| The Al-Mg-Si alloys are widely used in transportation industry includingautomobiles and high-speed trains, due to their favorable mechanical properties suchas medium strength, low density and excellent formability. As typicalprecipitation-hardened alloys, their properties are closely related with the nature，involving type, size and distribution, of the nano-sized precipitates formed duringheat treatment, as well as the evolution of these precipitates, which depends largelyon alloy composition and aging conditions. Alloy composition, especially Mg/Si ratio,has a significant influence on the aging kinetics of the Al-Mg-Si alloys. To improvethe performance of the Al-Mg-Si alloy, multi-step aging concept by combining naturalaging with artificial aging or several steps of artificial aging at various temperatureshas been introduced recently. However, the effects of alloy composition and agingconditions, multi-step aging in particular, on the evolution of the precipitates are notwell understood. In this thesis, the influences of alloy composition and multi-stepaging on the hardening behaviors and precipitation processes in the Al-Mg-Si alloyswere investigated using micro-hardness tester, scanning electron microscopy (SEM)and transmission electron microscopy (TEM). The conclusions are summarized asfollows:(1) The softening characteristic of over-aged alloys didn’t occur for the alloysaged at60℃and120℃even after a very long duration. The effect of Mg/Si ratio onthe aging kinetics was pronounced when the alloys were aged at180℃, with thehardening rate decreasing with Mg/Si ratio. The age-hardening behaviors changedslowly with the decrease of Mg/Si ratio after the Mg/Si value approached1.0.Compared with the samples aged at180℃, the effect of Mg/Si ratio on the agingkinetics was larger for the ones aged at250℃, especially in the under-aged stage. Thepeak hardness increased with Si content. TEM observations reveal that the formationof β’-phase in the over-aged samples was related with the Mg/Si ratio. The β"-phaseprecipitates mostly formed in alloys with a Mg/Si ratio below1.0when aged at180℃.When aged at250℃, Type B phase was observed to form in the alloy with the Mg/Sicomposition ratio of about1.0. However, three types of precipitates, Type A, Type Band Type C, respectively, were found in the alloy with a Mg/Si ratio of about0.5whenaged at250℃. (2) The natural aging (NA) at room temperature had a negative effect on thesubsequent artificial aging (AA) for the Al-0.57Mg-0.51Si (wt.%) alloy. Incomparison with the alloys directly aged at180℃after quenching, the NAsignificantly decelerated the aging kinetics in the early-stage of AA and the obtainedpeak hardness also reduced. The size distribution and average size of the precipitatesformed in the subsequent AA became broader and bigger, respectively, with the NAtime prolonging. Although the size and distribution varied significantly, theprecipitates were the same monoclinic β".(3) The higher the pre-aging temperature, the less time the alloys needed to reachthe peak hardness by multi-step aging. The hardness curves changed little with theinterrupted aging time when pre-aging conditions were the same. Besides, the peakhardness of the alloy experienced a multi-step aging was nearly the same as one-stepaging. The precipitates existed in the alloys treated via pre-aging followed byinterrupted aging were mainly GP zones or precursors of β". These early-stageprecipitates may act as the nucleation sites of β" and promote the precipitation ofmore strengthening particles. |
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