Grain refinement mechanisms of alloying molybdenum with carbon manufactured by laser powder bed fusion

[Display omitted] •Microstructure control in additively manufactured molybdenum alloys from planar to cellular to columnar dendritic growth.•Grain refinement through increased thermal undercooling and reduced solidification front velocity by alloying with carbon.•The influence of solute element concentration on grain size in the absence of extrinsic nucleating particles for the columnar grain growth regime is shown.•Near-eutectic molybdenum-carbon alloy (2.2 wt% C) free of cracks with high density (> 99%) processed by LPBF.•Tensile strength is improved by 1300% to 650 MPa for molybdenum alloyed with 0.45 wt% C compared to pure molybdenum manufactured by LPBF. Pure molybdenum produced by additive laser powder bed fusion (LPBF) processes has a coarse, epitaxially-grown columnar grain structure with cracks located at high-angle grain boundaries, which are embrittled through oxygen segregation. The tensile strength of these pure molybdenum specimens is only 50 MPa, or 11% of the strength of powder metallurgically sintered, deformed, and recrystallized molybdenum. Alloying molybdenum with carbon can reduce cracks, increase density to 99.5%, and thus increase tensile strength to 650 MPa. The addition of carbon prevents embrittled grain boundaries and high residual porosity by outgassing oxygen, trapping any residual oxygen in the oxygen-soluble Mo2C carbide, and increasing the grain boundary surface on which the oxygen is distributed. In this study, we investigated the grain size, tensile strength, and hardness of five different molybdenum-carbon alloys produced via LPBF at constant process parameters. Our alloys ranged from pure molybdenum to 2.2 wt% carbon, which corresponds to a near-eutectic composition. In the absence of extrinsic nucleant particles in the melt, the relationship between grain size and solute concentration—and thus the growth restriction factor—follows an inverse exponential Arrhenius-type equation. We attributed grain refinement at higher carbon contents to increasing thermal and constitutional supercooling. The supercooling allows grains to nucleate and grow on the underlying solidified layer, where the fastest growth direction 〈100〉 is better aligned to the thermal gradient than that of the epitaxially growing grains.