| Al-Mg-Si alloys have received a great deal of attention because of their low specific density,comparatively good formability,better corrosion resistance and high final strength after the automotive paint bake cycle.Recently,AA6014 alloy sheet was paid attention as a new automotive sheet due to excellent hemming performance and good energy absorption.However,there are still some problems through the preparation process,i.e.the cracking during hemming and insufficient strength after paint baking.In the current study,the effect of Mg and Si contents,aging treatment,i.e.natural aging,pre-aging and artificial aging on the microstructure,texture,mechanical properties and hemming performance of AA6014 alloy sheets has been investigated and the aim of this work is hopefully to provide a guide to design superior aluminum alloys with optimized strength and hemmability.In the current study,four alloys with different Mg and Si content were designed.Three-dimensional atom probe(3DAP),Transmission Electron Microscope(TEM),Scanning Electron Microscope(SEM)with Electron Back-Scattered Diffraction(EBSD)and tensile testing and three-point bending test were used to systematically study: 1)Effects of Mg and Si contents on the evolution of microstructure through as-cast,homogenization,hot and cold rolling and solution treatment in AA6014 alloys;2)Effect of Mg and Si contents on the clusters,GP zone and ?" precipitates through natural aging NA,pre-aging PA/T4 P and artificial aging T6 P of AA6014 Al-Mg-Si alloys;3)Effect of Mg and Si contents on the mechanical property through natural aging NA,pre-aging PA/T4 P and artificial aging T6 P of AA6014 Al-Mg-Si alloys;4)The experimental study and finite element simulation on the hemming performance of AA6014 alloy sheet.The main conclusions are shown as below:(1)The as-cast microstructure consisted of several types intermetallic phases with different morphology,i.e.ALPHA phase,Al Fe Si_Beta phase,Mg2 Si phase,Si and Q-Al5Cu2Mg8Si6 phase,forming a continuous network distribution.After homogenization,most platelet-like Al Fe Si_Beta phase breaks up into small-sized rounded ALPHA particles.Besides,the precipitation of Al(Fe Mn)Si dispersoids,Mg2 Si and Q phases were happened during the air cooling process after homogenization.In addition,an increase in Si content raises the number density and area fraction of dispersoids.After hot and cold rolling,the Fe-containing primary phases break up and tend to homogeneous distribution.The number density and area fraction of Mg2 Si particle increased by raising Mg and Si contents and the texture is typically ?-fibre in cold rolled Al-Mg-Si alloy.After solution treatment,a completely recrystallized microstructure consisting of fine grains is observed and the high solute content alloy exhibit finest average grain size.The textures in recrystallized Al-Mg-Si alloys are composed of the Cube texture,together with rotations,i.e.Cube-ND and Cube-RD orientation,and P orientation.The intensity of recrystallization texture increases with increasing Mg and Si contents,and the relationship between texture component and r-value was calculated by Visco-plastic self-consistent(VPSC)model.(2)The formation of Mg1Si1 type clusters is encouraged during pre-aging treatment regardless of the alloy composition.Natural aging following directly water quenching promotes the formation of a large number density of small-sized Si-rich clusters in Si-rich alloy,whereas relatively low density of cluster with scattered Mg/Si ratios distribution in Mg-rich alloy.The concentration of the excess vacancies and the binding energy between vacancy and Mg and Si atoms play important roles in the precipitation kinetics of cluster during different aging processes.The nanoscale precipitates distributed in alloys are composed of clusters with size less than 1 nm,GP zone with 1-2 nm and ?" precipitate with more than 2 nm in T6 P temper.The clusters and GP zone have a wide range of Mg/Si ratios from 0 to 5.0,while ?" precipitate exhibit a narrow distribution of approximately 1.2-1.4.The clusters are the most numerous and the number density of clusters increased with increasing Mg and Si content,while the GP zone and ?" precipitate exhibit little change.(3)The yield strength of Si-rich alloy is increased compared to Mg-rich alloy after natural aging for up to seven days,while the strength is similar for both Mg-rich and Si-rich alloys after pre-aging and the following room temperature storage.The appropriate pre-aging treatment can alleviate the deleterious NA effect and keep the samples stable in room temperature.The yield strength and ultimate tensile strength increased while elongation decreased with increasing Mg and Si contents in both T4 P and T6 P tempers.The strength is dominated by the number density of clusters in T4 P temper,together with GP zones and ?" precipitates in T6 P temper.The total elongation is dominated by uniform elongation,which is related to the work hardening exponent.For the same combined Mg and Si content,the Si-rich alloy has better elongation than Mg-rich alloy due to higher work hardening exponent.The cluster and solid solution atoms statistics are utilized to predict the yield strength of the alloys in different aging conditions by using Hall-Petch equation,Ashby-Orowan equation and Areal glide model.(4)The hemming performance decreases with raising Mg and Si contents of AA6014 alloy in T4 P temper.The hemming performance of the PA alloys is improved comparing with NA alloys.This improvement is significant for Si-rich alloy,while is slight for Mg-rich alloy.The hemming performance is related to the grain boundary precipitates,fracture strain and Lankford coefficient r-value.The formation of shear band which related to the crystallographic orientation,precipitates and solute content in the matrix is responsible for the difference on the hemming performance through the alloys with different alloy composition and aging treatment.The hemmability is sensitive to the strength,and an increased strength leads to inferior hemmability.The transgranular fracture mode is mainly account for the bending of AA6014 Al-Mg-Si alloys.Crack initiation occurs around Fe-containing constituent particles and the propagation of cracks are promoted by the combined effects of shear bands and voids growth.The finite element model shows the outer surface area undergo severe plastic deformation and shear bands appear at about 45° to the bending axis are found.The cracks propagated along shear band from outer to inner through the thickness of Al-Mg-Si alloy sheet.In addition,the plastic strain in outer area after bending to 108° is close to the maximum plastic strain after final hemming to 180°. |
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