The influence of Mg content on the microstructures and mechanical properties at room temperatures of A1-3.5Cu- （0.71-1.81）Mg alloys was studied. Precipitation phases in the alloys were identified by TEM and HRTEM. The results show that when Mg contents increase from 0.71 to 1.81 wt%, the precipitates are transformed from S, S＂, 2, and 0 phases to S and St phases, and f2 phase is first observed in A1-3.48Cu-0.71 Mg alloy with Cu/Mg mass ratio of 5 during the conventional aging heat treatment （190 C/12 h）. Regard to aging hardness effect of the tested alloys, the hardness of the alloys improves with the increase of Mg content, but the increases become slow when Mg content is greater than 1.35 wt%.
Experiments were conducted to investigate the cooling manner of an ultra-thick hot aluminum alloy plate during multistage quenching. Cooling curves and heat flux curves of different rapid quenching flux varied from 23 to 40 L min-1 and were analyzed in detail. In this investigation, cooling process was divided into the following four steps： （Ⅰ） starting step, （Ⅱ） rapid cooling step, （Ⅲ） slow cooling step, and （Ⅳ） stopping step. Based on the curves, the calculation method for surface transfer coefficient was provided, and the effects of coefficient on surface temperature and quenching flux were discussed. Results showed that the transfer coefficient disagreed with heat flux and that it is a nonlinear function of surface temperature. The highest coefficient was observed not in the rapid cooling step with the largest heat flux but in the slow cooling step with lower heat flux. The coefficient increased with surface temperature ranging from 480 to 150℃, and a coefficient peak appeared in the temperature range of 150- 100℃. The coefficient also increased with quenching flux. Finally, a simulation was performed using the finite element method to verify the reliability of the coefficient results, which showed good agreement with the measurement values.
FAN ShiTongDENG YunLaiZHANG YongHUANG XinYueWU PengFei
通过电导率、硬度、拉伸、电化学腐蚀以及透射分析等实验,研究了回归再时效(RRA)中预时效温度和第三级时效温度的改变对Al-Zn-Mg铝合金型材的力学性能和电化学腐蚀性能的影响。研究了其两个阶段温度的变化对实验结果产生的相互作用以及对实验结果产生的影响。结果表明:110℃/24 h + 185℃/105 min + 120℃/24 h合金拉伸强度最高,腐蚀性能一般。110℃/24 h + 185℃/105 min + 150℃/24h合金耐腐蚀性能最好,但拉伸强度最差;较高的预时效温度会加快晶内析出η相,回归时效时不能溶解η相,从而加速合金在第三级时效产生过时效,获得较好的耐腐蚀性能。
The effects of sub-grain boundaries on the quenching sensitivity and the precipitation behavior in Al-7.01Zn-1.26Mg- 1.43Cu alloy were investigated by an end-quenching test. Specimens were solution treated at 440 ℃ and 480 ℃ to get different recrystallization fractions, respectively. The results show that the maximum hardness value of the Al-Zn-Mg-Cu alloy can be improved by the sub-grain boundaries, but the depth of age-hardening layer decreases significantly. The precipitation temperature and the activation energy are reduced by the changes of surface energy, which is induced by sub-grain boundaries. So, the precipitation process from η phase to η phase becomes much easier. In this way, an increase in the number of sub-grain boundaries promotes the precipitation of MgZn2 particles, especially η＇-MgZn2.
