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实验中使用的飞秒激光器型号为PharosPHI-20,该激光器输出光束波长为1026nm,脉冲宽度290fs,最大输出功率20W,最大脉冲重复频率100kHz。通过控制系统可以直接调节飞秒激光器的脉冲能量、重复频率、扫描速率、填充间距等参数。激光束通过焦距160mm的透镜聚焦后形成的光斑直径大约为44μm。飞秒激光加工系统如图 1所示。
实验中所使用的材料为2.5维编织、经由先驱体浸渍裂解工艺制备而成的SiC/SiC复材厚板,试样尺寸为30mm×15mm×4mm。复材中单根SiC纤维直径12μm,SiC纤维的体积分数为0.45。SiC/SiC复合材料试样的密度为2.3g/cm3,比热容为1002J/(kg·K),热导率为12.75W/(m·K),气孔率约为10%。
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制孔加工采用光束同心圆填充扫描方式进行,如图 2所示。光束扫描从最外圈轨迹开始,最外圈轨迹圆直径为700μm,然后以相同的扫描速率加工到最内圈,完成一次扫描。扫描100次完成一层加工,光斑按照设定进给量沿垂直方向(z向)进给,进行下一层扫描,直到完成进给次数。
加工参数变化范围为:脉冲能量E为65μJ~130μJ, 脉冲重复频率f为20kHz~100kHz, 光束扫描速率v为100mm/s~300mm/s, 线重合度δl为9%~77%, 进给距离h为0.1mm~0.5mm。上述参数中,脉冲能量、重复频率、扫描速率和线重合度均与光束扫描面积内的能量密度Q有关,Q值(单位为J/mm2)采用下式进行计算:
$ Q=\frac{E f s}{\pi R^2 v} $
(1) 式中,s为按照加工轨迹扫描一次激光束移动的距离,R为理论加工孔径。
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加工后的试件经超声波清洗烘干,在扫描电子显微镜(scanning electron microscope,SEM)下观测小孔的加工表面及入口直径,并利用工业断层扫描(computerized tomography, CT)技术检测小孔锥度。小孔锥度θ采用下式进行定义:
$ \theta=\arctan \left(\frac{D-d}{H}\right) $
(2) 式中, D为入口直径,d为出口直径,H为板材厚度(通孔状态)或小孔实际深度(盲孔状态)。
飞秒激光加工SiC/SiC复合材料厚板的孔型特征研究
Hole shape characteristics in femtosecond laser drilling of SiC/SiC composite thick plate
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摘要: 为了探究飞秒激光加工SiC/SiC复合材料厚板的孔型特征, 采用光束同心圆填充扫描方式对厚度为4mm的SiC/SiC复合材料进行制孔实验, 分析了飞秒激光加工参数对入口直径、孔深、锥度等孔型特征的影响规律和影响机理。结果表明, 脉冲能量、重复频率、线重合度以及扫描速率对小孔入口直径影响较小, 但对孔深和锥度影响较大; 上述实验参数与光束扫描面积内的能量密度密切相关, 小孔锥度随能量密度增大而减小, 小孔深度则反之; 当采用最大脉冲能量130μJ、最大重复频率100kHz、最小扫描速率100mm/s、最大线重合度77%以及最小进给量0.1mm时, 小孔锥度达到最小值12.38°; 上层材料对光束的遮挡以及排屑困难导致深孔加工锥度不易控制。该研究可以为今后SiC/SiC超快激光制孔应用提供参考。
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关键词:
- 激光技术 /
- 孔型特征 /
- 飞秒激光 /
- SiC/SiC复合材料
Abstract: In order to evaluate the hole shape characteristics in femtosecond laser drilling of SiC/SiC composite, an experimental study has been conducted on 4mm thick specimen using beam concentric scanning mode. The effect of process parameters on entry diameter, hole depth, and hole taper has been analyzed. The results show that pulse energy, pulse frequency, line overlap ratio, and scanning speed have almost no influence on entry diameter. However, these factors significantly affect hole depth and hole taper. The pulse energy, pulse frequency, line overlap ratio, and scanning speed markedly influence energy density per unit area that during scanning of the laser beam. Higher energy density results in lower hole taper and greater hole depth. The experiment reached a minimum taper angle of 12.38° using the maximum pulse energy 130μJ, maximum repetition frequency 100kHz, minimum scanning speed 100mm/s, maximum line overlap 77% and minimum feed distance 0.1mm. It is concluded that the taper angle can hardly be controlled because the upper material will shelter part of the laser beam and there is difficulty to evacuate chips. This study can provide a reference for the future application of ultra-short pulse laser drilling of SiC/SiC composites.-
Key words:
- laser technique /
- hole shape characteristics /
- femtosecond laser /
- SiC/SiC composite
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