Fully dense and crack free molybdenum manufactured by Selective Laser Melting through alloying with carbon

Pure molybdenum processed by Selective Laser Melting (SLM) shows a coarse–grained, columnar microstructure interspersed with cracks. The aim of this study was to process molybdenum by SLM with mechanical properties of powder metallurgically produced material by employing a novel alloying concept for molybdenum powder. Alloying of molybdenum with 0.45 wt% carbon leads to constitutional supercooling, thereby changing the solidification mode from planar to cellular. A refined microstructure with interlocking grains could be achieved. Cracking was entirely suppressed, and porosity was reduced to almost zero. A cellular microstructure with a misorientation angle between the individual cells <0.5° was found. The alpha–Mo phase is surrounded by a closed network of Mo₂C. The oxygen content was lowered during SLM processing due to outgassing of CO. The typically intergranular fracture mode of pure SLM molybdenum was changed to transgranular for Mo – 0.45 wt% C. Comparing Mo – 0.45 wt% C to pure molybdenum, the relative density, hardness and bending strength were increased by 1.9%, 65% and 340%, respectively.