Microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging

The microstructure evolution and precipitation behavior of Al-Mg-Si alloy during initial aging were studied using hardness testing, conductivity testing, differential scanning calorimetry(DSC), and high resolution transmission electron microscopy(HRTEM). The results show that the precipitation sequence of the Al-Mg-Si alloy during initial aging can be represented as: supersaturated solid solution → spherical Mg/Si clusters → needle-like Guinier Preston(GP) zone → β″. Clusters are completely coherent with the Al matrix. The GP zone with relatively complete independent lattice parameters that differ slightly from the Al matrix parameters, is oriented along the direction of <111>Aland lying on {111}Alplane. The strength of the Al-Mg-Si alloy is greatly enhanced by the threedimensional strain field that exists between the β″ phase and the two {200}Alplanes. After aging at 170 ℃ for 6 h, the hardness reaches the peak of 127 HV and remains for a long time. At this stage, the electrical conductivity keeps relatively stable due to the formation of coherent precipitates(Mg/Si clusters/GP zones) and the reduction in solute atom concentration in the Al matrix. The severe coarsening and decreased number density of the β″ phase during the over-aging stage result in a significant decrease in the hardness.

Quench sensitivity of Al-Cu-Mg alloy thick plate

The quench sensitivity of Al-Cu-Mg alloy was investigated at different thicknesses of the thick plate.The quenching process was simulated via finite element analysis(FEA);time-temperature-property(TTP)curves and time-temperature-transformation(TTT)curves were obtained through hardness test and differential scanning calorimetry(DSC)test;and the microstructural observation was carried out by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Experimental results exhibit that the quench cooling rate decreases dramatically from the surface to the center of the plate,and the inhomogeneous quenching causes the difference in microstructure.With the decrease in quench cooling rate,constituent particles are coarsening gradually;the quantity of T-phase(Al_(20)Cu_(2)Mn_(3))increases and the S-phase(Al_(2)Cu Mg)decreases.According to the precipitation kinetics analysis,the decrease in S-phase is caused by the increase in precipitate activation energy.So that the center of the plate shows the highest quenching sensitivity,which is consistent with the analysis of time-temperature-property curves and time-temperature-transformation curves.

Corrosion evolution and behaviour of Al-2.1Mg-1.6Si alloy in chloride media

Owing to the importance of finding better methods and technologies for protecting against corrosion of aluminium alloys,research into the electrochemical characteristics of the corrosion is an ongoing concern.In this paper,the corrosion behaviour of Al-Mg-Si alloys in 3.5 wt% NaCl solution was investigated using multiple methods,including scanning electron microscopy(SEM),transmission electron microscopy(TEM),potentiodynamic polarisation measurement and electrochemical impedance spectroscopy(EIS).The results suggest that corrosion begins with dealloying of Mg,followed by conversion of MgSi particles into Al-Fe-Mn-Si particles.The corrosion rate increased until it reached a peak and then decreased.The increase in the corrosion rate may be attributed to an increased electrochemical driving force,which is produced by anodic dissolution between the matrix and residual MgSi particles.Additionally,the thin re-deposited layer of inert elements also plays an important role in accelerating the anodic process of the corrosion pit.The passivated corrosion pit and narrowed intergranular corrosion(IGC)pathway cause the area of active dissolution to decrease,thus reducing the corrosion rate.