Microhole Drilling by High-Intensity Focused Ultrasound-Assisted Water-Confined Laser Micromachining

Abstract

Laser micromachining has several advantages such as the capability of flexibly producing very small features in both conductive and nonconductive materials. However, it may often suffer from induced defects, such as debris deposition on workpieces. To improve laser micromachining, a novel machining process, called “ultrasound-assisted water-confined laser micromachining” (UWLM), was proposed by the corresponding author. The ultrasound during UWLM can be applied through different approaches, such as an ultrasonic horn or a high-intensity focused ultrasound (HIFU) transducer, which can be called horn- and HIFU-based UWLM, respectively. This is the first paper (to the authors’ best knowledge) reporting experimental studies on microhole drilling using the novel HIFU-based UWLM process. In this study, drilled workpieces have been characterized; and in situ time-resolved shadowgraph imaging and pressure measurement during the UWLM process have been performed. Under the investigated conditions, it has been found that the microholes drilled by HIFU-based UWLM under suitable conditions appear reasonably clean without significant debris depositions often seen for a nanosecond (ns) laser ablation in air. The UWLM process can produce much larger average ablation depths per pulse than laser ablation in water without ultrasound (e.g., for copper, the former depth can be up to more than six times the latter). The study has revealed one important mechanism for the enhanced ablation depth, which is introduced in more details in the paper.