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激光束辐照特定工件,激光能量和被加工物质之间产生热效应作用及非热效应作用。其中热效应作用使材料表面产生熔化、蒸发现象,进而去除材料表面物质;而非热效应作用则是由于金属表面吸收激光能量使温度快速升高, 超过金属的熔点和蒸发温度使物质产生高度电离,导致金属的烧蚀[11]。由于温度不同,两种机理下产生的加工分别被称为热加工和冷加工[12]。
飞秒激光去除物质的机理主要基于冷加工,能在金属表面加工的同时,产生最小的热影响区,因而是一项微纳尺度内材料精密加工的新技术[13], 其加工过程如图 1所示。考虑到激光参量(功率、波长、脉冲持续时间等)、金属自身属性等因素,将飞秒激光加工金属的机理细分为熔化、热汽化、库伦爆炸、雪崩电离与多光子电离等几种作用机理[14],在实际加工中,通常是以雪崩电离与多光子电离为主要机理[15-16]。自由电子在吸收激光能量后产生较大的动能,与原子碰撞后产生多个自由电子,此过程不断反复,形成雪崩电离现象,如图 2所示。飞秒激光辐照金属表面时,雪崩电离和多光子效应使大量的金属价带电子转变为高温自由电子,当其积累到一定密度时,激光能量被材料大量吸收,形成的高温高压等离子体以喷射的形式被剥离母材表面,达到材料去除的目的[11, 16]。
为了研究超短脉冲激光加工金属材料过程中的温度变化,ANISIMOV等人[17]在1974年提出双温模型(two temperature model, TTM)[2, 18],该模型主要研究金属材料表面的电子温度Te和晶格温度Tl随时间的演变:
$ {C_{\rm{e}}}\left( {{T_{\rm{e}}}} \right)\frac{{\partial {T_{\rm{e}}}}}{{\partial t}} = \frac{\partial }{{\partial z}}\left( {{\kappa _{\rm{e}}}\frac{{\partial {T_{\rm{e}}}}}{{\partial z}}} \right) - g\left( {{T_{\rm{e}}} - {T_1}} \right) + S(z, t) $
(1) $ {C_1}\left( {{T_1}} \right)\frac{{\partial {T_1}}}{{\partial t}} = g\left( {{T_{\rm{e}}} - {T_1}} \right) $
(2) 式中, Ce和Cl分别为电子温度热容、晶格热容;κe为电子热导率;g为电子-晶格耦合常数;S(z, t)为单位体积内的激光热源辐照的功率密度。
飞秒激光与铜作用时表面电子温度和晶格温度随时间t的演化曲线如图 3所示。电子温度在数百飞秒的时间内迅速升高,快速到达峰值12000K,而此期间的晶格温度基本没有上升。这就表明激光辐照金属表面时,材料的去除主要是大量的激光能量被自由电子接收使其温度升高而导致的。当辐射结束后,电子温度与晶格温度以耦合的方式达到平衡状态[19]。
图 3 表面电子温度和晶格温度随时间的演化曲线[19]
飞秒激光在金属微加工中的应用
Application of femtosecond laser in metal micromachining
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摘要: 作为一种新型的减材加工技术,飞秒激光在材料微加工中具有独特优势。介绍了飞秒激光加工的机理,分析了飞秒激光加工效率和加工质量的影响因素,阐述了飞秒激光加工工艺参量及表面质量的预测方法,对飞秒激光与增材制造的结合应用作了展望。飞秒激光加工的效率与精度影响因素众多,要真正在金属加工领域精准大规模应用这一精细技术,尚需对飞秒激光及其与不同特性金属材料间的交互作用进行更为深入系统的研究。Abstract: As a new type of subtractive processing technology, femtosecond laser has unique advantages in material micromachining. The mechanism of femtosecond laser processing was introduced, and the factors that influence the femtosecond laser processing efficiency and processing quality were analysed. The prediction method of femtosecond laser processing parameters and surface quality was expounded. The combined application of the femtosecond laser and additive manufacturing was thenprospected. There are many factors affecting the efficiency and accuracy of femtosecond laser processing. To truly apply this fine technology on a large scale in the field of metal processing, more in-depth study of the system of femtosecond laser and its interaction with the different properties of the metal material is still needed.
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图 3 表面电子温度和晶格温度随时间的演化曲线[19]
图 4 不同激光能量密度下单脉冲烧蚀得到的烧蚀坑模拟结果[23]
a—2J/cm2 b—5J/cm2 c—8J/cm2
图 5 整个加工过程中的形貌演变和温度曲线[28]
图 6 离焦量对加工表面质量的影响[41]
图 7 不同激光入射角加工表面形貌[44]
a—激光入射角60° b—激光入射角45°
图 8 扫描速率对加工表面质量的影响[12]
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