  The mechanical performance of Directed Energy Deposition Additive
Manufactured (DED-AM) components can be highly material dependent. Through in
situ and operando synchrotron X-ray imaging we capture the underlying phenomena
controlling build quality of stainless steel (SS316) and titanium alloy (Ti6242
or Ti-6Al-2Sn-4Zr-2Mo). We reveal three mechanisms influencing the build
efficiency of titanium alloys compared to stainless steel: blown powder
sintering; reduced melt-pool wetting due to the sinter; and pore pushing in the
melt-pool. The former two directly increase lack of fusion porosity, while the
later causes end of track porosity. Each phenomenon influences the melt-pool
characteristics, wetting of the substrate and hence build efficacy and
undesirable microstructural feature formation. We demonstrate that porosity is
related to powder characteristics, pool flow, and solidification front
morphology. Our results clarify DED-AM process dynamics, illustrating why each
alloy builds differently, facilitating the wider application of additive
manufacturing to new materials.
