Zhi Hui Li

Effect of Artificial Aging Temper on the Microstructure and Properties of 7B04 Pre-Stretched Thick Plate

One- and two-step artificial aging had been performed on the 7B04 pre-stretch thick plate, whose nominal composition is similar to 7075 alloy with lower Fe and Si content. The effect of aging temper on the microstructure and properties has been studied. The research results show that T6 temper can improve the mechanical properties of the alloy greatly, yet the alloy have lower electron conductivity which is no more than 19Ms·s-1. When T73 and T74 tempers were performed on the alloy, the mechanical properties of the alloy decreased about 6~10% of the T6 strength while the electron conductivity was improved obviously. With the increasing of the aging temperature, the time that the alloy needs to get peak aging becomes shorter and the mechanical properties at peak aging status are lower. The electron conductivity of the alloy, however, becomes higher with the increasing of aging temperature. The main strengthen phase of 7B04 alloy is also η(MgZn2) phase which is same to other 7xxx series alloys.

Investigation of Quench Sensitivity of an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr Alloy

In the present work, quench sensitivity of an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy is investigated by temperature-time-property (TTP) curve and TEM analysis, comparing with traditional AA 7B04 and AA 7150. The results indicate that the nose temperature of TTP curve and the corresponding incubation period of the alloy, AA 7150 and AA 7B04 are about 290°C/4.5s, 320°C/2.6s and 335°C/0.1s, respectively, The nose temperature of the alloy is the lowest among three alloys and the critical time at the nose temperature is the longest for the alloy, which is obvious that the alloy has lower quench sensitivity. Further TEM analysis shows that, with the prolongation of keeping time at the nose temperature, quench-induced precipitation phenomenon becomes obvious.

Effects of Heat Transfer Coefficients on Quenching Residual Stresses in 7055 Aluminum Alloy

The quenching process can produce great residual stresses in 7055 aluminum alloy plates. The main factor that affects the quenching residual stresses is the heat transfer coefficient in the quenching process. In this paper, the heat transfer coefficients of spray quenching under different spray water flows were measured by using the inverse method, and the heat transfer coefficients of immersion quenching under different water temperatures were measured by the iterative method. The heat transfer coefficient increases as the spray water flow increases while decreases as the water temperature increases. The basic differences of water temperatures/spray water flows/quenching methods are the different heat transfer coefficients. According to the heat transfer coefficients results of immersion and spray quenching, an orthogonal test was carried out to study the effects of heat transfer coefficients in different temperature regions on the quenching residual stresses. The heat transfer coefficients in the range of 100oC ~200oC have a great influence on the quenching residual stresses, especially for the heat transfer coefficient near 150oC.

Improvements in Strength and Electrical Conductivity in 7B04 Aluminum Alloy Pre-Stretched Thick Plates Via Low-Temperature Retrogression and Re-Ageing Heat Treatments

The effect of RRA treatment with using low retrogression temperatures between 170°C to 190°C on the strength and electrical conductivity (as an indicator of corrosion resistance) of 7B04 aluminum alloy thick plates was investigated. The research results showed that the low-temperature RRA heat treatment provides a means for improving electrical conductivity of the aluminum alloy 7B04 pre-stretched thick plates without sacrificing the mechanical strength. The RRA temper with retrogression at 180°C for 60 min improved electrical conductivity remarkably (reached 21.0MS/m), with only a 3% reduction in strength below T6 temper. Furthermore, TEM observations showed that the microstructure of RRA treated alloy was a very fine distribution of η′ MgZn2 precipitates in the aluminum matrix, similar to T6 condition and η MgZn2 precipitates on grain boundaries distributed similarly to T73 temper.

Quantitative Investigation on Nano-Scale Precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr Alloy under Various Aging Tempers

In the present study, a systematic study of nanoscale precipitates in an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy have been carried out for various typical aging tempers, including T6, T76, T74, T73 and RRA treatments, by combining synchrotron-based small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) techniques. Based on the TEM observations, quantitative and statistical structural information, including precipitate size, volume fraction, number density and inter-precipitate distance have been extracted from SAXS data through model fitting. The results show that the T6 peak-aged alloy with the smallest precipitate size has the highest number density and lowest volume fraction of precipitates. Under two-step T7X over-aged tempers, with the deepen of aging degree, the precipitate size, volume fraction and inter-precipitate distance increases, but the number density decreases. The size distribution of precipitates for the RRA-treated alloy is in between that of T6 and T76. The results also show that as the degree of over-aging deepens, the precipitate size distribution interval becomes broader.

Microstructure and Mechanical Properties of Laser Beam Welded AA7021 Aluminum Alloy

In this study, a laser beam welds in a Al-Zn-Mg alloy were characterized by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and tensile tests. It is found that the joint of the alloy contained three distinctive regions, i.e. fusion zone, heat affected zone and base metal region. The fusion zone consists of small grains, whose size is heterogeneously distributed. Extensive microhardness measurements were conducted in the weld regions of the nuggets exhibited a hardness loss in the fusion zone due to the loss of strengthening phases. Microstructural examination of the joints revealed typical eutectic structure was appeared in the heat-affected zone due to relative low cooling rate. Tensile properties of the joints were obtained by testing flat transverse tensile specimens, and the results indicated that tensile strength of these welds approached 76.8~77.3% of the base metal.

Microstructure and Properties of an Al-7.5Zn-1.5Mg-1.4Cu-0.12Zr Alloy Forging

In this present work, the microstructure and properties of an Al-7.5Zn-1.5Mg-1.4Cu-0.12Zr alloy forging in T7452 condition has been investigated by means of OM, TEM and EBSD analysis and varied properties test. The results indicate that there are obvious differences in grain morphology, precipitation characteristics and recrystallization degree along the thickness direction of the forging. The strength of the alloy forging is stable and varies less than 10% along the thickness direction. The alloy forging in the T7452 condition shows high strength, high toughness and good corrosion resistance, with the UTS, TYS, elongation and electrical conductivity values being 500~530 MPa, 460~500 MPa, 7.0~15.0% and 23.9~24.2 MS/m, respectively, and the fracture toughness in L-T, T-L and S-L direction being 33.5~39.5, 25.5~27.0, 23.0~24.5 MPa.m1/2, respectively, and the EXCO rating being EA.

DSC and TEM Study of Al-Cu-Mg-Ag Alloy Ageing at Low and Elevated Temperature

Microstructural characterization of Al-Cu-Mg-Ag alloy ageing at low and elevated temperature were investigated by differential scanning calorimeter (DSC) and transmission electron microscopy (TEM). The results show that the precipitation sequence is significantly influenced by the ageing temperature. With prolonging of ageing at 65°C, G.P. zones on the plane of {100}α separates out with no precipitation of Ω phase. After 10h ageing at 160°C,the phase of Ω separates out and dominant the microstructure, while a small quantity of θ′separates out as well.

Microstructure Evolution and Properties of 7A56 Aluminum Alloy by Homogenization

The microstructure evolution and properties of Al-matrix in homogenized 7A56 alloy were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), electrical conductivity and hardness test. The second phases in as-cast 7A56 alloy consisted of AlZnMgCu, Al2Cu and Al7Cu2Fe. With the homogenization temperature increasing, more non-equilibrium phase AlZnMgCu was dissolved into Al-matrix. The diffusion of alloying elements from AlZnMgCu phase into Al-matrix leads to a decrease of electrical conductivity and an increase of hardness. The lattice constant of α-Al has an increases of 0.0019 Å, 0.0032 Å and 0.0053 Å after 380°C/24h,430°C/24h,and 470°C/24 h treatment,respectively.

Effect of Solution Treatment on Microstructure and Mechanical Properties in an Al-9.3Zn-2.0Mg-1.8Cu Alloy

The microstructure solution treated by various temperatures of 2h in as-extruded Al-9.3Zn-2.0Mg-1.8Cu alloy was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) analysis. The mechanical properties treated at 465oC for various times were tested by room temperature tensile mechanical properties test. The results indicated that second phase of the as-extruded alloy mainly consists of Mg (Zn,Cu,Al)2 and Fe-rich phases. Mg (Zn,Cu,Al)2 phase completely dissolved into the matrix solution treated at 465oC or higher for 2h while residual phase was mainly Fe-rich phase. The mechanical properties treated at 465oC for various time were tested and optimized solution treatment parameter was chosen as 465°C/1.5h.