| 6xxx series aluminum alloys are widely used in automobile body panel materials because of their low density,medium and high strength and good formability.Adding trace alloying elements is an effective method to improve the mechanical properties of the alloys.The addition of alloying elements is mainly to improve the performance of the alloys by changing the aging precipitation behavior of the alloys,so it is of great significance to understand the influence of alloying elements on the precipitation evolution in the alloys.During the manufacturing process,the alloys will naturally age,which will adversely affect the subsequent artificial aging process.The addition of trace alloying elements to reduce the negative effects of the natural aging of the alloys has a positive significance for improving the performance of the alloys.In this work,through the mechanical properties test,conductivity test,differential scanning calorimeter test,transmission electron microscope observation and other means,the effects of Zn addition on the aging precipitation behavior and natural aging effect of Al-Mg-Si alloys were investigated.The mechanism of the influence of alloy composition on the natural aging effect of Al-Mg-Si-Zn alloys was discussed.For the Mg-rich Al-Mg-Si alloy directly artificially aged at 180℃ and 250℃,the peak hardness of the alloy is significantly increased by the Zn addition,especially for the high-temperature artificial aging case.The corresponding microstructure characterization results show that the addition of Zn refines the size of the precipitates and changes the precipitate type of the alloy.When aged at 180℃,the main strengthening precipitates of the Zn-free alloy are theβ"-precipitates,while a small number of lath-like precipitates(Q"-precipitates and C-precipitates)will be precipitated in addition to theβ"-precipitates in the Zn-added alloy.When aged at 250℃,the main strengthening precipitates in the Zn-free alloy are the coarseβ’-phase precipitates,while those in the Zn-added alloy are still theβ"-precipitates.Under high-temperature artificial aging,the addition of Zn changes the precipitate type in the Zn-free alloy,resulting in precipitation reversal.Natural aging has a negative effect on the Al-Mg-Si alloys.The addition of Zn can reduce the aging kinetics of the alloys during natural aging.In the subsequent artificial aging,the addition of Zn reduces the negative natural aging effect of the Mg-rich alloy.For the Si-rich alloy,the addition of Zn makes the negative natural aging effect disappear.The microstructural characterization results show that after applying natural aging to the Mg-rich alloy prior to artificial aging,theβ’-precipitates become dominant over theβ"-precipitates,whereas theβ"-precipitates are still prevailing with adding Zn.The addition of Zn causes the precipitation reversal induced by natural aging in the Mg-rich alloy to disappear.In the Si-rich alloy,all the precipitates are theβ"-precipitates for both the Zn-free case and the Zn-added case.The addition of Zn and natural aging do not change the precipitate type of the alloy,though the fomer improves the size and distribution of theβ"-precipitates.Characteristics of the natural aging clusters in the Al-Mg-Si-Zn alloys with different Mg/Si ratios are significantly different.The natural aging clusters formed in the Mg-rich alloy possess higher stability than those formed in the Si-rich alloy,and only a small fraction of them are prone to dissolve during artificial aging.Among the undissolved clusters,a small amount of them are capable of acting as nucleation sites for the strengthening precipitates,and the remaining clusters neither dissolve nor grow in artificial aging,which consume the solutes for nucleation of the precipitates.Consequently,the number of nuclei of the precipitates decreases,resulting in coarsening of the precipitates.Nevertheless,most of the natural aging clusters formed in the Si-rich alloy dissolve in the subsequent artificial aging,and re-precipitation of solutes from the dissolved clusters happens,resulting in homogeneous nucleation of the precipitates in the matrix. |
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