Powder bed fusion – Laser beam in reactive atmospheres – Ignition limits for Fe and Ti-6Al-4V powder blends in CO2 and N2

Abstract

Powder bed fusion – laser beam (PBF-LB) in reactive CO₂ and N₂ atmospheres affect material properties, such as ductility and strength due to their uptake during processing. This can be exploited to manufacture in-situ particle reinforced materials. In Fe-based materials, in-situ precipitation is limited, thus Ti is added to increase the material-gas-interaction. However, the fraction of Ti in Fe-Ti blends must be limited, as laser exposure in these reactive atmospheres can lead to a strongly exothermic and self-sustained combustion reaction in the powder bed. In this study, the occurring combustion reaction and ignition limits are investigated. The laser power, speed, and spot size, as well as heat accumulation influence the onset of the combustion reaction. Based on our results, a lower limit for which no ignition occurs was determined to ca. 23 wt% Ti. At higher Ti concentrations, reasonable PBF-LB parameters may lead to ignition. The combustion products contain high numbers of oxides and carbides and consist of a combustion zone and a molten and sintered area below. These results show that undesired reactions in the powder bed can be controlled, and that Ti-containing powder blends are safely processable in CO₂ and N₂ atmosphere if the Ti fraction is limited.