Research On Experiment And Numerical Simulation Of Micro Holes Drilling On Superalloy By Millisecond Laser

Laser drilling technology is widely applied in manufacturing industry with its unique advantages compared with traditional micro hole machining methods,especially in the aerospace,electronics,cars,and the biomedical sciences.Millisecond laser has become one of the most important drilling tools in the field of aerospace because of its large power,high machining efficiency,accuracy and Reliability.Based on the processing of gas film cooling hole on aero-engine hot-end components,this paper summarizes the application of micro hole in aero-engine and the significance of film cooling hole for aircraft engine.besides,it also introduce the research of millisecond laser drilling at home and abroad and the application of numerical simulation technique.Finally,the experiment and numerical simulation were carried out after the analyzing of the defects in the laser drilling process.The experimental material is super alloy GH4169 which was used commonly for aero-engine,the single factor experiments was carried out to research the effect of pulse energy,frequency,pulse width,defocus,beam expanded ratio and pulse number on the morphology,diameter and taper of holes.It shows that pulse energy,frequency,pulse width have a great effect on morphology of holes by changing the laser power density.Defocus and beam expanded ratio make an effect on drilling quality by controlling the focused condition.The shape of holes can be repaired by multiple pulses,and realized the control of diameter,taper and depth-diameter ratio.Due to the feature of high power and long pulse width,the spatter accumulated inevitably around the hole.A group of optimized parameters have been obtained: average power 90 W,pulse width 1ms,frequency 30 Hz,defocus 0mm,beam expanded ratio 4.The influence of millisecond laser thermal effect on metallographic structure and elementary composition has been detected in laser ablation zone and heat affected zone.Within the scope of about 200 micron from the holes edge,the grain size increases obviously,as thermal influence range extending from the edge of the hole to substrate,the grain size decreases and closes to substrate.Molten metal solidified in smaller super-cooling environment and the grains grown up abnormally,reduced the nucleation interface of ? phase which decided the alloy notch sensitivity,and led to notch sensitivity of recast layer.Laser thermal effect causes the result of approximate solid solution and aging on heat affected zone,and the transformation of strengthening phase ?? has occurred,therefore alloy strength decreased in this area.The component analysis indicated that the elements of Ni,Fe,Cr,Nb,Al,Si decreased in laser ablation zone and heat affected zone,but increased for O,Si.As a consequence of this,the strengthening performance of alloy degraded,and the stability of the organization and performance can't be guaranteed.Besides,the defects generated in millisecond laser drilling process such as micro cracks and spatter accumulation has been studied.The research shows that the micro cracks on the wall of holes can be reduced with the auxiliary gas such as nitrogen,argon gas,and the spatter accumulation can be reduced distinctly by the method of applying organic layer on process surface.In order to verify the reliability of experimental regularity,the numerical simulation of millisecond laser drilling on super alloy has been done.Laser drilling has been simplified two-dimensional symmetric structure,and the heat transfer temperature field mathematical model has been established.Furthermore,the problems of nonlinear transient heat transfer and the phase-change have also been analyzed.The finite element software ANSYS10.0 and parametric design language have been used to simulate and analyzed the process of drilling by millisecond laser on GH4169,the map of temperature field distribution and outline forming under different pulse have been achieved.The temperature field distribution shows that the temperature field was symmetrical with respect to laser beam axis,and the temperature reaches the highest point in central area and diminishes gradually to around,the simulated result satisfied the energy distribution law of Gaussian beam and actual drilling condition.The diagrams of contour forming process show hole forming dynamic process with the increase of laser pulses.The simulation value of hole depth and hole diameter keep the same change rule with experiment results.In consideration of the complexity of drilling process,the model has simplified some actual conditions,which makes the simulation value is slightly larger than the experimental value,but still can provide regular guidance for practical production.