Natural Aging And Its Effect On Subsequent Artificial Aging In Al-Mg-Si-Cu Alloys

Al-Mg-Si-Cu aluminum alloys are widely used to produce automotive body panels in light-weight vehicles due to their good corrosion resistance, excellent formability and high strength to weight ratio. In practical manufacture, before they are given an artificial aging (AA) treatment, these alloys are unavoidably stored for a period at room temperature (RT) and thus natural aging (NA) would happen. Generally, NA has a detrimental effect on subsequent AA, namely "negative NA effect".In this work, three Al-Mg-Si-Cu alloys with different Mg/Si ratios (0.5,1.0 and 2.0) experienced various NAtime before they were given AA.The effects of NA on subsequent AA (180℃,215℃ and 250℃) were systematically studied by Vickers micro-hardness measurements, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM).As for bake hardening (BH), which is a short time AA process, the conclusions are summarized as following:NA shows negative effect on subsequent BH and extending NA time exacerbates the negative effect. With the same NA time, NA of alloy with smallest Mg/Si ratio has a less deleterious effect on BH. However, three alloys with different Mg/Si ratios have a similar negative NA effect on BH when the NA time is very long. Meanwhile, no BH effect can be observed in the three alloys. The unstable NA clusters dissolve during BH and thus decreases the proportion of Pre-β" phases with larger sizes. Usually, Pre-β" phases with larger sizes have better strengthening effect, and their quantity reduction gives rise to the negative NA effect on BH.As for peak-aging of 180℃ AA, the conclusions are summarized as following:(1) With regard to short-term NA, NA of alloys with larger Mg/Si ratios has a strong negative effect with the prolonging of NA time before AA. In contrast, alloy with the smallest Mg/Si ratio possesses a negligible negative effect with the extending NA time. As for long-term NA, the reversal of negative NA effect occurs in the alloys with lager Mg/Si ratios. However, alloy with the smallest Mg/Si ratio has a serious negative NA effect with the extending NA time.(2) NA shows no obvious effect on lath-like Q"/L phases during subsequent AA. NA influences subsequent AA mainly by affecting the nucleation and growth of needle-like β" phase. A mechanism considering the stability of NA clusters during AA is used to explain the reason why NA has negative effect on subsequent AA. The stabilities of NA clusters during AA are related with the Mg/Si ratios of alloys and the NA time.(3) Two types of NA clusters, namely Cd1 and Cd2 respectively, are observed by DSC. Heterogeneous nucleation of β" phase on a few stable NA clusters and re-precipitation of partitioned solutes from unstable NA clusters could induce a broad precipitate length distribution and the appearance of large-sized precipitates. This scenario may be responsible for the negative effect on AA. When the NA time is very long, most of NA clusters become unstable and dissolve during AA, β" phase re-precipitates homogeneously. Thus, the precipitate length distribution gets narrow again and the appearance of coarsened precipitates is restricted, leading to the reversal of negative NA effect.As for AA at high temperature (215℃ and 250℃), the conclusions are summarized as following:Being similar with A A at 180℃, NA influences subsequent AA at high temperature mainly by affecting the nucleation and growth of needle-like precipitates. Nevertheless, NA has no negative effect on subsequent AA at high temperature. It can be explained by two reasons. In one hand, AA at high temperature induces the precipitation of coarsened precipitates. On the other hand, NA clusters dissolve rapidly during AA at high temperature, which can reduce the negative NA effect caused by NA clusters.