水导激光微加工机理与研究进展

Mechanism and Research Advances of Water-Jet Guided Laser Micromachining

随着我国航空航天、微电子和医疗等领域的快速发展,组成元件逐步迈向微型化和精细化,这对零部件的切割以及微米级孔、槽等结构的加工质量和加工精度提出了更高要求。与传统的机械加工、电火花加工和电化学加工相比,激光加工可实现“非接触”加工,加工材料的适应性好、柔性大。但单一的激光加工得到的切口存在明显的锥度,且易出现热影响区、毛刺、裂纹等缺陷。水导激光加工技术利用水射流“同轴”全反射导引激光,兼具高质量和高效加工的优势,因而得到了广泛研究和应用。为对水导激光加工技术进行较为详尽的综述,本文首先讨论了水射流的形成、衰减和发散过程,然后从水的光学特性、全反射形成条件以及耦合能束传输等方面对激光-水射流耦合进行了分析,最后介绍了水导激光在金属、半导体以及复合材料等领域的应用现状,并针对现有技术的不足,对其发展趋势进行了展望。

Significance Withthe rapid development of the national aviation,aerospace,communications,instrumentation,and medical fields,components such as fuel nozzles,solar silicon light panels,semiconductor chips,and heart stents tend to be miniaturized and sophisticated.The quality requirements for structures,such as holes and grooves processedon related materials are increasing,which correspondingly translates into higher processing technology requirements.At present,scholars have developed a variety of processing methods,including but not limited to mechanical machining,electrical discharge machining(EDM),electrochemical processing,and laser processing.Duringthe mechanical machining process,the tool is in direct contact with the workpieces,resulting in significant stress.EDM is suitable for conductive materials.There is no obvious force during the machining process,but the machiningefficiencyis generally slow.Electrode loss exits,and the corner radius is limited.The electrochemical processingefficiencyis relatively high,and cathode loss is absent.But the processing stability is poor,and the electrolysisproducteasilyresults in environmental pollution.Compared with the above processing methods,laser processinghas obvious advantages.It has been widely used for drilling,grooving,and cutting operations in the aerospace,microelectronics,precision medical,instrumentation,and other industries.The continuous laser and the long-pulselaser have high processing efficiency.The continuous laser and the long-pulse laser have high processing efficiency.However,the generation of heat-affected zones and recast layers cannot be ignored.To achieve the eruptionand removal of the material,the ultrashort pulse laser directly converts the material into a plasma state.The ultrashort pulse can theoretically achieve the effect of"coldprocessing"but the processing efficiency is low.Nanosecond-level short pulse lasers have lower acquisition costs and a higher material removal rate than ultrashort pulselasers,but obvious defects such as heat-affected zones,recast layers,and micro-cracks still cannot be avoided.To overcome the thermal defects in the"drylaser"process,domestic and foreign researchers attempt to develop a compositesystemthat combines laser and water.Compared with"drylaser"processing,water-jetguidedlaser(WJGL)has many advantages—large working distance;no obvious cone,neat cut,and no burrs;small heat-affected zone;almost no thermal deformation and thermal damage;and high processing quality.This work has provided a relativelycompleteoverview of water-jet guided laser processing technology,allowing us to deeply understand the mechanism of water-jet guided laser processing technology,exert its processing advantages,and broaden its applicationfields.Progress Thework first analyzes the water beam fiber’sformation mechanism,including the formation of a stable water jet,the influence of the nozzle on the water jet,and the water jet’sattenuation and divergence process.Then the influence of the coaxial gas on the stability of the water jet and the situation after the jet hits the surface of the workpiece are analyzed.This work systematically elaborates on the optical properties of water and the conditions of total reflection formation to interpret the coupling process of the laser and the water jet.The factors affecting the coupledenergybeam’sstability,as well as the influence of the coupling error on energy distribution and jet stability,are explained.The status of water-jet guided laser applications in aerospace,semiconductor,medical,and other fields is reviewed.Based on the summary of the shortcomings of the current technology and the emergence of new requirements and challenges,the future development trend of water-jet guided laser processing technology has prospected.Conclusion and prospect Thiswork reviews a series of literature on water-jet guided laser and systematically expoundson its formation mechanism and its application potential.1)The formation of water jets is discussed.The"cone-down"nozzle makes it easy to form a stable"retracted flow"water jet.Factors,such as environment and nozzle geometry,are related to the breakage of the water jet.The introduction of an auxiliary atmosphere can increase the stable length of the jet,and the jet will form a liquid film after impacting the processing surface.2)The coupling process of the laser and water jet is discussed.The linear absorption of laser by water is an importantfactor in energy loss,and the laser energy exceeding the threshold causes stimulated Raman scattering.The laser transmission in the jet can be divided into two types:meridian transmission and oblique ray transmission.The coupling error determines the coupling efficiency and energy distribution,and together with the loss of energy,affects the stability of the coupled energy beam transmission.3)The excellent performance enables water-jet guided laser to be used in aerospace,chip manufacturing,precisionmedicine,and other fields to process various difficult-to-process materials such as metals,semiconductors,and composite materials.Modeling and simulation provide appropriate help for understanding the physical mechanisms involved in laser ablation and promote the conduct of related experiments and the extension of the applicationrangeof water-jet guided laser.A large number of studies on water-jet guided laser have strongly proved its application value.However,the processingcapabilitiesof water-jet guided laser are still limited under processing conditions such as high-quality processingrequirementsand small working spaces.At the same time,the processing technology for difficult-toprocessmaterials such as diamonds,sapphire,and super-hard ceramics still needs to be further explored.To meet these requirements,the possible research directions of water-jet guided laser in the future are as follows:1)Reduce water jet diameter and transmission energy loss of high-intensity input laser.2)Research on the law of focus movement due to the thermal interaction between the laser and water during the couplingprocess.3)A thorough examination of the interaction principle between laser,water jet,and material during water-jet guidedlaser processing.4)Research on the law between the energy distribution of the laser on the machined surface and the evolution of the surface topography.