| Magnesium alloy has the characteristics of low density,good damping performance and good heat dissipation performance,etc.It can achieve the effects of weight reduction,energy saving and emission reduction when applied to automobile wheels.The forming technology of hub is mainly divided into casting method and plastic forming method.At present,magnesium alloy wheels produced by casting method can not meet the use requirements of automobile wheels,and plastic forming method can obtain better mechanical properties.However,at present,the back extrusion forming method often produces micro-cracks at the rim,uneven structure and coarse structure at the spokes and flanges,which seriously affects the wheel hub yield,increases the production cost and hinders the market popularization and application of products.Therefore,it is of great significance to study the back extrusion plastic forming process of magnesium alloy wheels.In this paper,the indirect extrusion process of AZ80 magnesium alloy wheel hub is studied by finite element simulation.Based on the analysis of macro and micro structure defects of AZ80 magnesium alloy wheel hub produced by an enterprise,the geometric model of die and blank is established by Creo,the constitutive equation and recrystallization model of AZ80 magnesium alloy are introduced into Deform-3 D software,and the appropriate ductile fracture criterion is selected.The temperature field,stress field,flow velocity field,damage field and the average grain size of the hub are simulated,the causes of defects in the hub forming process are analyzed,and the methods of die structure optimization and processing parameters optimization are put forward.Combined with the indirect extrusion experiment and numerical simulation results of AZ80 magnesium alloy hub,Normalize Crockroft-Latham ductile fracture criterion can better predict the deformation defects of the hub.The microstructure of different parts of the hub is quite different.The extrusion streamline at the rim is obvious,the dynamic recrystallization degree is large,and the grains are fine and uniform.However,the degree of dynamic recrystallization of spokes is small,and defects such as coarse recrystallization structure,uneven structure,cracking or microcrack appear in flange,lower rim,upper rim and the interface between spokes and rim.Numerical simulation shows that by optimizing the die structure,i.e.,increasing the spoke thickness,reducing the spoke inclination angle,increasing the punch fillet and the fillet at the upper rim,etc.,the fluidity of metal can be effectively improved and the blank filling can be smoother.Comparison shows that the optimized die structure is as follows:spoke thickness increases by 3mm,inclination angle decreases by 2°,punch fillet increases by 2mm,and upper rim fillet increases by 2mm.At this time,the temperature at the bottom of hub decreases and distributes uniformly,stress concentration at the joint between spoke and rim is relieved,mold filling at the upper rim is complete,and macroscopic tearing phenomenon disappears.Based on the optimization design of die structure parameters,the influence of forming process parameters on the macroscopic physical field and microstructure distribution of AZ80 magnesium alloy hub by indirect extrusion was studied.The results show that increasing the extrusion temperature can reduce the uneven degree of deformation stress during indirect extrusion.Proper reduction of extrusion speed can improve the uniformity of deformation temperature,but too low extrusion speed will increase the unevenness of deformation stress.Reducing the ratio of height to diameter of billet can reduce the damage value(crack tendency)of hub,but too small ratio of height to diameter will lead to coarsening of grain at the bottom of hub.The suitable process window is as follows:extrusion temperature 360～400℃,extrusion speed 3～5mm/s and blank height-diameter ratio 0.7～1.0.The research also shows that the macroscopic physical fields are similar with different hub sizes,so the reliability of numerical simulation results can be verified by the experimental study of small-sized hub. |
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