Impact Of Scanning Electron Beam Energy Density On Polished Composite Strengthening Of 45 Steel

Due to its excellent mechanical and cutting characteristics,45 steel is frequently employed.Surface strengthening and polishing treatment are necessary in order to prolong the operating life of 45 steel components under alternating loads.However,current electron beam polishing composite strengthening technology still has issues with real-world use.Surface imperfections,a thin polishing strengthening layer,calculations of the energy density that are problematic,and confusing melting pool evolution rules are some of these problems.In order to address the frequent technical issues of "volcano pits and micro-cracks defects" and "thin polishing strengthening layer" in existing electron beam polishing,the dissertation developed a scanning electron beam ring-down method and proposed a method for calculating the energy density of scanning electron beam micro-melting composite strengthening,exploring surface morphology evolution and control mechanism.This project takes 45 steel after quenching and tempering treatment as the research object.In order to create a highly fitted electron beam thermal source model that closely resembles the actual processing procedure,the Gaussian thermal source model was first superimposed on the circular ring using mathematical modeling.The peak energy coefficient was then included.The temperature field simulation approach was used to assess the distribution pattern of the beam under various peak energy coefficients and the change in surface temperature of the sample in order to study how the peak energy coefficient affects the energy distribution of the scan electron beam.The energy density was computed for micro-melting on the surface and a particular molten zone thickness based on the thermal source model from the previous chapter.First,a numerical model of the temperature and energy density of the micro-protrusion was constructed.Then,using the temperature range of the micro-protrusion as the calculation base,the appropriate range of the energy density for electron beam micro-melting polishing was determined.Second,a semi-infinite solid heat transfer model was developed to investigate the method of calculating the average heat flux density in the downward beam ring area as well as the relationship between the time factor corresponding to various sampling points in the scan ring and the depth of thermal action.In order to determine the energy density range of the micro-melting composite strengthening electron beam,the intersection between them was finally solved.Using the enhanced least squares method,a two-dimensional geometric model was created to fit the original milling surface shape.For machining surfaces,a molten pool flow and solidification model was built.The impact of thermal capillary force on the velocity transition point and the influence of eddy currents on the behavior and morphology of molten metal accumulation during molten metal flow were also studied.It was made clear how thermal capillary force operated inside the molten pool.Second,the distribution characteristics of grain size and type in various locations of the molten zone were clarified by analyzing the deviation law of the solid-liquid interface of the molten pool and comparing the solidification crystal parameters under various energy densities.The approach of controlling surface morphology by electron beam energy density was covered in the previous chapter and was based on the findings on the relationship between thermal capillary force and surface roughness.An analytical formula for the relationship between the normalized average displacement and electron beam parameters was established after first analyzing the relationship between the normalized average displacement of liquid particles within the molten pool and the degree of thermal capillary flow.After polishing,the distinctive characteristics of micro-roughness features were retrieved,and the evolution of these parameters at various energy densities was examined.The link between the macro and micro surface roughness was established,and the normalized average displacement and slope parameter were fitted.An energy density-to-surface-roughness-to-average displacement mapping chain was then built.The impact of surface grain refinement under the influence of electron beams on the mechanical properties of the reinforced layer was disclosed through studies involving microstructure monitoring and performance testing.The results demonstrate that the thermal action area is enlarged and the temperature difference between locations in the ring downward beam region is the least when the peak energy coefficient is 1.The issues of surface imperfections and a thin strengthening layer brought on by an uneven energy distribution during EB polishing can be efficiently resolved by this thermal source.The energy density needed to melt micro-protrusions is made up of three components: energy needed to heat the micro-protrusion to its melting point,energy conveyed to the base material,and energy absorbed by the latent heat of melting.The needed average energy density of the electron beam changes according on the scan frequency,and the smaller the scan frequency,the higher the required energy density for micro-melting composite strengthening.The approach and conclusion used in this chapter to solve the energy density of the electron beam are crucial for choosing the experimental parameters involved with the electron beam.The primary driving factor for horizontal movement within the molten pool is thermal capillary force,which directly causes the formation of velocity transition sites on the surface and influences changes in surface roughness.The average surface roughness and projected numerical errors when combined with polishing experiments are both within 7%,and the molten pool morphological evolution model and experimental findings are very congruent.The deviation of the solid-liquid interface has a major impact on grain growth,and identical rules are seen for the crystal parameters of solidification at various work-piece movement rates.Fine equiaxed crystals make up the top of the molten zone,fine columnar crystals make up the middle,and extremely fine cell-like crystals make up the bottom.The normalized average displacement is influenced by the electron beam current,the molten pool action time,and the beam spot width,with the latter two having the smallest effects.The beam spot width had the biggest influence.Both the normalized average displacement and the surface micro-roughness slope conform to the exponential function fitting relationship.The thermal capillary flow can be adjusted within a tolerable range by altering the energy density,allowing for customizable surface shape.For the same scanning frequency,the hardened layer thickness and surface hardness are typically positively associated with the electron beam energy density,and that the enhancement of surface hardness is due to the finer grain size in the melt zone.According to the conclusion drawn from the solidification model of the melt pool,the observed melt zone is made up of fine equiaxed grains close to the surface,fine elongated columnar grains in the middle,and extremely fine cellular grains close to the bottom.After being subjected to scanning electron beam treatment,the sample’s surface hardness and wear resistance both considerably increase.The sample’s surface hardness now exceeds 720 HV0.1,or almost three times that of the substrate.In the early stages of wear,particle wear predominates,while in the latter stages,adhesive wear predominates,with typical peeling and ripping morphologies seen in the wear markings.The overall mechanical characteristics of the composite reinforced layer are significantly enhanced by fine crystal strengthening.