Evaluation Of Energy Field Control Parameters For High Precision Nanosecond Laser Cleaning Of 7075 Aluminum Alliy

7075 aluminum alloy is the main material for aerospace parts and precision devices,due to it has the advantages of light weight and high hardness.However,its chemical properties are active,its surface is easy to oxidize,and its electrical properties are reduced.But the melting point of the oxide layer on the surface of aluminum alloy is much higher than that of the substrate.Then,the laser energy is too low to break through the influence of the oxide layer and achieve effective cleaning;and too high laser energy is more likely to damage the aluminum alloy substrate,resulting in over ablation and secondary oxidation.It is difficult to achieve smooth surface cleaning of 7075 aluminum alloy.It is necessary to research controlling the laser cleaning energy on the surface of 7075 aluminum alloy more accurately.In this thesis,nanosecond pulsed fiber laser is used as the main equipment of laser cleaning test.Through a large number of blind box tests,the laser energy parameters acting on the surface of 7075 aluminum alloy are studied and analyzed.A smooth surface with a roughness of 0.15?m is obtained after a single cleaning(along the scanning direction).The intensity distribution of the focused spot is obtained indirectly from the incident beam information.A new method for calculating the energy density proportional to the intensity distribution is proposed.Based on the above optimal processing parameters.the threshold range of single pulse laser energy density for removing oxide layer on 7075 aluminum alloy and the threshold range of cumulative laser energy density for obtaining smooth surface are discussed.This provides a parameter reference for large-scale machining.Finally,it is found that there are different color and brightness changes in the cleaning process with different laser parameters.Combined with the principle of grating diffraction,analyzed this phenomenon,and a new in-situ detection method for 7075 aluminum alloy cleaning quality evaluation is established.The main contents of this thesis are as follows:(1)The laser cleaning principle device is built,the maximum power of the device is 200W,the maximum scanning rate is 14000mm/s,the repetition rate is200-2000k Hz,the field mirror focal length is 254mm,the pulse width is 106.8ns,and the wavelength is 1064nm.The confocal microscope was used to observe the surface after laser cleaning,which provided basic research devices for the subsequent laser spot energy distribution.(2)A large number of experiments are carried out to analyze the influence of laser power,laser scanning rate,defocusing amount,laser pulse repetition rate and other parameters on the quality of laser cleaning.Among them,the laser power determines whether the single pulse laser energy can completely remove the oxide layer on the aluminum alloy surface;the laser scanning speed determines whether to obtain a laser cleaned smooth surface;the variance of defocusing amount affects the laser cleaning quality by changing the laser energy density;The repetition rate of laser pulse affects the laser output power and spot spacing at the same time,which leads to the research falling into a nonlinear cycle.Therefore,the laser power and galvanometer scanning rate are selected as the research parameters,and the defocusing amount and laser pulse repetition rate are fixed.the setting is the laser power of 80W,the laser scanning speed of 1000mm/s and the repetition rate of500k Hz.Under this condition,a high-precision surface with roughness of only0.15?m is obtained,on which the oxide layer is totally removed.These data provide basic experimental parameters for the following accurate analysis and calculation of energy threshold.(3)A calculation method of laser energy density proportional to light intensity distribution is proposed.However,the focus spot energy is too strong to measure its intensity distribution directly.Through the analysis of the focusing optical system,the relationship between the field distribution of the incident pupil and the field distribution of the focal plane is obtained,and the intensity distribution of the focused spot is calculated indirectly from the information of the incident beam.According to the calculation method of laser energy density which is proportional to the light intensity distribution,the above optimal processing parameters are calculated.When the laser single pulse energy density is 6.24J/cm~2?6.97J/cm~2,the oxide layer on the surface of 7075 aluminum alloy can be effectively removed.When the threshold value of cumulative energy density of laser spot reaches 148.59 J/cm~2?176.77 J/cm~2,a smooth laser cleaning surface can be obtained.(4)According to the principle of grating diffraction,the mechanism of color change after laser cleaning is analyzed.An in-situ on-line detection device for 7075aluminum alloy surface cleaning quality evaluation was built,and a rapid evaluation software based on HSL(H-hue,S-saturation,L-Luminance)was developed.The software is used to analyze the experimental results of laser power parameters and galvanometer scanning rate parameters.When the laser power changes,the test data corresponding to the high-quality surface is unique and can be effectively distinguished from the data under other powers.When the scanning rate of the galvanometer changes,the test data corresponding to the high-quality surface is unique and can be effectively distinguished from the data at other rates.The experimental results show that the method is effective.