| This paper focuses on the investigation for AZ31 magnesium alloy sheet in the tensile mechanical properties,fracture morphology,microstructure evolution and simulating of magnetic pulse bulging under different temperature and strain rate,which provide a theoretical basis for the high-speed plastic forming process of AZ31 magnesium alloy sheet with warm temperature.The stress-strain curves at different temperatures and different strain rates were obtained by quasi-static(using INSTRON universal testing machine)and high strain rate(using Split Hopkinson Tensile Bar setup)tensile tests at different temperatures.The results show that tensile strength and yield strength of AZ31 magnesium alloy sheets were increased with the increase of strain rate under quasi-static condition.The elongation decreased with the increase of strain rate.With the increasing of temperature,the tensile strength and yield strength of AZ31 magnesium alloy sheets were decreased and the elongation rate increased significantly.Under high speed conditions,with the increase of strain rate,the flow stress of AZ31 magnesium alloy increased gradually and it was obviously higher than that under quasi-static.The yield strength and tensile strength increased slightly and the elongation increased significantly.With the increase of temperature,the flow stress of AZ31 magnesium alloy decreased mildly.The thermal effect for AZ31 sheet in high speed tension had limit role to influence the formability.The yield strength,tensile strength decreased and the elongation had no evident difference.The Johnson-Cook constitutive equation of AZ31 magnesium alloy sheet at different temperature was obtained by the quasi-static and high-rate tensile test of AZ31 magnesium alloy sheet.The equation is:The fracture surface morphology and metallographic morphology of AZ31 magnesium alloy sheet were analyzed under different tensile test conditions.The results show that the microstructures of AZ31 magnesium alloy sheet specimen have the characteristics of river-like tearing edge,very few dimple and cleavage steps,which are mainly brittle fracture at room temperature and quasi-static condition.At 373 K,the number of dimples were higher than that of the ductile dykes.At 473 K,there was a thin river-like tearing edge,showing a typical ductile fracture.With the increase of strain rate,the dimples in the fracture surface of the specimens decreased significantly and becameshallower,which indicated that the fracture morphologies of AZ31 magnesium alloy plates showed some sensitivity to the strain rate.The fracture pattern of the high-speed dynamic tensile specimen showed tough-brittle fracture.There were dimple features in a certain area,and in the dimple round with the tongue cleavage pattern.Cleavage and cleavage steps could be observed on the micro-fracture,and mixed with a quantitative tearing edge.The fracture morphology of high rate dynamic tensile specimen is not sensitive to the change of strain rate.At room temperature and quasi-static condition,finer grain was observed with lower strain rate,and more twins existed.At 373 K,the grain elongation showed strip along the tensile direction.And larger strain rate caused more twin crystal.At 473 K,the grain was obviously elongated in the stretching direction,and a large number of fine grains appeared in the large grain.At 523 K,an obvious dynamic recrystallization appeared.At high strain rate conditions,the presence of twin crystals can be observed throughout the test temperature.With the increase of experimental temperature,twins did not decrease significantly.Under the condition of high rate tension,the twinning density increased significantly compared with the quasi-static tensile condition.The high strain rate significantly prolonged the existence of twins in the tensile test and delayed the dynamic recrystallization.Under the condition of high rate,the effect of temperature on the softening effect of AZ31 magnesium alloy sheet is small.The 2D magnetic pulse bulging model with warm temperature of AZ31 magnesium alloy sheet was established by ANSYS software using loose coupling method.The influence of temperature and strain rate on the deformation was analyzed.The simulation results show that the magnetic flux density on AZ31 magnesium alloy sheet decreased with the increase of temperature,and the electromagnetic force decreased at the same time.The maximum velocity in forming process become smaller,and the maximum displacement height decreased firstly and then increased.The common effects from softening,hardening and inertial influenced the formability of AZ31 sheet in magnetic pulse forming with warm temperature.The high strain rate becomes the mainly factor affecting the magnetic pulse bulging process with warm temperature of AZ31 magnesium alloy sheet. |
