Molybdenum and tungsten manufactured by selective laser melting: Analysis of defect structure and solidification mechanisms

In this work, processing of molybdenum and tungsten by Selective Laser Melting (SLM) is analyzed. The study reveals the impact of the oxygen content of the powder, the process atmosphere and the temperature of the substrate plate on the structural and mechanical properties of the processed material. For clarifying the causes and mechanisms for the formation of defects in molybdenum and tungsten processed by SLM, the samples were examined by x-ray, scanning and transmission electron microscopy including elemental distribution maps and crystallographic analyses by electron backscatter diffraction. Impurities, mainly oxygen, were identified as cause for the predominant defect structure comprising cracks and residual porosity. During processing, oxygen in the form of molybdenum/tungsten oxide, segregates at the grain boundaries, thereby inducing hot cracking. This is due to the lower melting point of the eutectic compared to the matrix phase. Moreover, the oxygen impurities were found to weaken the grain boundaries and thus increasing the risk for cold cracking and leading to a higher Ductile-to-Brittle Transition Temperature (DBTT). Subsequently, the combination of cracks through hot cracking at planar solidified grain boundaries and cold cracking along weakened grain boundaries during rapid cooling from the melting point creates the crack network generally found in molybdenum and tungsten processed by SLM. Also a substrate plate temperature of 1000 degrees C does not prevent the formation of cracks in tungsten caused by oxygen segregations.