New solution for machining deep high-aspect-ratio micro holes based upon picosecond laser layered trepanning

Deep high-aspect-ratio micro holes have been required urgently for manufacturing heat-resistant aero-engine components. Although ultrafast lasers have been proposed and used for machining such holes, the hole depth drilled using a traditional ultrafast laser is quite small. In this work, a new solution, which is based on picosecond laser layered trepanning technique cooperatively modulated by an airflow and an E-field, is accordingly developed to well solve such a challenging problem of small hole depth with a high aspect ratio. Using this hybrid technique, a comprehensive study was systematically conducted to trepan deep high-aspect-ratio micro holes in titanium alloy TC4 workpieces, and the hole-machining performance with mechanism analysis was also demonstrated. It was reported that high-quality deep high-aspect-ratio micro through holes were successfully machined in a workpiece of 5.6 mm thickness and the extreme blind-hole aspect ratio of around 20.8:1 was also achieved by using the new solution proposed, demonstrating its notable potential to greatly increase laser hole-machining depth and aspect ratio. Both horizontal/vertical airflow and E-field further improved the extremely machined depth. The airflow and E-field modulation further increased the extreme hole depth while suppressed oxidation. The airflow plus E-field modulation significantly suppressed residual stress with a maximum averaged reduction percentage of 88.0 %. The airflow used was more effective than the E-field applied to improve microstructure and micro hardness, and the cooperative modulation of an air flow and an E-field further improved micro hardness and microstructure. The problem of notable taper for the extreme through hole trepanned was well solved by using double-side picosecond laser layered trepanning.