In present work, the formation, evolution, and distribution of the primary Fe-rich phase in an Al–Mg–Si–Cu–Zn–Fe–Mn alloy are coupling controlled by ultrasonic melt treatment (USMT) and thermomechanical processing (TMP). It is shown in the results that the size of grains and Fe-rich phase in the as-cast state can be greatly reduced by the applied optimum USMT at 680 °C. Additionally, the transformation rate of β-Fe-rich phase to α-Fe-rich phase can be also enhanced. After the coupling control of USMT and TMP, the number density and distribution uniformity of multiscale Fe-rich particles can be greatly increased or improved, which contributes to the fine-grained recrystallization microstructure and weakened texture. Finally, compared with the 6xxx series Al alloys (such as AA6016 and AA6111), the alloy sheet in the pre-aging state exhibits substantially improved bendability and strength (the plastic strain ratio and tensile strength are 0.67 and 304 MPa, respectively). The effect of USMT on the formation and transformation of primary Fe-rich phase and the mechanisms of improved bendability and strength are deeply discussed.

A new non-isothermal pre-aging treatment was proposed and utilized in Al-Mg-Si-Cu-Zn alloys, together with natural aging and artificial aging. The influence of cooling rates on subsequent precipitation behaviors was investigated by experimental and thermodynamic simulations. The results show that by controlling the formation of clusters/GP zones through changing pre-aging cooling rates, i.e. PA-0.2, PA-0.3 and PA-0.4 (°C/min, from 80 °C to 40 °C), an excellent bake hardening increment and natural aging stability can be obtained. The highest bake hardening increment can reach 180 MPa for PA-0.4 sample, which is twice higher than those of Al-Mg-Si-(Cu) alloys. The microhardness remains almost unchanged within NA for 14 days at a lower level of approximately 85 HV_0.2. Thermodynamic simulations estimate the solvus temperatures and chemical composition for GP zones, revealing the strengthening and stabilising mechanisms behind: a) Mg-Zn- clusters formed during pre-aging can suppress Mg-Si- clusters formation in the natural aging process, b) non-isothermal hinders the precipitates growth, a faster cooling rate leads to smaller and softer Mg-Zn- clusters, and c) the formation of a heterogeneous microstructure contributes to the high bake-hardening response without changing the type of strengthening phase β″. Finally, the clustering and aging process was illustrated and explained.

The effect of Sn micro-alloying on microstructure evolution, formability and precipitation behaviour of Al-Mg-Si-Cu-Zn alloys were systematically studied by experimental techniques and theoretical calculations. Results show that Sn addition can accelerate both the precipitation and re-dissolution of the Fe-rich phase during casting and homogenising treatments, which thereby determined the final microstructure. A significant retarding effect to natural ageing precipitation was observed with increasing Sn content in quenching samples, but this effect was weakened in pre-aged samples, as explained by DSC and simulations. The different number densities of the strengthening phase β″at the same artificial aging state are mainly attributed to the changed activation energy of the β″ phase affected by the formed Sn-containing Mg-Zn clusters and Mg-Si clusters. Trace Sn participating in the formation of GP zones, Sn-containing MgZn₂ phase and new precipitating sequences during ageing were proposed for the first time.