| The Al-Mg-Si(Cu) alloys are widely used in automobile industry, due to their favorable mechanical properties such as low density, high corrosion resistance and excellent weldability. These alloys are heat-treatable strengthened alloys,and their properties are closely related with the nature of the precipitated nano-particles from the supersaturated solid solution Al matrix,including shape, types,amount, size and distribution.Recently, only rare reports about the effects of ageing treatments on the age hardening behaviors and microstructures can be found.This problem will be studied in detail in this work.The investigations were carried out on three kinds of Al-Mg-Si-Cu alloys,which had different Cu contents:0.0%,0.15% and 0.8%(wt%),named as A, B,C alloy respectively. The hardening behaviors and the corresponding microstructures were investigated by means of micro-hardmeter, Scanning Electron Microscopy(SEM), and Transmission Electron Microscopy(TEM).This work was divided into two major parts.The first part was focused on two-step ageing, and the second part was emphasised on T6I6 ageing and modified T6I6 ageing, especially for the evolution of precipitates in the C alloy. It was needful to introduce the modified T6I6,which was developed from T6I6 technique by introducing a natural ageing after solution heat treatment and quenched.The results of the first part show that:Compared with one-step ageing, the peak-value of the hardness decreases in the A alloy experienced two-step ageing; The Cu addition depresses the negative effect of natural aging on subsequent artificial aging, which is attributed to the increase of the number density of needle-shaped precipitates in the samples without natural ageing.Onlyβ" precipitates are observed in the B alloy. Bothβ" and Q" precipitates are observed in the B and C alloys. Compared with the A, B alloys,the occurrence time of the peak hardness for the C alloy is delayed due to the formation of Q" precipitates.This phenomenon is related to the Q" precipitates,because the Q"phase requires additional time to precipitate.The results of the second part show that:Compared with the T6,the T6I6 and modified T6I6 ageing can lower the peak hardness of the A alloy, and the hardness obtained aged for about 1200 min declines.The T6I6 ageing does not lower the peak ageing hardnss of B and C alloys,furthermore, it can shorten the ageing time to reach peak hardness.The modified T6I6 aging lowers the peak hardnss of B and C alloys, while the hardness obtained aged for about 1200 min does not decline. With the additions of Cu element, the peak hardnesses of the B and C alloys increase aged under the conditions of T6I6 and modified T6I6.Compared with the peak aged samples in the C alloy, the number of the lath-like Q" precipitates increases,on the other hand, the number ofβ" precipitates decreases. Different aging treatments do not change the types of precipitates,but their proportion has been changed.By contrast, the precipitates in samples peak-aged under the condition of T6I6 ageing are the smallest, and the quantity is the most.However, the precipitates in samples peak-aged under the condition of modified T6I6 are the most heterogeneous. |
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