Numerical Simulation And Experimental Study Of Microscale Laser Shock Processing On Metallic Target

Microscale Laser Shock Processing(μLSP) is one new technique importing shock wave induced by micro-level laser beam into metallic material to improve its performance. The fatigue life of metallic structure is prolonged by times through changing the stress distribution.The hardness and strength are increased remarkably and corrosion resistance is improved afterμLSP.The technique provides one feasible method for solving the problem of failure and reliability of micro electromechanical systems(MEMS).The selected sizes in micro level have higher efficency and flexibility when processing the unregular surface,such as channel,slot and hole.Firstly,the reason of choosing excimer laser and spot size of 20～200μm is briefly given basing on introducing the principle ofμLSP.The characteristic analysis ofμLSP is presented systematically,including microscale effect,elastic-plastic deformation theory, fluild theory,non-linear of material and the necessity of importing tangential shocked force.Next,the numerical simulation is investigated for existed liquid phase duringμLSP with confined medium.The influence of liquid phase on shocked effect is researched.The hydrodynamic model was established irradiated by pulse laser according the basic equations of mass continuity,momentum conservation and energy conservation,and numerical calculation for pressure is carried out.The results show the influence of liquid phase on peak pressure is existent,which need to be considered under the condition of accurate caculation for peak pressue.Then,the infinite element is importedμLSP calculation comparing with pure finite element.The judgement conditions of spall are considered for the near top and rear surface of target,especially the adhesive strength of interface between target and coating layer.The semi-infinite finite elment body model is assumed only in radial direction.The raidal shock force is inported;especially the 2-D plasma pressure model considering tangential force is presented firstly,which improves the simulation precision.The relax time is discussed for multiple impact.The influence of different parameters on shocked effect is analyzed and presented.The affected range and the fluctuation scope of stress increase with increasing laser radius;the plastic deformation is sensitive to laser intenstiy and increases with it.For the same laser radius,the pulse number has great influence on plastic deformaiton under larger intenstiy. And then,the shock wave propagation in metal material is simulated forμLSP.The effects of selected path mode & length,beam radius,laser intensity and pulse number on propagation rule are discussed systematically.The caculations show the selected path mode of node list or point list is appropriate,and the attenuation of stress wave is independent with the path length.For larger laser radius,the stress curves show the obvious propagaton property of wave;for smaller radius,the stress wave attenuates quickly.The stress peak increases with increasing intensity and pulse number before decreasing to HEL(Hugoniot Elastci Limit);the elastic-plastic wave attenuates to elastic one after the stress peak decreasing to HEL.Henceforth the stress peak is independent with pulse number and propagates as acoustic wave.The idea thicknesses of coating layer are presented for the parameters used in this thesis.Finally,the experiments are carried out for selected laser and beam sizes.The pure mechanical effect is proved through experiment for target with appropriate thick coating layer inμLSP.The pulse energy,pulse number and thickness of coating layer are the main factors of affecting the depth of plastci deformation.The surface and in-depth residual stress are improved afterμLSP,which enhances the surface hardness of the whole shocked region and the average hardness of all shocked area.The pulse number has great influence on surface hardness.The wearing and corrosion resistance of Al and Cu sample are improved after electroechemical measurement and thinning experiment.The researches enrich the theory and experiment investigation ofμLSP,which makes the study ofμLSP cover the whole micron scale and provides the helpful reference for optimal selection ofμLSP parameters on MEMS components.The investigation also establishes basis for the further research ofμLSP.