Papers by Author: Ji Chang Yang

Abstract: The principle of laser peen forming (LPF) is introduced, the loading model of laser shock wave is established. This paper focuses on applying finite element analyses, instead of a complicated experimental procedure, to predict the development, magnitude and distribution of residual stresses induced by laser impacts on a metal plate, and dynamic process of laser continuous peen forming is realized with the FEM code ABAQUS. Based on the numerical analysis, the laser processing parameters can be optimized and the deformation contour of metal plate can be analyzed. The results calculated by the finite element method are correlated well with the available experimental results. The simulated results also reveal that adjusting the laser energy appropriately can result in an anticipated shape of plate in LPF process.

Abstract: Micro-flowing technique gained popular applications in microdevices of microelectromechanical systems (MEMS), and the performance of micro-devices is greatly determined by the properties of micro-flow. This paper studied the characteristics of different viscosity fluid flowing over microchannels with different diameters and lengths under low pressure driving, and the influence of scale effects on the flowing characteristics of low viscosity fluids was also examined. The experiments studied the flow rate–pressure characteristics of distilled water flowing over microchannels with diameter of 13 μm, 20 μm, and silicon oil flowing over microchannels with diameters of 50 μm, 100 μm. The results indicate that, when the diameter of micro-channel is more than 20 μm, the flowing characteristics of distilled water and silicon oil agrees well with conventional flow theory, and when the diameter of microchannels is 13μm, the flowing characteristics are related to the length of micro-channel. When the length is relatively shorter, the flowing characteristics are almost in agreement with the conventional flow theory. When the length reaches 100 mm, the flow rate is much higher than the values predicted by theoretical calculation when the length reaches 100 mm. It is obvious that scale effect arises when the length arrives to 100 mm and the velocity slippage results in the great increase of flow rate.

Abstract: Laser peen forming of sheet metal is a new plastic forming technique based on laser shock waves, which derives from the combination of laser shock processing and conventional shot peening technique, it uses high-power pulsed laser replacing the tiny balls to peen the surface of sheet metal, when the laser induced peak pressure of shock waves exceeds the dynamic yield strength of the materials, the sheet metal yields, resulting in an inhomogeneous residual stresses distribution in depth. The sheet metal responds to this residual stress by elongating at the peened surface and effectively bending the overall shape. On the basis of analyzing the mechanism of laser peen forming, the line-track-peening experiments of 45 steel sheets with 2 mm thickness were carried out; a curved sheet metal with deep layer of residual compressive stress was obtained. The preliminary experiment result shows that laser peen forming can offer desirable characteristics in shaped metals and is a valuable technique for producing components for a range of industries.

Abstract: In terms of the principle and characteristics, this paper proposes a new method to implement laser shock forming for large region based on the STL model. The method allows the build-up of laser shocking track to be accomplished automatically and therefore resolves the problem of continuous machining in course of the laser shock.

Abstract: MS2(M=Mo, W, S=S, Se) are often used as solid lubricants in high/low temperature, high load, vacuum, in which oil lubrication is failed. Although WS2 is much more expensive than MoS2, it has better high temperature properties than MoS2. Thermal decomposition method is used to produce fullerence-like WS2 nanotube on Al2O3 template using (NH4)2WS4. The fabricated WS2 nanotube’s diameter is about 100nm and its length is longer than 2µm.The morphology, microstructure and tribological behavior of WS2 nanotube are investigated by means of SEM, TEM, XRD and AFM. The friction force between Si3N4 probe of AFM with WS2 nanotube is lower than that with MoS2 powder. Similar results are also found in pin-disc tester by GCr15 pin and lubricants coating disc. The results of HETEM show that WS2 nanotube has rolling debris between the two antagonist surfaces, and MoS2 powder only has flattened debris. It may be the fullerence structure brings the chemical stability and leads to low friction.