Liang Dong

Two kinds of alloys with and without the addition of Cu, Al-7%Si-0.3%Mg-0.3%Cr and Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu, are studied in this work. The addition of Cu can notably improve notably the strength of Al alloy but it reduces its corrosion resistance. In this study, the electrochemical workstation is used to measure the open circuit potential and polarization curve of alloys and immersion corrosion is carried out. SEM and TEM images are taken before and after immersion corrosion to observe the pitting and intergranular corrosion of the alloy. Results show that the addition of Cu accelerates the immersion corrosion rate of Al alloy by 26.8% to 269.2%. This affects the peak ageing and overageing samples the most. The influence is less evident for underaged samples. At the same time, the addition of Cu aggravates the aggregation of pitting corrosion in the primary step of corrosion of Al alloy and the intergranular corrosion around and within the pitting hole. ${\beta }^{″}$-Mg₅Si₆ precipitates and ${\theta }^{″}$-Al₃Cu precipitates are observed in Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu alloy. The Cu atoms occupy Si3/Al site of ${\beta }^{″}$ and segregate at the edge of ${\beta }^{″}$. It is believed that the deterioration of corrosion performance essentially is attributed to the Si-enriched particles, Al₁₃Cr₄Si₄ phase and the Cu-enriched precipitates, ${\beta }^{″}$-Mg₅Si₆ precipitates and ${\theta }^{″}$-Al₃Cu precipitates.

In this work, the effect of Cu additives and heat treatment on the precipitation sequence of an Al-Si-Mg-Cr alloy has been systematically studied by means of advanced spherical aberration-corrected electron microscopy. Cu atoms tend to gather at the interface between the precipitates and the matrix at the beginning of the aging process. Then, Cu atoms diffuse into the precipitates. Two types of GP zones are formed in the first stage of precipitation: one is the type I GP zone and the other is the type II GP zone. The type I GP zone ${\mathsf{\beta }}_{\mathrm{Cu}}^{″}$ evolved into the Q′ phase, while the type II GP zone evolved into the θ′ phase during the aging process. The aging sequence of the Al-Si-Mg-Cr alloy can be determined as a supersaturated solid solution (SSSS) → GP zones → β″→ β′/B′(→β). The aging sequence of the Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu alloy can be determined as a supersaturated solid solution (SSSS)→GP zone→${\mathsf{\beta }}_{\mathrm{Cu}}^{″}$→Q′ + θ′(→Q + θ).