Rapid Preparation of Iris and Color Composite Patterns by Laser Direct Writing
Objective This study explores the utilization of laser technology to alter the surface structure of metals,producing rainbow-colored effects.Beyond aesthetic purposes,this method has potential applications in data storage and anti-counterfeiting measures.Laser modification of metal surfaces presents several advantages over alternative techniques:reduced processing time,user-friendly operations,rapid molding speeds,durable structures,and an environmentally-friendly process that produces no pollutants.However,present strategies for creating rainbow structures suffer from sluggish laser scanning rates,leading to slower molding speeds.Many of these approaches rely on laser polarization and interference,which produce relatively basic patterns.Storing intricate data requires multiple scanning passes.To address these challenges,this study employs nanosecond lasers to inscribe grating patterns directly onto stainless-steel surfaces,generating vibrant rainbow hues.A comprehensive statistical analysis is conducted to examine the influence of factors,such as laser power,scanning speed,scanning interval,and repetition frequency,on the shape transformations of the samples.By melding a range of rainbow effects,insights into the laser interaction with stainless-steel surfaces are obtained.Through systematic parameter adjustments,the mechanisms behind laser-induced rainbow coloration on stainless-steel surfaces are identified.Then,a composite structure,which combines grating diffraction rainbow hues,thin-film interference color,and the metal inherent color,is developed.This composite showcases a dynamic dominant color shift among its three colors,depending on the viewing angle.This investigation offers valuable insights for potential industrial implementations,seeks to enhance molding efficiency,and introduces innovative solutions for custom metal surface coloration.Methods In this study,the relationship among the scanning speed,scanning interval,and repetition frequency is investigated to improve the molding speed.The variation trends among them are calculated and analyzed.Scanning speed of 400 mm/s,repetition frequency of 20 kHz,interval of 0.02 mm,and energy percentage of 55％-65％are chosen to observe the corresponding structural changes,and the effect of laser energy on the experimental results is analyzed.The scanning speed of 400 mm/s,repetition frequency of 20 kHz,energy percentage of 80％,and scanning interval of 0.04-0.07 mm are selected to observe the trend of structural changes and to analyze the effect of the scanning interval on the results.The experimental data are summarized,and the forming effect of the proposed scheme is verified.The parameter range required to achieve rainbow colors is determined,and the mechanism behind the changes in the stainless steel surface structure is deduced.Finally,a composite structure is formed after a single scan.Results and Discussions We speculate that this rainbow-colored structure forms due to laser-induced effects(Fig.5).Laser pulses superimpose on each other,and the parts that do not overlap,due to the absence of subsequent energy input,cool down.The overlapping parts maintain a high-temperature state because of the relative temperature difference,leading to wavy structures in the melt pool.Meanwhile,the translation mode of the laser causes the relative displacement between the pulses to be minimal,creating horizontal overlapping lines along the continuous upper and lower edges.The second pulse continues to sweep over the previous pulse,partially overlapping it.This portion of the energy wave either disrupts the previously formed wavy structures,creating new structures,or if the energy is too high,prevents the wavy parts from cooling down promptly.At the same time,the upper and lower edges keep forming horizontal intersection lines,and subsequent laser beams continuously superimpose in vertical and parallel directions of the laser,resulting in a rainbow effect(Fig.6).At a scanning interval of 0.1 mm,a composite structure consisting of a grating diffraction iris,film interference structure color,and intrinsic color forms(Fig.7).Conclusions Using a standard nanosecond fiber laser,the surface of stainless-steel plate is directly etched to realize a high-speed rainbow structure.The results demonstrate that the rainbow structure is influenced by the laser energy parameters and scanning path.Under optimal conditions,structures are formed quickly.An increase in laser energy is found to compromise the rainbow structure from the outset.The ripple shape of the rainbow is determined by the laser scanning path,repetition frequency,and speed.By adjusting the laser pulses,a melt-pool boundary that adheres to the Bragg interference condition is formed,producing a wavering effect that spans the entire visible spectrum.Simultaneously,a composite structure that includes the grating diffraction iris,film interference structure color,and intrinsic color is constructed using a specific process.In this structure,alternating dominant colors for the three hues at different angles are achieved,offering a new personalized surface color scheme.
laser techniquelaser opticssurface structurelaser direct writingstructural color
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