Papers by Author: Xin Ming Zhang

Abstract: The hot deformation behaviors of 7A55 aluminum alloy were investigated by compression tests at temperatures ranging from 270°C to 450°C and strain rate ranging from 0.1s⁻¹ to 25s⁻¹. Tha rResults show that the flow stress increased with increasing strain rate and decreasing temperature. A two-stage constitutive equation was established and the hot deformation activation energy was 140 kJ/mol. EBSD observations show that the fine and equiaxed grains with the misorientation angle above 15° nucleated at the initial grain boundaries under high temperature and low strain rate conditions. It is concluded that the softening mechanism of 7A55 aluminum alloy is dynamic recovery (DRV), together with a partial dynamic recrystallization (DRX). The nucleation mechanism of DRX could be explained by the strain induced grain boundary migration (SIBM). The DRX softening model was established based on the dislocation density theory finally.

Abstract: The evolution of through-thickness strain gradients during snake rolling (SR), which introduces an horizontal offset between upper and lower rolls, of Al-Mg-Si-Cu sheets was investigated with the inscribed lines in side including rolling and normal direction of sheets. The complete pole figures were used to investigate the influence of such rolling on texture evolution. The results revealed that the SR rolled sheet developed a continuous through-thickness shear strain gradients and shear textures. The surface in contact with the slower roll (the lower roll) developed the largest shear strain and the strongest shear texture. While for the conventional symmetric rolling (CR), the sheet developed a small shear strain gradient which is symmetrical about the centerline of the sheet with nearly negligible shear strain and texture at the center of the sheet.

Abstract: The effect of severe plastic deformation at ambient temperature on microstructures and mechanical properties of aluminum alloy 2519 was investigated by means of tensile test, micro-hardness test, optical microscopy and scanning electron microscopy. The results showed that tensile strength of as-queched 2519 alloy was greatly enhanced to nearly 550MPa (ultimate tensile strength, UTS) and 520MPa (yield strength, YS) by severe cold rolling or equal channel angular processing (ECAP) while the elongation decreased to 5%. The 2519 alloy could obtain quite well mechanical properties as much as 80 % and 12 passes cold rolling deformation. This indicated that pre-deformation by ECAP is effective in improving the mechanical properties of 2519 alloy by grain refinement, strain aging and high density dislocations.

Abstract: Nano-indentation technique was used to survey creep stress exponent of ZM6 (Mg-2.8Nd-0.7Zn-0.6Zr) alloy at room temperature. The results showed that average press creep stress exponent of ZM6 alloy was about 89.75, and independent of strain rate and hardness, which has been verified by linear relationship of the double logarithmic plots between strain rate ( ε& ) and hardness (H ) measured by a nano-indentation equipment with constant load of 500mN.

Abstract: Microstructures and properties of rolled Mg-9Gd-4Y-0.6Mn（nominal wt%）sheets solution treated at 793K for 12h, and aged at various combinations of temperatures and time were investigated by hardness measurement, tensile test and TEM. The evolution of the precipitates and their effect on the mechanical properties of the pre-, peak-, and over-aged tempers were determined, and the optimal yield tensile strength (YTS) and ultimate tensile strength (UTS) were 276MPa and 287MPa at room temperature, respectively. It has been found that the addition of Mn into Mg-Gd-Y system contributes to block behavior of matrix diffuse and increase potential of heat resistance of the alloy.

Abstract: The effect of pre-rolling reduction prior to ageing on the size and distribution of the precipitates, the width of precipitation free zones (PFZ) along grain boundaries and intergranular corrosion (IGC) of aluminum alloy 2519A were investigated by hardness tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that the time for peak-age shortened when the reduction increased, which resulted in refining the precipitatates and distributing homogeneously within the grains and hence the IGC decreased. With increasing pre-rolling reduction, the PFZ along grain boundaries became narrower and precipitates changed from continuous chains to discrete distribution, which resulted in high IGC resistance.

Abstract: 2024-T6 Al alloy sheet s were modified by bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) silane film to improve the corrosion resistance. Fourier-Transform Reflection Absorption (FTIR-RA) spectroscopy was used for structural characterization of BTESPT silane film formed on surface of the sheet. Potentiodynamic polarization and immersion test in 3.5% NaCl solution were used for evaluating the corrosion performances of the silane film. The results showed that the film formed after curing at 120 °C for 40 min was cross-linked through Si-O-Si and that it was covered on the entire surface of the sheet. The content of elements S and Si on the Al2CuMg particles is a little higher that of on the matrix. The strong peak at 1032 cm-1 indicated that the film was linked to the sheet by Si-O-Al. Compared to the untreated case, the corrosion current density of the sheet treated with the silane film was reduced by close to 2 orders. Treatment of BTESPT silane can provide about 670 h protection of corrosion for the sheet in 3.5% NaCl water solution.

Abstract: 2519A aluminum plate was shot obliquely by an incendiary projectile with the diameter of 7.62mm at impact velocity of 818m/s. The penetration inhomogeneities of the crater of 2519A aluminum target plate were investigated with optical microscopy (OM), scanning electron microscopy (SEM). Melted substance, a mixture of the target material and the projectile was found at the interface of target and projectile. Different microstructural characteristics were observed along the crater depth. Grains were elongated near the entrance area, while severely kinked grains and adiabatic shear bands were present in the middle region, and microbands, as well as abnormal grown grains, appeared at the crater bottom. The failure form of target is mainly ductile expanding.

Abstract: The particle swarm optimization (PSO) algorithm is introduced into ghost correction, which is also compared with the NMS algorithm. With linear regression correlation factor as evaluation parameter, it is found that both algorithms have the same quality for model ODF, but when it comes to complicated textures, the PSO algorithm shows high ODF fitting quality. It is also demonstrates that the ghost peaks in the reduced ODF can be excluded out in the true ODF from PSO components with both even and odd terms in the series expansion method.

Abstract: The effect of forward extrusion temperature and rate on the formability and microstructure of 7075 aluminum alloy was investigated by means of scanning electron microscopy (SEM), optical microscopy (OM) and hardometer. The results showed that the outflow rate was in the range of 6~2m/min with the extrusion temperature from 340°C to 420°C. Higher temperature resulted in lower critical extrusion force, decreased permissible extrusion rate and obvious drop in the formability. Lower temperature resulted in improvement in the formability but higher critical extrusion force, which might lead to shuttingdown of the extruder. There were no obvious effects of extrusion temperature and rate on the microstructure and hardness of the as-extruded alloy. It suggests that dynamic recovery or partial recrystallization occurred in the alloy during extrusion.