Effect Of Microstructure On High Frequency And High Cycle Fatigue Behavior Of AZ31 Magnesium Alloy

Magnesium alloys have been widely used in aerospace,automobile,machinery and other fields due to their excellent physical and chemical properties.With the continuous development of the automotive industry,magnesium alloy has become the preferred material for the production and processing of automotive parts in order to meet the increasing demand for lightweight in the automotive industry.Magnesium alloy sheet shows great advantages in manufacturing high performance magnesium alloy parts for lightweight automobile.In the realization of lightweight,we should also pay attention to the safety performance of the vehicle in the process of use,auto parts will inevitably fail in the service process.Therefore,it is very important to explore the fatigue fracture mechanism of wrought magnesium alloy.In this paper,hot-rolled AZ31 magnesium alloy sheet with different grain sizes was taken as the object,and the actual service conditions of automobile parts were simulated to study the high-cycle fatigue behavior of sheet with different grain sizes and textures at different frequencies(f=3 Hz and f=30 Hz),and to explore its fracture mechanism under high frequency loads.It provides scientific guidance for the production of AZ31 magnesium alloy sheet automobile parts with good service performance.The average grain size of hot rolled AZ31 magnesium alloy sheet is about 8.9?m,the tensile strength of hot rolled AZ31 magnesium alloy sheet is about 255 MPa,and the fracture elongation is about 30%.The average grain size of hot rolled AZ31 magnesium alloy thick plate is about 27?m,the tensile strength is about 230 MPa,and the fracture elongation is about 20%.After annealing,the average grain size decreased from 27?m to 20.3?m,the number of twins decreased gradually,the fracture elongation and tensile strength increased from 20%to 24.52%,the tensile strength increased from 230 MPa to 239 MPa,and the yield strength decreased.Each sample has no obvious anisotropy along the three directions of the plate surface.Compared with the thick plate,the magnesium alloy thin sheet exhibits better fatigue performance,which is related to the finer and uniform microstructure.The high cycle fatigue life of the annealed specimen is lower than that of the rolled specimen,which is related to the reduction of yield strength caused by the disappearance of textural weakening twins in the annealed process.The experimental results show that the fatigue test cycles of magnesium alloy thin plate and annealed magnesium alloy thick plate and annealed magnesium alloy specimens exceed 10~6when the loading stress is 90 MPa,and no fracture failure occurs when the loading stress is 70 MPa.With the increase of cyclic stress,the number of{10-12}twins in the sample increases.High cycle fatigue of AZ31 magnesium alloy has no obvious frequency effect.The number of crack sources increases with the increase of cyclic stress.Compared with low frequency,when the loading frequency is f=30 Hz,the number of crack sources increases,which accelerates the crack growth rate and increases the distance between the glows.With the increase of stress,the area of the fatigue crack propagation zone decreases gradually,and obvious fatigue striations are observed in the fatigue crack propagation zone.The surface of the final transient fault zone is relatively rough,mainly with dimples,ridged ridges and secondary cracks.