With the 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 processed on 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. During the 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 machining efficiency is generally slow. Electrode loss exits, and the corner radius is limited. The electrochemical processing efficiency is relatively high, and cathode loss is absent. But the processing stability is poor, and the electrolysis product easily results in environmental pollution. Compared with the above processing methods, laser processing has 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-pulse laser 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 eruption and 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 "cold processing" but the processing efficiency is low. Nanosecond-level short pulse lasers have lower acquisition costs and a higher material removal rate than ultrashort pulse lasers, but obvious defects such as heat-affected zones, recast layers, and micro-cracks still cannot be avoided. To overcome the thermal defects in the "dry laser" process, domestic and foreign researchers attempt to develop a composite system that combines laser and water. Compared with "dry laser" processing, water-jet guided laser (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 relatively complete overview 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 application fields.
The work first analyzes the water beam fiber’s formation mechanism, including the formation of a stable water jet, the influence of the nozzle on the water jet, and the water jet’s attenuation 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 coupled energy beam’s stability, 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.
This work reviews a series of literature on water-jet guided laser and systematically expounds on 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 important factor 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, precision medicine, 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 application range of water-jet guided laser.
A large number of studies on water-jet guided laser have strongly proved its application value. However, the processing capabilities of water-jet guided laser are still limited under processing conditions such as high-quality processing requirements and small working spaces. At the same time, the processing technology for difficult-to-process materials 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 coupling process.
3) A thorough examination of the interaction principle between laser, water jet, and material during water-jet guided laser processing.
4) Research on the law between the energy distribution of the laser on the machined surface and the evolution of the surface topography.
