Papers by Author: Ding Fei Zhang

Abstract: Based on characteristics of direct extrusion for magnesium alloy seamless tube and continuous equal channel angular pressing (ECAP) of tubes, a new composite extrusion process of tubes including direct extrusion and many steps ECAP was invented firstly, which are shorten for TES process in this paper. A three-dimensional finite element thermo-mechanical coupled model and conditions for TES process were established. The extrusion process and cumulative strains evolution during TES process were simulated. The results show that TES process can improve the cumulative strains significantly. The microstructures observations of longitudinal sections for tubes fabricated by direct extrusion and TES process were carried out. It was found that when the extrusion temperature was 400°C, and extrusion ratio was 8.4 and the channel angle was 150°, TES process can refine the microstructures of tubes effectively.

Abstract: To investigate pinholes in 8079 Aluminum foil, optical microscopy and scanning electron microscope with energy dispersive spectrometer were adopted. Results show that four types of pinholes in 8079 aluminum foil, (1) small size, rod-shaped, (2) pinhole in groove, (3) hole with big size, (4) crumpled pinholes. Big compounds along with all rolling and heat treatment processes cause the small rod-shaped pinhole. The big compound contains elements of Al-Fe-O.

Abstract: This paper deals with a finite element model (FEM) for the extrusion of AZ31 alloy and extrusion experiments at different temperatures. The simulation is utilized to predict the extrusion force and strain at different temperatures. ES (extrusion-shear) process and direct extrusion are investigated at 370, 400 and 420°C to discuss the effects of different temperatures on the microstructures and mechanical properties with the extrusion ratio of 11.56. The simulation results obtained show that the extrusion force of ES process is much larger than direct extrusion because of the two corners. ES process could effectively refine the grains, especially the microstructure of the central region compared with direct extrusion. The bar extruded at 370°C shows the best microstructures and mechanical properties.

Abstract: A series of tensile tests of AZ61 magnesium alloy were conducted using Gleeble-1500 thermal-mechanical material testing system to learn the effect of the test temperatures and strain rates on the mechanical properties of the alloy. It is indicated that the higher the temperature, the lower the ultimate strength and fracture stress, and the larger the plasticity. It is also revealed that the larger the strain rate is, the higher the ultimate strength of the specimens will be, and the larger the plasticity of the specimens will be. The failure mechanism of the material under high temperature was also analyzed based on the fracture observation. It shows that the high temperatures will induce microvoids or microflaws in the material.

Abstract: The effects of strain rate (SR) and heating rate (HR) on the mechanical behaviors of the tensile specimens of magnesium alloy AZ61 were experimentally investigated using a Gleeble-1500 thermal-mechanical material testing system. It showed that the higher the temperature is, the lower the ultimate strength of the specimens will be. The higher the heating rate is, the higher the ultimate strength of the specimens will be. The metallurgraphs of the fracture section of the specimens were also experimentally investigated for exploring their failure mechanism under different temperatures and heating rates. It showed that the high temperatures and high heating rates will induce microvoids in the specimens. The microvoids make the specimens failure under relative low loads.

Abstract: In this work, the galvanic corrosion behavior of AZ31 magnesium alloy in contact with A6N01S-T5 aluminum alloy in NaCl solution was investigated by electrochemical method, the salt immersion method, SEM. And three protection techniques: (a) rubber gasket technique ,(b)organic coating technique and (c) anodizing technique were used to inhibit the galvanic corrosion. The results of open circuit potential and polarization curve method were explained the corrosion behavior of AZ31 and A6N01S-T5.It was found that AZ31 as anode was accelerated corrosion and A6N01S-T5 as cathode was also corrode in salt immersion experiment. With the increasing of immersion time, the corrosion rate of AZ31 and A6N01S-T5 was gradually decreasing. Then, the efficacy of different anti-corrosive technologies was compared. Fluorine coating technique was an effective way to protect Mg alloy and Al alloy. It was possible to guide the anti-corrosion of magnesium alloy in mobile industry.

Abstract: The mechanical response and failure of the specimens of magnesium alloy AZ61 with different heating rates (HR) and loading rates (LR) were investigated by a Gleeble-1500 thermal-mechanical material testing system. It was found that heating rate has markedly effect on the strength and plasticity of the specimens. The higher the heating rate is, the lower the strength and the smaller of the plasticity of the specimens will be. There is the relatively small effect of the loading rates on the strength and plasticity of the specimens. The metallographs of the failed specimens were also observed. It shows that there are many microvoids in the specimens near the fracture sections. These microvoids may come from the local thermal and stress inconsistency under high heating rate and loading rates and degrade the strength and plasticity of the specimens.

Abstract: The effect of different factors on the corrosion performance of the anodic film formed on AZ31 was studied by using electrochemical impedance spectroscopy (EIS) and Tafel polarization curves techniques. The result of EIS showed that the oxide film formed at 20 mA·cm^-2 had the highest corrosion resistance and the optimum temperature was 40°C. The similar conclusion was drawn by the Tafel polarization curves. Moreover, the equivalent electrical circuit R(C(R (QR))) (CR) was used to analyze the EIS data.

Abstract: This paper deals with an environment-friendly anodizing oxidation process for AZ31 magnesium alloy. Good anti-corrosion anodizing oxidation coating was prepared on the surface of AZ31 magnesium under 10 mA/cm2 constant current at 20°C after 20 min. The anodic film of AZ31 was ivory-white, dense, uniform, compact and porous structure. The anodic film of AZ31 mainly consisted of MgO, Mg2SiO4.According to Tafel curve and EIS,the anodic film of AZ31 had better anti-corrosion property than the substrate AZ31. This anodic film could increase the anti-corrosion property of AZ31 effectively.

Abstract: With the aim of evaluating corrosion inhibition for various inhibitors, a new qualitative method for corrosion inhibition effect test for magnesium and magnesium alloys was developed. The change of polarization current density of AZ61 magnesium alloy in electrochemical polarization experiments in alkali aqueous solution with 5mmol/L sodium dodecylsulphate(SDS), phytic acid(PA), ethylenediamine tetraacetic acid(EDTA), p-nitro-benzene-azo-resorcinol(PNBAR), acidum tannicum(AT) or stearic acid(SA) were tested. The SEM-EDS techniques and deposition experiment method were used for further confirmation of the corrosion inhibition. The results showed that those organic compounds which could form the inhibitor-magnesium precipitation in aqueous solution could be used as corrosion inhibitors for magnesium alloys to inhibit the increase of polarization current density as well as the dissolution and oxidation of magnesium alloys effectively.