纳秒激光烧蚀钛合金微坑形貌的数值模拟分析

Numerical Simulation of Micro-pit Morphology of Titanium Alloy Ablated by Nanosecond Laser

钛合金微坑边缘的火山口形貌会显著影响干细胞的黏附形态和增殖。本文采用数值模拟方法研究了钛合金烧蚀温度场随时间的演变规律,分析了表面张力和马兰戈尼效应等因素对烧蚀区速度场的作用,预测了钛合金微坑的几何尺寸,通过实验验证了预测模型的可靠性。结果表明:在重力、浮力、反冲压力和表面张力的作用下,熔融液滴以最高0.3m/s的速度向烧蚀边缘迁移;当考虑马兰戈尼效应后,最高流速达到14m/s,马兰戈尼效应显著影响液相的迁移和表面形貌的形成。熔化、气化、液相迁移、溅射以及凝固是纳秒激光烧蚀钛合金的主要物理过程。随着烧蚀次数的增加,通过气化去除和迁移的材料体积增加。然而,随着烧蚀次数的增加,在烧蚀边缘的水平方向,因较低的表面张力和反冲压力导致孔径无显著增长。

Objective The titanium alloy microstructure processed by pulsed laser has been widely used in orthopedics or dental implants.It has been an essential method in the surface modification of medical titanium alloys.The surface microstructure processed by the short-pulse laser can promote the adhesion and proliferation of stem cells and inhibit bacterial colonization.The shape of the crater at the edge of the ablated micro-pit significantly affects the osseo integration and antibacterial properties of the implant.However,nanosecond laser ablation process of titanium alloy is a complex multiphysics problem,and the ablation process occurs in nanoseconds.The ablation process experiment cannot fully reveal the influence of process parameters on the formation of surface morphology.Therefore,this study employed the finite element method to simulate the forming process of titanium alloy micro-pit morphology.The changes in the temperature field,velocity field,and morphology with ablation time were also investigated.This research will promote the application of nanosecond laser modification technology in the field of titanium alloy implant surface treatment.Methods The Osher-Sethian level set method was used to describe the surface tension using the curvature of the level set isosurface of the free boundary.Additionally,the surface tension was introduced into the source term of the Navier-Stokes equation.Considering the effect of surface tension,Marangoni effect,and recoil pressure on the liquid phase under the action of Gaussian beam,a two-dimensional finite element model of nanosecond laser ablation of titanium alloy micro-pit was established.The evolution of the ablation temperature field with the ablation time was investigated to analyze the effect of surface tension and Marangoni on the velocity field of the ablation zone.The dynamic formation process of the titanium alloy micro-pit structure was clarified.The scanning electron microscopy was used to observe the morphology of the micro-pits,and a video microscope was used to obtain information on the optical topography and geometric dimensions of the surface.Finally,the reliability of the simulation model was verified by comparing simulation and process experiments.Results and Discussions Under the action of a Gaussian beam,the temperature field has an elliptical distribution(Fig.3),and subsequent pulses cause the temperature field to have a crescent-shaped distribution(Fig.5).In a pulse period,under the combined action of the Marangoni effect,recoil pressure,and surface tension,the liquid phase at the edge leaves the matrix at a certain speed and splashes around;thereby,forming droplet sputtering features around the ablation(Fig.11).The formation of surface morphology has successively undergone melting,vaporization(Fig.8),liquid phase migration,sputtering(Fig.9),and solidification(Fig.10).The cooling process is mainly affected by heat conduction.In contrast,air convection and heat radiation have little effect on the cooling process in a single pulse period(Fig.4).The liquid phase gradually solidifies from the ablation edge to its center when the ablation temperature is lower than the melting point,forming the crater morphology(Fig.10).The depth of micro-pits and height of the crater significantly increased as the number of ablation increased(Fig.13).However,the width of the groove and crater did not increase significantly with the number of ablation increased(Fig.15).Conclusions This study employed a level set method to establish a two-dimensional finite element model of nanosecond laser to simulate the forming process of titanium alloy micro-pit structure covering heat transfer,fluid flow,surface tension,and Marangoni effect.In a single pulse period,the Marangoni effect caused by the surface tension gradient significantly affects the formation of the crater structure during melting,vaporization,liquid phase migration,and solidification of the material.The formation of sputtered droplets arises from the combined effect of recoil pressure,surface tension,and thermal capillary force on liquid phase migration and sputtering.In the multipulse period,the differential surface tension of the ablation edge in the horizontal and vertical directions causes the height of the crater to increase significantly with the number of ablation;however,the width of the crater does not change significantly.Because the influence of plasma shielding on the incident laser was not considered in the model,it was only suitable for predicting the geometric size of the micro-pits when the number of ablation was less than 15times.