Research On Numerical Simulation Of Extrusion Process Of Aluminum Profile With Large Cross-section And High Wall Thickness Ratio

Al profile is one of the most widely applied light--weight components,and it has the advantages of light-weight,high strength,easy deformation,beautiful appearance,good corrosion resistance and so on.Al profile has been widely applied in the fields of transportation,aerospace,electric appliance and construction,especially in the field of rail traffic.Currently.with the broaden application of light-weight A1 alloys,the demand on Al profiles has been growing in many fields,and the requirements on dimensional accuracy and mechanical properties of Al alloys also become higher.In recent years,it has become a rapid trend for Al profile to have larger cross-section,higher ratio of wall thickness and more specifications.The extrusion technology on Al profile with large cross-section and high wall thickness ratio represents the highest level of Al processing industry.It is difficult to control the extrusion of Al profile with large cross-section and high wall thickness ratio,and there are many factors affecting the material flow,dimensional accuracy,microstructure and mechanical properties.The problems that have not been solved are summarized as follows:(1)The material during extrusion of Al profile with large cross-section and high wall thickness ratio is complex,and the uniformity of the flowing velocity on the cross-section of the profile is poor,which easily cause the twisting,warping and cracks of the Al profile,resulting in high rejection rate.(2)The temperature difference on the Al profile with large cross-section and high wall thickness ratio is obvious,resulting in inhomogeneous microstructure and different grain size.Hence,it is necessary to control the extrusion temperature field.(3)The transverse weld is an inevitable phenomenon during the continuous extrusion process,resulting in the loss and waste of material.The problem of transverse weld is more serious in the extrusion of Al profile with large cross-section and high wall thickness ratio,resulting in a substantial increase of manufacturing costIn the light of above problems,this study establish the numerical modeling on the steady and transient states of extrusion process of Al profile with large cross-section and high wall thickness ratio.The optimization of extrusion process parameters,optimization of extrusion die structure,the variation of extrusion temperature field,and the evolution of transverse weld were studied.The optimized mold structure and process parameters were achieved,which efficiently improve the homogeneity of flowing velocity and temperature distribution,and also reduce the length of transverse weld.The main contents and conclusions of this study are summarized as follows(1)The Al profile of the skin of high-speed train has large cross-section and high wall thickness ratio.The orthogonal tests were designed based on Taguchi method,and the effects of process parameters on distribution of flowing velocity of the cross-section at the die exit,distribution of temperature distribution and extrusion pressure were studied.The results showed that when the billet diameter was 600mm,the extrusion speed was 2mm/s,the billet preheating temperature was 510?,the die temperature was 470?,the container preheating temperature was 450?,the most uniform distribution of flowing velocity and temperature on the cross-section of the profile were achieved.After optimization,the relative mean square error of velocity on the cross-section of profile was 0.1426 mm/s,and it was reduced by about 56.8%compared to the initial scheme.The relative mean square error of temperature on the cross-section of profile was 0.006?,and it was reduced by about 67.9%compared to the initial scheme.When the bar diameter was 600mm,the extrusion speed was 0.5 mm/s,the billet preheating temperature was 510?,the die temperature was 470?,the container preheating temperature was 470?,the extrusion pressure of 8161.42 tons was the minimal,and it was less than 9858.17 tons of the initial scheme.(2)For the skin of high-speed train with large cross-section and high wall thickness ratio,the initial scheme of die structure was designed and the material flow was analyzed.It was found that the profile was distorted due to the large difference of material flowing velocity.Furthermore,the outline of porthole,outline of welding chamber and the length of bearing were optimized and adjusted,and the optimal structure and shape of the die were determined.After optimization,the mean square error of the velocity of the profile at the die exit was reduced from 12.46 mm/s to 1.46 mm/s.In addition,the difference between maximum and minimum temperature of the profile cross-section was reduced from 46.7? of the initial mold structure to 21.9? after optimization.It indicated that the temperature distribution on the profile cross-section was more uniform,and the die optimization had obvious effects.(3)For large cross-section and hollow Al profile,the transient analysis model of extrusion temperature field was established,and the effects of process parameters on the profile temperature distribution was clarified.Furthermore,the gradient heating of billet was introduced,and the effects of profile gradient heating of billet on the profile temperature distribution under single-cycle and multi-cycle extrusion were studied.The greater the extrusion speed,the higher the billet temperature,the higher the mold temperature,the overall temperature of the profile would rise.When the extrusion speed was 1mm/s,the effects of gradient heating of billet on the extrusion temperature field and profile temperature distribution was slight.When the extrusion speed was 5mm/s,the gradient heating of billet was helpful on improving the uniformity of the profile temperature distribution.For multi-cycle extrusion,the temperature had sudden variation between the cycles.When the gradient heating was large during multi-cycle extrusion,the maximum temperature was reduced,and the minimum temperature was increased,both of which were helpful on achieving better distribution of profile temperature.(4)For large cross-section and hollow Al profile,the transient analysis model of transverse weld evolution was established.The effects of extrusion process parameters and mold structure on the length of transverse weld was studied based on Box-Behnken test design.The optimum combination of process parameters was billet diameter of 380.17mm,extrusion speed of 2.93mm/s,billet temperature of 504.89?,and the length of transverse weld was reduced from 16404.0mm of the initial scheme to 11031.7mm.The optimal combination of mold structure parameters was porthole angle of 10.04°?height of welding chamber of 2.04mm?corner radius of welding chamber of 14.80mm,and the length of transverse weld was reduced form 16404.0mm of the initial scheme to 15395.8mm.