Papers by Author: Qiang Zhang

Abstract: Aluminum syntactic foams were fabricated by pressure-infiltrating liquid pure aluminum into packed preforms of cenosphere fly ash. The morphology, true density and porosity of fly ash microballoons were characterized. The microstructure of the syntactic foams demonstrated uniform distribution of the microballoons in the aluminum matrix and seldom infiltration of cenosphere fly ash. These foams were subjected to quasi-static uniaxial compression tests and behaved like high strength aluminum foams under compressive deformation, exhibiting an extended plateau region in the stress–strain curves. With the decreasing of fly ash diameter, the plateau stress and absorbed energy of the syntactic foams increased. X-ray microcomputed tomography was used to examine the foam microstructures after interrupted compression and reveal the damage evolution. The current work provides a better understanding on the structure and mechanical properties of aluminum matrix syntactic foams.

Abstract: The effect of magnetic fields on the molecular configuration of liquid crystalline polymers (LCPs) under simple shear flows is numerically simulated using the Doi theory when the initial state of the director is out-of-plane. A method of effectively enhance the order parameter is discussed in this paper. The simulation results show that the degree of molecular alignment can be greatly increased along the direction of the magnetic field. It is an efficient way to improve the performance of LCP materials.

Abstract: The effect of magnetic field on the flow-phase diagram of discotic nematic liquid crystalline polymers (LCPs) is analyzed with the extended Doi theory in which the molecular shape parameter is defined at negative one. The evolution equation for the probability function of the LCP molecules is solved without any closure approximations. The transition among flow-orientation modes, such as tumbling, wagging and aligning defined similar to the rodlike LCPs, are strongly affected by the magnetic fields parallel to the flow direction.

Abstract: Synchrotron X-ray microtomography (SPring-8, Japan) has been used for the microstructure characterization in a closed cell Al-Zn-Mg alloy foam. Some sophisticated microstructure features, such as micropores and intermetallic particles inside the cell wall, were visualized and quantified three dimensionally(3D) by the high-resolution phase contrast imaging technique. By microtomographies tuned to energies above and below the Zn K-absorption edge, the 3D quantitation of Zn distribution was obtained using subtraction imaging technique. It has been clarified that the Zn distribution was inhomogeneous in the cell wall of the foam. And the agglomeration of Zn-bearing particles was confirmed to induce the brittle fracture of cell wall. The distributions of Ti and Ca in the foam were also visualized by subtraction method. The current tomographic techniques provide novel solutions for the 3D microstructure analysis in the highly inhomogeneous foam materials.

Abstract: Maxwell 2D software is introduced in this paper to calculate the magnetic shielding effectiveness (MSE) properties of iron plate. The three-dimensional magnetic shield is thought isotropic and simplified as two-dimensional model to study its MSE properties by the finite element method. In this method, a uniform magnetic field is generated by two huge magnets and the MSE properties of iron plate, which is in the centre of the uniform magnetic field, is then calculated by the ratio of magnetic field intensity after and before magnetic shielding. All the results indicate that shape of shield materials affects the MSE properties much and the MSE properties of shield with square and circular shape with 3mm in depth are 39.3 and 53.5 dB, respectively. That means the shield shape with fewer bending is favorable to the conductivity of magnetic energy. It also shows that the MSE value decreased linearly with the distance between the magnetic shield and the centre of the magnetic field. That is, the increase of side length of magnetic shield will lead to the decrease of MSE properties of iron plate, which is agreement with the theoretical prediction of Lu H.M. model. Furthermore, the MSE properties of double layers shielding (iron plate with 2mm in depth and 3mm iron plate with 81% porosity) are also studied in this paper. The effect of places of iron plate with 2mm in depth is presented to play important role in double layers shielding and the MSE value increases with the distance between the two magnetic shields. Compared to that of shield with circular shape, the MSE properties are similar to each other when the distance of the two shields is 8mm. In addition, it also indicates that the MSE value is higher when the iron plate with 2mm in depth is inside of the other than that when it is outside.

Abstract: An inexpensive fly ash (FA), which is from a waste production, has been employed to fabricate fly ash/epoxy composites in our work. Three kinds of fly ash with the most probable diameters of 74"m, 119"m and 146"m were filled in the modified epoxy resin (EP). The purpose of this study is to characterize the dynamic mechanical properties of such composites, and the dynamic mechanical behaviors of the composites are investigated in the temperature range from -40 to 150oC using a tension-compression mode. The results indicate that the dynamic elastic moduli for the fly ash/epoxy composites are (1.4~2.0) GPa, and the peak values of loss factor (tanδ) for these composites can reach (0.79~0.90) in the test specification. In addition, a scanning electron microscope (SEM) has been used to observe the distribution of fly ash particles in the matrix, as well as the photographs of fracture surface of composites.

Abstract: High reinforcement content TiB2/2024Al composites (Vp=55, 65%) were fabricated by squeeze casting technology, and their microstructures as well as mechanical properties under quasi-static and dynamic loading were evaluated. For 55 vol.% TiB2/Al composite, the bending strength and elastic modulus were as high as 623.5MPa and 218.1GPa. When compressed at a strain rate of 1050s-1, both composites exhibited a higher compressive flow stresses and compressive moduli than those under quasi-static loading. The micro-damage of high reinforcement content composites was mostly dominated by the large particle cracking. In addition, some evidence of aluminum alloy melting was observed on the fracture surfaces of dynamic compression, it was ascribed to the adiabatic heat accumulated in a local region transferred by plastic work.

Abstract: In the present work, a SiCp/Al composite was fabricated by pressureless infiltration of aluminum alloy into loose-packed SiC particles preform, and its microstructure and hardness performance were investigated. The results showed that the composite was fully infiltrated and the particles were distributed uniformly in the composite. Interfacial reactions were found in the as-cast composite and the reaction product was identified as MgAl2O4 by TEM observation and XRD analysis. The interfacial reactions enhanced the wettability and promote the spontaneous infiltration process. The thermal exposure process increased the Brinell hardness of the composite. After the thermal exposure process, the block-like interfacial reaction products were distributed discretely, but the amount of the reaction products was increased.

Abstract: Aluminum matrix composites, reinforced by 0.15μm and 5μm Al2O3 particles with 40% volume fractions were fabricated by squeeze casting technique. The microstructure characterization near the interfaces of Al2O3p/1070Al composites was investigated by SADP and HREM techniques. Results showed that high-density dislocations were generated in the 5μm-Al2O3p/Al composite due to the thermal mismatch stress. In contrast, the matrix of the 0.15μm-Al2O3p/Al composite appeared to be nearly free dislocations and some “micro distortion areas” of 1-5nm were observed, which was attributed to the dispersion of fine sub-micron particles and uniform distribution of the stress near the interfaces.

Abstract: Two Al-Si alloys (Al-12Si and Al-20Si) and an industrial pure Al were reinforced with 70vol.% dual-sized SiC particles. The composites experienced annealing treatment, to investigate the effect of silicon addition and thermal history on the thermal expansion behavior of high SiC content aluminum matrix composites. The results showed that silicon additions led to a beneficial reduction in the coefficients of thermal expansion (CTEs) of the composites. In the temperature range between 20°C and 400°C, a continuous increase in CTEs with temperature was observed for SiCp/pure Al composite. However, the CTEs of SiCp/Al-12Si and SiCp/Al-20Si showed the maxima at 350°Cand 250°C respectively, then diminished at higher temperatures. This was related to the change of solid solubility of silicon in aluminum at elevated temperatures. The thermal expansion behavior of SiCp/Al composites was also influenced by thermal history. After annealing treatment, the CTEs were reduced when compared with those of as-cast composites. Annealing treatment reduced the original thermal residual stresses, and then altered thermal expansion behavior of the composites.