Evolution Of Transverse Welds During Porthole Extrusion Of 7N01 Hollow Aluminum Alloy Profile

In recent years,with the increasing attention of air pollution on the world,the importance of energy saving and emission reduction becomes increasingly prominent.In the field of high-speed trains,lightweight car body is one of the most important means to save energy and reduce emissions.Due to its advantages of light weight,high specific strength,good weldability,superior electrical and thermal conductivity,good corrosion resistance and easy of recycling,aluminum alloy is one of the most ideal materials to achieve the lightweight of high-speed train.Currently,the bearing structure of high-speed train are mainly manufactured by 7××× series aluminum alloy extrusion profiles.During continuous production of aluminum alloy profiles,the interface of two adjacent billets flows through die cavity and forms transverse weld in the profile.The region including transverse welds usually has poor mechanical properties and must be discarded,so it has great influence on the finished product rate of extrusion production.Due to the poor plasticity of 7xxx aluminum alloy compared to 6xxx aluminum alloy,so the length of transverse weld is increased apparently,especially for AA7××× hollow profile,which leads to poor product quality and low finished product rate.Therefore,it is necessary to investigate the evolution of transverse weld for AA7xxx hollow profile.In this study,the 7N01 aluminum alloy hollow profile used in high-speed train is chosen to investigate the evolution of transverse weld during the extrusion process by experimental analysis coupled with FE simulation.The main works and conclusions of this paper are as follows:(1)The macroscopic morphology and evolution of transverse weld in the AA7N01 hollow profile are obtained by extrusion experiments and the corresponding microstructure,texture and fracture appearance are analyzed with SEM and EBSD.The results show that there are obvious differences in grain size,texture content and fracture morphology on both sides of transverse weld.(2)The mechanical parameters and thermo-physical parameters of 7N01 aluminum alloy are examined by experiment.The stresses and strains data of AA7N01 under different strain rates and different deformation temperatures are obtained by hot compression tests.Then,the experimental flow stresses are corrected by considering the friction and temperature error.Based on the corrected experiment data,the strain compensation Arrhenius model and processing maps of AA7N01 are established.(3)Based on the mechanical parameters,thermophysical parameters and constitutive equation parameters determined in this work,the numerical model of transverse weld formation in the AA7N01 hollow profile is built with HyperXtrude software.The comparisons of nose-end shape,extrusion pressure curve,transverse weld shapes and transverse weld length show good agreements with experimental observations,which verifies that the accuracy of material property parameters,constitutive equation and numerical model.(4)The influences of extrusion process parameters and die structure on the length of transverse weld are studied systematically by using the established transverse weld simulation model.The results show that the transverse weld length can be reduced effectively by adjusting the extrusion ratio,ram speed,height of baffle plate,corner radius of welding chamber and sinking depth of port bridge.