Study On The Mechanism Of Melt Ejection,Recast Layer And Micro Crack Formation In Millisecond Laser Drilling

Based on applications of millisecond laser drilling,phenomena of melt ejection,recast layer and micro crack during laser interaction with metallic materials are studied via numerical and experimental validation methods,as well as the corresponding physical mechanisms.The main research contents and conclusions are listed as follows:During the millisecond laser drilling process,the drilling hole is generated by pressure,induced by metal melting and evaporating,which pushed molten liquid away.Suffered from recoil pressure,surface tension and marangoni force etc.,transitions among solid,liquid and gas phases occur simultaneously at hole surrounding.In order to quantitatively analyze the complex millisecond laser drilling process,in the dissertation,a hydrodynamic mathematical model used for laser drilling considering influences of latent heats and acting forces is constructed according to improved fluid dynamics equations and level set equation.Next,to certificate the proposed hydrodynamic model,numerical calculations were adopted on typical metallic materials as steel and aluminum.Through analysis on melt ejection,quasi-quantitative relation between quantity of melt ejection and thermal diffusivity of material was acquired with the newly proposed concept of liquid layer.Moreover,quantitative observation on melt ejection as well as its deposition was experimentally studied.The results indicate that experiments fit well with numerical simulation,proving the accuracy and feasibility of the proposed hydrodynamic model.Moreover,the liquid layer is extracted during laser drilling on aircraft engine materials as titanium alloy.The thermal-mechanical coupling mechanism of recast layer is studied via quantitative analysis on both temperature and flow fields of liquid layer.Additionally,the relationship between thickness of recast layer and laser pulse width is obtained,which is verified by direct measurements by scanning electron microscope.In the process of millisecond laser drilling,the severe changes of temperature of workpiece usually produce thermal stress and then induce micro crack in recast layer and workpiece.In this dissertation,a two-dimensional axisymmetric thermo-elastic plastic mathematical model of millisecond laser drilling using level set method to capture liquid/gas interface is proposed according to the theory of thermal conduction,structural mechanics,material expansion and soften.Using the experimental certificated model,the depth and length of micro crack in recast layer are quantitatively obtained with high accuracy.In conclusion,the proposed model and experimental testing approaches are significant references for both numerical and practical analysis on laser drilling with long pulses.