Femtosecond laser processing of controlled tapered micro-holes based on dynamic control of relative attitude

Femtosecond laser drilling of metal necessitates precise control over taper to ensure dimensional accuracy. However, conventional vertical incidence laser processing methods result in micro-holes with inherent taper and a beam waist at the midportion. To address this issue, this paper proposes a micro-hole machining method that involves adjusting the relative attitude between the laser beam and the workpiece, enabling controlled machining of micro-holes with varying tapers. The feasibility of the method is validated by simulating theoretical hole profile using an isophote model, which exhibit good agreement with experimental results. Experimental findings on nickel-based alloys demonstrate a linear correlation between the attitude angle and hole taper. By manipulating the attitude angle, it is possible to achieve controllable machining, ranging from positive taper to negative taper. This approach eliminates the waist within the micro-hole and enhances chip removal space, resulting in improved hole wall quality. Moreover, employing multi-attitude angle and dynamic attitude angle processing, in addition to single attitude angle control, effectively eliminates the core generated during drilling. This method prevents the exacerbation of multiple reflections caused by the core and the reduction of chip removal space, leading to enhanced energy coupling between the laser and the material in the ablation area. As a result, the maximum depth of the hole is increased, allowing for the realization of no tapered micro-holes on 5 mm thick nickel-based alloys. Furthermore, laser processing introduces a significant amount of dislocation structures in the microstructure near the hole wall. This study introduces a novel approach to achieve controllable taper in femtosecond laser drilling. It offers both theoretical and experimental support for understanding the hole shape evolution process in femtosecond laser drilling.