Research On Coloring Mechanism And Quality Evaluationof Nanosecond Laser Induced Surface Of Titanium Alloy

With its small density,strong corrosion resistance,excellent thermal stability and high specific strength,titanium alloy is widely used in civil industry such as biomedical materials,machinery manufacturing,aerospace,and sporting goods as well as national defense industry.On the basis of the traditional laser processing technology,laser coloring gradually becomes a hot area of research because of its,various advantages including simple process,high production efficiency,facile automation,and environmental protection nature.An experiment of laser coloring was conducted on Ti-6A1-4V titanium alloy by Nd:YAG nanosecond laser,including the rules and effects of laser parameters on coloring surface,the optical properties,coloring mechanism and the quality of the coloring surface.Different color on the surface of titanium alloy were conducted by Nd:YAG nanosecond laser.The thermal effect of laser coloring on titanium alloy surface was divided into single pulse,line scanning and overall scanning stages on the basis of heating effects derived from the interaction between laser and metal.The influencing rule of processing parameters including current intensity,scanning speed,Q frequency,filling space on the color parameters(L*,a*,b*)were analyzed from aspects including single pulse energy density,pulse overlapping ratio and scanning overlapping ratio.A BP neural network is adopted to build a predictive model between the above processing parameters and color parameters to achieve effective prediction on laser coloring effect to guide production practice.As to the explanation of the laser coloring mechanism,a definite conclusion has not be drawn yet.A series of researches about optical properties,color parameters,and microstructure adopting instruments including spectrophotometer,optical profiler,and scanning electron microscope and X-ray energy spectrometer were conducted on the samples of yellow,orange and blue at different scanning speed.By using three layers model(air/oxide film/Ti),it was confirmed that different colors were caused by interference effect such as the thickness of outer layer film because of small difference between the experimental value and theoretical value from reflectivity and color parameters.As there is no uniform evaluation system on coloring quality,stability and uniformity of the coloring samples were chosen as the evaluation standard in the paper.A Lab color difference formula AE was used as the evaluative indicator of stability of coloring to achieve the laser processing parameters in the stable ranges of all color samples.The standard deviation of color parameters(L*,a*,b*)was adopted as the evaluative index of coloring uniformity to obtain the optimal parameters combination for processing best uniform coloring samples using the orthogonal test.