The quality of parts manufactured by laser powder bed fusion is closely related to the uniformity and density of the powder bed. In this work, the discrete element method is used to simulate the powder spreading process by different spreader geometries with rough substrate surfaces. The results indicate that reducing the spreader inclination angle significantly increases the number of force chains, enhances compaction, and consequently improves the quality of the powder bed. Studies also show that optimizing the bottom structure of the spreader can effectively reduce exposed areas. An arc-shaped structure promotes particle packing and filling, improving the powder distribution characteristics. A narrow spreader significantly affects the packing density of the powder bed at low layer gaps, whereas a wide spreader is relatively less constrained. At high spreading speeds, the spreader with an inclination angle of 135° produces the highest quality of the powder bed. R1000 performs excellently at larger layer gaps. The above findings provide valuable guidance for optimizing powder spreading strategies in the laser powder bed fusion process.

Powder bed fusion additive manufacturing has been applied to the fabrication of functionally graded materials. A new design that allows the material composition to change along the direction perpendicular to the powder spreading has been reported in the literature. Based on this design, this work examines the quality of the graded spread powder layer with two powders, which have a large difference of density. The results reveal that during the spreading of graded powders, the volume of particles on the heavy powder side is deposited less than that on the light powder side, indicating that heavy particles diffuse to the light powder side. This diffusion is affected by the spreading speed, but not much by the layer gap. Large spreading speed causes more significant deviation. The results also show that particle size affects diffusion, indicating that decreasing the particle size of the heavy powder may be a solution to reduce diffusion. [Figure not available: see fulltext.]

Understanding and controlling the composition segregation during powder spreading is of key importance in the additive manufacturing (AM) of composite materials. Under this circumstance, the segregation behavior of WC/316 L composite powders during spreading in laser powder bed fusion (LPBF) AM was numerically investigated by the discrete element method. The effects of process conditions (i.e., spreader velocity and geometry) and powder properties (i.e., size and shape of the WC powder) on the powder bed composition segregation and related characteristics were systematically analyzed. Corresponding mechanisms were identified from microscopic scale in terms of particle velocity, motion trajectory, mechanical behavior, and energy information. Finally, proper solutions in designing and constructing WC/316 L composite materials with desired gradient structures were proposed. The results show that the small blade velocity (V) will enhance the negative segregation, increase the average packing density ρ¯, and decrease uniformity ρ_vc in the WC/316 L composite powder bed. Compared with the blade, the roller can increase the negative segregation (Se_roller = −0.027 < Se_blade = −0.019) and the average packing density (ρ¯_roller = 0.31>ρ¯_blade = 0.20). When the WC/316 L size ratio increases from 25 μm/45 μm to 45 μm/45 μm, the negative segregation becomes weaker, and its value increases from −0.084 to −0.007. When the size ratio increases to 65 μm/45 μm, the powder behaves positive segregation with Se_max = 0.017; in this case, the packing density is the lowest (0.14), and the uniformity is the worst (0.17). In comparison with spherical shape, polyhedral WC powder can reduce the negative segregation of the powder bed (Se_sphere = −0.019 < Se_polyhedron = −0.008), while the WC shape has less effect on the packing density and uniformity. The density difference of the WC and 316 L powders leads to the difference in energy and force, resulting in different motion and segregation behaviors in the composite powder bed. For WC/316 L composite powder with a fixed composition, the condition of V = 0.025 m/s, WC/316 L size ratio = 25 μm/45 μm, roller spreader, and spherical WC can realize the proper composition gradient along the spreading direction in the composite powder bed.