Research On The Macro/micro Characteristics And Mechanism Of Metal Material Damage Caused By Near-solid Cavitation Under High Hydrostatic Pressure

BackgroundCavitation refers to the continuous formation and growth of cavitation bubbles in the liquid that collapses near the solid wall,causing the wall to be repeatedly impacted by huge pressures,which can cause fatigue damage or even surface erosion of the material.Cavitation seriously threatens the safety of hydropower stations,aircraft,and ships.In recent years,my country has vigorously developed deep-sea exploration,oceanographic research,naval ships,and aerospace.It is extremely important for national strategic needs to carry out theoretical and experimental studies on the cavitation resistance of metal materials that need to be served in liquids,especially in the deep sea(under high hydrostatic pressure).The cavitation effect will be accompanied by the generated high temperature,high pressure,shock wave and microjet.Cavitation is the result of the interaction between the extreme conditions and materials generated during the collapse of cavitation bubbles.Different cavitation conditions and the properties of the material will affect the cavitation results.At present,the damage process and mechanism of multi-cavity cavitation are still No unified theory was formed.Therefore,it is of great value to study the macroscopic and microscopic characteristics and mechanism of the damage to the solid wall caused by bubbles near the solid wall under high hydrostatic pressureObjectiveBased on the research on the damage of different metal materials,which produced by cavitation bubbles near solid wall under different hydrostatic pressures,explore the effect of cavitation intensity on the near-solid-wall cavitation-induced metal materials damage,and explore the effect of material properties on metal material damage caused by near-solid wall cavitation,clarify the macro and micro morphology and damage mechanism of metal material damage pits caused by near-solid wall cavitation under different hydrostatic pressures,and the relevant physical mechanism of ultrasonic cavitation under different hydrostatic pressures is analyzed in reverse.In order to establish a method for evaluating the cavitation resistance of metallic materials,and research on materials suitable for service under cavitation conditions provides the basisMethods1.Macroscopic morphology of metal materials damaged by ultrasonic cavitation under different hydrostatic pressureAt different hydrostatic pressures(3 MPa,6 MPa,10 MPa),electron multiplied charge-coupled devices were used to photograph the sonoluminescence of cavitation bubble groups in the free field of spherical focused ultrasonic transducers with open ends.The collected pictures are processed by MATLAB and the average light intensity of the light-emitting area is calculated.The frequency of the spherical focused ultrasonic transducer is 0.6 MHz,the opening radius is 219 mm,the inner diameter is 480 mm,and the height of the table is 400 mmCopper,17-4PH stainless steel and tungsten metal materials with different melting points are selected as the base material(the size is 20 × 6× 1 mm),and the surface is smoothed(grinded,polished).The mechanical properties of metal materials are measured by Vickers hardness measurement and tensile test at room temperature.And use Electron Back-Scattered Diffraction technology to analyze their grain size Ultrasonic cavitation under the conditions of hydrostatic pressures of 3 MPa,6 MPa,and 10 MPa was selected to act on metal materials,the action time was 1 s,and the electric power was 2000 W.A Confocal Laser Scanning Microscope was used to perform three-dimensional imaging of the cavitation erosion pits after the experiment.Then,the maximum pit depth and pit diameter were selected for measurement to obtain the corresponding pit depth and pit diameter values2.Study on the microscopic characteristics of metal material damage caused by ultrasonic cavitation under different hydrostatic pressureField Emission Scanning Electron Microscope was used to observe the microscopic morphology and structural characteristics of the surface damage of metal materials.X-rays diffraction and Energy Dispersive Spectroscopy were used to analyze the chemical composition of the surface damage of thin metal materials.Electron Back-Scattered Diffraction(EBSD)was used to observe the cross-section of cavitation pits with a grain size greater than 1 mm copper(size 10×8×1 mm)after the ultrasonic experiment.Then,the micro-morphology and damage mechanism of cavitation damage pits of metallic materials caused by near-solid wall ultrasonic cavitation under different hydrostatic pressures are analyzedResults1.Luminous intensity of cavitation bubble group under different hydrostatic pressureAt 3 MPa,6 MPa,and 10 MPa hydrostatic pressure,the luminous range of the cavitation bubble group at the center of the sphere decreases with the increase of hydrostatic pressure,and the brightness increases.This indicates that the increase of hydrostatic pressure causes the environmental pressure outside the cavitation bubble to increase,the cavitation threshold is increased,and the cloud of the cavitation bubble is compressed.The bubble cloud range becomes smaller,and the cavitation bubble cloud range is the smallest and most stable at 10 MPa.Under different hydrostatic pressures,the luminous intensity of the cavitation bubble group showed a decreasing trend from the center area to the edge area.Among them,the difference in luminous intensity within the cavitation bubble group was the most obvious under the hydrostatic pressure of 10 MPa2.Macromorphology of metal material damage caused by ultrasonic cavitation under different hydrostatic pressureThe depth of cavitation pits of copper,17-4PH stainless steel and tungsten gradually increased with the increase of hydrostatic pressure.The diameter of cavitation pits showed different trends.The diameter of the pits on the copper surface gradually decreased with the increase of hydrostatic pressure.The diameter of the pits on the surface of 17-4PH stainless steel did not change significantly with the increase of hydrostatic pressure.And the diameter of tungsten surface erosion pits gradually increased with the increase of hydrostatic pressure.Under the same conditions,the pit depth of the three metal materials is hCu>h17-4PH Stainless Steel>hw and the pit diameter is dCu>d17-4PH Stainles Steel>dW.When the hydrostatic pressure is 3 MPa,there are obvious erosion pits on the surface of copper and 17-4PH stainless steel,and there are no obvious erosion pits on the tungsten surface3.Microscopic characteristics and mechanism of metal material damage caused by ultrasonic cavitation under different hydrostatic pressureObserved by SEM,the damage morphology of the copper material erosion pit under the hydrostatic pressure of 3 MPa is relatively uniform,mainly in a honeycomb shape,there are micro-pits on the surface of about 20 ?m,which may be formed under the action of force due to the high temperature caused by cavitation.Under the hydrostatic pressure of 6 MPa,the copper material erosion pit is uneven than 3 MPa,and the edge of the structure inside the pit is smoother than 3 MPa.The micro-pits at the edge of the pit are more obvious,and the number of micro-pits in the central area is smaller.Under the hydrostatic pressure of 6 MPa and 10MPa,there are many molten spheres in the copper material erosion pits,and the size of the spheres is about 10 ?m to 40 ?m.The micro-morphology of the erosion pit on the surface of 17-4PH stainless steel has a uniform and small damage similar to the surface uplift when the hydrostatic pressure is 3 MPa.The pit in the center of the erosion pit under the hydrostatic pressure of 10 MPa increases in depth than the hydrostatic pressure of 6 MPa,but other area morphology is consistent,similar to the damage of copper material under hydrostatic pressure of 3 MPa,but the size of the micro-pits is smaller than that of copper material.Tungsten has no obvious erosion pits on the surface when the hydrostatic pressure is 3 MPa.As the hydrostatic pressure increases,the diameter and depth of the pits increase.Under the hydrostatic pressure of 6MPa and 10MPa,the micro-morphology of erosion pits is roughly the same,both of which are mainly fractures along grains,and there are obvious cracks at the grain boundaries.In addition to the fractures along the grains,there are a few penetrating fractures.XRD test results showed that no new phases were produced in the copper,17-4PH stainless steel,and tungsten materials after the ultrasonic cavitation treatment.The grain size of pure copper used in the experiment is about 25 ?m,the grain size of 17-4PH stainless steel is about 10 ?m.And the grains of pure tungsten include block and lamellar shapes,the size of the block grains is about 5 ?m.After the cavitation experiments of pure copper materials with grain size and millimeter grade,EBSD was used to observe the samples after ultrasonic cavitation under the hydrostatic pressure of 3MPa,6MPa and 10 MPa.It is found that the ultrasonic cavitation under the three conditions of hydrostatic pressure causes the phenomenon of color unevenness in the grains of pure copper materials,and the phenomenon becomes more obvious as the hydrostatic pressure increasesConclusion(1)Combining the grain size,melting point and mechanical properties of metal materials,it is considered that the cavitation resistance of metal materials is highly correlated with the melting point and hardness of materials.According to the melting point of the material,the temperature acting on the surface of the material after the collapse of the cavitation bubbles under the hydrostatic pressure of 3 MPa,6 MPa and 10 MPa are all greater than 1440?.As the melting point of the material increases,the effect of temperature on the damage to the material gradually decreases,mainly manifested by the effects of shock waves,microjets,etc.,and as the hydrostatic pressure increases,its effect on the material also gradually increases.(2)As the hydrostatic pressure increases,the luminous intensity and cavitation intensity of the cavitation bubble group at the sphere center gradually increase,and the range gradually decreases.The depth of cavitation pits increases with the increase of hydrostatic pressure.The damage area of cavitation pits is determined by the relationship between material properties and cavitation.The luminous intensity from the center to the edge of the cavitation bubble group gradually weakened,and the effects of forces such as temperature,shock wave,and microjet generated during cavitation gradually weakened.(3)The forces generated by ultrasonic cavitation at hydrostatic pressures of 3 MPa,6 MPa,and 10 MPa all caused the atoms in the grains in the pure copper material to be irregularly arranged at the center of the cavitation pit and cause dislocations.As the hydrostatic pressure increases,the dislocation density increases.However,in degassed deionized water with an oxygen capacity not exceeding 0.8 mg/L,cavitation does not cause the material to form new substances or oxides,nor does it change its phase.