| 6005A aluminum alloy is a typical Al-Mg-Si alloy with the advantages such as high specific strength,good plasticity,and easy recycling.Its extruded profiles are widely used in manufacturing vehicle bodies and important components for high-speed trains,subway trains,automobiles,and ships."Peripheral coarse grain" is a common microstructure defect of aluminum alloy extruded profiles.Its essence is that the grains on the profile’s surface grow abnormally,which not only efficiently reduces the profile’s strength,fracture toughness,and stress corrosion resistance,but also affects the subsequent processing of the profiles and the mechanical properties of aluminum matrix composites.Therefore,studying the formation law and mechanism of "peripheral coarse grain" and determining the best process parameters through mechanical property testing so as to effectively control the extrusion process and reduce the thickness of the coarse grain layer,is of great significance for improving the comprehensive performance of aluminum alloy industrial profiles.In this paper,an I-shaped 6005A aluminum alloy profile was taken as the research object.The HyperXtrude numerical simulation software was used to analyze the distribution characteristics of the temperature field,strain field,and strain rate during the extrusion process of the profiles.From the perspective of the physical field,the reasons for abnormal grain growth on the surface of the profiles were explained.The extrusion experiments were conducted under 15 sets of process parameter combinations.The grain morphology,orientation,and recrystallization of the surface and core of the extruded profiles under different extrusion process parameters were studied.The distribution characteristics of secondary phases and dislocations of extruded profiles were observed.The mechanism of abnormal grain growth on the surface and in the core of profiles was revealed from a microscopic perspective.The optimal combination of extrusion process parameters was obtained by conducting tensile and hardness tests on the extruded profiles.The abnormal grain growth characteristics during the solid solution process were studied by conducting solid solution treatment on the extruded profiles with different coarse grain layer thicknesses.The main research work and achievements of this paper are as follows:(1)By consulting literatures and the actual production process of 6005A aluminum alloy,15 combinations of extrusion process parameters were determined.The billet heating temperatures were set at 420℃,450℃,480℃,510℃,and 540℃,respectively,and the ram speeds were set at 0.5 mm/s,1.5 mm/s,and 2.5 mm/s,respectively.Based on the geometric characteristics of the I-shaped aluminum profile,an extrusion die was designed,and the unequal-length bearing was used to improve the uniformity of the metal flow rate.A threedimensional model of the extrusion die was constructed using UG modeling software and imported into HyperMesh for numerical modeling.15 combinations of extrusion process parameters were input for calculation,and the distribution of "field variables" such as temperature,strain,and strain rate were obtained in HyperView.The results showed that the temperature,strain,and strain rate at the bearing on the surface of the profiles were higher than those in the core.In this way,the physical factors leading to abnormal grain growth on the profile surface were obtained.(2)The samples used for microstructure observation were taken from the extruded profiles,and the observation surface was characterized using an electron microscope.The influence of extrusion process parameters on the thickness of the coarse grain layer,microstructure,and mechanical properties of the profiles was studied.The results showed that the thickness of the coarse grain layer increased with the increase of billet heating temperature and ram speed,and a lower ram speed could suppress the generation of coarse grains.Through transmission electron microscope observation,it was found that the 6005A aluminum alloy extruded profiles mainly contained β and AlFeMnCrSi phases.Tensile and hardness tests on the extruded profiles under 15 sets of extrusion process parameters were performed to obtain the tensile strength,yield strength,elongation,and hardness.It was found that with the increase of billet heating temperature,the strength and hardness of the profile increased,which was attributed to the solid solution strengthening effect caused by the dissolution of β phase into the matrix.As the ram speed slowed,the strength and hardness of the profile increased due to the fine grain strengthening effect caused by the decrease in thickness of the coarse grain layer.The billet heating temperature and ram speed have little effect on the elongation of the profiles.The primary fracture mode in the coarse-grained region is a combination of dimple fracture and cleavage fracture,while that in the fine-grained region is dimple fracture.When the billet heating temperature is relatively high(540℃)and the ram speed is relatively slow(0.5 mm/s),the comprehensive mechanical properties of the profile are the best under the combined influence of solid solution strengthening and fine grain strengthening mechanisms.(3)The microstructures on the surface and in the core of the profiles,containing the grain morphology and boundary,orientation,misorientation angle,and texture,were studied.The mechanism of abnormal grain growth on the surface and in the core of the profiles was revealed.The abnormally coarse grains on the surface of the profiles mainly exhibit specific orientations of<100>//ED and<110>//ED.The reason is that the surface of the profile contacts with the die during the extrusion process,and the surface grains undergo severe shear deformation,thus resulting in severe lattice distortion,which provides a driving force for dynamic recrystallization and grain growth.When the billet heating temperature is relatively high,dynamic recrystallization is stronger,and the Cube {100}<100>and R-cube {100}<110>grains undergo abnormal growth to form surface coarse grains.When the billet heating temperature is relatively low,the dynamic recrystallization is weakened,while the static recrystallization is strengthened.The Goss {110}<100>and R-Cube {100}<110>grains undergo abnormal growth to form surface coarse grains.Most of the abnormally coarse grains in the core of the profile show<100>//ED orientation,while a few show<110>//ED orientation.The Cube {100}<100>recrystallized grains are surrounded by Copper {112}<111>and Brass {110}<112>deformed grains,which results in significant grain boundary misorientation and strain gradients,thereby providing a driving force for the abnormal grain growth in the core of the profile.(4)The solid solution treatment experiments were conducted on the samples that were taken from extruded profiles under different extrusion process parameters,and the grain changes with extending the solution time were observed.The results showed that the samples with a thin initial coarse grain layer exhibited abnormal grain growth behavior both on the surface and in the core of the sample during high-temperature solid solution.However,for the samples with a thicker initial coarse grain layer,abnormal grain growth is only manifested as an increase in the size of coarse grains on the surface and in the core of the profile,and there are no new abnormally coarse grains generated.In addition,the coarse grains have a tendency to "protrude" outward.As the solid solution time prolongs,the coarse grains gradually absorb the small grains,making their size larger and larger.Finally,the fine grain areas are entirely engulfed by the coarse grains,resulting in a decrease in system energy and an improvement in stability. |
PDF Full Text Request