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

Abstract

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.

Keywords

• Laser Powder Bed Fusion
• Molybdenum
• Constitutional Supercooling
• Grain Refinement
• Additive Manufacturing
• Refractory Metal