Drilling with a high intensity energy beam can be incapable if the keyhole induced is collapsed. In this work, we identify the conditions for the keyhole collapse during high power density beam drilling from fundamental principles of thermal physics. The approach adapted is to probe the supersonic flow behavior of the two-phase vapor-liquid dispersion in a vertical keyhole of varying cross-section, paying particular attention to the transition between the slug and annular flows. It shows that the keyhole collapse occur from entrainment around the keyhole wall resulting in a severely deformed wavy shape of the inner liquid surface of the keyhole. This work provides a critical investigation to reveal incapability of drilling encountered in materials processing, packaging and manufacturing technologies.

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