Basic Research On Laser Assisted Machining Of Silicon Nitride Ceramics

Due to the high temperature strength, high hardness, low coefficient of thermal expansion, oxidation resistance and chemical corrosion resistance, advanced ceramic materials can remain stable performance and high mechanical strength in poor working conditions. However, the good properties of materials led to the difficulty of processing and low.processing efficiency. Laser assisted machining is an effective composite processing technology machining difficult to machine materials, which uses laser to heat local workpiece changing the processing performance with the advantage of low tool wear, good quality and high efficiency. Theoretical and experimental investigations were conducted for widening machining ideas, improving processing quality and reducing part costs in this paper. The research includes the following contents:The study on absorptivity of laser on silicon nitride and heating process perform the foundation for future study. Laser absorption and heat conduction process in laser assisted machining process were investigated, the effects of operating parameters on theoretical temperature value were analysed through multi-factor orthogonal analysis and empirical models were established.The absorptivity of silicon nitride for YAG laser was obtained by fitting simulation and experimental verification study. Combined with heat transfer theory, temperature field and stress field finite element models for laser assisted turning and laser assisted milling of silicon nitride were build and the model were verified by measuring test by infrared thermometer.The method of selecting operating parameters which influcing the machining results provided theoretical supports for the processing test. The temperature and stress fields were simulated by the established finite element model. Effects of operating parameters on the temperature of cutting zone and laser spot and the laser thermal stress caused by the incident laser were analysed. The method of parameters selection was proposed to achieve smooth processing of laser assisted turning and laser assisted milling for silicon nitride ceramic.Experiments on laser assisted turning and laser assisted milling of sintered silicon nitride were carried on to provide experimental foundation for the technical application. Laser assisted turning and laser assisted milling experimental system was established. "PC + motion control card" form was used in laser assisted milling system, in which milling machine and laser were combined, CNC system was developed to achieve the moving control, laser control, path and temperature monitoring. Laser assisted turning and laser assisted milling of sintered silicon nitride ceramics were performed using preselection parameters. The process was characterized in terms of cutting force, tool wear, chip morphology, surface roughness and subsurface damage for a variety of operating conditions. Material removal mechanisms were inferred from scanning electron microscopy observations of the chips and surface microstructure. In the process of laser assisted milling for silicon nitride ceramics, edge chippings were formed due to the sudden impact of the tool on the workpiece. The edge chipping fracture model was built to simulate the formation process of edge chipping. Based on the model and experimental study, the mechanism of edge chipping and the method of reducing edge chipping was investigated.The technology of laser assisted milling developed a new direction for the application in machining complex shape parts. Based on the characteristic of temperature distribution and cutting process, the feasibility of continuous path milling was investigated, a continuous path milling assisted laser heating system was established, a method of NC code conversion was found as well. Operating parameters for laser assisted milling of continuous path were optimized through temperature field analysis and continuous line and curve grooves were machined in silicon nitride ceramics. The good surface quality of workpiece proved the feasibility and a good prospec of the technology.