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Feng L.,Central South University | Pan Q.-L.,Central South University | Wei L.-L.,Central South University | Huang Z.-Q.,Guangdong Fenglu Aluminium Co. | Liu Z.-M.,Guangdong Fenglu Aluminium Co.
Journal of Central South University | Year: 2015

The through-thickness corrosion inhomogeneity of 7050-T7451 Al alloy thick plate was studied using immersion tests, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), slow strain rate testing (SSRT) technique combined with optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the through-thickness corrosion resistance is ranked in the order of T/2>surface>T/4. And the 75 mm-thick 7050 alloy plate presents better corrosion resistance than the 35 mm-thick plate. The results are discussed in terms of the combined effect of recrystallization and cooling rate in quenching. Alloy with lower volume fraction of recrystallization and smaller grain aspect ratio displays better corrosion resistance. The lower corrosion resistance caused by the slower cooling rate results from the higher coverage rate of grain boundary precipitates and larger width of precipitate free zone. © 2015, Central South University Press and Springer-Verlag Berlin Heidelberg.

Huang X.,Central South University | Pan Q.,Central South University | Li B.,Central South University | Shi Y.,Central South University | And 2 more authors.
Zhongnan Daxue Xuebao (Ziran Kexue Ban)/Journal of Central South University (Science and Technology) | Year: 2015

The stress corrosion cracking resistance, intergranular corrosion resistance, exfoliation corrosion resistance, mechanical properties, electrical conductivity and microstructure of Al-Zn-Mg-Zr alloy containing Sc after two-step ageing treatment were investigated by means of slow strain rate tention(SSRT), intergranular corrosion, EC experiments, room temperature tensile test, hardness and electric conductivity measurement, and transmission electron microscopy. The results show that the ultimate tensile strength, yield strength, elongation and electrical conductivity of the alloy are 553 MPa, 534 MPa, 12.0% and 22.3 MS/m, respectively after duplex aging (120 ℃/6 h+140 ℃/20 h). After the two-step ageing treatment, crystal dispersed within the metastable strengthening phases η′-MgZn2, and grain boundaries are not continuous equilibrium phases η-MgZn2, accompanied by a certain width PFZ, which reduces the tendency of intergranular corrosion and contributes to an evident improvement in the exfoliation corrosion resistant and stress corrosion cracking resistant, compared with single step aging treatment. The level of exfoliation corrosion is moderate exfoliation corrosion (EB). © 2015, Central South University of Technology. All right reserved.

Yan J.,Central South University | Pan Q.L.,Central South University | Li B.,Central South University | Huang Z.Q.,Central South University | And 3 more authors.
Journal of Alloys and Compounds | Year: 2015

The hot deformation behavior of Al-6.2Zn-0.70Mg-0.3Mn-0.17Zr alloy was studied using hot compression tests over deformation temperature range of 623-773 K and strain rate of 0.01-20 s-1. The flow stress behavior and microstructural evolution were observed during the hot deformation process. The results show the flow stresses and microstructure evolution are sensitive to deformation parameters. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by the Zener-Hollomon parameter Z in the hyperbolic sine equation with the hot deformation activation energy of 178.85 KJ/mol. The dynamic flow softening for the alloy is accounted for dynamic recovery and dynamic recrystallization. Under conditions of high ln Z values, only dynamic recovery occurred, and the main soften mechanism transformed from dynamic recovery to dynamic recrystallization at low ln Z values. With decreasing the value of ln Z parameter, the size of recrystallization grain becomes larger. According to the TEM evolution, it can easily observed that high densities of the fine and nano-scaled Al3Zr particles have precipitate in the aluminum matrix and grain boundary, which can effectively inhibit the dynamic recrystallization of experiment alloy. Based on dynamic material model and Prasad' instability criterion, the processing maps for the alloy are built at true strains of 0.3 and 0.5. The processing map at the strain of 0.5 exhibits the optimum processing conditions are in deformation temperature range from 703 K to 773 K and strain rate range from 0.03 s-1 to 0.32 s-1 with the maximum efficiency of 33%. © 2015 Elsevier B.V. All rights reserved.

Huang X.,Central South University | Pan Q.,Central South University | Li B.,Central South University | Liu Z.,Guangdong Fenglu Aluminium Co. | And 2 more authors.
Journal of Alloys and Compounds | Year: 2015

2.5 mm thick cold-rolled Al-Zn-Mg-Zr alloy plates with trace amount of Sc were subjected to metal-inert gas (MIG) welding with Al-Mg-Sc as the filler metal. The influence of Sc additions on microstructure and mechanical properties of the welded joints was investigated by means of optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Vickers micro-hardness test and room temperature tensile test. Experimental results indicated that trace amount of Sc had significant effect on microstructure and mechanical properties of the joints. In both cases, base metal consisted of a typical fiber structure, different width of the equiaxed zones (EQZs) with fine grains were observed in the weld metal at the fusion boundary, fusion zones exhibited dendritic microstructure and average size of Al-Zn-Mg-0.10%Sc-Zr was 120 μm, while in Al-Zn-Mg-0.25%Sc-Zr, grain size decreased drastically to 50 μm. The grain refinement is mainly caused by the extremely fine, coherent Al3(Sc, Zr) particles with L12 structure, which act as heterogeneous nucleation. The observed grain refinement resulted in an appreciable increase in fusion zone strength and micro-hardness. Tensile strength of Al-Zn-Mg-0.25Sc-Zr alloy welded joint was 460 MPa, approximately 11% higher than Al-Zn-Mg-0.10Sc-Zr alloy welded joint. Micro-hardness in the center of fusion zone rose from 95 HV to 110 HV. The main strengthening mechanisms of Al-Zn-Mg-Sc-Zr MIG welded joints are precipitation strengthening derived from η′ precipitates, dispersion strengthening and fine grain strengthening caused by coherent secondary Al3(Sc, Zr) particles. © 2015 Elsevier B.V. All rights reserved.

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