Experimental And Simulation Study On Jet Assisted Laser Electrochemical Composite Drilling Of DD6 Nickel-based Single Crystal Superalloy

With the continuous increasing in aero-engine performance demand,the machining technology of the air film cooling hole on turbine blade has gradually become one of the key technologies in aero-engine manufacturing.The processing quality of air film cooling hole has an important influence on improving the working temperature of turbine and protecting turbine blade.As a contactless processing method,laser machining is associated with high efficiency and high precision,and has been widely used in the machining of cooling holes.The appearance of ultra-short pulse laser has further improved the quality and precision of cooling hole.Picosecond laser can achieve high precision,high quality and low damage micro-machining,as well as acceptable processing efficiency,and therefore has potential in many engineering application prospects.However,due to the inevitable accumulation of thermal effect in laser processing,there exist recasting layer and micro-cracks on the side-wall of the cooling holes fabricated by the picosecond laser.In this thesis,based on the picosecond pulse laser drilling study,low pressure water jet and electrochemical processing are introduced for assistance to further improve the drilling quality.In order to reduce the micro-hole taper and remove the recast layer attached on the inner wall,the experimental study has been carried out to optimize the processing parameters.In addition,finite element simulation exploration has also been conducted for a better understanding of the complicated machining process.The research content includes the following aspects:1.The interaction mechanism between picosecond pulsed laser and material has been theoretically analyzed.On this basis,the material removal mechanism of water-jet assisted laser etching is discussed,in which the interaction between laser and solution is analyzed in detail.Finally,combined with the principle of electrochemical machining,the mechanism of jet assisted laser electrochemical composite machining is analyzed.2.The experimental study for film cooling holes fabrication on DD6 alloy is carried out by picosecond laser direct etching,water jet assisted laser etching and jet assisted laser electrochemical composite machining,respectively.Firstly,the results of cooling hole processing are compared and analyzed.The results show that there is more spatter slag on the surface of the cooling hole directly etched by laser,and the remelting material around the entrance is thicker and the inner wall quality is poor.The surface morphology and quality of the cooling hole are better when water jet is introduced to assist the laser etching process,as the hole taper becomes smaller and the inner wall quality is improved significantly.For jet assisted laser electrochemical composite machining,the roundness and aperture of hole outlet can be further improved,and the inner wall quality also becomes better.Secondly,the orthogonal experiment is conducted to study the influence of different laser parameters on the reduction of hole taper.The range analysis result suggests that the laser pulse repetition frequency and laser scanning speed have most important influence,while laser power and laser element number are associated slight influence.Finally,the parameters of water jet and electrochemical parameters are studied by single factor test.The influences of various factors on the surface morphology,inlet diameter and taper of cooling holes are analyzed.It is found that the hole taper increases with an increase in jet velocity,but decreases with the jet angle.In addition,an increase in electrochemical machining time or voltage decreases the taper of cooling hole.3.Mathematical models for laser etching,low pressure waterjet impingement and electrochemical dissolution are developed using the finite element software COMSOL.The two-temperature model of picosecond pulse laser etching,the flow field model of water jet impact and the current density distribution model of electrolyte jet are established successively.Simulation study by picosecond etching model shows that electron temperature increases rapidly first,and then lattice temperature rises,gradually reaching the thermal equilibrium state.Once laser pulse is off,both the electron and lattice temperatures gradually fall to room temperature.The ablation rate depends on the position and increases during the time of laser pulse.The largest ablation rate is achieved at the laser focus center,and decreases with an increase in the distance away from the focus center.The ablation is limited to 20 microns in the simulation study.The simulation results show that the velocity of water jet impingement is the highest at the impingement point.The flow velocity decreases with the increase of the distance from the impact center.The pressure is concentrated around the jet impact center,while the rest of the material surface has almost no pressure.The current density distribution model of the electrolyte jet flow field shows that the current density of the liquid flow reaches the maximum before the jet impact on the material surface,and decreases rapidly after the jet impact on the material surface.The current density is inversely proportional to the distance from the center of the jet.