Papers by Keyword: Zn/Mg Ratio

Abstract: The main alloying elements have a decisive influence on the type and quantity of the second phase of the Al-Zn-Mg-Cu alloy, and even on the dissolution of the second phase during solution treatment. The effect of Zn/Mg ratio on second phase dissolution during solution treatment of Al-Zn-Mg-Cu alloys was investigated by means of differential scanning calorimetry (DSC), scanning electron microscope (SEM) and electrical conductivity testing. The results showed that Mg (Zn,Cu,Al)₂ phase and Fe-rich phase existed in the as-deformed alloys. In addition, a small amount of Al₂CuMg phase was found in the low and medium Zn/Mg ratio alloy. The number of Mg (Zn,Cu,Al)₂ phases as the major second phase in the alloys was inversely proportional to the Zn/Mg ratio. Mg (Zn,Cu,Al)₂ phase essentially dissolved into the matrix after solution treatment at 465°C/2h. Increasing the solution temperature and time were both beneficial to the dissolution of the Al₂CuMg phase. With the increase of the solution temperature from 465°C to 475°C, the conductivity of the alloy showed a decreasing trend initially and then increased. As the solution time increased at 470°C, the electrical conductivity of the low-Zn/Mg ratio alloy decreased and then increased due to the more secondary phase. After the second phase was fully dissolved in the alloy, the electrical conductivity gradually increased with the increase of the solution time.

Abstract: The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.

Abstract: The influence of Zn content and Zn/Mg ratio on the equilibrium phase amounts of major ageing strengthening phase η’ (MgZn2) in lithium containing 7000 series aluminum alloys has been investigated by means of thermodynamic calculations. The results show that, comparing with Li-free 7000 series aluminum alloys, Zn/Mg ratio is no longer the determinant for the amount of η’ and only increasing Zn content that can increase the amount of η’. And the results were discussed with existing experiments.