Solute-induced grain refinement and defect suppression in boron-modified molybdenum manufactured via laser powder-bed fusion

• Published in: International journal of refractory metals & hard materials 2023-12, Vol.117, p.106384, Article 106384
• Main Authors: Kaserer, L., Brennsteiner, D., Braun, J., Goettgens, V., Letofsky-Papst, I., Singer, P., Kestler, H., Schafbauer, W., Leichtfried, G.
• Format: Article
• Language: English
• Subjects: Alloy development; Grain refinement; Laser powder-bed fusion; Mechanical properties; Microstructure; Molybdenum
• ISSN: 0263-4368
• DOI: 10.1016/j.ijrmhm.2023.106384

Molybdenum manufactured with laser powder bed fusion (LPBF) has an undesirable coarse-grained, columnar microstructure interspersed with intergranular cracks, high porosity, and poor mechanical strength. These defects result from a combination of the harsh LPBF process conditions and the disadvantageous properties of molybdenum, such as its high brittle-ductile transition temperature and low tolerance for oxygen impurities. In order to suppress these defect-forming mechanisms and improve the suitability for LPBF, alloy-side material adjustments with simultaneous process optimization are necessary. In this work, the effect of adjusting Mo by adding 3.5 at.% B is investigated experimentally. Mo-3.5 at.% B specimens can be produced entirely free of cracks, with a density of 99.8%. The specimens have a microstructure of fine, equiaxed grains with an average grain size of 31 μm and an aspect ratio of 1.3, thus achieving substantial refinement of the otherwise typically coarse-grained columnar, anisotropic microstructure of pure Mo in LPBF. Furthermore, the grains possess a honeycomb-like cellular subgrain structure. This structure is formed through the solute rejection effect of B during the solidification and consists of initially solidified pure α-Mo cells with a cell size