Microstructural Evolution And Mechanical Properties Of AZ31 Magnesium Alloy By Stagger Spinning

As a"green"engineering material in the 21st century,magnesium alloy has been widely used in aerospace,automobile,national defense industry and many other fields due to its many advantages such as low density,high specific strength and stiffness,and easy to process.However,AZ31 magnesium alloy has less independent slip system and poor plastic deformation ability at room temperature,which has become a major difficulty in the plastic deformation of magnesium alloy.This paper suggested that AZ31 magnesium alloy was processed by stagger spinning which can produce continuous local plastic deformation.In this paper,ABAQUS finite element software was used to simulate the spinning process of AZ31 magnesium alloy,and the influence of main parameters on the forming quality of magnesium alloy tube was studied.The microstructure of magnesium alloy at different spinning passes was observed by metallographic microscope,and the grain refinement mechanism of magnesium alloy was investigated by electron backscattering diffraction（EBSD）and transmission electron microscope（TEM）.The mechanical properties of magnesium alloy at different spinning passes were tested and analyzed by tensile test,and the micro-mechanical properties of magnesium alloy were studied by nanoindentation instrument,and the relationship between tensile strength and nano-hardness was discussed.The corrosion resistance of magnesium alloy was tested by electrochemical corrosion test,and the effect of grain size on corrosion resistance of magnesium alloy was analyzed.According to the finite element simulation result,the optimum technology of AZ31 magnesium alloy by spinning is:Magnesium alloy was processed by multi-passes spinning under the spinning temperature about 375℃,the speed of mandrel of 250 r/min,the feeding rate of 1.2 mm/r.In combination with the optimum process obtained by simulation,AZ31 magnesium alloy tube were successfully prepared in this paper,and its wall thickness decreased from 6 mm to 0.7 mm and its overall reduction rate to 88.3%.The microstructure of AZ31 magnesium alloy was observed and analyzed by metallographic microscope.The results showed that the microstructure of AZ31magnesium alloy was more uniform and grain size（from 625μm to 6.38μm）of AZ31 magnesium alloy was finer with the increase of spinning times.EBSD and TEM were used to investigate the grain refinement mechanism of spinning magnesium alloy.In the first and second passes,the main mechanism of grain refinement is that the dislocation density increases sharply due to the high spinning pressure,and dynamic recrystallization occurs after polygonization.In the third process,due to the fine grain after the second spinning,it is easy to release the stress in the way of dynamic recovery when subjected to the roller force.Therefore,a large number of subcrystal structures were formed to refine the grain of magnesium alloy.The tensile test was conducted to analyze the mechanical properties of AZ31magnesium alloy at different spinning passes.It was concluded that the tensile strength of magnesium alloy increased with the increase of spinning passes,and the tensile strength at the third spinning pass was up to 181.65 MPa.The micro-mechanical properties of AZ31 magnesium alloy were analyzed by nanoindentation instrument,and the relationship between tensile strength and nano-hardness was discussed.According to the nanoindentation test,the hardness and modulus of the magnesium alloy significantly increased after spinning.The nano-hardness increased from 0.172 GPa to 0.508 GPa.Compared with the base material,the hardness of AZ31 magnesium alloy increased by 195.35%.The elastic modulus increased from 24.85 GPa to 38.02 GPa.Compared with the base material,the modulus of AZ31 magnesium alloy increased by 53%.After analysis and discussion,the linear relationship between the tensile strength（σ_b）and micro-hardness（H）of magnesium alloy is H=2.26σ_b+94.17.The corrosion resistance of spinning magnesium alloy was tested by electrochemical corrosion test,and the influence of grain size on corrosion resistance was analyzed.The results showed that the corrosion resistance of AZ31magnesium alloy decreased after spinning,and the corrosion resistance of AZ31magnesium alloy decreased with the increase of spinning times.The base metal has the best corrosion resistance.The corrosion current was 8.3791×10^-6 A/cm²,and the corrosion potential up to-1.4128 V.The corrosion resistance of the third pass magnesium alloy was the worst,with corrosion current of 1.9908×10^-5 A/cm²and corrosion potential of-1.6229 V.