A femtosecond laser two-step helical drilling method was proposed, which realized the extremely high-quality machining of film cooling holes (FCHs) on nickel-based single crystal superalloy for turbine blades. The two-step helical drilling increased the exhaust capacity of machining residues, steam and laser-induced plasma, so it avoided the damage and defects, such as recast layers, microcracks and microvoids caused by laser energy shielding and heat accumulation. Furthermore, since more aggressive parameters can be adopted in the first step, and the amount of material removal in the second step was reduced, the two-step helical drilling improved the machining efficiency compared with one-time helical drilling. After two-step helical drilling, sub-wavelength nano-stripes were formed on the hole wall at a certain regular inclination angle along the hole depth direction. This characteristic feature was caused by the combined action of the periodic distribution of energy due to the interference of the laser and the plasmon, and the high-speed rotation and downward movement of laser beam. With this fundamental understanding of the mechanism, three processing parameters, i.e., laser output power (P), dwell time of single-layer scanning (T) and amount of focal plane drop for single-layer scanning (H), were coherently determined, such that the hole wall surface with nano-roughness without ablation can be realized.

提出了一种飞秒激光双步螺旋钻孔方法，实现了涡轮叶片镍基单晶高温合金气膜冷却孔（FCHs）的超高质量加工。两步螺旋钻孔增加了加工残留物、蒸汽和激光诱导等离子体的排出能力，从而避免了激光能量屏蔽和热量积累造成的重铸层、微裂纹和微孔等损伤和缺陷。此外，由于第一步可以采用更激进的参数，第二步的材料去除量减少，因此与一次螺旋钻孔相比，两步螺旋钻孔提高了加工效率。经过两步螺旋钻孔，亚波长纳米条纹沿孔深方向以一定的规则倾角在孔壁上形成。这种特征是由于激光和等离子激元的干涉引起的能量周期性分布，以及激光束的高速旋转和向下运动的共同作用造成的。有了对机制的这种基本理解，三个加工参数，即激光输出功率（P）、单层扫描停留时间（T）和单层扫描焦平面下降量（H）相干确定，从而实现纳米粗糙度孔壁表面无烧蚀。