Objective With the development of the aerospace field, high temperature resistant materials such as nickel-based alloys and gas film cooling holes are gradually applied to the hot end parts of aero engines, and the manufacturing requirements for the gas film holes of the hot end parts of aero engines are also increasing. Traditional processing methods such as EDM and electrochemistry can no longer meet the manufacturing requirements for machining high-quality microholes on nickel-based alloys. Due to its precision, low damage and cold processing characteristics, femtosecond ultrafast laser is increasingly suitable for the processing of nickel-based alloy microholes, but the problems such as thermal defects and insufficient taper in the processing of large depth to diameter microholes still need to be overcome. To solve the taper problem of nickel-based alloy material deep microhole machining, the femtosecond laser machining of nickel-based alloy without taper single hole experiment was carried out in this paper.
Methods A femtosecond laser numerical control machining system (Fig.1) is adopted to carry out non-taper single-hole machining experiments on nickel-based alloy materials based on the laser processing technology of rotating drilling of inclined workpieces. The influence rules of laser defocusing amount, laser repetition frequency, laser scanning radius and laser optical axis offset on single-hole morphology are investigated by using the control variable method, and then the process is optimized. Three processes of drilling, reaming and repairing are adopted (Tab.3), and an auxiliary device for side-shaft blowing is added (Fig.8).
Results and Discussions The effects of laser defocusing amount (Fig.4), laser repetition frequency (Fig.5), laser scanning radius (Fig.6) and laser optical axis offset (Fig.7) on the morphology of single hole are investigated, the optimal process parameters are obtained, and high-quality single hole machining with a taper of 0.04° is realized on nickel-based alloy with 2 mm thickness. On the basis of process optimization, three processes of drilling, reexpanding and hole repair are adopted, and auxiliary technology of side shaft blowing is added, thus deep microhole machining with a taper of 0.14° is realized on 6 mm thick nickel-based alloy (Fig.9).
Conclusions In order to solve the taper problem of deep microhole machining of nickel-based alloy materials, based on the laser processing technology of rotary drilling of inclined workpiece, the processing method and experimental principle are firstly described. Then, the experiment of single-hole machining without taper is carried out on nickel-based alloy materials. It is found that when the laser defocus quantity gradually changes from positive defocus to negative defocus, the inlet diameter of the microhole is basically unchanged, and the outlet diameter increases significantly. When the laser repetition rate is small, the outlet diameter of the microhole is smaller and the taper is larger. As the laser scanning radius increases, the diameter of the entrance and exit of the hole also increases, but the taper of the hole is basically unchanged. In addition, it is necessary to keep the bias between the laser and the workpiece rotation axis within 10 μm in order to realize non-tapering single-hole machining. Then, on the basis of optimization of process parameters, longitudinal feed and blowing are used to assist. The microhole machining with a large depth-to-diameter ratio of 0.14 has been successfully achieved on a 6 mm thick nickel-based alloy.
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