# Research on Microhole Processing Technology Based on the Femtosecond-Laser Spiral Trepanning Method
## 1 Introduction  
With the development of the aerospace industry, increasing the inlet temperature of the turbine is the most effective way to achieve a high thrust-to-weight ratio of new aero-engines [1; 2]. Therefore, it is necessary to machine film holes on the blade to cool and isolate the blade. By varying the size and taper of the film holes, the flow characteristics of the turbine can be changed to produce different film cooling effects [3]; therefore, the machining of film holes with different tapers has always been a key technology for machining aero-engine turbine blades. Besides, in the fuel injection structure, the micro-hole cone in the fuel injection nozzle has a very important influence on fuel atomization [4]. Microholes play an important role not only in the aviation [5] and automotive [6] industries but are also relevant to the field of microfluidics, where they are used in microfluidic chips [7], electronics [8], biomedicine [9], and other fields [10]. Therefore, it is of great significance to realize the controllable roundness, accuracy, and taper machining technology for engineering applications. At present, the main microhole machining methods include electrochemical machining [11; 12], electrical discharge machining [13; 14], electron-beam machining [15], ultrasonic machining [16], and ion-beam processing [17]. However, comparing with femtosecond-laser machining [18; 19], the electrochemical machining, electrical discharge machining and ultrasonic machining are complexto process microholes of different sizes and tapers. Besides, electron-beam machining is extremely expensive. Therefore, electron-beam machining is not suitable for practical industrial applications. The commonly used laser microhole processing methods are mainly divided into three categories (Figure 1): percussion drilling [20], ring cutting [21], and spiral trepanning [22]. In percussion drilling, the laser is directly focused on the material to make impact. Here, the hole type and accuracy depend on the beam quality after focusing, and the microhole depth-to-diameter ratio is not high; thus, it is difficult to process deep holes; moreover, the aperture needs to be adjusted to adjust the focusing lens and laser power, and the molten material can easily accumulate near the microhole, resulting in poor processing quality. For example, [PERSON] et al. [23] used a femtosecond laser to perform impact drilling experiments on a copper surface, and many recast layers and impurities occurred around the microholes. The method is difficult to achieve microholes of different sizes and tapers. The second method, ring cutting, involves repeatedly scanning the sample along a circular path; however, with the increase in the cutting depth, removing the material becomes increasingly difficult, which can lead to the repeated heating of the surface chip and the subsequent formation of a recast layer and a heat-affected zone. [PERSON] et al. [22] achieved annular drilling on K24 superalloy by femtosecond-laser processing, and the results showed that significant impurities occurred around the microholes. Besides, the ring cutting method cannot process different taper microholes. The third method, spiral trepanning, involves moving the focused spot scan along a certain trajectory, using an optical system. The spiral trepanning is conducted from the inside to the outside, and the material is removed step by step from the top to the bottom. The heat-affected zone and the recast layer are also smaller than those of the other processes, which can achieve the accurate control of the microhole size. The advantage of this method is the highly precise control of the hole diameter. The hole size can be arbitrarily adjusted, and a high depth-to-diameter ratio can be achieved. [PERSON] et al. [22] also achieved spiral trepanning on K24 superalloy by femtosecond-laser processing, and the quality of microholes had been greatly improved. However, the method could not achieve inverted an cone hole.