| Micro scale laser shock peening (μLSP) has been proposed to improve the fatigue durability, corrosion and wear resistance of metals with laser beam diameter at the order ofμm, pulse duration at ns and laser energy at pJ, during which shock wave pressure generated by the interaction between laser beam and material can introduce moderate residual stress distribution and enhance surface nano-hardness and elastic modulus. It fundamentally resolves the failure problem of micro metal components. Nevertheless, up to now, problems of scale effect, response of mechanical properties, efficient characterization of surface morphology, micro strengthening mechanism and technological criterion ofμLSP are still stacking. After detailed analyzing the strengthening mechanism ofμLSP, theory, numerical simulation and experiments were adopted to research the copper specimen treated byμLSP. The main work are as follows:(1) Theoretical analysis of the shock wave propagation in micro scale. According to differential geometry, detonation wave theory and stress wave basis theory,2-D shock pressure distribution was obtained.2-D pressure model was established based on the analysis of pulse laser-energy transfer medium-shock wave pressure model with high strain rate conditions and material property of nonlinear effects. Comprehensively considering properties of shock wave ellipsoidal propagation in time and spatial distribution and geometric features of ellipse, ratio of pressure in axial direction and radial direction K(t, r) were put forward, and spatial pressure distribution in radial direction at different time was derived.(2) Numerical simulation of copper treated byμLSP. Based on ABAQUS software, the finite element models ofμLSP under high strain ration condition was established. The special analytic module forμLSP was generated. The influence of material anisotropy was considered, material model for single crystal was introduced. Surface dynamic stress and plastic deformation under single spotμLSP were analyzed, and influences of processing parameters on residual stress were researched. Meanwhile, simulation of multi-spotμLSP was also conducted. Typical paths with profile across laser spot centre and 0.5R away from laser spot centre were selected to characterize the distribution of residual stress and plastic deformation according to characteristic of overlapping circle spot. Residual stress and plastic deformation distribution at surface and along depth direction were explored, and the influence of processing conditions on residual stress and surface morphology were also discussed.(3) Technical process control and parameters optimization. Shaper factorσSF was introduced to precisely describe shape feature of residual stress under different crystal orientations. Taguchi method of Minitab software was used to carry out experimental design according to derived simulated results, the characteristics of residual stress measured under various processing parameters were extracted and treated, the influence degree were determined by the analysis of signal-to-noise ratio and variance, which provides good guidance for optimizing the processing parameters and controlling residual stress duringμLSP.(4) Experiments ofμLSP were conducted. After measuring surface profile and roughness of single spotμLSP region, influence of processing parameters on copper plastic deformation were analyzed. Mechanical properties in single shot region, enhancement mechanism ofμLSP on copper nano-hardenss and elastic modulus afterμLSP were explored by discussing the measured nano-hardenss, elastic modulus and contact depth inμLSP region. Key factor of surface performance was proposed. Multi-spotμLSP experiments were conducted to study the influence of main processing parameters (laser energy and overlapping rate) on residual stress and surface morphology. Appropriate processing parameter and processing rule were obtained under different conditions.(5) The microstructure evolution of copper and micro strengthening mechanism in plastic deformation region treated byμLSP was investigated. The grain size and microstructure of copper in depth direction were observed by transmission electro microscopy (TEM) and optical microscope (OM) technique. The microstructure evolution mechanism of copper was investigated based on the analysis of microstructure change under multipleμLSP treatment. Grain refinement mechanism and micro strengthening mechanism was proposed systematically, and the effects of microstructure in strengthening treatment were discussed.The researches enrich the basic theory of pressure induced by laser, which is theoretical significant for microstructure evolution in dynamic plastic deformation under high strain rate, and also provide the helpful reference and guidance to evaluation of enhanced result. |
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