Microstructure control of AZ91 magnesium alloy by high density electropulsing treatment : mechanisms and application to AZ91 alloy strip manufacturing
高能電脈沖處理 AZ91 鎂合金微觀組織控制及其作用機理研究
Student thesis: Doctoral Thesis
On the basis of the self-made electroplastic-rolling mill in the research group, the effects of the high-density electropulsing treatment (EPT) on the microstructure evolution and the mechanical properties of the aged AZ91 magnesium alloy and the cold-rolled AZ91 magnesium alloy strip are systematically investigated, respectively. Some pertinent mechanisms on the effects observed are proposed. An appropriate temperature field model of EPT applied to metals and alloys is established, and the temperature calculated using the model is in good agreement with that measured by the thermoscope. This temperature field model can be used to effectively estimate the temperature field of the different metals and alloys during EPT, and one can effectively control the target temperature field by adjusting the appropriate electropulsing parameters. This job will give an important guidance to engineering application of EPT. The research discovers that compared with conventional heat treatment, EPT rapidly completes the spheroidization and dissolution of the β-Mg17Al12 phase in the aged AZ91 alloy strip at relatively low temperature, and such EPT-induced microstructural improvement results in a remarkably increase in elongation to failure without loss of their tensile strength. According to the experimental observations and theoretical deductions, EPT dramatically decreases the apparent solid solution temperature of the β-Mg17Al12 phase due to a reduction in the nucleation thermodynamic barrier. It also accelerates the kinetics of dissolution of β-Mg17Al12 phase significantly due to an enhancement of atomic diffusion resulting from the coupling of thermal and athermal effect, on the basis of the model for the dissolution kinetics of the β-Mg17Al12 phase under EPT. The research also reveals that compared with conventional heat treatment, EPT with optimized parameter of pulses rapidly completes the recrystallization process of the AZ91 alloy strip at relatively low temperature, suppressing the precipitation of the β- Mg17Al12 phase. EPT succeeds in obtaining fine microstructure of quasi-single-phaserecrystallized materials, and such EPT-induced microstructure changes improve the mechanical properties remarkably and weaken the intensity of the texture as well. According to the experimental observations and theoretical deductions, a model has been proposed to explain that EPT dramatically accelerates recrystallization kinetics. EPT can both increase dislocation climbing velocity and subgrain growth rate substantially due to an enhancement of atomic diffusion resulting from the coupling of thermal and athermal effects, which decreases the incubation period for recrystallization and increases the nucleation rate of recrystallization. Such effect of EPT results in fine microstructure of recrystallized materials at relatively low temperature in a very short time.
- Rolling (Metal-work), Microstructure, Magnesium alloys