Study On Flow Lines Control Of 7A04 Aluminum Alloy Inner Ring Bars During Extrusion Forming

The demand for lightweight and high performance equipment is increasing in military industry,aerospace and transportation fields.The forming research of inner ring ribs has attracted extensive attention due to its remarkable advantages in weight reduction and performance improvement.It is difficult to ensure the integrity of rib flow lines with traditional forming method and can't meet the requirements of high bearing capacity,which has become a bottleneck restricting the development of equipment.In the paper,focusing on the flow lines control problem in the extrusion forming of 7A04 aluminum alloy inner ring ribs,the effects of extrusion passes and extrusion direction on the mechanical properties of7A04 aluminum alloy are studied by finite element numerical simulation,macro and micro test and extrusion test,and the micro mechanism of anisotropy of mechanical properties is revealed.The influence of extrusion method and forming parameters on the flow lines distribution and bearing capacity of inner ring ribs is analyzed,and the flow lines control method of extrusion forming of inner ring ribs is proposed,which laid the foundation for the overall forming and flow lines reasonable distribution of aluminum alloy inner ring ribs.The extrusion passes and extrusion direction have obvious influence on the metal flow lines and grain morphology of 7A04 aluminum alloy,the differences of flow lines direction and density make the mechanical properties show obvious anisotropy.The grains of as-cast alloy are elongated along the extrusion direction by one pass extrusion and with obvious metal flow lines;the metal flow lines density increases significanly by two passes extrusion,and the flow lines of ED0 are still parallel to the extrusion direction,while the flow lines of ED90 deflects and has a certain angle with the extrusion direction.The yield strength(YS)and ultimate tensile strength(UTS)by two passes extrusion ED0 are the highest in 0°direction,follwed in 90° direction;the elongation(EL)in 0° and 45° directions is similar,and the EL in 90° direction is relatively low.The YS and UTS of ED90 along the three directions are similar,and the EL in 90° direction is significantly better than of 0° and 45° direction.Continuous dynamic recrystallization and discontinuous dynamic recrystallization occurred in the extrusion deformation of 7A04 aluminum alloy in different passes and directions.After two passes extrusion,the recrystallization ratios of different extrusion directions are obviously different,and the increase of discontinuous dynamic recrystallization ratio is greater.During the extrusion process of ED90,more intense intracrystalline dislocation and grain boundary movement are excited,resulting in an increase in the area percentage of grains formed by continuous dynamic recrystallization and discontinuous dynamic recrystallization.The discontinuous dynamic recrystallization caused by grain boundary movement is the main controlling factor.After one pass extrusion deformation,the samples showed typical “copper type” and “brass type” characteristic textures,and a small number of cubic textures.After two passes extrusion,the extruded samples in different directions show different texture types.With the increase of deformation passes,the overall texture weakens,and the texture weakening caused by dynamic recrystallization is an important factor.The flow lines evolution of inner ring ribs under different extrusion methods and parameters is compared and analyzed.Compared with radial extrusion,the flow lines of the inner ring ribs formed by upsetting extrusion are symmetrically distributed,and the eddy current degree of the metal flow lines is improved,the flow lines distribution at the inner ring ribs root is more uniform and the density is higher.The metal flow lines of the inner diameter model(the inner diameter of the billet is equal to the inner diameter of the cylinder wall)will change direction,and the metal flow lines will be disordered.The flow lines density of the upper and lower surfaces of the inner ring rib is different,and there are obvious outcrop defects.Increasing the aspect ratio can change the eddy current degree of the inner ring rib flow lines,and the eddy current angle decreases gradually with the increase of the aspect ratio,large aspect ratio(?3.5)is easy to produce folding phenomenon.The outer diameter model(the outer diameter of the billet is equal to the outer diameter of the cylinder wall)is more conducive to the metal flow filling of the inner ring ribs,the flow lines density of the upper and lower surfaces of the inner ring ribs is basically the same,and there is no obvious outcrop defect,the aspect ratio has little effect on the flow lines distribution.When the initial flow lines direction is 0°(parallel to the loading direction),the metal flow line is distributed along the shape of the inner ring rib in an eddy current state,and a small part of the flow lines is parallel to the height direction of ribs;When the initial flow lines direction is ±45°,the uneven distribution of flow lines is easy to occur at the corner of the inner ring rib,which is easy to cause turbulence;When the initial flow lines direction is 90°(perpendicular to the loading direction),the flow lines distribution of the inner ring ribs is uniform,most flow lines are parallel to the height direction of ribs,and the upper and lower surfaces of the ribs are prone to outcrops.The aluminum alloy inner ring ribs formed by upsetting extrusion process have good overall size and surface quality,and the inner ring ribs are filled fully without obvious folding defects,which verifies the feasibility of upsetting extrusion process.The flow line of longitudinal section of inner ring ribs formed by upsetting extrusion under different process parameters shows the same distribution law as the simulation results,which verifies the accuracy of finite element simulation.In this paper,the change law of the bearing capacity of the inner ring ribs formed under different process parameters is explored.Compared with the initial flow lines of 0°,the shear strength of the inner ring ribs formed with the initial flow lines of 90° is improved.The shear strength of the outer diameter model increases significantly with the increase of the aspect ratio of the billet.For the aspect ratio of 3.5,the shear strength of the outer diameter model is significantly improved compared with the inner diameter model;there is no significant difference in shear strength between as-cast and extruded blanks,indicating that the initial state of blank has little effect on shear strength.The fracture absorption energy per unit area of the inner ring reinforcement showed the same variation trend as the shear strength.The flow lines distribution of the inner ring reinforcement under different process parameters was consistent with the variation trend of the shear strength and the fracture absorption energy per unit area.