| Al-Zn-Mg series aluminum alloys have the advantages of high specific strength and strong corrosion resistance.Al-Zn-Mg series aluminum alloy seamless pipes are mostly used in working environments with high comprehensive performance requirements and long service time.In order to obtain high-precision seamless pipes with uniform longitudinal structure and properties,this paper studies the friction behavior and wear mechanism of Al-Zn-Mg alloys under different contact conditions during hot extrusion through ball-on-disk friction test and find a suitable friction model for the hot extrusion of Al-Zn-Mg series aluminum alloy.The finite element simulation calculation is carried out for the production of Al-Zn-Mg series aluminum alloy extruded pipe with this suitable friction model.This paper investigated the micro structure and mechanical properties of the seamless tube to provide theoretical basis for producting process.The main work are listed as follows:(1)Through the ball-disk friction test,the influence of temperature and load on the friction behavior of steel and aluminum was analyzed.The test temperature was increased from 25℃ to 500℃,the friction coefficient increased from 0.6 to 1.6,the volume wear rate increased,and the wear mechanism gradually changed from abrasive wear to adhesive wear and oxidation peeling wear.The normal load increased from 1N to 5N,the friction coefficient increased from 0.58 to 1.6,the area and depth of the exfoliation layer on the worn surface increased,and the volume wear rate increased.The subsurface layer of the worn sample has a mechanically mixed layer(MML)and a plastic layer with high oxygen content under higher load.During the sliding process,the cracks in the MML are connected and the oxides are broken,resulting three-body friction and aggravating the wear.(2)According to the mechanical model of ball-disk friction,the shear friction stress and contact stress under different contact conditions were calculated,and the friction factors of Siebel,Coulumb and Levanov friction models in the QFORM software were obtained by data fitting.The three friction models were used respectively to calculated the piercing force and extrusion force of Al-Zn-Mg series aluminum alloy seamless tube,which shows that the calculation result of Levanov model is closer to the actual extrusion.The relationship between shear friction stress,flow stress and contact stress is:τ=0.9σT/(?)(3)Using the Levanov model to analyze the temperature rise of aluminum alloy pipes under different extrusion process conditions,it shows that with the increase of ingot temperature and extrusion speed,the outlet temperature of the pipe increases,but the increase of temperature rise is negatively correlated with the ingot temperature and positively related to the extrusion speed,so the temperature rise of the pipe is more sensitive to the extrusion speed.Considering the outlet temperature and flow uniformity of the pipe,the ingot temperature is 450℃ and the extrusion speed is 2mm/s are more reasonable in extrusion processes.(4)The Al-Zn-Mg series aluminum alloy extruded tube has a fibrous structure along the extrusion direction,and the second phase particles are dispersed.After the intermediate annealing,static recovery occurs,the fiber structure becomes wider,the second phase particles are obviously coarse,and the plasticity promote.The grains of the cold-rolled pipe are flattened and elongated,the second phase is finely dispersed and distributed in a streamlined shape,and the plasticity is improved after annealing.The size and distribution of the dimples at the tensile fracture of the pipe at different stages correspond to the particles of the second phase.The particles of the second phase are fine and dispersed,the size of the dimples at the tensile fracture is uniform,and the pipe has good plasticity. |
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