Jianguo Tang

Precipitation of metastable phases and its effect on electrical resistivity of Al-0.96Mg2Si alloy during aging

Abstract The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg2Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy (TEM). The precipitates of the peak aged alloy include both β″ and β′, but the amount ratio of β″ to β′ varies with the aging temperature and time increasing. The precipitates during aging at 175 °C are dominated by needle-like β″ phases (including pre-β″ phase), the size of which increases with the time prolonging, but does not increase substantially after further aging. The evolution of electrical conductivity is directly related to such microstructural evolution. However, the hardness of the alloy stays at the peak value for a long term. When the alloy is aged at 195 °C, the ratio of β″ to β′ becomes the main factor to influence relative resistivity (Δρ) value. The higher the temperature is, the smaller the ratio is, and the faster the Δρ value decreases. Moreover, the hardness peak drops with the decrease of the ratio. With the size and distribution parameters measured from TEM images, a semi-quantitative relationship between precipitates and the electrical resistivity was established.

Effect of copper on precipitation and baking hardening behavior of Al-Mg-Si alloys

Abstract The effects of copper on the ageing precipitation behavior of as-quenched and pre-aged AA6016 aluminum alloy were studied by differential scanning calorimetry (DSC), Vickers hardness measurement and transmission electronic microscopy (TEM). The results indicate that the addition of copper facilitates the growth of clusters (GP I) to the critical size during pre-ageing. Therefore, the addition of copper accelerates the transition from GP I (pre-β″) to GP II (β″) during final artificial ageing, and finally results in the favorable paint-bake response. However, the one with the copper level of 0.3% does not show significant baking hardening response as expected. Pre-aging can also reduce the detrimental effect due to natural aging of copper-containing alloys.

Microstructure and Stress Corrosion Behavior of MIG Welded Joint Al–Zn–Mg Alloy

The microstructures and stress corrosion behaviors of the welded joint in a MIG welded Al–Zn–Mg alloy were investigated by slow strain rate test and OM, EBSD, TEM observation. The results indicated that the stress corrosion cracking index of welded joint is bigger than the base metal, which means the stress corrosion resistance of base metal is better than MIG welded joint. The stress corrosion of MIG welded joints mainly occur in the weld zone and heat affected zone, which indicates that they are the weak areas of welded joints. The fracture mode of the joint in 3.5% NaCl solution are intergranular brittle fracture and partial transgranular dimple fracture, while that of base metal is only transgranular dimple fracture. The microstructure is coarse equiaxed grain in welded zone, and there is virtually no strengthening phase in this zone. In the fusion zone, fine columnar grains and equiaxed grains are distributed at different sides. There is the fibrous phase in the base metal, and the microstructure of heat affected zone is similar to this zone. But in the heat affected zone, recrystallization occurs and the grains grow obviously. And the strengthening phases in the quenching zone are completely dissolved, but their particles in over aged zone became coarseobviously.

Effect of RRA Treatment on the Microstructure and Fatigue Behavior of 7020 Aluminum Alloy

The fatigue crack growth (FCG) behavior and fatigue strength of 7020 aluminum alloy in an over aged treatment and two retrogression and re-aging (RRA) treatments were investigated. The microstructures and fractographies were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The result showed that RRA-treated alloys have faster FCG rate than over aged alloy at lower ΔK region due to the dense non-shearable precipitates. And the RRA120 alloy re-aged at 120 °C/24 h has a relatively faster FCG rate than RRA150 alloy re-aged at 150 °C/12 h. RRA-treated alloys have higher fatigue strength than over aged alloy. The fatigue strength corresponding to 107 cycles of over aged, RRA120 and RRA150 conditions are 114, 129 and 118 MPa, respectively. The effect of RRA treatment on the fatigue property is mainly caused by its finer and more dispersed precipitates and narrower precipitate free zone (PFZ).