Research On Rheological Behavior Of Thin-walled Aluminum Alloy Tube Necking And Thickening On Hot Extrusion

Tube plastic forming has been widely used in aerospace,automobile,shipbuilding and other manufacturing industries.At present,a method of manufacturing the aircraft rod parts by riveting the threaded sleeve after the tube necking.But the connection strength is unstable,the manufacturing cost is high,and the products can’t achieve the purpose of weight loss.If the new way by tapping when the wall thickness increased after tube necking is used to contact,it will increase the connection strength and reduce the cost.The problem of the wall thickness increased is usually ignored after tube necking,and the material structure and properties were not considered.This is related to the quality and life of the aerospace parts.Aiming at these problems,LF2 M aluminium alloy tube with diameter of 22 mm and thickness of 1mm was used on the research object to study the material rheological behavior of thin-walled aluminium tube by hot extrusion for necking and thickening in this paper.Based on the theoretical analysis and experimental study,a method for measuring the properties of materials under hot extrusion is established.According to the material flow stress behavior,the accurate material flow stress model is calculated after selected.Analysis the material flow behavior and the influence of technological parameters,according to the numerical simulation.The main contents and results are as follows:At first,based on the existing methods to test the mechanical properties and formability of tube,to establish a method of thin-walled ring compression to test the tube properties under hot pressing.Through the stress analysis of the specimen during the compression process,there is closely relationship between the instability defect with the height of the specimen.The ratio of height to thickness(L/t)is used to describe the geometry of the ring compression specimen.The ratio of height to thickness value is an important factor leading to the occurrence of instability defects,it is necessary to effectively control,to accurate describe the flow stress state of the material.The Secend,the vacuum isothermal ring compression test at the conditions of the temperature 350℃ to 475℃,and the strain rate 0.1s-1 to 1s-1,then the experimental results of three kinds of the height to thickness ratio are analyzed.The result shows when the ratio of height to thickness is 1,the contact surface wrinkling due to the tangential compressive stress exceeding the critical value with the increasingcompression.When the ratio of height to thickness is 3,the compressive stress exceeds the critical value,the materials packing and the wrinkling occurs in the inner specimen.When the ratio of the height to thickness is 2,the above defects were not found after compression.The third,according to the thin-walled ring compression test,material flow to both sides along the neutral surface of the ring specimen,the fundamental performance parameters under hot pressing were obtained,and analysis the influence on the properties of the temperature and strain rate.The result shows that with the same strain rate,when the temperature was improved,the flow stress was obviously reduced.When the temperature was invariable,the strain rate was improved,the flow stress was obviously increased.The fourth,observe the microstructure of the ring compression specimen,the initial grains are equiaxed grains.The grains of the arc surface and longitudinal section were elongated along the two sides of the material flow.With the increase of the deformation temperature,the grains growth obviously,the grains distribution have regulations and the structure have higher stability.At low strain rate,the grains grow up uniformly and regular distribution,at high strain rate,the grains grow,the grains deformation degree is small and irregular distribution.The fifth,based on the experimental data of ring compression,the characteristics of flow stress variation under hot pressing were studied.According to the classical flow stress equation named Arrhenius,the model describe the flow stress relationship named Z is established which is proved to have high accuracy through verification.At last,according to the model of flow stress,based on the finite element analysis software ABAQUS,then the Finite Element Model about the thin-walled necking and thickening under hot extrusion is established.The flow rule of material and the influence of process parameters on the forming were analyzed.The result shows that the material of the forming parts with uniform distribution is obtianed by increasing the forming temperature,reducing the forming speed and the effective lubrication.