Melting modes in laser powder bed fusion

Depending on processing conditions, laser powder bed fusion (LPBF) is known to have two operational regimes - conduction mode and keyhole mode. Heat conduction is the dominant heat transfer mechanism for conduction mode melting, whereas heat convection is the dominant heat transfer mechanism for keyhole mode melting. In addition, there exists a transition mode, which lies between the conduction and keyhole mode, wherein the dominance of conduction or convection depends upon the processing conditions. In this work, normalized processing diagrams are obtained to visualize the three melting modes - conduction mode, transition mode, and keyhole mode. The normalized processing diagrams obtained from this work are shown to be independent of material for specific classes of materials, of LPBF system, of laser modulation, and of powder layer thickness. Additionally, an analytical model is proposed to robustly predict the threshold between the three melting modes for two different classes of materials, (i) materials with low reflectivity and low thermal conductivity such as titanium, ferrous, and nickel alloys, and (ii) materials with high reflectivity and high thermal conductivity such as aluminium alloys. The normalized processing diagrams, alongside the identified melting mode thresholds, can provide a useful tool in diagnosing the origins of porous defects and enable accelerated process optimization efforts towards tailoring material properties in LPBF.