| The 6xxx series aluminum alloys(Al-Mg-Si(-Cu)alloys)are widely used to produce automotive body panels because of their light weight,medium to high strength and excellent formability.In practical manufacture,the natural aging(NA)process can not be avoided and has an detrimental effect on the subsequent artificial aging(AA),namely“negative NA effect”.It can weaken AA kinetics and reduce maximum hardness.Therefore,it is hoped that the precipitation behavior of the alloy can be tuned by adding some trace alloying elements to suppress the negative effect of NA on AA.The present study investigates the effects of two kinds trace alloying elements(Sn and Zn)on the NA and AA of Al-Mg-Si(-Cu)alloys by means of hardness measurement and transmission electron microscopy(TEM),and discusses the mechanisms in detail.As for the effect of Sn on NA and AA of Al-Mg-Si alloy,the conclusions are summarized as follows:(1)When aged at 180~oC,the negative NA effect can be totally suppressed for a period of time(4d),and can be reduced upon long-term NA time(2w)in the Al-Mg-Si-Sn alloy.When aged at 250~oC,Sn can significantly enhance the age-hardening potential of the Al-Mg-Si alloy,and lead to the reversal of the positive NA effect on AA.(2)TEM observation revealed that the addition of Sn modifies the precipitation pathways of the alloy:when aged at 180~oC with natural aging for a long time,the?'-precipitates are dominant in the Al-Mg-Si alloy,while the?"-precipitates and?'-precipitates both exist in the Al-Mg-Si-Sn alloy.With regard to AA at 250~oC,when directly artificially aged,the addition of Sn leads to the precipitate types change from?'-precipitates to?"-precipitates.Whereas when the alloy is subjected to“natural aging+artificial aging”treatment,upon prolonged natural aging time,the addition of Sn leads to the precipitate types change from?"-precipitates to coexistence of?"-precipitates and?'-precipitates.The reason of the Sn-induced modification of the precipitation behavior is believed to be that Sn increases the effective Si-concentration in the matrix,consequently changes the precipitation pathways in the alloy.As for the effect of Sn on NA and AA of Al-Mg-Si-Cu alloy,the conclusions are summarized as follows:The addition of Sn reduces the negative NA effect on the peak hardness in subsequent AA at 180~oC,and NA shows no negative effect on the peak hardness in subsequent AA at250~oC in both the Sn-free alloy and the Sn-added alloy.The addition of Sn makes the proportion of lath-like Q"/L phases increase during subsequent AA whether there is NA or not.The NA effect on subsequent AA in the alloy is decided by the change of the size and size distribution of the needle-like?"phases and lath-like Q"/L phases.As for the effect of Zn on NA and AA of Al-Mg-Si-Cu alloy,the conclusions are summarized as follows:(1)In the Al-Mg-Si-Cu-Zn alloy,NA obviously decreases the hardness in the early-stage of AA,but slightly increases instead of decreases the peak-aged hardness.That is,NA has a negative effect on the bake hardening response(BHR)but has a positive effect on the age hardening potential.(2)NA influences the subsequent AA in the Al-Mg-Si-Cu-Zn alloy by affecting the nucleation and growth of the needle-like?"phases.As for the paint-bake-treated alloys,NA for 4m leads to the decrease of the proportion of pre-?"with larger size and better strengthening effect,which results in the decrease of the hardness of the alloy.For the peak-aged alloy,NA makes the average size of the?"phases decrease and the size distribution narrower,which leads to the increase of the hardness of the alloy.The reason could be that the majority of the NA clusters are unstable and will be dissolved at subsequent AA.The solutes from the dissolved NA clusters are not able to re-precipitate when the AA time is too short,but are able to when the AA time is long enough. |
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