| Al-Zn-Mg aluminum alloy belongs to the heat-treatable aluminum alloys and have been widely used in the field of aerospace and transportation as structural materials. The 7N01 aluminum alloy as typical commercial middle strength Al-Zn-Mg alloy is greatly used in the construction of high-speed trains and subways due to its good properties of formability, mechanics, weldability and corrosion resistance. But 7N01 aluminum alloy has a strong natural aging effect, and it is very difficult to completely avoid during the artificial aging treatment process. So make clear the effect of natural aging on subsequent artificial aging performance has an important engineering significance.In this paper, mechanical property tests, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and differential scanning calorimetry (DSC) analysis were employed to systematically investigate the effect of natural aging on microstructure and mechanical properties of artificial aging, and a three-step aging heat-treatment was designed to improve the mechanical properties 7N01 aluminum alloy. The main innovation points are summarized as follows:(1) In the early stage of artificial aging, the "reversion" of the GP zones formed by the natural aging is not only related to the artificial aging temperature, but also closely related to the natural aging time. Under the condition of 120? artificial aging, natural aging generated clusters with the prolongation of natural aging time change from "completely reversion" to "partial reversion". But the clusters which are produced by the full natural aging will not be "completely reversion" even in 140? artificial aging.(2) Natural aging pretreatment has positive effect on subsequent single stage artificial aging. In the artificial aging stage, the clusters formed by the natural aging whether it is "partial reversion" or "completely reversion", will not reduce the hardness of the peak-aged condition, even with the extension of the natural aging time there is a slight increase and also making the artificial ageing peak appeared significantly ahead of schedule.In addition, natural aging can inhibit the overaging tendency appeared in the higher temperature artificial aging.(3) In a certain range the peak strength obtained by NA(natural aging)+110?/10 h+ 140?/8 h three-step aging improvement with the natural aging time elongation. The stable state of natural ageing of 7N01 aluminum alloy after three-step artificial ageing treatment can obtain higher mechanical strength than that of the T6 treated, the yield strength (395.8MPa) and tensile strength (448.2MPa) are respectively higher 9.0% and 10.7% and without loss of elongation rate (13.8%), and the artificial aging time was control within 20 h.(4) GP zones and small ?'phase are the main strengthening precipitates in 7N01 aluminum alloy after three-step aging treatment. In the subsequent artificial aging stage, the clusters generated in natural aging have not completely dissolved, or as a core for phase transformation. It can be roughly divided into three categories from the transition paths in subsequent artificial aging process:(i) Part one can retained stability to the aging end; (ii) part two can promote precipitation by "reversion"; (iii) and part three can act as ?'phase nucleation core directly change to it. |
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