Laser powder bed fusion of molybdenum under various Ar–N₂ mixture build atmospheres

Laser powder bed fusion (LPBF) is a promising technology for the processing of refractory metals like molybdenum (Mo). Unfortunately, LPBF-processed Mo exhibits cracking when processed under typical conditions, limiting the application of the process. The influence of N content in a gas mixture used as the build atmosphere on the cracking tendency of LPBF processed Mo was studied. The effect of N partial pressure on grain structure, cracking characteristics and mechanical properties was evaluated under varying Ar–N₂ gas mixtures. The microstructure analysis revealed that the increasing content of N in the build atmosphere caused a shift from columnar grain structure which is expected for pure metals fabricated under LPBF, to irregular grain structure. Also, a decrease in average crack length was observed with the increase in nitrogen content in the samples. A linked trend of decreasing oxide content was observed at grain boundaries from fracture surface investigation. The reduced embrittlement of GB from the suppression of GB oxide formation is believed to hinder GB cracking in LPBF processed Mo. While cracks were observed in some samples, crack-free samples could be fabricated under high N partial pressure conditions, resulting in N content equal to or higher than 580 ppm.