Mechanistic influence of laser spot diameter on SS316L directed energy deposition revealed by in-situ plasma plume diagnostics

In laser-based directed energy deposition (DED), achieving stable melt-pool behaviour and consistent clad quality is challenging due to sensitivity to spatial energy distribution at the laser-material interface. Laser spot diameter governs this distribution, yet its mechanistic influence on plasma-plume behaviour and melt pool thermophysics remains insufficiently understood, as prior studies have mainly emphasized laser power and scanning speed. This study demonstrates the role of spot diameter as a design-relevant control parameter in SS316L DED by linking plasma plume dynamics to melt pool behaviour and clad morphology. An integrated in-situ diagnostic framework combining optical emission spectroscopy (OES), CMOS-imaging, and two-colour pyrometry was applied across four spot diameters (0.8, 1.3, 2.1 and 3.5mm) under constant processing conditions. OES measurements revealed a non-monotonic dependence of plasma plume intensity fluctuation on spot diameter, reflecting changes in vaporization intensity and melt pool stability. A small spot diameter (0.8 mm) produced high plume fluctuations (~ 42.5%) and melt-pool instability, while a large diameter (3.5 mm) resulted in weak plume activity and poor bonding. An intermediate diameter (1.3 mm) yielded a stable plasma-plume with reduced fluctuation (~ 21.5%), uniform clad-morphology and favourable microstructural-characteristics. OES-derived plasma-plume intensity and fluctuation provide a physics-based, in-situ measure of spot-diameter-induced melt-pool stability in DED.