飞秒激光烧蚀面齿轮材料的形貌特征研究 下载: 818次
Objective Face gear has been widely used in aviation, construction machinery, and other industries. With advancements in science and technology, the accuracy of face gear consequently demands higher requirements. Machining has met a certain bottleneck; therefore, femtosecond laser microcorrection face gear has been proposed as a new machining technology. Herein, the morphological characteristics of femtosecond laser ablated face gear material, such as pit diameter and depth, are investigated to provide a technical basis for improving the precision-machining quality of face gear.
Methods FemtoYL-100 all-fiber laser is used to generate 828 fs laser to ablate 18Cr2Ni4WA. A digital three-dimensional (3D) video microscope is used to observe the micromorphology, diameter, and depth of ablated pits. The experiment of single pulse femtosecond laser ablation gear material is conducted. Combined with the theoretical and experimental results, the ablation threshold is obtained from the quantitative relationship between the ablation pit diameter and laser power. Furthermore, the material absorption coefficient is obtained from the quantitative relationship between the ablation pit depth and laser power. As a result, the theoretical ablation model of multipulse femtosecond laser face gear material is established, and the effects of energy accumulation and variable defocusing are considered. The effect of variable defocusing amount suggests that with the increase in the number of pulses, the depth of ablation pits increases and the defocusing number of a single pulse femtosecond laser changes continuously. It implies that the defocusing amount of the central position of the laser spot changes with the change in the number of pulses. According to the theoretical model, changing the number of pulses and laser power, Matlab is used to simulate the change in the diameter and depth of femtosecond laser ablation pits. Then, the multipulse femtosecond laser ablation experiment is conducted to verify the accuracy of the theory.
Results and Discussions According to the linear relationship between the square of the diameter of the ablation pit and the logarithm of laser power (Fig.5), the ablation threshold of 18Cr2Ni4WA is 0.1383 J/cm 2. According to the linear relationship between the depth of ablation pit and the logarithm of laser power (Fig.6), the absorption coefficient of 18Cr2Ni4WA is 0.5188 μm -1. The results showed that when the pulse number is greater than 20, the diameter of ablation pits tends to be stable (Fig.10), and the energy accumulation coefficient is 0.9967. High power laser produced more liquid materials; thus, liquid materials will remain at the bottom or wall of the ablation pits and solidify to form molten materials, resulting in an uneven morphology of the ablation pits (Fig.11). With an increase in laser power, the residual molten material in the pits ablated by single-pulse femtosecond laser begins to appear, changing from a stripe structure to an irregular hole structure, mainly located at the bottom of the pits (Fig.12(a)). In the multipulse femtosecond laser processing, when the vaporized material pushes the liquid material away from the ablation pit, the liquid material, which fails to discharge the ablation pit, solidifies into a larger convex structure (Fig.12 (b)). The high temperature of the vaporized material and the accumulated energy in the material will lead to the second ablation of the material on the surface of the ablation pit, forming a smaller convex structure on the surface of the ablation pit (Fig. 12(b)). The smooth and highly reflective surface of the convex structure will make it difficult to be ablated and hinder the discharge of subsequent materials (Fig. 12(b)). With an increase in laser power, the residual melt in the multipulse ablation pits will increase, the quality of pit morphology will decrease, and the convex structure in the pits will be higher than the material plane (Fig.13). The multipulse femtosecond laser ablation pits with laser power of 1 W have better quality (Fig.13). However, when the pulse number is greater than 20, the second pulse number has no obvious effect on the quality of the pit morphology (Fig.13). By comparing the experimental and theoretical values of ablation pit depth under different power and pulse number, it can be seen that the theoretical model is reasonable, and the residual melt in the ablation pit generated by high power multisecond femtosecond laser will reduce the ablation pit depth (Fig.14). Under the condition of ensuring ablation depth and machining quality, it is more suitable to set the laser power of machining face gear material to 1 W (Fig.14).
Conclusions Herein, the ablation threshold and material coefficient of 18Cr2Ni4WA are determined. The quantitative relationship between the ablation pit diameter and pulse number is also determined. Furthermore, the quantitative relationship between the ablation pit depth and pulse number is established and verified. Based on the ablation depth and machining quality, it is found that the machining effect of multipulse femtosecond laser with 1 W laser power is the best. It provides parameters and theoretical references for femtosecond laser surface modification technology.
1 引言
面齿轮具有的重载、平稳传动特性,使其广泛应用于航空器等领域中。随着航空技术的发展,人们对面齿轮精度的要求也越来越高。但面齿轮齿形复杂、技术要求高、制造困难,机械加工精度已经到达瓶颈。飞秒激光具有脉冲宽度短、峰值功率高等特点,对于周围材料的热影响很小,几乎不存在长脉冲激光加工中的等离子体屏蔽效应[1-3]。此外,微纳级别的加工精度可突破衍射极限,使飞秒激光实现对各种材料的超精密加工[4-7]。因此,研究功率和脉冲数对飞秒激光烧蚀面齿轮材料18Cr2Ni4WA烧蚀凹坑直径和深度的影响具有重要意义。
目前,飞秒激光加工金属的研究主要集中在单质金属、成分比较简单的合金和半导体等,对于18Cr2Ni4WA的研究较少。彭元钦等[8]研究了激光功率以及离焦量对飞秒激光烧蚀钛铝合金TC4表面深度、形貌的影响,并在钛铝合金表面制备了一种二维周期结构,提高了钛铝合金表面的硬度,降低了其表面的磨擦系数。Genieys等[9]使用单脉冲飞秒激光辐照4种金属(铝、铜、镍和钨),并测量其烧蚀深度和直径随激光入射能量的变化情况,从而确定单脉冲状态下的烧蚀阈值和烧蚀速率。明瑞等[10]研究了烧蚀面齿轮材料18Cr2Ni4WA的电子亚系统和晶格亚系统的能量耦合作用,建立了双温模型,采用有限差分法仿真分析了激光脉冲宽度、平均功率对电子和晶格温度的影响规律,并通过实验观测了平均功率和脉冲数对材料微观形貌的影响。
本文结合理论和实验得到激光的烧蚀阈值和材料的吸收系数,并以此为基础研究变离焦量效应和能量累积效应,建立能量吸收模型,探索飞秒激光烧蚀过程中的能量吸收规律。此外,还研究了激光功率和脉冲数对面齿轮材料18Cr2Ni4WA烧蚀凹坑深度和直径的影响,并与飞秒激光实验进行了对比分析。
2 材料及方法
实验材料18Cr2Ni4WA是一种高强度的合金渗碳钢,除主要成分Fe外,还含有质量分数分别为4.25%、1.5%、1.0%的Ni、Cr、W等化学成分。为方便实验并提高实验的准确性,采用DK7725E型线切割机床切取试样,并对试样的待加工表面进行打磨,以去除表面应力和降低粗糙度。
表 1. 面齿轮材料18Cr2Ni4WA的参数
Table 1. Parameters of the face gear material 18Cr2Ni4WA
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表 2. 实验平台的运动参数
Table 2. Motion parameters of the experimental platform
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表 3. 飞秒激光烧蚀18Cr2Ni4WA的实验参数
Table 3. Experimental parameters of the femtosecond laser ablation of 18Cr2Ni4WA
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飞秒激光加工系统的原理如
实验采用的激光器为FemtoYL-100全光纤激光器,最大功率116.4 W,激光束的质量因子M2为1.259,能产生中心波长为1030 nm的脉冲激光,脉冲宽度为300 fs~6 ps,重复频率为25~5000 kHz,实验中的激光参数如
烧蚀实验示意图如
图 2. 飞秒激光烧蚀实验示意图。(a)单脉冲烧蚀;(b)多脉冲烧蚀
Fig. 2. Schematic diagram of femtosecond laser ablation experiment. (a) Single pulse ablation; (b) multipulse ablation
实验完成后,对烧蚀形成的凹坑进行检测,检测设备为数字式3D视频显微镜HIROX KH-7700,最大放大倍数为7000倍,测量精度为0.001 μm,其实物图如
3 单脉冲飞秒激光的面齿轮材料烧蚀特性
3.1 面齿轮材料的烧蚀阈值
烧蚀阈值指激光对材料造成烧蚀的临界能量密度,由材料本身的性质决定[11],即飞秒激光对材料具有固定的烧蚀阈值[12]。激光到达材料表面前会损失部分能量,为了提高实验和理论的准确性,实验中的激光功率均指激光达到材料表面时的功率,已考虑飞秒激光在光学系统和空气中的损耗,但未考虑材料反射的损耗。
飞秒脉冲激光的能量在空间上呈高斯分布,光斑的能量密度分布如
在光斑截面上对能量密度进行积分,可得到单脉冲能量Ep。也可通过激光功率P和脉冲重复频率fn得到单脉冲能量Ep,从而确定光斑中心能量密度F0,单脉冲能量Ep可表示为
若单脉冲烧蚀凹坑的直径为D,烧蚀凹坑边缘处的密度刚好足够发生烧蚀,定义距离光斑中心D/2处的能量密度为材料的烧蚀阈值Fth,则
将(2)式代入(3)式,整理得到
可以发现,烧蚀凹坑直径的平方D2与功率的对数值ln P满足线性关系,其斜率k=2
当烧蚀凹坑直径无限接近于0 μm时,光斑中心的能量密度就是材料的烧蚀阈值,即Fth=F0=2P/(π
3.2 面齿轮材料的吸收系数
目前关于18Cr2Ni4WA材料的实验研究较少,缺少可参考的18Cr2Ni4WA材料吸收系数,因此,尝试用铁的参数代替。由文献[ 13]可知,材料吸收系数b可以根据其消光系数κ和激光波长λ得到,即b=4πκ/λ。根据文献[ 14]给出的消光系数得到铁的材料吸收系数为50.5827 μm-1,而文献[ 15]中给出铁的材料吸收系数为71 μm-1,为了确认材料吸收系数的准确性,进行了理论和实验验证。
由于能量随传播距离的增加呈指数衰减[16],则距离材料表面H处的能量密度可表示为
式中,β为材料的吸收率。使R=0 μm,F(H,R)=βbFth,得到光斑中心位置处的烧蚀凹坑深度hmax为
可以发现,烧蚀凹坑深度hmax和功率的对数值ln P满足线性关系,其斜率k=b-1。根据不同功率P下的单脉冲烧蚀凹坑深度hmax绘制散点图,结果如
4 多脉冲飞秒激光的面齿轮材料烧蚀模型和仿真
4.1 多脉冲飞秒激光的能量累积效应
能量累积效应指前一个脉冲激光作用于材料结束后,一部分热量损失在外部环境中,而大部分热量被吸收后传递并累积在材料内部[17]。多脉冲飞秒激光加工时,能量累积效应导致低能量密度区域的能量密度随脉冲数不断累积,从而达到烧蚀阈值,使低能量密度区域材料被烧蚀。
设s为18Cr2Ni4WA的能量累积系数,可表示材料中能量累积效应的程度。当s=1时,材料中不存在能量累积效应。由文献[ 18]可知,飞秒激光脉冲的间隔时间越长,能量累积效应越弱。为了保证能量累积系数s相对固定,加工中保持脉冲频率不变。将脉冲激光按累积程度在材料内部残留的能量等价为本次脉冲激光的能量,则材料内部距离表面H处、第N个激光脉冲辐照后的能量密度可表示为
4.2 多脉冲飞秒激光的变离焦量效应
离焦量对微孔加工的影响如
图 7. 离焦量对微孔加工的影响。(a)正离焦量;(b)无离焦;(c)负离焦量
Fig. 7. Effect of defocusing amount on micro-hole processing. (a) Positive defocus; (b) no defocus; (c) negative defocus
变离焦量的示意图如
从高斯激光的聚焦方式可知,激光光束横截面半径ω(Δf)可表示为
可以发现,激光光束横截面半径ω(Δf)的增大分散了激光能量、降低了能量密度。由(1)式和(8)式得到光斑中心位置R=0 μm、离焦量为Δf处光斑的能量密度为
4.3 面齿轮材料对多脉冲飞秒激光的能量吸收模型
结合能量累积效应和变离焦量效应,由(7)式和(9)式得到材料吸收能量后内部的能量密度分布为
用Matlab仿真得到不同激光参数的烧蚀凹坑剖面轮廓及能量分布如
图 9. 烧蚀凹坑剖面轮廓及能量分布。(a) P=1 W;(b) P=3.2 W
Fig. 9. Profile and energy distribution of ablation pits. (a) P=1 W; (b) P=3.2 W
为了得到烧蚀凹坑直径和脉冲数之间的关系,使(10)式中的F(H,R)=βbFth,H=0 μm,即N个脉冲后距离光斑中心位置R处达到烧蚀阀值,得到烧蚀凹坑直径D=2R与脉冲数的定量关系为
可以发现,(11)式符合文献[
12]描述的烧蚀凹坑直径与脉冲数的关系。为了保证能量累积系数s的可靠性,在激光功率P=1 W,重复频率fn=200 kHz时,在脉冲数N=20、100、200、500、1000、2000、3000、4000、5000、6000的条件下烧蚀18Cr2Ni4WA并测量烧蚀凹坑直径D,绘制(N,D)散点图,同时通过仿真得到理论拟合曲线,结果如
烧蚀凹坑的深度会受变离焦量效应和能量累积效应的影响,若烧蚀凹坑的深度hmax位于光斑中心,即R=0 μm,H=hmax,F(H,R)=βbFth时,得到烧蚀凹坑深度hmax与脉冲数N的关系为
由文献[ 16]可知,电子和晶格温度的温度平衡时间在皮秒量级,且平衡温度受激光功率的影响,之后晶格温度仍旧持续降低。实验中的飞秒激光脉冲间隔时间为微秒量级,在下一个脉冲到达前,材料的晶格温度已远远下降至熔化温度以下,即烧蚀凹坑表面在脉冲间隔时间内就完成了冷却凝固。而飞秒激光烧蚀后的凹坑表面材料主要成分为铁的氧化混合物,即第1个脉冲烧蚀的是18Cr2Ni4WA,需考虑18Cr2Ni4WA的材料吸收率β1;之后的脉冲则需要考虑铁氧化混合物的材料吸收率β2。由于氧化混合物的厚度远远低于烧蚀凹坑的深度,因此,可忽略其对材料吸收系数的影响。此时,烧蚀凹坑深度hmax与脉冲数N的关系为
5 实验结果与分析
分别在激光功率P=1,1.7,2,2.7,3.2 W的条件下,用脉冲数N=20,100,200,500,1000,2000,3000,4000,5000,6000的多脉冲飞秒激光烧蚀材料。随着距离材料表面深度的增加,材料的能量密度呈指数衰减,当能量密度达到烧蚀阈值时,材料温度达到气化温度,材料会直接气化;当能量密度低于烧蚀阈值时,材料的温度低于气化温度但达到熔化温度,材料会熔化成液态;当能量密度继续降低时,材料的温度低于熔化温度,材料仍为固态。用飞秒激光烧蚀时,材料到达气化温度的时间很短,从而在烧蚀凹坑底部形成较大的气压差,气化材料带动液态材料沿烧蚀凹坑坑壁反向排出烧蚀凹坑外。随着激光功率的增大,熔化成液态的材料增多,导致烧蚀凹坑深度增加,液态材料排出烧蚀凹坑外所需的动能也逐渐增大,此时会有液态材料残留在烧蚀凹坑内并凝固形成熔融物,从而影响烧蚀凹坑的形貌。烧蚀凹坑内熔融物的残留形式如
图 11. 烧蚀凹坑内熔融物的残留形式。(a)残留在坑底;(b)残留在坑壁
Fig. 11. Residual form of melt in ablation pits. (a) Remain in the pit bottom; (b) remain in the pit wall
烧蚀凹坑内的残留熔融物形貌如
图 12. 烧蚀凹坑内的残留熔融物形貌。(a)单脉冲烧蚀凹坑;(b)多脉冲烧蚀凹坑
Fig. 12. Morphology of residual melt in ablation pits. (a) Single pulse ablation pits; (b) multipulse ablation pits
不同脉冲数和激光功率下的烧蚀凹坑形貌如
图 13. 不同激光功率下的烧蚀凹坑形貌。(a)1 W;(b)1.7 W;(c)2 W;(d)2.7 W;(e)3.2 W
Fig. 13. Morphologies of ablation pits under different laser powers. (a) 1 W; (b) 1.7 W; (c) 2 W; (d) 2.7 W; (e) 3.2 W
不同脉冲数N和激光功率P下烧蚀凹坑深度hmax的仿真与实验结果如
6 结论
开展了多脉冲飞秒激光烧蚀面齿轮材料18Cr2Ni4WA的烧蚀凹坑形貌研究,建立了多脉冲飞秒激光烧蚀18Cr2Ni4WA的能量吸收模型,并探究了不同脉冲数和激光功率下烧蚀凹坑直径和烧蚀凹坑深度的变化规律。实验结果表明,面齿轮材料18Cr2Ni4WA的烧蚀阈值为0.1383 J/cm2,吸收系数为0.5188 μm-1;脉冲频率为200 kHz的飞秒激光在面齿轮材料18Cr2Ni4WA上的能量累积系数s=0.9967,根据该累积系数建立的烧蚀凹坑直径与脉冲数的定量关系是可靠的;经20个激光脉冲后烧蚀凹坑深度可达到稳定,且多脉冲飞秒激光加工质量受脉冲数的影响较小,但随着功率的增加会有明显下降。在兼顾烧蚀深度和加工质量的前提下,激光功率为1 W的多脉冲飞秒激光加工效果最好。
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Article Outline
林嘉剑, 明瑞, 李学坤, 赖名涛, 马玉龙, 明兴祖. 飞秒激光烧蚀面齿轮材料的形貌特征研究[J]. 中国激光, 2021, 48(14): 1402017. Jiajian Lin, Rui Ming, Xuekun Li, Mingtao Lai, Yulong Ma, Xingzu Ming. Study on Morphology Characteristics of Femtosecond Laser-Ablated Face Gear Materials[J]. Chinese Journal of Lasers, 2021, 48(14): 1402017.