Research On Cavitation Strengthening Mechanism Of Laser-induced Cavitation Buble And Its Chemical Effect

Cavitation is commonly seen in hydraulic machinery and affects the normal operation of equipment.Its influence mechanism comes from two aspects:mechanical impact of shock wave and water jet and chemical corrosion.Cavitation bubbles frequently collapse near hydraulic machinery equipment,and the surface of the equipment will be repeatedly impacted by shock waves and water jets,resulting in fatigue,wear and even erosion of surface materials.Long-term collapse of bubbles will also cause a large number of water molecules to tear.Strong oxidant hydroxyl radical（·OH）,so that hydraulic equipment is also subject to oxidative corrosion when subjected to mechanical shock.For many years,research on cavitation has focused on preventing cavitation from causing damage or erosion.However,how to turn harm into benefit and rationally use the huge energy released by cavitation collapse is a new way to study cavitation.Compared with other methods,laser induced cavitation technology is easy to control,accurate positioning,and can produce single bubble with good spherical symmetry,which greatly promotes the development of cavitation research.Based on the mechanical impact and chemical corrosion caused by cavitation,this dissertation takes laser cavitations as the research object,proposes a machining method of Laser cavitation peening（LCP）,and uses the chemical effects of cavitations to characterize the machining effect of materials.The main research contents and conclusions are as follows:Dynamic characteristics and mechanical effects of near-wall laser-induced cavitation bubble:The effects of laser energy and dimensionless parameterγon the radius and pulsation period of laser-induced cavitation bubble are studied by high-speed camera,and the changes of pressure field,flow field and temperature field in the process of bubble pulsation under differentγwere analyzed by ANSYS Fluent software.Asγincreases from 0 to 1.2,the maximum radius of bubble decreases by nearly one third（from 3.01 mm to 1.15mm）,and the pulsation period decreases by about half（from 500μs to 255μs）.The pressure and temperature of the cavitation bubble decreases obviously during the second and third pulsations,and the maximum pressure and temperature during the expansion of the cavitation bubble appear on the enclosing wall of the bubble.The shock wave signal in the process of laser cavitation is detected by hydrophone,and the impact of cavitation water jet is simulated by Autu Dyn software,and the mechanism of laser cavitation impact is revealed.The mechanical effects of laser cavitation impact mainly come from the laser shock wave in the laser breakdown process,the bubble collapse shock wave and the water jet in the first pulsation process.Shock wave plays a dominant role in laser cavitation impact,and the impact pressure is much greater than that of water jet.Shock wave is a transient impact process while water jet is a continuous impact process.Strengthening mechanism of mild steel impacted by laser cavitation impact:The surface morphology and roughness,residual stress in depth direction and microhardness of Q235 steel are studied by combining experiment and Abaqus simulation software.Q235 steel is based on the plastic deformation caused by laser shock wave,and the continuous impact of cavitation rupture shock wave and water jet doubles the strength of the material.After laser cavitation treatment,plastic deformation and residual compressive stress are generated on the surface of the material,and the microhardness is improved.The residual stress and microhardness of Q235 steel increased significantly with the increase of the number of shocks and pulse frequency,and the maximum value appeared on the top surface of the material and is released with the increase of depth.The microstructure and microstructure evolution of mild steel after laser cavitation were studied.The microstructure evolution and grain refinement law of low carbon steel surface layer induced by laser are revealed.The effect of laser cavitation reduces the grain size of Q235 steel and increases the depth of grain refinement layer significantly.Dislocation structures are formed within grains and near grain boundaries,and the stacking of dislocation lines forms dislocation entanglement,dislocation walls and dislocation cells.The high density of dislocation entanglement and the sharing of dislocation cells lead to the refinement of the original coarse grain into equiaxed fine grain,and the continuous dynamic recrystallization occurs at the subgrain boundary,and the final grain refinement.There is a lot of dislocation accumulation in ferrite and grain boundaries,while there is almost no dislocation accumulation in cementite.Corrosion resistance of laser cavitation-strengthened mild steel:The electrochemical corrosion behavior and stress corrosion performance of Q235 steel after laser cavitation treatment are studied,and the mechanism of electrochemical corrosion and stress corrosion of Q235 steel impacted by laser cavitation with or without coverage layer is revealed.Laser cavitation peening can significantly improve the electrochemical corrosion resistance of Q235 steel.The surface of the material treated by laser cavitation peening without coverage layer has better corrosion resistance in a short corrosion time.With the increase of soaking time,the degree of electrochemical corrosion becomes more and more serious.The final corrosion product of Q235 steel after electrochemical corrosion is the combination of Fe（OH）₂,Fe（OH）₃and Fe₃O₄.The corrosion product will hinder the polarization reaction and resist the invasion of Cl ions.The yield strength and ultimate strength of Q235 steel are improved by laser cavitation impact,but the elongation is reduced,and the fracture behavior of the material is changed from ductile fracture to fracture combining toughness and brittleness.The coverage layer has excellent ductility to buffer the shock wave and water jet impact and transfer the impact to the material surface,but reduces the degree of plastic deformation caused by laser cavitation impact.Hydroxyl radical production by laser cavitation and its mapping relationship with strengthening of mild steel:The yield and rate of hydroxyl radical induced by laser cavitation in infinite domain are studied under different process conditions,and the mechanism of hydroxyl radical production by laser cavitation and the energy consumption are analyzed.The production yield of hydroxyl radical increases with the increase of laser energy and pulse frequency,and the optimal temperature for hydroxyl radical production is 35℃-45℃.The production velocity of hydroxyl radical increases first and then decreases significantly,and the maximum production velocity time under various working conditions is concentrated in 10-20min.The energy efficiency of hydroxyl radical generation increases with the decrease of pulse energy and pulse frequency.The optimal conditions for the generation of hydroxyl radical by laser cavitation are 25m J laser energy and 1Hz pulse frequency.The increase of residual stress and microhardness after laser cavitation treatment is proportional to the production yield of hydroxyl radical,and the optimum defocusing amount of hydroxyl radical production is 1 mm.A new concept of using chemical effects of laser cavitation to characterize the strengthening effect of materials is proposed for the first time.Based on the mapping relationship between hydroxyl radical content produced by laser cavitation and residual stress and microhardness of mild steels under different laser parameters,the corresponding fitting equation ias established.