Modification of Nb powder for laser powder bed fusion additive manufacturing: Experimental and theoretical analysis

Pure niobium and niobium alloy components with complex shapes are especially used in aerospace and biomedicine fields. However, the cost of preparing them using traditional methods is high. Laser powder bed fusion technique has the potential to fabricate these components at a low price. However, expensive spherical powders instead of cheap irregular-shaped powders are required to be the feedstock because of the poor flowability of the latter. Planetary ball milling modification technique has been proven to be an effective method to improve the flowability of low-cost irregular-shaped powders, enabling them to replace the high-cost spherical powders. This study used this technique to improve the flowability of irregular-shaped pure niobium powder and examined the powder properties before and after modification. The relative density of bulk samples fabricated by laser powder bed fusion with the as-modified powder were measured using Archimedes’s method, and their mechanical properties were obtained with a tensile test machine. The microstructure of these samples were also characterized and analyzed by XRD, OM, SEM and EBSD techniques. After the modification, the apparent density of the powder increased by 16.7 %, the Hausner ratio and Hall flow rate dropped from 1.27 to 1.24 and from 26.3 s to 24.4 s, respectively, and the roundness increased from 74.1 % to 80.8 %. The highest relative density of the samples fabricated with the as-modified powder achieved 99.0 %, and the average yield strength, ultimate strength, and tensile ductility of these samples were 426 MPa, 506 MPa, and 25.3 %, respectively. The microstructure of these samples showed a unique mixture consisting of primarily large grains and a small fraction of fine grains, which was caused by partial recrystallization.