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.

The alloy design and homogenization processes are intimately associated with the microstructure, phase composition and performance for Al-Zn-Mg-Cu alloys. The microstructures and phase composition of a series of Al-Zn-Mg-Cu alloys before and after the homogenization treatments were investigated along with thermodynamic calculation to understand the underlying relationship. The eutectic microstructures (α-Al + M (Mg(ZnAlCu)₂)) are dominating with Cu-enriched [AlCuMgZn] particles, both depending on the Zn:Mg ratio and (Cu + Mg) content, in addition to minor constituent θ (Al₂Cu) and Al₇Cu₂Fe phases in the as-cast alloys. The optimal homogenization process was suggested based on the analysis of the residual phases (i.e., the S (Al₂CuMg) phase) since all (for low/mediate-(Cu + Mg) alloys) or partially (for high-(Cu + Mg) alloys (∼>4.24 wt%)) S (Al₂CuMg) particles were dissolved during the homogenization. This residual S phase may be transformed from the primary M and/or Cu-enriched [AlCuMgZn] phases. The homogenization kinetics calculation results agreed well with above experimental results. A critical (Cu + Mg) level and a linear correlation between Cu and Mg concentrations were revealed based on the thermodynamically modelling, which can be conductive to determine the optimal homogenization process. Furthermore, the solubility limit and stoichiometric balance principles based on controlling the homogenized microstructures can guide the composition design for advanced high-strength aluminum alloys.

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 retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.

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.

Synergy of Ni micro-alloying and thermomechanical processing on the phase distribution, formability and bendability of Al–Mg–Si–Cu–Zn–Fe–Mn alloys was systematically studied in this paper. With the addition of micro-alloying Ni, the Ni-containing Fe-rich phase can be formed, which not only serves as nucleation sites of Mg–Si precipitates (such as, Q phase) during the casting process, but also improves the uniform distribution level of Fe-rich phases after homogenization. The formability and bendability of Ni-containing alloy can be both improved to a certain level due to the positive effect of Ni micro-alloying. In comparison, if increasing the cold rolling deformation between hot rolling and annealing, the distribution of multi-scale Fe-rich phases can be significantly improved based on the synergy of Ni micro-alloying and thermomechanical processing. And finally, this improvement further results in the great improvements in the microstructure, texture, formability (average r = 0.688, △r = −0.09) and bendability of the alloy together. Based on the microstructure evolution, the synergy mechanism of Ni micro-alloying and thermomechanical processing is put forward in this paper.

Hot tearing is one of the most severe and irreversible casting defects for many metallic materials. In 2004, Eskin et al. published a review paper in which the development of hot tearing of aluminium alloys was evaluated (Eskin and Suyitno, 2004). Sixteen years have passed and this domain has undergone considerable development. Nevertheless, an updated systematic description of this field has not been presented. Therefore, this article presents the latest research status of the hot tearing during the casting of aluminium alloys. The first part explains the hot tearing phenomenon and its occurrence mechanism. The second part presents a detailed description and analysis of the characterisation methods of the mushy zone mechanical properties and hot tearing susceptibility. The third part presents considerable data pertaining to the mushy zone behaviour, including those of the linear contraction and load behaviour during solidification, semi-solid strength and ductility, and characteristic points related to hot tearing. The fourth part examines the effect of the composition and casting process parameters on the hot tearing susceptibility of aluminium alloys. The fifth part describes the hot tearing simulations and the associated criteria and mechanisms. Finally, recommendations for the further development of hot tearing research are presented.

The coupling control of quenching rate and pre-aging and its positive effect on the age-hardening response of Al–Mg–Si–Cu–Zn–Fe–Mn alloy was systematically investigated. The larger and more stable solute clusters can be formed in alloy with fast age-hardening response by using the lower quenching rate (5.3 °C/min) and an appropriate pre-aging, in which the deterioration of natural aging also can be obviously suppressed. Additionally, the highest bake hardening increment of the alloy can reach 145.2 MPa, which is much higher than those of traditional Al–Mg–Si–(Cu) alloys (such as, 6016 and 6111 alloys). Based on the detailed precipitation behavior characterization of alloys with different quenching rates and the same pre-aging, the quenching rate change can result in the significant differences in the size, number density of precipitates in the both paint baking and peak aging states, but the type of precipitates basically keeps the same, i.e., Mg–Si precipitates, and no Mg–Zn precipitates can be observed. Finally, the related mechanisms of coupling control of quenching rate and pre-aging were also discussed in this paper. The developed coupling control method shows great potential and could significantly increase applications of Al–Mg–Si–Cu–Zn–Fe–Mn alloys with a fast age-hardening response.