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根据扭摆微冲量测量方法[10],冲量
$I$ 可以表示为:$$I = \frac{{J{\omega _n}}}{{{L_f}}}\exp \left( {\frac{\zeta }{{\sqrt {1 - {\zeta ^2}} }}\arctan \frac{{\sqrt {1 - {\zeta ^2}} }}{\zeta }} \right){\theta _{\max }}$$ (1) 式中:
${L_f}$ 为烧蚀力臂;${\omega _n}$ 为固有频率;$J$ 为转动惯量;$\zeta $ 为阻尼比;${\theta _{\max }}$ 为最大扭转角。脉冲激光烧蚀产生的微冲量所导致的扭摆横梁摆动,满足小角度假设条件,因此有:
$${\theta _{\max }} \approx \sin {\theta _{\max }} \approx \frac{{{d_{\max }}}}{{{L_m}}}$$ (2) 式中:
${d_{\max }}$ 为高精度位移传感器测量得到的扭摆横梁最大位移;${L_m}$ 为测量臂。因此,冲量$I$ 与${d_{\max }}$ 的关系为:$$I = \frac{{J{\omega _n}}}{{{L_f}{L_m}}}\exp \left( {\frac{\zeta }{{\sqrt {1 - {\zeta ^2}} }}\arctan \frac{{\sqrt {1 - {\zeta ^2}} }}{\zeta }} \right){d_{\max }}$$ (3) 式中:阻尼比
$\zeta $ 、无阻尼振动频率${\omega _n}$ 、转动惯量$J$ 为系统参数,需要在冲量测试前进行标定获得。典型的扭摆横梁扭转角随时间的变化如图5所示。 -
由于环境干扰的存在,造成位移传感器测量获得的位移数据在平衡位置上下波动,给直接计算冲量带来了干扰,因此需要对实验测量数据进行预处理,以减小测量噪声对冲量计算结果的影响。对于这类噪声,通常采用最小二乘拟合方法寻找试验数据点
$({x_i},\;{y_i})(i = 0,\;1,\; \cdots ,\;m)$ 的平均位置。在实际的烧蚀冲量测量中,拟合整个数据采集区间内的实验数据时,在波峰波谷处出现了拟合误差较大的情况,因此采用由局部到整体的拟合方法。对于需要拟合的每个实验数据点
$({x_i},\;{y_i})$ ,在其附近选取$2k + 1$ 个数据点$({x_{i - k}},\;{y_{i - k}})$ ,···,$({x_i},{y_i})$ ,···,$({x_{i + k}},\;{y_{i + k}})$ ,利用最小二乘法进行拟合。从采样的起点到终点,均进行上述局部拟合,最终获得整个采样区间的拟合结果,如图6所示,该方法能够较好的对实验数据进行平滑降噪处理。 -
图7给出了Al 5A06铝靶、TC4钛合金靶、30CrMnSiA不锈钢靶三种金属材料在不同能量密度下的冲量。实验中能量密度范围为2.25~25.81 J/cm2,对应的功率密度为0.26×109~2.97×109 W/cm2。可以看出三种材料的冲量均随着能量密度的增加线性增加。并且在相同能量密度下,钛合金TC4对应的冲量是三种材料中最大的,略大于烧蚀30CrMnSiA不锈钢靶获得的冲量,明显大于烧蚀Al 5A06铝靶材获得的冲量。这是因为当入射激光为1064 nm时,钛合金对激光的吸收率较高,而此时铝的吸收率较低[11],即在相同能量密度下,钛合金相对于其他两种金属材料将耦合更多的激光能量,因此将获得更大的冲量。
图 7 不同能量密度下,三种金属材料对应的冲量测量结果
Figure 7. The impulse of the three metal materials irradiated at different laser fluences
图8给出了Al 5A06、TC4、30CrMnSiA三种金属材料在不同能量密度下的冲量耦合系数。可以发现无论哪种金属材料,冲量耦合系数均随着能量密度的增加,先迅速增加到最大值,随后逐渐减小,并且普遍认为冲量耦合系数的下降是等离子体屏蔽效应导致的[12]。由于铝靶对入射激光的反射率较高,而钛合金和不锈钢对入射激光的反射率相对铝来说较低[13],烧蚀铝靶将损失更多的激光能量,能量耦合效率较低,因此,激光烧蚀TC4靶材获得的最大冲量耦合系数略大于烧蚀30CrMnSiA靶材,明显大于烧蚀Al 5A06靶材。通常我们称最大冲量耦合系数对应的能量密度为最优能量密度,可以发现,三种金属材料中,最大冲量耦合系数越大,对应的最优能量密度越小,即利用较低的脉冲激光能量,烧蚀TC4可以获得较好的冲量耦合效果。
图 8 不同能量密度下,三种金属材料对应的冲量耦合系数
Figure 8. The impulse coupling coefficient of the three metal materials irradiated at different laser fluences
能量密度为14 J/cm2时,由图7可知,烧蚀铝靶获得的冲量为3.5 μN·s,而前期研究中,烧蚀光斑尺寸为270 μm时,14 J/cm2的能量密度对应的冲量却只有0.4 μN·s[10]。这是由于在相同的能量密度下,烧蚀光斑越大,对应的脉冲激光能量就越高,沉积到靶面上的激光能量将烧蚀更多的靶物质,产生更强的等离子体羽流喷射,获得更高的反冲冲量。此外,图8中脉冲激光烧蚀铝靶获得的最大冲量耦合系数为20 μN·s/J,而前期研究中,烧蚀光斑尺寸为270 μm时,烧蚀铝靶获得的最大冲量耦合系数为30 μN·s/J[10],说明光斑尺寸越大,脉冲激光能量转化为靶材冲量的效率越低。这是由于光斑尺寸为270 μm时,等离子体羽流的喷射满足一维膨胀模型[14-15],并且羽流接近于细长圆柱形[9],即等离子体羽流主要沿着垂直于靶面的方向喷射,而当光斑尺寸为毫米量级时,羽流为近似半球形[16],即羽流除了沿着靶面法向喷射外,还会沿着平行于靶面的方向迅速膨胀,由于靶材冲量的获得来自于羽流喷射引起的反冲,因此,光斑尺寸为百微米时,脉冲激光能量转化为靶材冲量的效率更高,而且冲量耦合系数通常用来表示脉冲激光能量转化为靶材冲量的效率,即百微米聚焦光斑条件下获得的冲量耦合系数更大。
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烧蚀羽流吸收入射激光能量,导致到达靶面的激光能量减少,进而影响冲量耦合效应。等离子体羽流对入射激光的吸收能力可以用羽流对入射激光的透射率来表示。简化的羽流透射率表达式如下[17]:
$${\left( {\frac{{{C_{mi}}}}{{{C_{m1}}}}} \right)^2} = \frac{{\xi _1^2}}{{\xi _i^2}}\frac{{\left( {\ln {\tau _i} + \ln {\xi _i}} \right)}}{{\ln {\xi _1}}}\frac{{{\tau _i}{\xi _i} - 1}}{{{\xi _1} - 1}}$$ (4) 其中
$${\xi _1} = \frac{{{\phi _1}}}{{{\phi _0}}},{\xi _i} = \frac{{{\phi _i}}}{{{\phi _0}}}$$ (5) 式中:下标
$i$ 表示该量对应一确定的能量密度。${\phi _0}$ 、${\phi _1}$ 、${\phi _i}$ 分别表示产生靶蒸汽的能量密度阈值、实验中使用的最低能量密度以及某一确定的能量密度。${C_{m1}}$ 表示能量密度为${\phi _1}$ 时对应的冲量耦合系数,${\tau _i}$ 表示能量密度${\phi _i}$ 对应的透射率。基于公式(4)和公式(5)获得的羽流对入射激光的透射率及其对应的冲量耦合系数如图9所示。可以看出,随着能量密度的增加,三种材料对应的羽流透射率先是急剧降低,当冲量耦合系数达到最大值后,透射率减小趋缓。这是由于入射激光能量主要被羽流中的等离子体所吸收,而等离子体的密度主要与电离度有关,电离度随着能量密度的增加先急剧增加,随后逐渐饱和[18]。此外,当冲量耦合系数达到最大值时,透射率大约为0.3,即此时多数激光能量被等离子体羽流吸收。当能量密度大于最优能量密度后,羽流的透过率低于0.3,即产生了很强的等离子体屏蔽效应,激光能量无法到达靶面进行冲量耦合,进而导致冲量耦合系数逐渐下降。
Impulse coupling characteristics of typical metal materials irradiated by nanosecond laser with a millimeter-scale spot size
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摘要: 利用纳秒脉冲激光器对典型金属材料在毫米级烧蚀光斑尺寸下的冲量耦合特性进行了实验研究。基于典型扭摆测量系统测量激光烧蚀产生的冲量特性,采用局部最小二乘法平移拟合的方法,对扭摆振动产生的微小位移实验数据进行了预处理,避免了噪声对冲量测量的干扰。设计了一种毫米量级烧蚀光斑尺寸测量方法。在此基础上,实验获得了毫米量级光斑辐照金属靶材Al 5A06、TC4、30CrMnSiA的冲量,研究结果表明,在相同能量密度的情况下,钛合金TC4对应的冲量最大,TC4获得的最大冲量耦合系数大于Al 5A06和30CrMnSiA,其对应的最优能量密度却是三种材料中最小的。为了分析烧蚀羽流对冲量耦合特性的影响,估算了不同能量密度对应的羽流透射率,计算结果表明,当羽流透射率低于0.3时,大量的入射激光能量被羽流吸收,导致冲量耦合系数的下降。Abstract: A nanosecond pulsed laser was used to study the impulse coupling characteristics of typical metal materials with millimeter ablation spot size. The impulse characteristics of laser ablation were studied based on a typical torsion pendulum measuring system. By using the method of translational fitting of the local least square method, the experimental data of small-displacement generated by torsion pendulum vibration were preprocessed, and the interference of noise to impulse measurement was avoided. A measurement method of millimeter scale ablation spot size was designed. On this basis, the impulse of millimeter scale spot irradiating metal target Al 5A06, TC4, 30CrMnSiA was obtained in the experiment. The results showed that the impulse generated by irradiating titanium alloy TC4 was the largest at the same laser fluence. And the maximum impulse coupling coefficient generated by irradiating TC4 was greater than that of Al 5A06 and 30CrMnSiA. However, the optimal laser fluence of TC4 was the smallest among the three materials. In order to analyze the influence of the ablation plume on the impulse coupling characteristics, the plume transmittance at different laser fluences was estimated. The results indicated that a large amount of incident laser energy was absorbed by the ablation plume when the plume transmittance was lower than 0.3, which caused a decrease of the impulse coupling coefficient.
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