Papers by Author: Jun Wei

Abstract: Friction Stir Processing is a solid state process with the ability to modify microstructure and refine grain sizes of the material without melting and uniformly disperse reinforcement particles in the material matrix resulting in further improvements in the mechanical properties. In this study, it was used to disperse Al₂O₃ reinforcement particles of different sizes. Uniform dispersion of the reinforcements was achieved in the aluminium matrix with significant reduction in grain size were observed via SEM and EBSD. Improvement in Vicker’s micro hardness was observed after FSP.

Abstract: A fiber Bragg grating (FBG) temperature sensor with high pressure applied in down-hole is experimentally studied. The relationship between the central wavelength and the temperature and pressure of the fiber Bragg grating is derived. The experimental results show that the packaged FBG temperature sensor has improved linearity and good repeatability. Its linearity is more than 0.999. The good linearity of temperature and wavelength is also kept in the high pressure when the compression and decompression were carried out under the pressure of 0-20 MPa and temperatures of 0-100 °C. Besides, the structure is simple. It can be widely applied in down-hole and other occasions

Abstract: Direct metal thermocompression bonding is one of the key approaches used in creating interconnections in many heterogeneous devices. It has been reported that by coating a monolayer of alkanethiols on metallic surfaces such as gold or copper prior to bonding, the bonding temperature required for forming joints can be significantly reduced. In this paper, room temperature copper bonding is demonstrated successfully with the help of the organic monolayers. We also found that all alkanethiol- coated copper (CnH2n+1SH, at n = 6, 11, 18) exhibited superior bond strength (>25MPa) compared to that of the uncoated copper (<23MPa) at bonding temperatures from 25°C to 80°C. Further investigation shows that bond strength of copper joints increases with alkanethiol chain length (C18>C11>C6), which contradicts our previous finding in gold. We attribute this discrepancy to the difference in hardness between the two substrates.

Abstract: In this paper, the local approach based on the Weibull stress criterion was used to investigate the interfacial fracture behavior between LX88A coating and Q345 steel. LX88A coating was deposited by high velocity electric arc spraying technology (HVAS). The finite element method (FEM) was used to analyze the stress-strain fields of the coating specimen which consisted of three different specimen geometrics or modes of loading. It was found that the Weibull stress for all specimen geometries was almost identical under the same fracture probability when the interfacial fracture initiation occurred for different specimen geometries. It showed that the geometry dependence on the interface brittle fracture toughness data can be reduced through application of the local approach, and the local approach can be used to describe the interfacial fracture behavior.

Abstract: Mg-Y2O3 nanocomposites were synthesized by disintegrated melt deposition method (DMD). Tensile tests and constant stress amplitude fatigue tests were carried out on Mg nanocomposites with 0.5, 1.0 and 2.0 volume percentages of reinforcements. Yield strengths of the nanocomposites were found to increase with higher volume fraction of Y2O3. Both unreinforced Mg and the Mg nanocomposites were found to strain harden upon cycling. The hardening behaviour was more intense in the nanocomposites.

Abstract: In this study, both liquid and powder metallurgy techniques were employed to fabricate Mg nanocomposites. Comparisons were made on the physical and mechanical properties of Mg nanocomposites using these two different fabrication techniques. When similar weight fractions of carbon nanotubes (CNTs) were added to Mg using these two different techniques, the nanocomposites fabricated using powder metallurgy technique were found to be thermally more stable and has higher yield and tensile strengths than those produced using liquid metallurgy technique. The higher yield and tensile strengths were due to the presence of MgO particles which were pre-existent in the Mg powders. The MgO particles cause a hybrid reinforcement effect in the Mg-CNT nanocomposites, which surpasses the strengthening effect of the CNTs alone.

Abstract: In this study, lead-free composite solders were successfully synthesized, with varying amount of nanosized ZrO2 + 8 mol.% Y2O3 particulates incorporated into 95.8 Sn – 3.5 Ag – 0.7 Cu solder. These composite materials were fabricated using the powder metallurgy technique involving blending, compaction, sintering and extrusion. The extruded materials were then characterized in terms of their physical properties, microstructural development, thermal and mechanical properties. Experimental results revealed that with the addition of increasing amount of reinforcements, the density values of the composite solders decreased, but there was no influence on the melting point of the composite solders. Thermomechanical analysis of the solder nanocomposites showed that the use of reinforcements lowered the average coefficient of thermal expansion of the solder material. Moreover, the results of mechanical property characterizations revealed that the addition of reinforcements aids in improving the overall strength of the nanocomposite solder. An attempt is made in the present study to correlate the variation in volume percentages of the hybrid reinforcements with the properties of the resultant nanocomposite materials.