dislocation slipping at 573K and above.3. The processing map theory, which is based on dynamic material model, is introduced into the investigation on the deformation behavior of AZ91D magnesium alloy. According to the data obtained from hot compression experiments, the power dissipation maps, the instability maps and the processing maps of AZ91D magnesium alloy are plotted at true strain of 0.1-0.4, respectively. According to the change of power dissipation coefficient (η) and instability coefficient(ξ) with deformation conditions, respectively, the proper processing parameters of AZ91D magnesium alloy are approximately deformation temperature ranging from 600K to 660K and strain rate from 0.02s^-1 to 0.05s^-1.