Kevlar-29材料的飞秒激光制孔形貌及性能研究

Characteristics and Properties of Femtosecond Laser Drilling ofKevlar-29 Substrates

为提升飞秒激光加工芳纶纤维复合材料制孔质量,优化制孔加工参数,进行了制孔试验。用共聚焦显微镜与超景深显微镜观察圆孔边沿形貌,并测量了热影响区大小。分析了激光功率、扫描速度和重复频率对圆孔的形貌、热影响区大小以及几何精度的影响,对比了激光制孔与机械制孔后材料的拉伸强度。研究发现,一定参数范围内,激光功率增加、重复频率降低或扫描速度减小使热影响区尺寸整体上逐渐变大。但激光功率不足导致的烧蚀不充分也会产生较大的热影响区与表面损伤。热影响区最优状态对应的激光参数为激光功率5 W,扫描速度1050 mm/s,重复频率200 kHz。对比传统机械加工,飞秒激光加工后材料的拉伸强度波动性更小。结果表明,芳纶纤维复合材料的飞秒激光制孔加工不是完全的“冷”加工,由于芳纶纤维导热性差,热量累积造成纤维及纤维周边基材的热损伤,仍会导致较小的热影响区产生。采用合适激光参数的飞秒激光对芳纶纤维复合材料进行制孔加工,能有效提升制孔质量,满足相关领域精度和强度的要求。

Aramid Fiber Reinforced Plastics(AFRP)are with excellent physical properties,but defects such as burrs and damage may result from traditional mechanical drilling methods,whereas the potential to avoid these issues exists in laser drilling technology.However,thermal effects would induce severe thermal damage to AFRP components during the ablation process by continuous wave laser or nanosecond laser.It is suggested that ultra-short pulse width of ultrafast lasers can effectively suppress thermal diffusion.Accordingly,drilling experiments were conducted on one of the AFRP substrates,i.e.,Kevlar-29 by using femtosecond laser in this study,aiming to improve the quality of laser drilling,reduce thermal damage and thermal influence zones during laser ablation,enhance the geometric accuracy of circular holes,and optimize the processing parameters of femtosecond laser drilling.The experimental material is a 2 mm thick Kevlar-29 fiber-reinforced composite,the laser focal plane is set at half the thickness of the material,and the designated hole diameter is 6 mm.The laser wavelength is 1030 nm and the pulse width is 480 fs.Firstly,the drilling effect of two scanning strategies,cross-scan and concentric ring scan,was compared,and the scanning strategy with superior drilling effect was determined.Furthermore,the influence of laser processing parameters(laser power,laser repetition rate and laser scanning speed)on drilling quality was analyzed using orthogonal matrix experiments.Confocal microscopy and ultra-depth microscopy were used to observe the circular hole morphology after laser drilling,and the size of the thermal influence zone was measured.The effects of laser power,scanning speed,and repetition rate on circular hole morphology,thermal influence zone size,and geometric accuracy were analyzed.Finally,quasi-static tensile tests were conducted using a universal tensile testing machine,and the tensile strength of the samples after laser drilling was compared with those by using mechanical drilling processes.It has been found in this research that concentric ring scanning is more efficient than cross-scanning and the roundness of hole topography of concentric ring scanning is better.Within a certain femtosecond laser processing window,the overall size of the heat-affected zone will gradually increase as the laser power increases.Nevertheless,this increasing trend of heat-affected zone develops when the repetition rate is reduced,or the scanning speed is decreased.Moreover,a larger heat-affected zone and surface damage can also be produced if insufficient burning happens due to inadequate laser power.The optimal laser parameters for the heat-affected zone are a laser power of 5 W,a scanning speed of 1050 mm/s,and a repetition rate of 200 kHz.The decrease in the taper angle of the femtosecond laser drilled hole is proportional to the laser power increasing and repetition rate decreasing.When the laser repetition rate is 200 kHz and the laser power reaches 9 W,a minimum taper angle of the hole is obtained as 6.99°,meantime a relatively large heat-affected zone is produced under these process parameters.It is found that the optimal taper angle of the hole is 14.10°with the least heat-affected zone.The tensile strength of the material after femtosecond laser processing is similar to that of traditional mechanical processing,but the fluctuation of tensile strength test performance is smaller.It has been indicated by the results that femtosecond laser drilling of Kevlar-29 fiber-reinforced composite is not a completely“cold”process,as thermal damage to the fibers and the surrounding matrix material is seen which is caused by the poor thermal conductivity of aramid fibers,hence resulting in the appearance of a relatively small heat-affected zone.To summarize,the use of appropriate laser parameters for femtosecond laser drilling of Kevlar-29 fiber-reinforced composite can effectively reduce the size of the heat-affected zone and improve the geometric accuracy of the hole so as to meet the requirements for precision and strength in related application fields.