An Experimental Study on Material Removal Mechanisms in Quasi-Continuous Wave Fiber Laser Microdrilling of SS 304

This paper investigates the different material removal mechanisms that occur during quasi-continuous wave (QCW) fiber laser drilling using millisecond pulses on stainless steel 304 samples and studies their effect on hole quality. A high-speed imaging camera is integrated with an in situ laser setup to capture the material removal during the laser drilling process. Based on high-speed camera images, four different material removal mechanisms were observed, which include vaporization, melt-expulsion at the hole entrance, melt-ejection at the hole exit, and explosive boiling. Vaporization occurs at all fluences beyond a certain threshold and is followed by melt-expulsion after a particular laser fluence value, leading to material deposition at the periphery of the hole entrance. Explosive boiling occurs at higher fluences beyond a certain threshold, resulting in material removal in the form of vapor and liquid droplets. Besides, the high pressure involved in explosive boiling also causes melt-ejection from the hole's exit, leading to the formation of a through hole. Furthermore, it is observed that an assist gas plays a crucial role in effectively displacing the molten material, thus generating a uniform and through hole. This sequential evolution of mechanisms offers valuable insights into delineating the roles of each mechanism and developing process maps for the dimensions and quality of μ-holes produced.