Abstract
Dross formation is a phenomenon that is observed while printing metallic components using Laser Powder Bed Fusion ( L -PBF) and occurring primarily at down-facing surfaces that are unsupported and suffer inadequate heat removal. Naturally, dross formation causes dimensional inaccuracy, high surface roughness and also adversely affects the mechanical properties of printed components. Through simulation and experimentation, this study fundamentally elucidates the driving phenomenon behind dross formation. The simulation results, in terms of the degree of generated dross domain, well agree with the ones observed in the printed samples and the behaviour of the melt pool while moving from bulk material to the powder domain is clearly depicted in this study. The simulations show that due to the low thermal conductivity of loose powder and the inability to conduct heat away, the quasi steady state melt pool collapses while entering the powder domain and transitions to a keyhole-like melt mode which causes a pronounced drilling effect. This causes excessive melting known as dross that is seen both in the simulation and the experimental parts. This work also shows through simulation and experimentation the reasoning behind the production of larger and smaller dross domains while printing with high and low laser energy densities respectively. Additionally, through SEM imagery this study also explains the observed deep internal grooves and near-surface porosity that are present within this dross domain which can further affect mechanical properties such as density, fatigue strength etc.
Highlights
The laser powder bed fusion (L-PBF) process is one of the most popular AM processes and uses a high power laser beam to selectively melt layers of metal powder [1]
This paper has elucidated the mechanism of formation of dross at the down-facing overhang surfaces made with the Laser Powder Bed Fusion (L-PBF) process
The model has been applied for predicting the melt pool behaviour while manufacturing subsequent layers, using a computational fluid dynamics (CFD)-Discrete Element Model (DEM) approach
Summary
The laser powder bed fusion (L-PBF) process is one of the most popular AM processes and uses a high power laser beam to selectively melt layers of metal powder [1]. Upon solidification of the layer, the printing platform is moved down the distance of approximately the thickness one layer, and a new layer of powder is applied on top of the previous layer [2]. This process is repeated until the final desired component has been produced. Since AM allows for the manufacturing of components which have high strength and light weight, and are hard to process, using conventional manufacturing processes, such as Titanium and Nickel based alloys [3,4,5,6]. The L-PBF process is considered one of the most promising AM techniques and its impact is expected to grow in the future with even a wider application and adoption [7,8,9,10]
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