Effect of process gas composition on laser-powder-substrate interaction, powder degradation and build quality in Laser-Directed Energy deposition of IN718

This work investigates the impact of the deposition environment on laser-substrate-powder interactions during Laser-Directed Energy Deposition ( L -DED) of IN718, correlating powder characteristics and thermal history with bead quality and mechanical behaviour. Five gas environments were investigated by varying the centring, shaping and carrier gas compositions (all-Ar, all-N 2 , centre-N 2 , shape-N 2 and carrier-N 2 ). Using N 2 as centring gas minimized the Ar plasma plume, improving laser energy transfer to meltpool and increasing dilution, while bead width remained constant (∼1.6 mm) across conditions. Owing to its superior thermal conductivity, N 2 environments produced faster cooling rates, suppressing Laves phase formation (∼3 vol% in all-Ar vs. an overall range of 2–3 vol%) and promoting finer grain size. The gas flow configuration also affected ricocheted powder characteristics: N 2 as carrier gas minimized particle size deviation and satellite coarsening (∼57% smaller satellites) relative to as-received powder, whereas N 2 as centring gas promoted coarser satellites and broader size distributions. XPS analysis revealed Al-nitrides and Ti-oxides on ricocheted powder surface under all conditions, with all-N 2 showing the highest Ti-nitride (12.2% vs. 3.1–4.0% for other conditions) and AlN (59.2% vs. 17.3% for all-Ar) fractions. Multi-wall depositions confirmed superior build quality under N 2 , with lower porosity, higher ultimate tensile strength (∼840 MPa) and elongation (∼37%) than all-Ar (∼730 MPa, ∼22%), despite comparable hardness across conditions. These findings highlight the importance of selecting suitable deposition environment to minimize defects, powder degradation and cost rather than relying solely on inert gases.