Abnormal Grain Growth Behavior And Inhibition Method Of Deformed Sub-Rapid Solidification Al-Mg-Si Alloys

Age-hardened Al-Mg-Si series(6xxx series)alloys are widely used in the field of aluminum alloys for automobile bodies because of their advantages of low density,good formability and good corrosion resistanc.Owing to the higher cooling rate(ranging from?10² to?10³ K/s),the sub-rapid solidification(SRS)can significantly promote microstructure refinement and enhance solid solubility.So it shorten the homogenization time and has become one of the promising directions of fabricating Al-Mg-Si alloys for automobile body sheets due to high efficiency and low-energy consumption.However,we found that a few grains excessively grow by engulfing the neighboring ones in SRSed Al-Mg-Si alloy during subsequent solution after rolling,and their grain size are one order of magnitude higher than the matrix grain,which named abnormal grain growth(AGG).The AGG deteriorates the performance of Al alloy sheets and limiting the application of SRS.In this work,the AGG behavior of Al-Mg-Si alloys under SRS was studied.We revealing the mechanism of AGG in SRSed Al-Mg-Si alloys,which is verified to be closely related to the{100}<310>grain clusters based on quasi in-situ microstructure analysis and the Mg₂Si particles that can be easily dissolved.The influence of Mg content on the AGG behavior of SRSed Al-Mg-Si alloys was clarified.We propose a method to enhance the thermal stability of grains by microalloying.The main conclusions in this paper as follows:(1)The effect of solidification cooling rate on AGG behavior of Al-Mg-Si alloys was studied.AGG is prone to occur in SRSed Al-Mg-Si alloys during subsequent annealing after rolling(540?)under the same process.The incubation period for AGG in SRSed alloys grains is much shorter(less than 5 min)than that of the conventional solidification(CS)alloys(more than 30 min).It is closely related to the existence of{100}<310>grain clusters and fine dissoluble Mg₂Si particles in the SRS alloys.The{100}<310>grain clusters have higher mobility due to the low frequency of CSL grain boundaries compared with the other-oriented grains.The fine Mg₂Si particles originating from the SRS process with a high cooling rate in the SRS sample can dissolve quickly during annealing,which make the Zener pinning force in the SRSed alloys decrease much faster than that of the CSed alloyes.(2)We investigated the effect of Mg content on AGG behavior of SRSed Al-Mg-Si alloys.The most significant AGG behavior occurs in the alloys with Mg content of 0.45 wt.%.However,when Mg content is 0.20 wt.%and 0.80 wt.%,AGG is not obvious or even not occur.The results show that the microstructure is significantly different due to the different Mg content.Random-orientated grains can grow due to a few particels in 0.20 Mg alloy.In 0.45Mg alloy,the growth advantage of{100}<310>grain clusters makes it possible to get rid of pinning to preferential growth,which promote AGG.The{100}<310>grains in 0.80Mg alloy present a discrete distribution which weakens theirs growth advantage,and a large amount of particels improve the thermal stability of the grains.(3)It is found that the method of inhibiting the AGG in SRSed Al-Mg-Si alloys.By microalloying,that is,the introduction of Mn or Sc(0.05?0.20 wt.%)can inhibit the AGG and improve the mechanical properties in SRSed Al-Mg-Si alloys.The high melting point?-Al(Mn Fe)Si particles or Al₃Sc particles can effectively pingrain boundaries to restrain the AGG.And the effect on enhancing thermal stability of Mn is better than that of Sc.AGG could be effectively inhibit When Mn content?0.05wt.%,while the effective content of Sc is 0.1-0.2 wt.%.