Papers by Author: Yue Hui He

Abstract: t is challenging to achieve good surface quality and high efficiency simultaneously when machining thin film solar panels. In this study the machining characteristics of a multilayered thin film structure were investigated using diamond wire sawing and grinding. It was found that the efficiency in sawing was significantly greater than that of the employed grinding process, but the machined surface quality was much worse. The results indicated that grinding could still be the solution for such processing and sawing must be improved before this technology could be further progressed for machining thin film multilayered structures.

Abstract: A Fe-Cr-W-Mo-V overlaying alloy was prepared on Q235 steel by SMAW with different Fe-V powders added in electrode coating. Effects of vanadium on microstructure, hardness and wear resistance of the alloy were studied with SEM, OM, hardness and wear test. The results show that Fe-V powders content affects welding crack number, which increasing quickly at 0-2%(wt, as fellow), decreasing slowly at 2-20%, disappearing above 22%. With the increase of Fe-V, vanadium-rich carbides number increase, the grain size and hardness increase firstly then decrease above 2, at 20% gain the highest. Conversely, the wear loss decreases slowly first, then increases quickly above 2, wear resistance is the best at 20-22%.

Abstract: In-situ synthesized WC particles reinforced Fe-based composite coating was prepared by argon arc cladding on Q235 with mixture of tungsten powders, graphite and iron powders mainly. Microstructures of coating were investigated using SEM, EDS and XRD. Micro-hardness and wear resistance were also studied. The result showed that the main phases of coating were WC, Fe₃W₃C, martensite and residual austenite. The WC particles were well distributed in matrix and appeared significant growth and reunion from bottom to top of coating. Hardness of the coating ranged between HV_0.2 1102.2 to 1863.5. The wear loss of Q235 steel was 37.2 times than that of the coating.

Abstract: The microstructure evolution of TiAl intermetallics with different microstructures loaded under different strain rates and temperatures was investigated. The results showed that the deformation twinning dominated the deformation process under high strain rate, while dislocation slip was another dominating deformation mode under quasi-static loadings. The proportion of twinned grain increased with the increased strain rate. In Duplex TiAl, the plastic deformation was mainly found in equaxied grains and seldom found in lamellar grains.

Abstract: The microstructure evolution of Ti-46.5Al-2Nb-2Cr with different microstructure types loaded under a large range of strain rates and elevated temperatures is investigated by TEM. The results show that deformation twins are the main deformation mode under high strain rate loadings and both ordinary dislocation and super-dislocation are the additional modes under quasi-static loadings. The proportion of twinned grains increases with the increased strain rates.

Abstract: This paper reports the synthesis of porous titanium with a nominal composition of Ti-6 wt%Al-4wt%V through a press-and-sinter process. Blended elemental (BE) mixtures of Ti and master alloy Al-40V powders were uniaxially pressed and sintered in vacuum. Porosity of the sintered samples was determined in the range of 23vol. % to 37vol. % by the Archimedes method. Tensile strengths were found to range from 73 to 147MPa and Young’s moduli of the sintered samples varied from 3.4GPa to 13GPa. Both tensile strength and elastic modulus decreased with increasing porosity. Electrochemical assessment of the sintered porous samples showed deteriorated corrosion resistance, as compared to 95% dense Ti-6Al-4V prepared by sintering pre-alloyed powder. The challenge of using blended elemental powder sintering to fabricate porous Ti-6Al-4V alloys is discussed.

Abstract: The effect of temperature and strain rate on the mechanical behavior and microstructure evolution of Near Gamma Ti-46.5Al-2Nb-2Cr (NG TiAl) was investigated at temperatures ranging from room temperatures to 840 under strain rates of 0.001, 320, 800 and 1350s-1. The TEM analysis indicated that deformation twinning and stacking fault are the main deformation modes under dynamic loadings and dislocation slip is another important deformation mode under quasi-static loadings. The density of deformation twinning and/or stacking fault increases with the increased temperature and strain rate.

Abstract: The invention of functionally graded structure cemented carbide is a significant revolution. In this paper, graded structure cemented carbide with cubic carbide free layer (CCFL) was prepared. Using scanning electron microscope (SEM), metallographical microscope, electroprobe microanalyzer and microindentation, the gradient characteristics were investigated. The variation in elemental compositions from surface to inner is gradient, the concentrations of nitrogen and titanium are very low in surface layer, only element of tungsten, cobalt and carbon exist. A cobalt concentration peak occurs, which is higher than the average composition in bulk. Wherein binder phase piles up, its volume fraction is much higher than nominal value, resulting in a decrease in hardness and forming a tough layer.

Abstract: Carbon fiber reinforced CMCs (ceramic matrix composites) are promising high temperature structural materials for aeronautical and astronautical industries. But the engineering application of the materials strongly depends on the joining technique. As a fundamental research, high strength graphite can be used to simulate carbon fiber. The study of wettability of graphite by liquid metals contributes to the development of joining technique of carbon fiber reinforced CMCs. The wettability of graphite/Ni plus Cr system was investigated using the sessile drop method. The contact angle was determined by high temperature photography. The microstructure and composition of the interfacial zone were analyzed by SEM, EDX and XRD. The contact angle of graphite/Ni plus Cr system decreases with increasing the Cr content. It becomes zero, when the Cr content reaches 50wt%. The wetting of graphite/Ni plus Cr system is chemical wetting. The contact angle of the system decreases with increasing the wetting temperature or prolonging the holding time. The higher the Cr content, the more remarkable the effects of the temperature and the holding time on the contact angle are. Microstructure and phase analysis reveals that inter-diffusions and interfacial reactions take place in the wetting process. The reaction products include Cr3C2. The formation of Cr3C2 contributes to the interfacial bonding and results in the accumulation of Cr in the interfacial zone.

Abstract: Joining of ceramics is of importance from both technical and economical points of view. Brazing is a widely used process to join ceramics. In order to increase the working temperature and weld strength of joints, a high temperature brazing process using Ni-Cr-SiC powders (consisting of Ni, Cr and SiC powders) as filler to join recrystallized SiC ceramic has been investigated. The obtained optimized technological parameters are joining temperature of 1360°C, holding time of 5min and filler mass of 280mg. Under these conditions the maximum relative bending strength of joints, 70.5%, is achieved. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer, of which the major phase is Ni2Si, exists between the welding base material SiC ceramic and the filler reaction product layer, called as interlayer, of which the major phase is Cr23C6.