Cavity-induced bubbles and pore formation of laser direct energy deposited titanium alloy: Influence of powder melting degree

Porosity defects and their impact on fatigue performance have become one of the key issues in the widespread application of laser direct energy deposited titanium alloy components. The present work focuses on the cavity evolution and bubble generation process of unmelted, partially melted and fully melted powder entering the melt pool. Firstly, an equivalent experiment, ice particle with different melting degree impact water, was designed and conducted on the basis of similarity criterion in fluid mechanics. Secondly, a numerical model of powder impingement melt pool was established by using dynamic mesh method to simulate the evolution of cavity with different melting degree. Finally, a deposition processing with different melting states of the powder, by adjusting the angle and height of the powder feeding nozzle, was engaged to verify the influence of the melting states of the powder on the pore formation. The equivalent test and simulation results show that for the unmelted powder with large diameter and high velocity, the impacted cavity pinches off near the surface and induces gas entrainment and bubbles formation. For comparison, the fully melted powder enters the melt pool with a crater-shaped cavity which collapsed in a contraction mode and has not residual gas and bubbles. The surrounding liquid surface of partially melted powder changes the shape of the cavity and extended the time of collapse. With the increase of melting degree of powder, the collapse of cavity changes from the pinch-off to contraction mode, which avoids the generation of cavity-induced bubbles. The deposition experimental results show that both the pore size and porosity of the deposited specimen decrease with the increase of powder melting degree. This study is significant to clarify of the mechanism of porosity formation and decrease of porosity defects of laser direct energy deposited titanium alloy.