| Due to the excellent characteristic,such as high energy,high flexible,locality manufacturing and green environmental protection,the technologies of laser manufacturing have been applied in many industrial circles.However,because of the limitation of laser beam size,it always chooses an overlapping way by line or layer manufacturing to modify the integrated plane or to fabricate parts with three-dimensional structure.Hence,the ultimate performance of materials processed by laser is decided by the weakest point in the integrated fabricated area.During laser manufacturing,it must ensure there is no macro-defects or poorquality parts in each line or lay,so it is necessary to install real time monitoring system on the instrument of laser manufacturing.Recently,with the help of different kinds of real time monitoring signals,it can detect the location of macro-defects and reduce the error accumulation during layer-by-layer fabrication,which improve the quality of products prepared by laser.However,for the composition and mechanical properties of products processed by laser,it is always measured by conventional methods.the samples are token from the as-fabricated parts by a destroyed way,and then be processed into standard size,finally,they were test with related testing devices.In order to improve the accuracy and reliability of testing,the conventional measurements should take a longer testing period,moreover,the original product will become unavailable completely by a destroying way of sampling.Hence,it is impending to develop a system which can monitor the composition distribution and variation of mechanical performance during laser manufacturing.To satisfy the needs discussed above,an optical system was established to collect laser induced emission spectra during real-time laser processing,and a program was coded to process the original spectra.The correlations among spectra,microstructure,composition and performance of molten zone were researched during laser surface modification of AISI4140 steel.The main research item can be divided into four parts.(1)In this experiment,AISI4140 steel was treated by the laser under different laser parameters.The Vickers tester was used to measure the hardness of molten zone.The macro-morphology of cross-section of molten zone was observed by optical microscope,and the size of molten zone was counted by the software of Image-Pro Plus.The microstructure of molten zone was measured by the electron scanning microscope(SEM).The laser induced emission spectra were collected during laser processing synchronously,and the valid information was selected in original data.According to analysis the evolutions for both spectra and hardness of molten zone,the correlation between spectra and hardness of molten zone was established.(2)The hardness of substrate was changed by heat treatment.The properties of pre-treated substrates,such as electrical resistivity and latent heat of phase change,were measured by resistivity tester and differential scanning calorimetry(DSC).The same melting process was applied to the surface of the pre-treated substrates,and laser induced emission spectra were collected and analyzed by the same way.The effect of substrate hardness on the spectra was researched in the experiment.The correlation of spectra and hardness of substrate was established.(3),The finite element method was used to simulate the temperature distribution of surface of molten pool during the laser processing on AISI4140 steel.Depending on the simulation results,the Fe vapor flux density and temperature in the air mass above molten pool were calculated.(4)4 kinds of Ni-based alloyed coating were fabricated on AISI4140 by laser cladding,and the composition distribution was tested by EDS,additionally,the laser induced emission spectra were collected during real processing of laser cladding.To use the spectra to predict the composition in cladding layers,the internal standard method was used to draw calibration curves for monitoring the composition.By analysis of experimental data,the main results can be summarized as:The fluctuation of intensity of spectral lines becomes heavily when the laser power becomes strong or the laser scanning speed becomes slow.There is complex non-linear trend between mean intensity of spectral lines and micro-hardness of molten zone.when the laser power is lower than 1200W,the mean intensity reduces with the decreasing of laser scanning speed,however,when the laser power is higher than 1400W,with the descending of laser scanning speed,the mean intensity declines after its original increasing.By dimensionless processing among hardness of molten zone,intensity of spectral lines,thermo-physical parameter of molten zone and the laser processing parameters,a dimensionless laser power density is deduced.In this dimensionless system,there is a linear correlation between hardness and laser power density,and the mean intensity increases linearly with the enhance of laser power density firstly,and then the mean intensity becomes stable.By non-linear regression,the hardness of molten zone can be quantitatively expressed by a piecewise function with the variable as laser scanning velocity and Fe I spectral line intensity,and the mean error rate between the monitoring results and the real measured hardness is lower than 3%(laser power is 1700W and laser scanning speed is 10mm/s).The electron temperature of spectrum increases with the enhance of substrate hardness,when the hardness of substrate changes from 197HV to 687HV,and the max variation among the electron temperature can be 421K(laser parameters are 1800W and 20mm/s),however,the correlations among intensity of spectral lines,electron density of spectrum and micro-hardness of substrate are weak.The reasons caused the phenomenon can be contributed that the influence of laser parameters on the intensity of spectral lines is seriously,which causes the large error during collection,moreover,the ionization degree of gas above the molten pool is low,and the electron density do not reflect the change of substrate.If the different between upper level energy of spectral lines is positive,the intensity ratios between these two spectral lines increases with the enhance of substrate.The max relative variation of intensity ratio among spectral lines can arrive at 21.3%.The direct reasons for the increase of electron temperature are contributed to the increase of metallic particle density and temperature of mixed gas above molten pool.The metallic particle density and temperature have a close relationship with heating process of molten pool.By the analysis of physical parameters,the max variation of thermal conductivity of substrate is around 50%after heat treatment,and the lower conductivity causes a higher heating rate during laser melting,which can make a higher temperature on surface of molten poor.The results of calculation are shown,when the laser power is 1600W and the laser scanning speed is 10mm/s,the surface of molten pool becomes gasification state.The density of Fe atoms in the mixed gas shows an exponential increasing trend with the increase of hardness of substrate(y=a-be-cx,a,b and c are positive),correspondingly,the temperature of Fe atoms shows a linear increasing trend.The Fe flux density above the molten pool of quenched substrate is 16.7%higher than that of annealed substrate,moreover,the temperature of Fe atom is 14.9%higher than that of annealed substrate.Therefore,the change of thermo-physical parameters is the root reason causing the variation of spectral signals.For the difference among saturated vapor pressure of elements,the ablation mass of low melting point elements is higher than that of high melting point elements in during laser cladding.Hence,the weight ratio between low melting point elements and high melting point elements declines with the increase of laser power density,oppositely,it shows an increasing trend in the mixed gas above molten pool.A corrected parameter is deduced with the saturated vapor pressure of elements,measured composition in cladding layer and the laser parameters,the intensity of spectral lines is revised by the corrected parameters,which ensures the element proportions are same in both metallic gas and laser cladding layers.When the element weight percent is higher than 3%,the mean error rate between monitoring results and EDS measured data is lower than 8%,however,if the incomplete melting is found during laser cladding,the mean error rate is bigger than 10%.To conclusion,the correlations among laser induced emission spectra,hardness of molten zone,hardness of substrate and composition of cladding layers are remarkable.There is a nonlinear correlation between the intensity of spectral line and the hardness of molten zone.The intensity ratio between spectral lines shows an exponential relationship with hardness of substrate.The intensity ratio of spectral lines correlates with weight ratio of element linearly in cladding layer.It offers a valid way to predict the performances of molten zone,the properties of substrate and the elements distribution in cladding layer by analyzing the emission spectra during laser processing.Therefore,the laser induced emission spectrum has the potential to be a signal to monitoring the properties of products fabricated by laser processing... |
PDF Full Text Request