Precipitates Evolution And Strength-ductility Meachanisms Of 6061 Aluminum Alloy Processed By Equal Channel Angular Pressing After Post-aging

In recent years,great progress has been made in the preparation of ultrafine/nano-grained 6000 series aluminum alloys processed by severe plastic deformation?SPD?technique which is one of the processing methods that can produce three-dimensional,large-size,sub-micron and nanocrystalline materials.Ultrafine grained age-hardenable 6000 series Al alloys with both high-strength and high-ductility have been successfully fabricated by SPD techniques.However the integrated toughening and strengthening mechanisms as well as the quantitative relationship between the structural parameters and properties in these alloys are not clear.When the grain size is refined to ultrafine/nano-grain,the density of grain boundary will increase greatly and the aging precipitation sequence and kinetics etc.will be changed signifacntly.As a result,the microstructures,properties and strength-ductility mechanisms in the alloys are quite different.Therefore,it is significant to study aging precipitation behaviors,microstructure evolution and strength-ductility of the ultrafine/nano-grained Al-Mg-Si-?Cu?aluminum alloys fabricated by using combination of SPD and aging treatment.In this work,6061 aluminum alloy was taken as the matrix material and post-aging samples were carried out after ECAP different passes at room temperature.The aging behaviors,precipitate evolution and strength-ductility in post-aging alloy were systematically investigated by using tensile test,hardness measurement,conductivity test,X-ray diffraction?XRD?,differential scanning calorimeter?DSC?and transmission electron microscopy?TEM?analysis.The main conclusions include:?1?DSC analysis show that the main precipitate in the post-peak aging after ECAP one pass at room temperature is?"precipitate.A large amount of??precipitate and a small amount of??precipitate are main precipitates in the post-peak aging after ECAP two passes at room temperature.Majority of the GP zones have been dissolved in the post-peak state after ECAP three passes,the??precipitate has also partially dissolved.In addition,the??precipitate has begun to precipitate in a large amount.?2?The XRD pattens show that precipitates are not detected in the alloy processed by ECAP different passes at RT after post-aging,but the main strengthening phase that??precipitate is in a large amount during the post-aging.Under the same aging condition,as the stain rise sharply,the deformation degree of the material increases,the grain refinement becomes obvious and the dislocation density increases greatly.In this study,the grain refinement of alloy processed by ECAP three passes at RT is the best.The grain size of this sample is only 168 nm,lower than one pass?292nm?and two passes sample?211 nm?,the dislocation density is as high as 1.3×10¹³m^-2.?3?TEM analysis show that the distribution of??,??,Q precipitates and the interaction between precipitates and dislocation can be observed in T6 samples of6061 aluminum alloy,but the dominant position are occupied by the??precipitates.After ECAP two passes at RT and then aging at 170°C×3 h,the precipitates are easy to gather near the dislocation,the high strain produced in the deformation of ECAP promotes the formation of dislocation and accelerates the dynamic precipitation of precipitation.The existence of precipitates is beneficial to refine grain and pile up dislocations during the process of deformation.?4?The hardness measurement and tensile test show that the hardness of the alloy decreases gradually when post-aging time extends.With the increasing of the degree of deformation,the hardness of the alloy will be further improved and the time to reach the post-peak hardness will be greatly shortened.The highest hardness 195 HV appear in the sample with post-aging at 80°C with 5 min after ECAP three passes.The combination between plastic deformation technology and the post-aging treatment contribute to the improvement of strength greatly in the case of sacrificing the lower plastic toughness?the uniform elongation rate?_u is more than 10%?.In this work,the best tensile property is post-aging at 80°C with 20 min after ECAP two passes,i.e.,the highest tensile strength and uniform elogation are 514 MPa,489 MPa and 12%,respectively.?5?The conductivity results show that the conductivity of the materials is connected to the aging temperature and time,the highest conductivity is post-aging at170°C for 3 h after ECAP one passes,it's value is 40%IACS.The ultrafine grain Al-Mg-Si-?Cu?alloy precessed by severe plastic deformation technology and aging treatment is very sensitive to the grain boundaries,dislocations,solute atoms and precipitates.High density dislocations and lattice distortions lead to rapid emergence of precipitates and decrease the concentration of solute atoms,which increase the conductivity of the post-aging alluminum alloy.