Materials Science Forum Vols. 532-533

Abstract: To improve the edge detecting effect of the pressed character, a novel method based on facet model and topographic structures is proposed. Firstly, the discrete gray scale image is approximated by a bivariate cubic function, then, the concerned features of the continuous function are computed to describe the discrete image. After every pixel in the image has been defined to Peak, Ridge, Saddle, Flat, Ravine, Pit or Hillside, the edge of the character is detected. To reduce the time in computing the coefficients of cubic function, an efficient separable algorithm is presented. In addition, an image enhancement method prior to edge detection is adopted to improve the detecting effect. The tests show that the proposed method is more suitable for complex edge detecting problems than common methods.

Abstract: The effect of filler shape and volume fraction on the micromechanical damage behavior of particulate-reinforced dental composites was investigated using the finite element method. Threedimensional unit cell models for various filler volumes and shapes, using hydroxyapatite in Bis- GMA as a model system, were used. Young’s modulus and stress concentration factor were calculated. The effects of filler shape on the ease of initiation of strain damage in the matrix and onset of particle-matrix debonding are discussed.

Abstract: Nanocrystalline diamond film is an excellent material for high-tech fields because of its excellent properties. In this paper, research was carried out on the synthesis and deposition mechanism of nanocrystalline diamond thick film in the self-made double bias hot filament chemical vapor (HFCVD) deposited system. Double biases including positive bias on the grid electrode on top of hot filaments and negative bias on the substrate were added to the HFCVD system. During the deposition, nucleation and growth stages were applied alternately to control crystalline size and improve quality. The mechanism of the deposition is discussed in detail in this paper. The positive grid bias increases the active, decomposition and ionization of hydrogen and methane molecules, while negative substrate bias helps positive carbon-containing ions bomb the substrate that leads to the high nucleation density of the diamond. Raman results show that the film prepared has high quality. And SEM results show that the film’s grain size is less than 100nm with thickness of 50μm.

Abstract: Accurately material constitutive model is essential to understand and predict machining process. High temperature split Hopkinson pressure bar (SHPB) test system is used to investigate flow stress behavior and characteristics of 7050-T7451 aluminum alloy. Specimens are tested from 200°C to 550°C at intervals of 50°C and room temperature, at strain-rate of 2800s-1. The experimental results show that flow stress is strongly dependent on temperature as well as strain rate, flow stress decreases with the increase of temperature, while increase with the increasing of strain rate. The material parameters are determined for both Johnson-Cook constitutive equation and modified Johnson-Cook constitutive equation. The modified JC equation is more suitable for expressing the dynamic behavior of 7050-T7451 aluminum alloy.

Abstract: The theory of composite material patch winding is proposed to determine the winding trajectory by a meshed data model. The bridge condition on the concave surface is considered in this work. This paper puts forward the judgmental principles and corresponsive solutions by applying differential geometry theory and space geometry theory. To validate the feasibility of the patch winding method, the winding control code is programmed. Furthermore, the airplane inlet winding experiments are carried out.

Abstract: Magnetorheological finishing (MRF) is a novel precision optical machining technology. MRF utilizes magnetic particles, nonmagnetic polishing abrasives in carrier fluid, and a magnetic field to finish optical materials. Owing to its flexible finishing process, MRF eliminates subsurface damage, corrects surface figure errors and the finishing process can be easily controlled by computer. To achieve deterministic finishing, it’s necessary to know the mechanism of material removal. Different magnetorheological fluids are used to finish optical glass on the same machining condition. The material removal and surface quality are examined after finishing with no polishing abrasive, aluminium oxide and cerium oxide. The results show that the hardness of polishing abrasive is not the main factors to affect material removal.

Abstract: The laser shocking to the Al2O3 ceramics was proceeded, and the fracture microphology that formed from the strong laser shock processing (LSP) was analyzed by the Scanning electron microscopy (SEM). It was discovered that the feature of ceramics responds differently when the laser energy was changed. The brittle fracture that consists of intergranular fracture and cleavage fracture was the main mode under high energy laser shocking (laser pulse enegry: 42J); the macroscopical fracture characteristic was the radial crack. When the laser energy reduced to a fit level (25J), the brittle fracture of ceramics appears to the characteristic of plastic deformation, its fracture microphology appears lots of slippage lines, and the macroscopical feature of radial crack under 42J become subulate crack. While the energy reduced to 15J, the Al2O3 ceramics did not fracture, its micro-hardness ascended, a feature of micro-plastic deformation was existed under the low energy. The reason of the brittle materials appears to the feature of plastic deformation was analyzed.

Abstract: Guide rolls play an important role in controlling both the ring circularity and the stability of cold ring rolling process. However, it is difficult to predict and control the motion of the guide rolls due to the complexity of process associated with the coupled effects of multi-factors. In this paper, a reasonable controlling model of the guide rolls is proposed, and the functional relationship between the motion track of the guide rolls and their setup parameters and process ones is established, by which the guide rolls motion track can be determined. On this basis, a 3D-FE simulation model for cold ring rolling is developed under the ABAQUS software environment and the effects of the initial position and motion track of the guide rolls on the forming stability, ring circularity, rolling force, and oscillating are investigated. Taking the forming stability and ring circularity as objects, the optimum initial position and motion track of the guide rolls are obtained.

Abstract: Based on the magnetorheological (MR) effect of abrasive slurry, this paper presents an innovative superfine machining method. In this technique, the particle-dispersed MR fluid is used as a special instantaneous bond to cohere abrasive particles and magnetic particles so as to form a dynamical tiny-grinding wheel. This tiny-grinding wheel can be used to polish the surface of brittle materials in millimeter or sub-millimeter scale. The characteristics of the machined glass surfaces examined by the scanning electron microscope (SEM) and the Talysurf roughness tester confirmed the effectiveness of the finishing technique. The machined surface with convex center and concave fringe demonstrates that the material removal process is dominated by the synergy of the applied pressure and the relative velocity between the abrasives and workpiece. In the case of glass finishing, the mode of material removal is found to be plastic, and controlled by the abrasive-wear mechanism.

Abstract: Cone spinning is an advanced but complex metal forming process under coupled effects of multi-factors. Understanding the deformation mechanism, i.e., the stresses, strains, and metal flow in the deformation zone during the process is of great significance for optimizing the spinning process and controlling the product quality. In this paper, based on ABAQUS/Explicit, a reasonable FEM model for cone spinning with a single roller has been established, and the features of stress, strain and wall thickness during the process have been obtained. The results show the following: (1) In the beginning, large stress, large strain and the acute thinning of wall thickness localize at the small region below the roller, then the region extends into a small ring, further it becomes a large ring, and finally the ring will become uneven if the wrinkling occurs in the flange. (2) After spinning, the acute thinning region locates at the midst of the wall near the bottom of the workpiece. (3) At earlier stage of cone spinning, as a result of the acute thinning of wall thickness in the wall zone, the unevenness of wall thickness increases sharply to a value, then it almost keeps the value at the stable stage, and finally it will slowly increase again if the wrinkling appears in the flange. The results are helpful for determination and optimization of process parameters of cone spinning.