The additive manufacturing technology Powder Bed Fusion-Laser Beam (PBF-LB) enables the resource-efficient production of components with highly complex geometries. Such complex components made of the refractory metal molybdenum are of great industrial interest as they allow to optimize functionalization for use in various applications.
The disadvantage of PBF-LB manufactured Mo components is that it is currently impossible to produce pure Mo components that achieve a similar strength and quality compared to their traditionally powder-metallurgically produced counterparts. Pure Mo PBF-LB components suffer from a coarse-grained, columnar, and cracked microstructure owing to the unique processing conditions of a steep temperature gradient G = 10³–10⁶ K/m, a high cooling rate 10³–10⁶ K/s and a high solidification rate in PBF-LB. In addition, residual oxygen impurities, which segregate and drastically reduce grain boundary cohesion, play a significant role in defect formation. The development of adapted Mo alloys that enable grain refinement and control of grain boundary segregation is necessary to improve component quality.
This study discusses and compares alloying strategies we have developed using the elements C and B in Mo. Both strategies result in significant grain refinement due to the solute rejection effect and lead to the development of a cellular subgrain structure of α-Mo cells and intercellular Mo₂X (X=C or B). The mechanism by which oxygen impurities are bound in a form that is not detrimental to grain boundary cohesion is a clear difference between the two alloying strategies. In Mo-C alloys, oxygen impurities are dissolved in the Mo₂C carbide. In Mo-B alloys, oxygen impurities are trapped as nanometer-sized boron oxide inclusions that are located within the Mo₂B boride phase. The results show that both alloying strategies allow to manufacture components with competitive bending strength at room temperature and superior strength at elevated temperature compared to PM produced Mo and Mo alloys.
This study shows that the development of adapted alloying strategies for PBF-LB of Mo makes it possible to develop materials that exceed the mechanical strength of conventionally produced Mo alloys, thus opening up completely new application possibilities.