超快激光对金属材料烧蚀的超快动力学及超快结晶过程的机理研究

Ultra-fast laser has extremely high peak power by which it can treat materials with very high precision. Currently, most of the processing is using low energy pulses to guarantee precision by reducing heat affected zone (HAZ). However, the processing speed is pretty low by using low energy pulses. Considering the multi-photon absorption effect, it is possible to realize very high processing precision that even less than diffraction limit by modulating the high energy ultra-fast laser pulses in the center part of the laser spot. Meanwhile, the processing speed can be significantly improved due to much higher pulse energy. But the treatment process is involving complicated optics, mechanism, and thermodynamics. It is critical to get deep understanding of these physics to realize fast precise processing by ultra-fast lasers. This project is intending to build a Critical Point Model which can describe the optical properties of metals such as gold, copper, and aluminum both at room temperature and very high temperature. By incorporating this model into the Two-Temperature Model, the interactions between metals and ultra-fast lasers can be described and the non-thermal ablation induced by low energy pulses and thermal ablation induced by high energy pulses can be studied. Besides, the ultra-fast crystallization process driven by “under-cooling” will be investigated based on phase field model to reveal how the laser parameters influence microstructure of the materials. In order to close to the real processing, a three dimensional continuum model will be set up. Based on this model, how the ablation process and crystallization are influenced by laser parameters will be studied to provide direction of using ultra-fast lasers in more fields.

超快激光具有极高的瞬时功率，可以实现对材料的精密加工。为了减少热影响区，保证加工精度，目前大多数加工过程采用低能激光脉冲，但是低能脉冲加工效率低下。考虑材料对超快激光的多光子吸收效应，对高能超快激光进行调制，可以在光斑中心区域实现多光子吸收，获得小于衍射极限的加工精度，同时提高加工效率。本项目针对上述问题，建立能在室温和高温下描述金属光学性质的Critical Point Model，为了尽量接近实际加工过程，本课题将突破以往普遍采用的一维模型，建立三维连续介质模型，研究超快激光与材料之间的相互作用机理，包括材料对光的非线性吸收、冲击波导致的非热性烧蚀和高能激光脉冲带来的热性烧蚀，弄清楚激光参数与材料去除量之间的关系。同时，本项目还将采用相场模型研究过冷效应带来的超快结晶过程，揭示激光参数对材料表面微观组织的影响，为超快激光在医疗、微电子、航空等行业的应用提供坚实的理论依据。

本项目研究了超快激光作用下金属材料的光学性质变化，将热弹性模型与双温模型耦合，研究了激光能量的沉积规律和材料的热学与力学响应，采用相场法研究了超快激光作用下材料的超快相变过程，探讨了激光参数对晶粒形状和尺寸的影响规律。此外，本项目初步探讨了纳米金属粒子的局域电场增强效应对水的击穿阈值的影响。研究得到以下结论：.（1）提出的Critical Point Model可在200-1000 nm范围内描述铜和铝在常温下的表面反射率和吸收系数，并采用该模型研究了高温下铜和铝的光学性质的变化。研究发现，超快激光作用时，电子温度急剧升高，电子弛豫时间变短，电子可在短时间内吸收更多的能量，表面反射率减小，吸收系数减小。激光能量密度越大，光学性质变化越剧烈。.（2）由于光学性质的变化，在较低的激光能量密度下，热电子爆发力形成的冲击波会在材料内部会产生较高的拉应力，甚至超过材料的抗拉强度，造成材料的破坏，此时材料的温度低于熔点，不会产生明显的熔化现象和热影响区，该机制可以用于材料的精密加工。.（3）相场法研究表明，熔池底部具有较大的温度梯度和较小的冷却速度，熔池顶部则相反；减小激光能量密度、脉冲宽度或减少脉冲个数会获得较浅的熔池，熔凝区冷却速度加快，温度梯度变大。超快激光作用下铜的微观组织呈细长的柱状晶，二次枝晶臂间距小于1μm。减小激光能量密度、脉冲宽度或减少脉冲个数可使局部微观组织更微小，二次枝晶臂间距可小至0.2 μm。此时，部分二次枝晶臂的生长被抑制，微观组织由枝晶向胞晶转变。.（4）建立了飞秒激光与水中铝纳米粒子相互作用的光学击穿模型。利用该模型对纳米粒子与飞秒激光的相互作用进行了理论研究。研究发现，具有Al-SiO2核-壳结构的纳米粒子的最大外电场是纳米粒子内电场的4倍以上。对于二聚体和三聚体结构，在每个击穿阈值处的晶格温度都低于铝的熔点。在飞秒激光照射过程中，纳米结构可以使水电离而本身不发生消耗。该模型可帮助进一步理解铝纳米粒子介导下水的光学击穿现象。