Experimental Study of Ultrasound-assisted Water-Confined Laser Micromachining and Double-Pulse Laser Micromachining

Ze Liu, Purdue University

Abstract

Laser micromachining based on short-pulsed lasers has many current or future potential applications in the fabrication of micro-scale parts or products with small features that need to be micromachined. The traditional laser micromachining processes, which are often carried out in air, may frequently suffer from drawbacks or defects, such as debris depositions, recast layers, and/or heat-affected zones with harmful residual thermal effect(s). These defects or drawbacks could not only significantly degrade the quality but also reduce the efficiency of the machining process. A novel machining process, called ultrasound-assisted water-confined laser micromachining (UWLM), has been investigated in this dissertation. In UWLM, a laser beam is used to machine the front surface region of a workpiece that is immersed in water, and in-situ ultrasounds are also applied. The in-situ application of the ultrasound could be achieved through approaches such as using an ultrasonic horn or high-intensity focused ultrasound (HIFU) transducer. The drilling of microholes and the production of microgrooves on metallic workpieces by using the novel UWLM process have been experimentally studied. The research results show that under the conditions studied: (1) the novel UWLM process can produce microholes and microgrooves that have much less debris deposition and/or recast material than those produced by laser micromachining performed in the ambient air environment; and (2) using similar laser pulses emitted from the laser, the average machining depths per pulse by UWLM can be much higher than those for laser micromachining performed in water without ultrasound. Time-resolved and in-situ shadowgraph imaging observations of the machining processes suggest that the applied in-situ ultrasound (with suitable parameters) during UWLM can help clean away (from the laser beam path) many bubbles and/or material particles in water, which are produced and left by the machining process of the preceding laser pulse(s), and which may otherwise stay in the laser beam path long enough to harm the effective laser-workpiece energy coupling for the subsequent laser pulse(s). Previous studies in the literature show that laser drilling using a double-(laser) pulse format with suitable inter-pulse separation time within each pulse pair may often lead to enhanced material removal rates. However, the prior research work reported in the literature about laser drilling using double nanosecond laser pulses often utilize two pulses of equal or similar pulse energies in each pulse pair. In this work, experimental studies have been conducted on microhole drilling in air using double nanosecond laser pulses, where the pulse energies within each pulse pair differ by over 10 times. From the research work, it has been found that under the conditions studied, the low-high double-pulse format under suitable conditions, where the low-energy pulse precedes the high-energy pulse in each pulse pair, can produce average material removal depths per pulse that are over several times higher than those by the high-low double-pulse format, or those by the single-pulse format, where each single pulse has about the same energy as the total energy of each pulse pair in the double pulse format. Research work has also been performed on the manufacturing of hourglass-shaped and through holes in a metal through a combination of the double-pulse laser drilling method and a subsequent laser-induced plasma-hole interaction process.

Degree

Ph.D.

Advisors

Wu, Purdue University.

Subject Area

Mechanical engineering

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