Shengdan Liu

Effect of step-quenching on microstructure of aluminum alloy 7055

Abstract The effect of step-quenching on the microstructure of aluminum alloy 7055 after artificial aging was studied by hardness testing and transmission electron microscopy (TEM). Step-quenching leads to decomposition of solid solution and heterogeneous precipitation of equilibrium phase mainly on dispersoids and at grain boundaries; thus lower hardness after aging. Prolonging isothermal holding at 415 °C results in coarser and more spaced η phase particles at grain boundaries with wider precipitates free zone, and lower density of larger η ′ hardening precipitates inside grains after aging. Isothermal holding at 355 °C results in heterogeneous precipitation of η phase both on dispersoids and at grain boundaries. Isothermal holding at 235 °C results in heterogeneous precipitation of η phase first, and then S phase. Precipitates free zones are created around these coarse η and S phase particles after aging. Prolonging isothermal holding at these two temperatures leads to fewer η ′ hardening precipitates inside grains, larger and more spaced η phase particles at grain boundaries and wider grain boundary precipitates free zone after aging.

Effect of Zr content on quench sensitivity of AIZnMgCu alloys

Abstract The effect of Zr content on quench sensitivity of AIZnMgCu alloys was investigated by mechanical properties testing and microstructure observations. The results show that with the increase of Zr the quench sensitivity relative to hardness and strength increases, while that relative to elongation decreases. From hardness and strength viewpoints, the low quench sensitivity is observed for the Zr-free and 0.05% Zr-containing alloys, which is quite quench sensitive from the ductility viewpoint. The largest quench sensitivity relative to hardness and strength is observed for 0.1% Zr-containing alloy, this is mainly due to large amount of high angle grain boundaries and incoherent Al 3 Zr dispersoids caused by recrystallization, which may efficiently promote heterogeneous precipitation during air quenching. More than 0.05% Zr can significantly decrease the quench sensitivity relative to ductility, which can be primarily attributed to recrystallization inhibiting and grain refining effects of Zr.

Evolution of grain structure in AA2195 Al-Li alloy plate during recrystallization

Abstract The evolution of the grain structures in AA2195 Al-Li alloy plate warm-rolled by 80% reduction during recrystallization annealing at 500 °C was investigated by electron backscatter diffraction, scanning electron microscopy and transmission electron microscopy. It is found that the elongated grain structures are caused by the lamellar distribution of recrystallization nucleation sites, being lack of large second phase particles (> 1 μm), and dispersive coherent particles (such as δ′ and β′) concentrated in planar bands. The recrystallization process may be separated into three stages: firstly, recrystallization nucleation occurs heterogeneously, and the nuclei are concentrated in some planar zones parallel to rolling plane. Secondly, the grain boundaries interacted with small particles concentrate in planar bands, which is able to result in the elongated grain structures. The rate of the grain growth is controlled by the dissolution of these small particles. Thirdly, after most of small particles are dissolved, their hindrance to migration of the grain boundaries fades away, and the unrecrystallized zones are consumed by adjacent recrystallized grains. The migration of high angle grain boundaries along normal direction leads a gradual transformation from the elongated grains to the nearly equiaxed, which is driven by the tension of the grain boundaries.

Exfoliation corrosion behavior of 7050-T6 aluminum alloy treated with various quench transfer time

The exfoliation corrosion (EFC) behavior of 7050-T6 aluminum alloy treated with various quench transfer time after solution heat treatment was investigated by standard EFC immersion tests, strength loss measurements after EFC tests and electrochemical impedance spectroscope (EIS) technique. The results showed that EFC resistance of the alloy decreased with increasing quench transfer time. Backscattered electron scanning electron microscope (SEM) together with transmission electron microscope (TEM) observations revealed that the coverage ratio and microstructure of precipitates at grain boundary area are the most important factors which influence the EFC susceptibility of the alloy, while precipitate-free zone (PFZ) near grain boundary has no or only a minor effect on it. In addition, galvanostatic measurements of the alloy present a good correlation between EFC resistance and transients in potential. The cumulated number of transients in potential can be used to evaluate EFC resistance of the alloy.

Influence of cooling rate after homogenization on microstructure and mechanical properties of aluminum alloy 7050

The influence of cooling rate (0.009–220 °C/s) after homogenization on the microstructure and mechanical properties of high strength aluminum alloy 7050 was investigated by tensile testing, optical microscope, X-ray diffraction, scanning electron microscope, and transmission electron microscope. A lower cooling rate after homogenization resulted in lower mechanical properties after aging. The drop in strength was significant when the cooling rate was decreased from 0.5 °C/s to 0.1 °C/s. A lower cooling rate gave rise to a larger amount of remnant S(Al2CuMg) phase and a higher fraction of recrystallization after solution heat treatment. Consequently, the increase in strength after aging due to precipitation hardening and substructure hardening was less significant in the case of slow cooling. This was supposed to be responsible for the lower mechanical properties due to a lower cooling rate after homogenization.

Effect of homogenization time on quench sensitivity of 7085 aluminum alloy

The effect of homogenization time on quench sensitivity of a cast 7085 aluminum alloy was investigated by means of end-quenching test, optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that with the increase of homogenization time from 48 h to 384 h, quench sensitivity increased slightly as the largest difference in the hardness was increased from 5.2% to 6.9% in the end-quenched and aged specimens. Prolonging homogenization had little effect on the grain structure, but improved the dissolution of soluble T phase and resulted in larger Al3Zr dispersoids with a low number density. Some small quench-induced η phase particles on Al3Zr dispersoids were observed inside grains during slow quenching, which decreased hardness after subsequent aging. The change in the character of Al3Zr dispersoids exerted slight influence on quench sensitivity.

Oxide distribution and microstructure in welding zones from porthole die extrusion

The oxide distribution and microstructure in longitudinal and transverse welding zones during the billet-to-billet extrusion process through porthole die were adequately investigated by means of finite element method, scanning electron microscopy and optical microscopy. The results indicate that the oxides exist at the interface between the matrix and transverse welding zone rather than longitudinal welding seam. The longitudinal welding zone reveals a darker band including the largest grain with irregular shape due to the abnormal grain growth under the heavy shear deformation and high temperature. The transverse welding zone consists of equiaxed recrystallized grains which are a little finer than those in the longitudinal welding seam.

Rate controlling mechanisms in hot deformation of 7A55 aluminum alloy

Abstract The hot deformation behavior of 7A55 aluminum alloy was investigated at the temperature ranging from 300 °C to 450 °C and strain rate ranging from 0.01 s −1 to 1 s −1 on a Gleeble–3500 simulator. Processing maps were established in order to apprehend the kinetics of hot deformation and the rate controlling mechanism was interpreted by the kinetic rate analysis obeying power-law relation. The results indicated that one significant domain representing dynamic recrystallization (DRX) existed on the processing maps and lying in 410–450 °C and 0.05–1 s −1 . The conclusions of kinetic analysis correlated well with those obtained from processing maps. The apparent activation energy values calculated in the dynamic recrystallization (DRX) domain and the stability regions except dynamic recrystallization (DRX) domain were 91.2 kJ/mol and 128.8 kJ/mol, respectively, which suggested that grain boundary self-diffusion and cross-slip were the rate controlling mechanisms.

Dynamic recrystallization behavior of 7085 aluminum alloy during hot deformation

Abstract The dynamic recrystallization behavior of 7085 aluminum alloy during hot compression at various temperatures (573–723 K) and strain rates (0.01–10 s −1 ) was studied by electron back scattered diffraction (EBSD), electro-probe microanalyzer (EPMA) and transmission electron microscopy (TEM). It is shown that dynamic recovery is the dominant softening mechanism at high Zener–Hollomon ( Z ) values, and dynamic recrystallization tends to appear at low Z values. Hot compression with ln Z =24.01 (723 K, 0.01 s −1 ) gives rise to the highest fraction of recrystallization of 10.2%. EBSD results show that the recrystallized grains are present near the original grain boundaries and exhibit similar orientation to the deformed grain. Strain-induced boundary migration is likely the mechanism for dynamic recrystallization. The low density of Al 3 Zr dispersoids near grain boundaries can make contribution to strain-induced boundary migration.

Anisotropy of localized corrosion in 7050–T7451 Al alloy thick plate

Abstract The corrosion anisotropy of 7050–T7451 Al alloy thick plate in NaCl solution was investigated by immersion tests, slow strain rate testing (SSRT) technique, potentiodynamic and anode polarization measurements, optical microscropy (OM) and scanning electron microscopy (SEM) observations. The results show that the thick plate exhibits severe corrosion anisotropy due to the microstructure anisotropy. The observations of immersion surfaces together with the analysis of polarization curves reveal that the differences of the corrosion morphologies on various sections in this material are mainly related to the area fraction of the remnant second phase, and higher area fraction displays worst corrosion resistance. The stress corrosion cracking (SCC) susceptibility of different directions relative to the rolling direction is assessed by SSRT technique, ranked in the order: S direction > L direction > T direction. The result show that the smaller the grain aspect ratio, the better the corrosion resistance to SCC.