When a laser beam scans once across the surface of a metallic powder bed, the resulting track may be continuous with a crescent or an elliptic cross-section, irregularly broken, balled or only partially melted. This paper reports what laser powers and scan speeds lead to what types of track, for a CO2 laser beam focused to 0.55 mm and 1.1 mm diameters, scanning over beds made from M2 and H13 tool steel and 314S-HC stainless steel powders. Beds have been made with particle size ranges from 300 μm to 150 μm, from 150 μm to 75 μm, from 75 μm to 38 μm, and less than 38 μm. Measurements are also reported of bed physical properties that are used in a finite element model to predict melt pool dimensions and temperatures. Boundaries between regions of different track formation are explained in terms of melt surface temperature gradients, melt pool length-diameter ratio instabilities, and transitions from partial to complete melting. Implications for building metal parts in powder beds without supports are considered. The modelling is briefly extended to multi-track and multi-layer processing, to conclude that bonding by remelting between layers, while still maintaining control of the melt flow, places severe constraints on the maximum allowable layer thickness.
Carbon Particle In-Situ Alloying of the Case-Hardening Steel 16MnCr5 in Laser Powder Bed Fusion
