The present article is dedicated to the study of the evolution of the keyhole obtained during the drilling of samples of Zinc by YAG laser beam. Two experimental methods are used to observe the shape of the keyhole. The first one is the Direct observation of Drilled hOle, that lets to obtain the shape of the keyhole for different laser powers and different pulse durations. The observations are carried out after drilling. The second one needs to clamp the zinc sheet with a quartz glass. This glass is chosen because of its excellent thermal shock resistance and its high light transmittance. The observation of the keyhole during the drilling shape is then realized through the quartz with a high-speed CCD camera. The evolution of the shapes of the keyhole obtained by both methods lets to highlight the role of the multiple reflections on the shape of the keyhole. When multiple reflections occur, the absorption of the energy by the wall of the keyhole increases and consequently the depth of the keyhole increases. This phenomenon was verified experimentally: in the first method, the multiple reflection of the laser beam on the wall of the keyhole occurs, whereas, for the second experimental set-up, it can be considered that there is only one impact of the laser beam on the keyhole walls. This result was theoretically proved by calculating the absorption for different shapes of the keyhole, in both cases: one reflection and multiple reflections, using the ray tracing method.

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