Influence of process parameters on bubble formation and melt pool dynamics during laser directed energy deposition via in-situ synchrotron X-ray imaging

Abstract

Process parameters play a crucial role in the formation of pore defects in laser directed energy deposition of titanium alloy components; these defects significantly compromise the fatigue performance of the components. The pore defects always originate from trapped bubbles. However, most studies focused on post-analysis for pore defects after solidification. There are limited investigations on the influence of process parameters on bubble formation and retention during deposition using real-time observation. In this paper, the influence of laser power and scanning speed on both bubble formation and melt pool dynamics was studied via in-situ synchrotron X-ray imaging and numerical simulation. The formation, escape, and retention of bubbles during deposition process were quantitatively analyzed. Results show that the number of introduced and residual bubbles increased with increasing laser power (from 300 W to 400 W), but the residual bubble ratio initially increased and subsequently decreased. This trend is attributed to the enhancement of both Marangoni flow and heat input caused by increasing laser power. The stirring effect induced by the Marangoni flow not only enhances the introduction of bubbles but also facilitates their escape. Meanwhile, higher heat input prolonged the retention time of the melt pool, which benefits bubble escape. The residual bubble number was high under slow scanning speed of 500 mm/min, which can be attributed to the sustained downward inner gas pressure exerted by the laser beam on the bubbles.