北京理工大学机械与车辆学院激光微纳制造实验室, 北京 100081
Objective As a typical metal microstructure, the metal microgroove structure is widely used in electronics, communications, aerospace, biomedicine, and other fields. With more applications of metal microgrooves in key parts, higher requirements are put forward on the quality and accuracy of microgrooves. For example, in the micro-heat exchange device, the heat transfer pipe with a rectangular cross-section microgroove array structure has better heat transfer performance than other shapes of the microgroove array structure, and the microgrooves with a width less than 50μm show better heat exchange efficiency. In addition, in the field of biomedicine, rectangular microgroove arrays smaller than 50μm have been proven to have better cell orientation effects than rectangular microgroove arrays of 60μm. The precision manufacturing of microgroove structure often requires high machining accuracy (below 100μm), and the machining edge is free of burrs. Besides, the microgroove structure is often processed on many difficult-to-machine materials with high hardness, toughness, and wear resistance. Traditional metal processing methods, such as traditional cutting, electric discharge machining, and electrochemical machining, are often troubled by insufficient precision or difficult material processing when processing high-precision metal microstructures. Ultrafast laser processing has the advantages of high instantaneous power density, low heat-affected zone, and a wide range of materials that can be processed, and it is playing an increasingly important role in precision processing. Ultrafast laser processing has become an essential processing method, especially for difficult-to-process materials such as ceramic materials, superalloys, and superhard materials. However, due to the uneven light field distribution and shielding effect caused by the processing process, the processed micro groove wall is often accompanied by a certain slope, which affects its application performance. Therefore, reducing the groove wall slope while ensuring accuracy is an urgent problem to be solved. In the present study, we adopt an spatial shaping ultrafast laser processing system, based on the principle of beam shaping, built to modulate the Gaussian beam before focusing on a rectangular flat-top light and explore the influence of spatial shaping light on the microgroove structure and reduction of taper.
Methods In this study, the laboratory's existing femtosecond laser processing experimental system was used to conduct experimental investigations on nickel-based superalloys. The spatial light modulator (SLM) was used to phase-shape the femtosecond laser, and the Gaussian light was shaped into a rectangular flat-top light and the processing experiment of the microgroove was on the nickel-based superalloy. Then, the surface morphology and three-dimensional morphology of the microgrooves processed by the spatial shaping light were analyzed by scanning electron microscope (SEM) and three-dimensional white light interferometer. In the next step, by adjusting the laser parameters, the processing parameters of the microgroove using the spatial shaping light was studied, and the microgroove and through groove with variable width were processed. In addition, the surface morphology and chemical composition of the microgroove were analyzed by SEM and energy dispersive X-ray spectroscopy (EDX). In addition, the effect of femtosecond laser processing on the oxidation of the microgroove was studied by EDX mapping.
Results and Discussion Compared with the Gaussian light, the slope of the microgroove obtained by the spatially shaped light is significantly reduced, and the groove wall profile is straighter. By comparing the effects of different scanning speeds at the same energy, it is found that at a scanning speed of 2000μm/s, as the scanning time increases, the depth of the microgroove gradually increases, and the depth change rate first increases and then decreases. At scanning speeds of 1000μm/s and 500μm/s, the depth of the microgroov gradually increases with the increasing scanning speed, and the depth change rate gradually increases. The analysis shows that with the increase in the scanning speed,the number of pulses deposited in the unit area of the superalloy decreases. After reaching a certain depth, as the processing debris at the bottom of the groove increases, the shielding effect increases, so that the average removed amount of a single pulse is reduced (Fig. 5). As the depth of the microgroove increases, the slope of the groove wall gradually decreases and the minimum slope of the groove wall can reach 1° or less (Fig. 6). In addition, deep grooves with width of 10, 20, and 150μm were processed using spatial shaping light, and the groove wall slope of the deep grooves with a width of 150μm reached 0.63° (Fig. 8 and Fig. 9). The elemental analysis and characterization of the groove wall found that the microgroove wall did not undergo significant oxidation, which should be attributed to the excellent cold working ability of the femtosecond laser (Fig. 10).
Conclusions In this study, the strategy of spatially shaped femtosecond laser is adopted and the Gaussian beam is formed into a rectangular flat-top beam by the SLM for metal microgroove processing. Compared with the processing result of the Gaussian beam, the slope of the groove wall fabricated by flat-top beam is significantly reduced. In addition, the method has been used to realize the processing of micro-deep grooves ranging from 10μm to 100μm, indicating that the method has a wide range of processing dimensions. Elemental analysis and morphological observation of the deep groove wall section were carried out. No obvious element changes and laser heat-affected zone were found, indicating the excellent processing ability and great application potential of this method.