Study On Free Bulging Deformation Of AA6061 Aluminum Alloy Extruded Seamless Tubes At Elevated Temperatures

In order to meet the need of light weight,aluminum alloy and other light alloys are widely used in automobiles.In order to solve the problem of poor forming performance of aluminum alloy tubes at room temperature,the hot gas forming technology was developed.The technology can be used to form complex pipe fittings of light alloy such as aluminum alloy and magnesium alloy.In the process of hot gas forming,the forming temperature and the pressure increasing rate determine the forming performance of the tube.For this reason,the distribution and variation rules of stress,strain and wall thickness were studied in the thermal free bulging process of 6061 aluminum alloy extruded tube under different temperatures and pressurization rates.The contour shape and peak wall thickness of the free bulging region under different tube lengths and die parameters were emphatically studied.First,the stress-strain relationship and mechanical properties of 6061 aluminum alloy tubes were obtained by axial unidirectional tensile test at the range of 350?⁵00?and the strain rate within the range of 0.001s^-10.1s^-1.The Field-Backofen model was used to establish the high temperature plastic constitutive relation of the tested pipe.Under the same strain rate,the peak stress of 6061 aluminum alloy tube decreases with the increase of temperature.At the same temperature,the peak stress of 6061 aluminum alloy pipe increases with the increase of strain rate.The strain hardening index n is related to the temperature and the pressurization rate,which decreases with the increase of the temperature and increases with the increase of the pressurization rate.The strain rate sensitivity index m increases with the increase of temperature.According to the results of high temperature tensile test,the parameters of thermal pressure bulging of aluminum alloy tubes,such as bulging temperature,initial bulging pressure,gas quantity and pressurization rate,were determined.Second,through Abaqus numerical simulation,the distribution and variation of stress,strain and wall thickness of 6061 aluminum alloy tube under different temperature and loading conditions were analyzed.The results show that when the temperature is the same,the overall stress increases,the wall thickness increases and the strain decreases at the same height.When the charging speed is the same,with the increase of temperature,the overall stress in the bulging region decreases,the wall thickness of the vertex increases,and the strain decreases.For the same temperature and charging rate,the vertex wall thickness decreases linearly with the bulging height,and the strain increases linearly with the bulging height.Then,through numerical simulation,the effects of initial wall thickness,initial outer diameter,length-diameter ratio and die constrained fillet diameter on the contour of the bulging area and the thickness of the vertex wall were analyzed when the bulging temperature was 450?and the pressure velocity was 2MPa/s.The results show that at the same bulging height,the contour of the free bulging region of the billet with different initial wall thickness is basically the same.With the increase of the initial outer diameter,the bulge on the contour of the bulging region moves outward to the point of the same height,and the thickness of the vertex wall increases.With the increase of length-diameter ratio,the bulge on the contour of the bulging region moves outward to the point of the same height,and the thickness of the vertex wall increases.With the increase of the circular corner diameter of the mold,the bulge on the contour of the bulging area moves outward to the point of the same height,and the thickness of the vertex wall increases.For the pipe blank with different initial parameters,the decreasing speed of the vertex wall thickness is proportional to the initial wall thickness,the decreasing speed of the vertex wall thickness decreases with the increase of the initial outer diameter,and the decreasing speed of the vertex wall thickness decreases with the increase of the length-diameter ratio.At last,the device of free bulging of light alloy tube under hot pressure was established,which can realize uniform heating of the tube billet and study the free bulging deformation of aluminum alloy tube under different conditions.The device can heat the tube blank to be tested to 500?,the air pressure control system can control the air pressure up to 16MPa,the control precision is±0.1Mpa,can achieve reliable gas sealing and input,cooling device can effectively prevent the heat transfer to the press mold.With this device,the thermal pressure free bulging experiment of AA6061 aluminum alloy tube was preliminarily carried out.In the tube billet,different internal mandrel filling methods are used to change the amount of gas required in the tube billet bulging,so as to control the tube billet bulging speed in the later stage.Bulging temperature 450?,bulging pressure 2.85Mpa,bulging time 8s;Bulging temperature 500?,bulging pressure 3MPa,bulging time 1.17s.The experimental results show that the higher the temperature is,the higher the pressurization rate is and the more uniform the wall thickness distribution is.The establishment of the device can realize accurate control of deformation conditions such as temperature and pressurization rate,which provides experimental conditions for further study of thermal free bulging deformation of aluminum alloy tubes.