Research On Fatigue Properties Of AZ80 Magnesium Alloy Wheels By Expansion-closed Extrusion

Light weight has become an important issue in the current era of increasingly prominent resources and energy issues.Magnesium alloys have the characteristics of low density,high specific strength,specific stiffness,high specific modulus,good shock absorption,and strong immunity to electromagnetic interference.This makes magnesium alloys widely used in the automotive industry.In the course of operation,the wheel hub must bear the impact force caused by the unevenness of the road surface.It will work under conditions of large friction,high strength,and high load for a long time,and it will easily crack at the spoke part and fail.However,the deformation of spokes of traditional extruded magnesium alloys is small,which results in non-uniformity of the microstructure,and its fatigue strength needs further improvement.For this reason,a method for forming the extruded hub based on the method of a hollow billet is proposed.This process has the advantages of increasing the deformation of the spokes and reducing the forming force.However,the microstructure and fatigue properties of the spokes have not been systematically analyzed.In this paper,the AZ80 magnesium alloy was used as the experimental material.We compared the hub forming process,heat treatment and the high and low cyclic fatigue performance of the spokes of the two processes.The main conclusions are as follows:(1)Numerical simulation was calculating by Deform-3D software the results show that the deformation of the spokes of the hollow billet expanding and pressing hub is larger than that of the traditional solid billet extrusion hub,and the equivalent strain is increased at least twice.The physical experiment results show that the microstructure of the traditional solid billet extrusion spoke is a mixed crystal structure with an average grain size of 23.4μm.The microstructure of the hollow billet expanding and extruding spoke is almost uniform and equiaxed.The average grain size is 11.2μm.(2)Using an Imager.A1 m metallographic microscope,a SU50000 Scanning Electron Microscope,and an Instron3382 tensile testing machine,We observed the as-cast,extruded,under-aged,and peak-aged microstructures and measured tensile properties.The results show that the tensile properties of the as-extruded state are better than those of the as-cast state.With increase of aging time,the strength and hardness of the AZ80 magnesium alloy increase,and the plasticity decreases by the two processes.However,the mechanical properties based on the expansion-closed extrusion process are higher than the traditional solid billet extrusion process,with finer grains,stronger texture,and more uniform second phase distribution..(3)Instron8801 fatigue tester was used to study the under-aged and peak-aged high-cycle fatigue performance of the two processes.Using a process of Conventional solid billet extrusion hub The results show that the fatigue strength of the peak-aged is higher than 6.7MPa of the under-aged.Using a expansion-closed extrusion process of hollow billet,the results show that the fatigue strength of the peak-aged is higher than 5.4MPa of the under-aged.Under under-aged and peak-aged conditions,the fatigue strength of expansion-closed extrusion process is 5.3MPa and 4MPa higher than that of the traditional solid billet extrusion process.And the fatigue life of the expansion-closed extrusion process hub based on hollow billet is also higher than that of traditional solid billet extrusion process.(4)Using SU5000 scanning electron microscopy to analyze high-cycle fatigue fractures,The sprouting area consists of a similar worm-like structure,the crack propagation area is a fan-shaped area formed by many layers of different sizes.The short-break zone consists of dimples of varying sizes,cleavage steps,and torn edges.(5)Using the same fatigue tester and scanning electron microscope,we investigate the under-aged and peak-aged low-cycle fatigue performance of the two processes.The results show that under the total strain of 0.4%,the stress amplitude of the peak aged is higher than that of the under-aged,and there is a cyclic hardening phenomenon.The average stress response diagram shows that: tensile-compressive asymmetry in magnesium alloy gradually reduce with the extension of the aging time.The hysteresis loop curve shows that the fatigue hysteresis loop is clearly symmetrical and the twinning does not start.Fracture observations show that the source of fatigue cracks appears on the surface,and the crack propagation areas show a radiating,divergent texture.With the extension of the time limit,the area of the crack propagation areas is decreasing.The short-break zone is composed of dimples,cleavage steps,and torn edges.The fracture morphology is closely related to its plasticity and toughness.