Yong An Zhang

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.

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 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.

Measurement of Residual Stress in As-Quenched 7055 Aluminum Plate by Various Methods

The accuracy of different residual stress measurement methods has always been the research focus from the beginning of research on residual stress. In this study, both conventional and newly-developed methods were applied to measure the residual stress in as-quenched 7055 aluminum plate. Methods such as hole drilling, X-ray diffraction based on sin2Ψ and cos α approaches, crack compliance method and neutron diffraction method were used. In the meanwhile, finite element simulation was used to obtain the residual stress distribution as a comparison. The results showed that among the methods studied, X-ray diffraction method has the greatest test error due to its shallow test depth. However, if the measurement condition was well controlled, the error could be acceptable. The absolute values of residual stress obtained by X-ray diffraction method were slightly greater than hole drilling method. If calculated with the reasonably chosen crack compliance function, the test result was similar to neutron diffraction method. Under different quenching conditions, all the studied methods showed that the greater the quenching cooling rate, the greater the absolute value of residual stress.

Flow Behaviors and Corresponding Constitutive Equation of the Al–9.4Zn–1.9Mg–2.0Cu Alloy

Isothermal compression tests of the Al–9.4Zn–1.9Mg–2.0Cu alloy were carried out at the temperature ranging from 300 to 460 °C and the strain rate from 0.001 to 10 s−1, and the deformation degree was 70%. Flow stress curves show that the flow stress decreases with the increasing deformation temperature and the decreasing strain rate. The measured flow stress was corrected because of the effect of friction. The corresponding corrected stress values are lower than measured stress values. The effect of friction is far greater when hot-deformations occurred at lower temperatures or higher strain rates. A constitutive equation considering the effect of strain on material constants (i.e. α, n, Q and A) are established based on the Arrhenius-type equation. Compared with the experimental results, the flow stresses calculated by the constitutive equation have a high precision with the correlation coefficient of 0.95. Results show that higher deformation temperatures and lower strain rates are beneficial for hot deformation of the Al–9.39Zn–1.92Mg–1.98Cu alloy.

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.

Research on Microstructure Evolution in Al-9.8Zn-2.0Mg-1.8Cu Alloy during Solution Treatment

The microstructure of as-extruded Al-9.8Zn-2.0Mg-1.8Cu aluminum alloy and its evolution during solution treatment were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC) analysis and electron back-scatter diffraction (EBSD). The results indicated that second phase of the as-extruded alloy mainly consisted of Mg (Zn, Cu, Al)2 and Fe-rich phases. After solution treated at 475°C for 4h, Mg (Zn, Cu, Al)2 phases were dissolved into the matrix, while Fe-rich phases still existed. Fe-rich phases cannot dissolve by prolonging solution time. The room temperature tensile strength gradually increased by prolonging solution time at 475oC. The ultimate tensile strength of the alloy reached 700MPa after both single and two-step solution treatments.

Processing Map and Microstructural Evolution of Isothermal Compressed an Al-Mg-Si-Cu-Zn Alloy

The hot deformation behavior of an Al-0.92Mg-0.78Si-0.20Cu-0.60Zn alloy was studied by isothermal compression in the temperature range from 350 to 500 oC with strain rates of 0.01-10s-1 on Gleeble-1500 thermo-mechanical simulator. The microstructural evolution during hot deformation was investigated by electron back-scatter diffraction (EBSD). The results show that the strain rate and deformation temperature have significant influence on flow behavior, and the flow stress increases with increasing strain rate and decreasing deformation temperature. Processing map at the strain of 0.7 is obtained and exhibits three peak efficiency domains (380-420 oC at 0.01s-1, 480-500 oC at 0.01s-1 and 450-500 oC at 0.1-0.32s-1). According to the processing map and microstructure observation, the optimized processing condition of hot deformation for the alloy is at 450-500 oC and strain rate of 0.1-0.32s-1. The homogenized ingot is hot rolled at 480 oC with a strain rate of 0.1s-1 based on optimized deformation parameters. The fraction of high-angle grain boundary is 35.2%, which is in accord with microstructure after hot deformed at 500 oC with a strain rate of 0.1s-1.

Numerical Simulation of Quenching and Pre-Stretching Residual Stress in 7085 Aluminum Alloy Plate

The finite element method (FEM) was used to study the quenching and pre-stretching residual stresses of the 7085 aluminum alloy plate, combining with experimental measurement. Quenching residual stress simulation results showed that the stress of the rolling-direction and transverse-direction in the center were 16.5MPa and 9.2MPa, respectively. The stress of the rolling-direction and transverse-direction on the surface were-33.8MPa and-40.4MPa, respectively. The quenching residual stress simulation results were in good agreement with the X-ray diffraction experimental results. As the pre-stretching ratio increases, the quenching residual stress reduction ratio increases. The ratio of 2.0~3.0% pre-stretching can effectively release the quenching residual stress in aluminum alloy plates. After 3.0% pre-stretching, the stress of the rolling-direction and transverse-direction in the center were 0.9MPa and 1.8MPa, respectively. The stress of the rolling-direction and transverse-direction on the surface were-3.6MPa and-7.1MPa, respectively. The quenching residual stress reduction ratio reached above 80%.

Correlation between Fracture Toughness and Quantitative Characterization of Microstructure in 7055 Aluminium Alloy

Microstructure of high strength aluminum alloy have determinant effect on the properties, thus an effort has been made to investigation the relationship between fracture toughness and quantitative characteristics of microstructure in high strength aluminum alloy. Fracture toughness was tested for aluminum alloy specimens with various microstructure. The corresponding microstructure was observed by optical metallography and electron back-scattered diffraction, and quantitative characterization was conducted by further analysis of result obtained. Correlation between fracture toughness and parameters included grain size, percentage of recrystallization, substructure content and area fraction of residual phases was investigated. It was shown that percentage of recrystallization was a crucial factor for the fracture toughness, and correlations were established with proper and reasonable correction.