Papers by Author: Qing Fen Li

Abstract: Dental diamond bur is now a regular rotary tool, with its head made of diamond particles embedded into nickel coating, and its shank made of stainless steel. There are strong demands from the dentist on prolongation of usage life and avoiding of breakage. To solve this problem, on the one hand, since diamond is hard to be wetted under the condition of normal temperature and pressure due to the high interfacial energy between diamond and general metals and alloys. Diamond particles coated with titanium layer was used for the preparation of composite electroplating with the intention of improving the interfacial adhesion between diamond and metal matrix; on the other hand, superelastic biomedical NiTi alloy was used as the substrate to improve the flexibility and prevent the breakage. In this study, the optimal preparation parameters of Ni/surface-modified diamond electroplating were determined by orthogonal test, and the bonding conditions between the diamond particles and the NiTi alloy substrate were studied by scanning electron microscope. Further performance comparison of Ni/modified and Ni/un-modified diamond composite electroplating was conducted on a pin-on-disc wear machine under the dry sliding condition, and the material removal volume was used as the evaluating criterion of wear resistance. The results showed that the binding strength between diamond particles and NiTi alloy substrate could be enhanced, as well as the wear resistance, which may give direction on the future design of dental bur.

Abstract: In this paper some results of 3D-finite element analyses of a modified CTS-specimen with an inclined crack plane are presented. It will be shown, that through the inclination of the crack plane, even under pure tension loading of the specimen, a superposition of all fracture modes I, II and III can be generated along the straight crack front of the inclined crack plane. Furthermore, mixed-mode I, II and III loading conditions can also be generated if this modified CTS-specimen is subject to an in-plane tension/shear loading. The computational fracture analysis is based on the calculation of separated energy release rates GI, GII and GIII along the crack front by the numerically highly effective modified virtual crack closure integral(MVCCI)-method and for the finite element(FE)-modelling the commercially available FE-code ANSYS is utilized.

Abstract: Characteristics of deformation-induced transformation (DIT) in the refractory low alloy steel 2.25Cr1Mo were experimentally studied. Effect of different controlled-rolling and controlled-cooling process on the steel microstructure and mechanical properties were investigated and the mechanism was discussed. Results show that the grain size and the ferrite volume fraction were obviously affected by the rolling and cooling processes. Proper DIT technique may significantly accelerate the transformation of austenite to ferrite in the steel and improve the steel strength.

Abstract: The microbiological influenced corrosion (MIC) behavior of a marine pipeline Cu-Ni alloy in the sterile seawater and sulfate-reducing bacteria (SRB) solution was investigated. Results show that severe pitting corrosion appeared on the specimens in the SRB solution. The corrosion potential of specimen in the SRB solution was much lower than that in the sterile seawater and the polarization resistance of specimen in the SRB solution decreased quickly after a period immersion and became much lower than that in the sterile seawater. Besides, the results of EDS and XRD show that the content of element Ni and Fe of the Cu-Ni alloy decreased greatly and the high content of element S appeared after 30 days immersion in the SRB solution. It was concluded that the SRB accelerated the corrosion process of the Cu-Ni alloy greatly. The MIC mechanism of the alloy in marine environment is discussed.

Abstract: An expert system (ES) prototype in structural component design based on fracture mechanics analysis was developed recently. The system consists of several main functional parts. Among them, the inference engine is the control structure of ES which allows the expert to use searching strategies to test different hypotheses and arrive at ES conclusions. A general inference engine which can perform different inference by loading up different knowledge bases and a set of track explanation mechanism which ensure the inference process visible and reliable were established and discussed in this paper.

Abstract: Fracture behavior and the microstructure of Ti3AlC2 ceramics prepared by SHS/PHIP method were studied. Stress-strain curves at different temperature with a strain rate of 1×10-3 s-1 were obtained. Fracture toughness, flexural strength, crack propagation behavior and compressive deformation of specimens were investigated. Results show that the microstructure of the large size Ti3AlC2 ceramics prepared by SHS/PHIP method has typical layered feature of ternary carbide compound. The bridge-link phenomena induced by the flaky grains occurred in three-point bending test. It restrained the crack propagation and improved the fracture toughness of the material. Cylindrical specimens under axial compression usually smashed into chips for most of the ceramics materials, however, for the Ti3AlC2 ceramics prepared by SHS/PHIP, shear fracture along 45º incline of the specimen occurred at room temperature, and bulging deformation without any crack exhibited when temperature was high. It is concluded that the Ti3AlC2 ceramics prepared by SHS/PHIP has better fracture-resistance properties.

Abstract: The solute segregation to grain boundaries may be classified into equilibrium and non-equilibrium segregation. The models and kinetics calculation equations were proved in previous work. However, the computational task for grain-boundary segregation kinetics process is complex and cumbersome as it can involve a vast amount of numerical data. It is therefore necessary to develop an easily usable computational program which can provide the researchers with a powerful tool in grain-boundary segregation kinetics process analysis in addition to having a sound theory. A computational program of non-equilibrium grain-boundary segregation (NGS) kinetics of solute is therefore developed in this paper. It includes programs for critical time calculation, effective time calculation and diffusion coefficients calculation, the program of Auger Electron Spectroscopy test data disposal, the program of curve fitting and the program of NGS kinetics simulation. A simulation example by using the computation program of NGS kinetic equations is in good accordance with the experimental observation of phosphorus in steel 12Cr1MoV. The computational program of NGS is therefore proved to be appropriate and helpful.

Abstract: In this study, the effect of combined environmental factors such as ultraviolet ray, high temperature and high moisture on mechanical and thermal analysis properties of glass fabric and phenolic composites are evaluated through a 2.5KW accelerated environmental aging tester. The environmental factors such as temperature, moisture and ultraviolet ray applied of specimens. A xenon-arc lamp is utilized for ultraviolet light and exposure time of up to 3000 hours are applied. Several types of specimens - tensile, bending, and shear specimens that are warp direction and fill direction are used to investigate the effects of environmental factors on mechanical properties of the composites. Mechanical degradations for tensile, bending and shear properties are evaluated through a Universal Testing Machine (UTM). Also, storage shear modulus, loss shear modulus and tan δ are measured as a function of exposure time through a Dynamic Mechanical Analyzer (DMA). From the experimental results, changes in material properties of glass fabric and phenolic composites are shown to be slightly degraded due to combined environmental effects.

Abstract: The martensitic transformation behavior and shape memory effect (SME) have been investigated in a Ni-rich Ti29.6Ni50.4Hf20 high temperature shape memory alloy (SMA) in the present study. After aging, the transformation temperatures of Ti29.6Ni50.4Hf20 alloy increase obviously due to the precipitation of (Ti,Hf)3Ni4 particles. And the transformation sequence changes from one-step to two-step. When the experimental alloy is aged at different temperatures for 2h, the transformation temperatures increase rapidly with increasing the aging temperature and then change slightly with further increasing the aging temperature. Most of the martensite variants preferentially oriented in the aged Ti29.6Ni50.4Hf20 alloy. The aged Ti29.6Ni50.4Hf20 alloy shows the better thermal stability of transformation temperatures than the solution-treated one because the precipitates depress the introduction of defects during thermal cycling. In addition, the proper aged Ti29.6Ni50.4Hf20 alloy also shows the larger SME than the solution-treated one since the precipitates strengthen the matrix strongly.

Abstract: In this paper, the anticorrosive performances of fastener coatings containing PTFE and/or nano aluminum particles were investigated by using electrochemical impedance spectroscopy, scanning electron microscope. The impedance spectroscopy was interpreted by means of analysis equivalent electrical circuits. The coating capacitance of the films was monitored with the immersion time to establish the water and ions permeability of these paint films. By comparison with clear coating, PTFE and PTFE- nano aluminum pigmented coatings, the results indicated that PTFE lowers the protective properties of the waterborne epoxy film although it can provide lubricating effect. The nano aluminum powder presented the best beneficial effect and resulted in a significant increase in corrosion resistance of the PTFE- nano aluminum composite coatings. The corrosion-resistant mechanism of the effect of PTFE and nano aluminum particles in the coating is discussed.