| Not only owing to their superior thermo-physical properties such as high specific modulus and low thermal expansion, but also in the density, thermal conductivity and thermal deformation coefficient superior to that of SiC, SiC particles reinforced aluminum matrix (SiCp/Al) composites with high volume fraction containing more50%SiC particles are being widely used to replace the conventional materials such as glass, quartz glass and SiC material in space mirror material. Because of SiC particles and Al alloy matrix in SiCp/Al composites with distinct characteristics, compared to the monolithic alloys and ductile material, its processing is more difficult and its grinding mechanism is complex, which leads to much special surface morphology. Conventional surface roughness of machined surface is also difficult to give reasonable evaluation. Therefore, the studies on technology of the precision and ultra precision machining, machining mechanism and surface integrity have important theoretical and practical significance. Aiming to solve the precision and ultra precision grinding machining problems existing in processing SiCp/Al composites with high volume fraction, this paper makes a research on the precision and ultra precision grinding mechanism, surface defect characteristics, the evaluation methods of machined surface, high surface integrity formation mechanism and conditions, the main studies are as follows:1. The grinding removal model of SiCp/Al composites was established. The surface defects, chips morphology and type were observed with scanning electron microscope (SEM), and its formation mechanism was put forward. Based on indentation experiments and fracture mechanics theory, the effect of indentation depth, indentation position and the shape of SiC particle on crack formation of SiC particles have been investigated. And the ductile-regime removal mechanism and conditions of SiC particles were put forward. Three-dimensional nonlinear finite element model of ductile-regime grinding condition of SiCp/Al composites was established to analyze the influence of edge radius of grain and indentation depth on the ductile-brittle transition of hard and brittle SiC particles. 2. A series of experiments for the surface grinding process were conducted on SiCp/Al composites with high volume fraction by diamond grinding wheel, in order to investigate the effect of different grinding conditions (wet, dry, cryogenic and ELID grinding conditions) and various grinding parameters on the grinding force and grinding force ratio during grinding of SiCp/Al composites. These studies further revealed the grindability and removal mechanism of SiCp/Al composites.3. The surface formation mechanism of high integrity surface in ductile grinding under cryogenic and ELID grinding conditions was studied. The formation mechanism of high integrity surface was studied under cryogenic grinding condition on the basis the analysis of micro-area hardness, two-phase residual stress and interface combination status before and after freezing treatment. Micro edge and equal high properties of wheel grains, passive film and micro-cutting effect of grains revealed formation mechanism of high integrity surface under the ELID precision grinding conditions. The grinding mechanism and theoretical research of high integrity surface under cryogenic conditions is a new exploration.4. According to surface morphology characteristics and typical defects of the SiCp/Al composites, comparative analysis of characterization method of two-dimensional roughness and three-dimensional roughness, power spectrum density and surface fractal dimension were made in this paper. The objective characterization parameter system of SiCp/Al composites grinded surface was established. The three dimensional evaluation parameters of surface roughness and surface power spectrum density respectively indicate the amplitudes characteristics and spatial wavelength distribution characterization of SiCp/Al composites grinding surface. Based on the characterization system, machined surface quality of SiCp/Al composites was evaluated. The influence of grinding parameters and grinding conditions on machined surface quality was analyzed, which contributes to references and theoretical guidance for practical application. |
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