Research On The Mechanism Of Laser Induced Surface Coloring On Titanium Plate

Laser color marking is a marking method that uses high-energy density laser to irradiate the workpiece locally and leave permanent color marks.There are still many technical problems in laser metal coloring technology,such as the lack of reasonable quantitative relationship model between process parameters and color,less color types,dark sample brightness and so on,which have not been perfectly solved.It is of great significance for these problems to study the influence of various parameters and parameter coupling on the coloring results and the properties of the film formed on the surface,and to construct a clear coloring mechanism.Compared with the mainstream research objects(such as stainless steel),the components of oxide layer formed in the coloring reaction of titanium are relatively simpler,which is more conducive to the analysis of the formation of coloring film.Therefore,based on TA2 industrial pure titanium,the MOPA fiber laser with wavelength of 1064 nm and maximum output power of 20 W is selected for laser marking experiment.The color and visible spectrum of color samples are collected and analyzed by spectrophotometer.By analyzing the coloring law produced by various parameters and parameter coupling,the oxide coloring model is further established.The main research results obtained in this paper are as follows:The influence of process parameters on laser coloring of titanium plate was studied by controlling variable method.The changes of line spacing,scanning speed,laser power and pulse repetition frequency directly affect the marking results:With the increase of line spacing and scanning speed,the color change is consistent with the color change caused by film thinning in the interference phenomenon;With the increase of laser power,the color change is consistent with the color change caused by the increase of interference film thickness;With the increase of pulse repetition rate,the color change of laser-induced surface first shifts red and then blue.The change of pulse width in the range of 13～27 ns has no obvious effect on the marking effect.However,the ratio of power to pulse width determines the threshold of the change trend of surface color when the pulse repetition frequency increases.The coupling relationship between parameters is explored.By analyzing the cumulative number of laser pulses acting on the titanium plate,the parameter coupling relationship is simplified,and then the cumulative injection step ΔQ(the cumulative injection step is the one-time change step of cumulative injection)model is proposed.By exploring the method of changing the cumulative injection step length,this paper obtains a color plate with complete color(the color plate is the color library obtained by laser marking on titanium plate).By analyzing the change curve of the color difference of the color board with the increase of cumulative fluence,it is found that the color change is not gradual,but will produce two color mutations.Through analysis,it can be seen that the interference level has changed by three levels in the whole process.According to the phenomenon that the color changes twice with the increase of cumulative fluence and combined with XRD phase analysis,a "three-stage three-layer oxide"model is proposed:The first stage is the double-layer model of "transparent oxide layer titanium plate substrate";The second stage is a three-layer model of "transparent oxide layer absorption layer titanium plate substrate",and the thickness of the absorption layer is 73 nm;The third stage is a three-layer model of "transparent oxide layer absorption layer titanium plate substrate",and the thickness of the absorption layer is 114 nm.The predicted color difference based on the theoretical model is less than 6(which is within the acceptable range of human eyes)which can be used to predict the film thickness corresponding to different colors.In addition,the influence of surface morphology on the prediction model is analyzed.The uneven surface will lead to the deviation of the prediction results of the model from the experimental results.