| In recent years,with the advance of China's industrialization process,the demand of aluminum alloy for electronics,transportation and other industries showed an upward trend.Among them,aluminum alloys are widely used for automobile lightweight.In addition,new aluminum alloys with high strength and toughness which can meet the needs of military industries are probably a main research direction in the future.Interestingly,ultrafine grained(UFG)materials prepared by severe plastic deformation(SPD)technology can significantly improve the mechanical properties of 6000 series aluminum alloys.Among various SPD techniques,equal channel angular pressing(ECAP)is considered to be the most promising way to produce bulk UFG materials.However,the potential of strengthening of 6000 series aluminum alloys fabricated by SPD hasn't been fully exploited.Therefore,it is very important to study the influences of the combination of SPD technology and aging treatment on the properties and microstructures of 6000 series aluminum alloys.In this paper,6013 Al-Mg-Si-Cu aluminum alloy was taken as the research material and the post-aging treatment was carried out for the samples with different ECAP passes at room temperature.The influence of microstructure on the performance of 6013 aluminum alloy processed by post-aging was systematically investigated by differential scanning calorimetry(DSC),X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)analysis.Furthermore,the change of strengthening mechanism after post-aging is predicted based on the microstructure characterization.The results are summarized as follows:(1)DSC analysis shows that the main precipitates are GP zones when the one-pass ECAPed materials aged at 110 ?.Meanwhile,a few ?? precipitates are arisen during the samples aged at 191 ?.In addition,a large amount of GP zones and ?? precipitates have been emerged when the two-passes ECAPed alloys aged at both 110 ? and 191 ?.Similarly,the main precipitates are a large number of GP zones,?? and ?? precipitates when the three-passes ECAPed alloys processed by post-aging.(2)The results of XRD show that the precipitation rate and quantity of precipitates have been developed due to the increasing passes of ECAP during post-aging.Under the same passes,the occurrence of precipitation will be accelerated with the increasing aging temperature.In the equal aging time,with the increase of ECAP passes,the difference among grain size of the materials with same ECAP passes will be decreased at different aging temperatures gradually.Higher aging temperature not only accelerates the strain reduction but also increases the degree of strain reduction.(3)During the post-aging treatment with the same passes of ECAP,the grain coarsening is accelerated and the dislocation density is reduced due to the higher temperature in the early aging stage.For overall post-aging process,the dislocation density is more effectively decreased by aging at high temperature compared with low temperature.With the increase of ECAP passes,the reduction rate of dislocation density is also increasing.Dislocation is more sensitive to temperature than grain size in materials after ECAP.(4)TEM analysis shows that the grains significantly refined after ECAP and the grain size distribution is uneven.Stacking faults and dislocation cell structures have been detected in the materials processed by ECAP combined with post-aging.A large number of dislocation structures are remained in the initial aging process.(5)The ??,L and Q? precipitates are detected in 6013 aluminum alloys after aging treatment.However,the dominant precipitates are occupied by the ?? precipitates.The interaction between precipitation and dislocation which has played a crucial role in strengthening of the materials always presents during all the investigated post-aging process.(6)The peak hardness of the post-aging samples are 169.5 HV,176.4 HV and 175.7 HV which ECAPed for 1,2 and 3 passes at room temperature and subsequently aging at 110 ? for 30 min,10 min and 1 min,respectively.The sample processed by 2-passes ECAPed at room temperature and post-aging at 110 ? for 10 min reaches the highest hardness during post-aging,which is 8.9 HV higher than the as-ECAPed material.The time of which materials reach the peak hardness is shorten with the increasing of deformation.(7)The ductility of the materials is increased by post-aging treatment.The highest elongation of post-aging sample is 37.4%,which is produced by two-passes ECAP and subsequently aging at 110 ? for 1 min.The ductility and toughness of sample processed by two-passes ECAP combined with post-aging are generally higher than that of sample with a single ECAP process.The ductility of the ultrafine grain materials is significantly influenced by grain size after post-aging.(8)The conductivity results shows that the highest conductivity is 41.8% IACS when the sample produced by two-passes ECAP at room temperature and subsequently aging at 191 ? for 2 h,which is 11.7 % IACS higher than that of the as-ECAPed sample.For ultrafine-grained materials,the rapidly appearance of precipitates is caused by high density dislocations and heavy lattice distortion which are induced by severe plastic deformation.Therefore,the conductivities of the materials are decreased by precipiattes in partial post-aging stage.In addition,the conductivities of the materials are developed with the increasing of aging temperature and aging time during the whole process.(9)The effect of dislocation strengthening on improving strength and hardness of materials is more obvious with the increasing passes of ECAP.The ultimate tensile strength of sample processed by two-passes ECAP at room temperature is 518 MPa.During the post-aging process,the lack of grain size strengthening and dislocation strengthening can not be compensated by the precipitation strengthening gradually. |
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