| Although unremitting efforts have been made in the study and manufacture of high strength Al-Zn-Mg-Cu series alloys in China, there are still lots of obvious deficiencies relating to alloy design, microstructural control, and strengthening-toughening mechanisms. The effects of alloy composition on strength, toughness and corrosion resistance are the critical areas for the industrial production that the related information is rarely reported. Systematic studies on the composition-microstructure-property relationships have great practical significance and academic value to the assimilation of foreign advanced alloy designs as well as the development of high-performance Al-Zn-Mg-Cu series alloys with independent intellectual property rights. Presently, with considering the strong effects of Mg and Cu elements on phase components at high temperatures, a series of high Zn-containing Al-Zn-Mg-Cu alloys (Zn= 8.5 wt%) are designed to study the effects of Mg and Cu contents on the microstructures during the whole manufacturing process (casting, homogenization, rolling, solid solution and ageing) as well as on the mechanical properties. Also, the computational thermodynamics/kinetics are utilized to simulate and analyse some important experimental phenomena, and the regularities obtained from simulation could agree well with those from experiments. The main conclusions are as follows:The real solidification paths of all designed alloys fall in between the Scheil model and the equilibrium conditions, but tend to the former. The amounts of the massive a phase and (σ+θ) pockets in the as-cast alloys are determined primarily by Mg content, i.e., the more the Mg content, the more the massive a phase and the less the (σ+θ) pockets. For the alloys with similar Mg content, the more the Cu content, the more the massive a phase, and generally the more the (σ+θ) pockets. However, it’s difficult for the alloys with high Cu content to form (σ+θ) pockets if they also contain high Mg content.The phase components of the designed alloys after 460℃/168 hours and (460℃/24 hours+475℃/24 hours) homogenizations are generally consistent with the calculated isothermal sections of Al-Zn-Mg-Cu quaternary phase diagram with 8.5 wt% Zn at 460℃ and 475℃. It shows at 460℃ the equilibrium phase components are greatly influenced by Mg content:the alloys with low Mg content can be likely to be in single Al phase field, even if they contain high Cu content; at 475℃ the equilibrium phase components are dominated primarily by (Mg+ Cu) content, except the alloys with (Mg+Cu)≥4.35 wt%, all designed alloys are in single Al phase field.Low Mg content or high Zn:Mg ratio can accelerate the ageing kinetics so as to speed up peak ageing and overageing, and this can be attributed to the promotion of GPII zone nucleation during the first stage ageing 120℃/6 h, at which stage the nucleation of GPI zone will be promoted with high Mg content or low Zn:Mg ratio. Due to the weaker thermodynamic stability of GPI compared with GPII, for the alloys with low Mg content or high Zn:Mg ratio, less GP zones can be survived in the ramp from 120℃ to 160℃ and grown as nuclei for η’ phase in the succedent ageing at 160℃. The effect of Cu on the ageing kinetics is not obvious.Under same ageing treatments, the conductivity, hardness, strength and toughness of the designed alloys are primarily determined by Mg content:the higher the Mg content, the higher the hardness and strength, but the lower the conductivity and toughness. Increasing Cu content can produce a similar phenomenon, but with weaker effects compared with Mg. The experiments and thermodynamic simulation indicate that, in underaged condition, the volume fraction of precipitates can be increased obviously with increasing Mg content (the precipitate sizes are little changed) so as to improve the strength and hardness; in overaged condition, with increasing Mg content the volume fraction of precipitates also can be increased obviously while their sizes can be somewhat decreased, both leading to higher strength and hardness. With increasing Cu content the volume fraction of precipitates can be increased to a certain extent, and therefore the strength and hardness can be improved.With increasing Mg content the area fraction of the grain boundary precipitates and the yield stress contrast between grain interiors and precipitate free zones (PFZs) at grain boundary can be increased greatly, consequently promoting the intergranular fracture and decreasing the toughness. But Cu weakly affects the toughness. For the alloys with low/middle Mg content (e.g.,1.5/2.0 wt%), increasing Cu content can increase the yield stress contrast between grain interiors and PFZs as well as the recrystallization degree, so that the intergranular fracture will be promoted for toughness reduction. For the alloys with high Mg content (e.g.,2.5 wt%), the increased undissolved phases induced by high Cu content will promote fracture at/near coarse constituent particles, favoring further toughness reduction. |
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