Melt pool oxidation and reduction in powder bed fusion

Abstract

In this work, the mechanism of melt pool oxidation and reduction during the powder bed fusion was studied. To elucidate the mechanism, the effect of process parameters such as laser power, scan speed and shield gas flow speed on the variation on oxygen, nitrogen and carbon content in the printed material was investigated. In 316L stainless steel, melt pool reduction and oxidation occurred simultaneously during the process involving CO and CO2 gas evaporation at the depression zone. An increment of energy density and shield gas flow speed led to an increase in melt pool reduction rate and a decrease in porosity. It was found that nitrogen and CO gas could be a major constituent of vapor when oxygen and nitrogen content in the powder was high. Uneven solidification texture and melt pool instability were observed when the speed of shield gas flow was similar to the laser scan speed. Porosity, melt pool dimension, and solidification texture also changed under different shield gas flow speeds. Higher oxygen content and porosity was found in bi-directional scanning pattern than in uni-directional scanning pattern. Among uni-directional scanning pattern cases, scanning along the flow direction resulted in higher oxygen content and porosity compared to opposite and perpendicular scanning. Interaction between shield gas flow and laser plume was appreciably engaged in melt pool oxidation and reduction rate.