Qinglin Pan

Effect of minor Sc and Zr on the microstructure and mechanical properties of Al–Mg based alloys

A series of Al–Mg based alloy plates with thickness of 4 mm containing minor Sc and Zr were prepared. Tensile properties and microstructures of the alloys were studied. The results show that adding 0.2% Sc and 0.1% Zr to Al–5Mg alloy, the strength of the alloy increased by 150 MPa. Strengthening effect is the most outstanding among all minor alloying elements in aluminum alloys. Strength increment caused by adding minor Sc and Zr is attributed mainly to fine grain strengthening, precipitation strengthening of Al3(Sc, Zr) and substructure strengthening.

Microstructural evolution of ultra-high strength Al-Zn-Cu-Mg-Zr alloy containing Sc during homogenization

The microstructural evolution and composition distribution of an Al-Zn-Cu-Mg-Sc-Zr alloy during homogenization were investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that severe dendritic segregation exists in Al-Zn-Cu-Mg-Sc-Zr alloy ingot. There are a lot of eutectic phases at grain boundary and the distribution of the main elements varies periodically along interdendritic region. The main eutectic phases at grain boundary are Al7Cu2Fe phase and T (Al2Mg3Zn3). The residual phases are dissolved into the matrix gradually during homogenization with increasing temperature and prolonging holding time, which can be described by a constitutive equation in exponential function. The overburnt temperature of the alloy is 473.9 °C. The optimum parameters of homogenization are 470 °C and 24 h, which is consistent with the result of homogenization kinetic analysis.

Modeling of strain hardening and dynamic recrystallization of ZK60 magnesium alloy during hot deformation

Abstract The flow stress behavior of ZK60 alloy at elevated temperature was investigated. The strain hardening and dynamic recrystallization of the alloy were modeled by Kocks-Meching model and Avrami equation, respectively. A new constitutive equation during hot deformation was constructed to predict the flow stress considering the dynamic recrystallization. The results show that the flow stress curves predicted by the proposed equation have high correlation coefficients with the experimental data, which confirms that the developed model is accurate and effective to establish the flow stress equation of ZK60 magnesium alloy during hot deformation. Microstructure observation shows that dynamic recovery occurs in the initial stage of hot deformation. However, the microstructure turns to recrystallization structure as the strain increases.

Recrystallization of Al-5.8Mg-Mn-Sc-Zr alloy

Abstract Al-5.8Mg-0.4Mn-0.25Sc-0.1Zr (mass fraction, %) alloys were prepared by water chilling copper mould ingot metallurgy processing which was protected by active flux. The recrystallization temperature and nucleation mechanism of the alloy were studied by means of hardness tests, observations of optical microscopy and transmission electron microscopy. The results show that the anti-crystallization ability can be significantly improved by adding minor Sc and Zr into Al-Mg-Mn alloy. This can be proved by a much higher recrystalliztion temperature (450 °C) than Al-Mg-Mn alloy without Sc and Zr (150 °C). The main reason of the great increase of recrystallization temperature can be attributed to the strong pinning effect of highly disperseded Al3(Sc,Zr) particles on dislocations and sub-grain boundaries. The recrystallizing process reveals itself the nucleation mechanism of the alloy involving not only the sub-grain coalescence but also the sub-grain growth.

Microstructural evolution during homogenization of DC cast 7085 aluminum alloy

Abstract The microstructural evolution of a DC cast 7085 alloy during homogenization treatment was investigated by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectrometry (EDS), differential scanning calorimeter (DSC) and X-ray diffraction (XRD). The results showed that serious dendritic segregation existed in the as-cast 7085 alloy. Numerous eutectic microstructures and phases were observed at the grain boundary. During homogenization process, eutectic α (Al)+ T (AlZnMgCu) microstructure gradually was dissolved into matrix. Intermetallic S (Al 2 CuMg) phase formed and grew along the eutectic microstructure and disappeared into the matrix completely when it was homogenized at 460 °C for 24 h. It could be found that the evolution of primary eutectic structure of 7085 alloy consisted of three processes, dissolution of eutectic α + T microstructure, phase transformation from T phase to S phase and the dissolution of S phase. The optimum homogenization parameter was at 470 °C for 24 h.

Microstructural evolution of Al−Zn−Mg−Zr alloy with trace amount of Sc during homogenization treatment

Abstract The microstructural evolution of Al–Zn–Mg–Zr alloy with trace amount of Sc during homogenization treatment was studied by means of metallographic analysis, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and differential scanning calorimetry (DSC). The results show that serious dendritic segregation exists in studied alloy ingot. There are many eutectic phases with low melting-point at grain boundary and the distribution of main elements along interdendritic region varies periodically. Elements Zn, Mg and Cu distribute unevenly from grain boundary to the inside of alloy. With increasing the homogenization temperature or prolonging the holding time, the residual phases are dissolved into matrix α (Al) gradually during homogenization treatment, all elements become more homogenized. The overburnt temperature of studied alloy is 476.7 °C. When homogenization temperature increases to 480 °C, some spherical phases and redissolved triangular constituents at grain boundaries can be easily observed. Combined with microstructural evolution and differential scanning calorimeter, the optimum homogenization parameter is at 470 °C for 24 h.

Hot deformation behavior and microstructural evolution of Al–Zn–Mg–0.25Sc–Zr alloy during compression at elevated temperatures

The hot deformation behavior of Al–Zn–Mg–0.25Sc–Zr alloy and its microstructural evolution were investigated by isothermal axisymmetric hot compression tests at temperatures from 340 to 500 °C and strain rates ranging from 0.001 to 10 s−1. The steady flow stress increased with increasing the strain rate or decreasing the deformation temperature, which can be described by a hyperbolic-sine constitutive equation with the deformation activation energy of 150.25 kJ/mol. The tendency of dynamic recrystallization enhanced at high deforming temperatures and low strain rates, which corresponded to low Z values. With decreasing Z value, the main softening mechanism of the alloy transformed from dynamic recovery to dynamic recrystallization, correspondingly, the subgrain size increased and the dislocation density decreased.

Flow behavior of Al–6.2Zn–0.70Mg–0.30Mn–0.17Zr alloy during hot compressive deformation based on Arrhenius and ANN models

Abstract The hot deformation behavior of Al–6.2Zn–0.70Mg–0.30Mn–0.17Zr alloy was investigated by isothermal compression test on a Gleeble–3500 machine in the deformation temperature range between 623 and 773 K and the strain rate range between 0.01 and 20 s−1. The results show that the flow stress decreases with decreasing strain rate and increasing deformation temperature. Based on the experimental results, Arrhenius constitutive equations and artificial neural network (ANN) model were established to investigate the flow behavior of the alloy. The calculated results show that the influence of strain on material constants can be represented by a 6th-order polynomial function. The ANN model with 16 neurons in hidden layer possesses perfect performance prediction of the flow stress. The predictabilities of the two established models are different. The errors of results calculated by ANN model were more centralized and the mean absolute error corresponding to Arrhenius constitutive equations and ANN model are 3.49% and 1.03%, respectively. In predicting the flow stress of experimental aluminum alloy, the ANN model has a better predictability and greater efficiency than Arrhenius constitutive equations.

Characterization of Fracture and Fatigue Behavior of 7050 Aluminum Alloy Ultra-thick Plate

The microstructure, mechanical property, fracture toughness, and fatigue behavior of 7050 aluminum alloy pre-stretched ultra-thick plate were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test, fracture toughness test, and high-cycle fatigue test. The results showed that the microstructure of the ultra-thick plate consisted of recrystallized grains, subgrains, constituent particles, precipitated phases, and precipitate-free zone. Mechanical tests indicated that anisotropy of fracture toughness existed in L-T, T-L, and S-T orientation. Fractographic features suggested that this anisotropy was significant due to the difference of recrystallized grain on different metallographic planes. Compared to 7050 aluminum alloy plate in less thickness, the ultra-thick plate showed deterioration on fracture toughness due to the increase of recrystallized grains but improvement on fatigue property ascribed to the less densely populated particles. Fractographic observations showed that fatigue initiation of this ultra-thick plate was primarily related to the constituent particles and promoted by increase of the stress amplitude.

Effect of aging time on precipitation behavior, mechanical and corrosion properties of a novel Al-Zn-Mg-Sc-Zr alloy

Abstract The precipitation behavior, mechanical properties and corrosion resistance of a novel Al—Zn—Mg—Sc—Zr alloy aged at different time were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile tests, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that with increasing aging time at 120 °C, the hardness and tensile strength of the alloy increased rapidly at first and then slightly decreased. The resistance of exfoliation corrosion (EXCO) and intergranular corrosion (IGC) increased gradually with increasing aging time. The same trend of corrosion properties was demonstrated by electrochemical polarization curves and EIS test. The characteristics of grain boundary precipitates and precipitate free zone (PFZ) had a significant influence on the mechanical and corrosion behaviors of the studied alloy. On the basis of TEM observations, the size of grain boundary precipitates and the width of PFZ became larger, and the distributed spacing of grain boundary precipitates was enhanced with increasing aging time.