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

Mechanistic influence of laser spot diameter on SS316L directed energy deposition revealed by in-situ plasma plume diagnostics | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Mechanistic influence of laser spot diameter on SS316L directed energy deposition revealed by in-situ plasma plume diagnostics Mohit Singh, Misba Amin, Manoj J, Ravi K R This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8623620/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Apr, 2026 Read the published version in Progress in Additive Manufacturing → Version 1 posted You are reading this latest preprint version Abstract 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. Laser spot diameter Directed energy deposition In-situ diagnostics Plasma plume dynamics Optical emission spectrometer CMOS imaging Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Additive manufacturing (AM) enables the fabrication of complex metallic components with high design flexibility and localized control over material deposition [ 1 ]. Among the various AM technologies, direct energy deposition (DED) has emerged over the past decade as a powerful method for manufacturing and repairing metallic components with high geometric adaptability [ 2 , 3 ]. However, the reliability and quality of DED components are governed by melt pool stability, which is highly sensitive to the spatial distribution of laser energy at the laser material interface [ 4 ]. Extensive research has established that process parameters such as laser power, scan speed, and powder feed rate strongly influence melt pool dynamics, defect formation, and microstructural evolution across steels, aluminium, titanium, and nickel-based alloys. These studies have primarily focused on defining stable deposition conditions under fixed laser beam configurations [ 5 – 8 ], leaving limited insight into how variations in beam spot size influence melt pool behaviour. Beyond conventional process parameters, recent studies have specifically investigated laser spot diameter as a means to tailor melt pool behaviour and deposition outcomes. Ma et al. [ 9 ] showed that variations in spot size directly influence microstructural development, and mechanical performance in SS316L. Similarly, Kies et al. [ 10 ] reported that reducing the spot diameter to 0.6 mm during DED of high-manganese steel refined dendritic microstructures, suppressed ε-martensite formation, and reduced elemental segregation due to increased solidification rates and reduced melt pool volume. Antony et al. [ 11 ] further identified an optimal spot diameter range (300–500 µm) for achieving process stability and improved mechanical strength. Kong et al. have reported that smaller spot diameters promote grain refinement, whereas larger diameters (≈ 1.3–3 mm) enhance powder catchment efficiency captured using high speed camera. Despite these advances, existing studies predominantly rely on post-deposition characterization of geometry, microstructure, or mechanical properties. As a result, the underlying thermophysical mechanisms by which laser spot diameter through its direct control of spatial energy density governs vaporization onset, melt pool instability, and deposition behaviour remain insufficiently resolved. This limitation is especially critical when laser spot diameter is varied within a single build to balance geometric resolution and deposition efficiency, as shown by Arias et al. via post-process correlation with clad morphologies [ 13 ]. In such strategies, smaller spot diameters enable the fabrication of thin-walled features and fine geometries, whereas larger spot diameters increase powder catchment efficiency and deposition rate, thereby reducing overall build time, as demonstrated by Fillingim et al. [ 14 ]. In the absence of in-situ, physics-based indicators capturing spot-diameter-induced melt pool process conditions, reliable implementation of such adaptive strategies remains challenging. To overcome the limitations of post-deposition analysis and enable in-situ insight into melt pool stability, a range of in-situ monitoring techniques have been explored in laser-based AM [ 15 – 19 ]. High-speed imaging has been widely used to visualize melt pool and spatter dynamics with high temporal resolution, providing qualitative insight into transient instabilities. However, the large data volume requirements limit its applicability for in-situ feedback and control [ 16 , 20 ]. Alternatively, CMOS-based imaging offers a more compact and data-efficient approach and has been employed to track plasma plume morphology and oscillatory behaviour during deposition [ 15 , 19 ]. Thermal sensing techniques, such as single- and two-colour pyrometry, have further been used to estimate melt pool surface temperature and cooling trends, yet they provide limited information on vaporization-driven dynamics. Importantly, the laser spot diameter directly alters surface energy density and vaporization intensity, diagnostics capable of probing plasma plume behaviour are uniquely suited to reveal the underlying thermophysical mechanisms linking spot diameter to melt pool stability. As plasma plume emissions predominantly occur in the UV–VIS spectral range, spectroscopic sensing techniques are particularly well suited for their detection and characterization. Optical emission spectrometer (OES) has proven particularly effective in assessing AM quality, enabling the detection of process anomalies, estimation of excitation temperature, and indirect assessment of melt pool conditions based on spectral emissions [ 23 , 25 ]. Despite its demonstrated utility, most prior OES studies have focused on conventional process parameters such as laser power or scan speed, leaving the role of spot diameter in in-situ process monitoring and control underexplored. Prior studies have employed single-wavelength monitoring, line-to-continuum ratios, and element-specific intensity ratios to detect process anomalies, identify lack-of-fusion defects, and estimate plasma excitation temperature [ 23 , 24 ]. These approaches demonstrate the capability of OES to capture transient plasma plume dynamics and provide spectrally resolved insights into vaporization and energy transfer during AM [ 25 , 26 ]. Although OES is commonly used for process monitoring, its potential to mechanistically interpret spot-diameter effects on melt pool behaviour via plasma plume analysis remains underexplored. The present study introduces a multi-sensor, in-situ diagnostic framework to resolve the mechanistic influence of laser spot diameter on melt pool behaviour during SS316L directed energy deposition. The integrated setup employs synchronized OES, CMOS imaging, and a two-colour pyrometer to monitor plasma plume dynamics, excited-state atom populations, and melt pool surface temperatures at various spot diameters ranging from 0.8 mm to 3.5 mm. The study establishes a dynamic understanding of the role of spot diameter on melt pool behaviour and resulting clad morphology by correlating emission intensity, thermal field evolution, and surface energy density. This approach lays the foundation for in-situ process optimization and the implementation of data-driven, closed-loop control strategies in laser-based AM. 2. Materials and methods The SS316L powder used in this study was characterized for morphology, particle size distribution, and elemental composition before experimentation. Powder morphology was examined using scanning electron microscopy (SEM; Carl Zeiss EVO 18), while elemental composition was determined by X-ray fluorescence spectroscopy (XRF; Malvern Panalytical Epsilon 4). The experiments were carried out using a custom laser-based DED system equipped with a 2 kW diode laser (Laserline) operating in the 900–1070 nm wavelength range. The laser delivered a uniform top-hat beam profile to maintain consistent spatial energy distribution at the laser–substrate interface. A schematic of the experimental setup and integrated in-situ diagnostics is shown in Fig. 1 a. During deposition, the optically emissive plasma plume was imaged using a CMOS camera (Basler acA1920-40gc, 75 mm lens) at 42 fps, while time-resolved spectroscopic diagnostics were obtained using an OES (Avantes ULS4096CL-EVO) operating over 482–571 nm with 0.05 nm resolution at a 300 µs integration time and 700 µs acquisition interval. A probe-guard-protected collimating lens (Indian Patent Application No. 202411021830; Design Registration No. 438737-001) enhanced signal collection, while a discrete powder splitter (Design Registration No. 451912-001) [ 27 ] stabilized powder flow and minimized background noise for reliable three-dimensional spectral acquisition. Melt pool surface temperature was measured using a two-colour pyrometer (Micro-Epsilon CTRM-2H1SF100-C3) to support thermal and surface-tension-related analysis, with a temperature range of 550–3000°C and a sampling rate of 1 kHz. The physical basis of plasma plume formation and atomic emission during laser interaction with SS316L, along with the spectroscopic analysis approach adopted is schematically illustrates in Fig. 1 b. Plasma plume 2D morphology is captured using a CMOS camera, while time-resolved OES records wavelength- and intensity-resolved plume dynamics during DED. Three-dimensional OES spectra were reconstructed from continuously acquired frames over the 510–550nm range, enabling tracking of transient excitation behaviour. Indexed emission peaks correspond to Fe, Cr, and Mo species, identified using the National Institute of Standards and Technology (NIST) atomic spectra database and literature [ 21 , 23 ]. Following prior systematic validation (Singh et al. [ 28 ]), the Fe I 520.79 nm line is employed as a single-wavelength proxy due to its high signal-to-noise ratio, spectral stability, high transition probability, negligible self-absorption, clear separation from nearby Cr I lines (with 0.05 nm spectrometer resolution) and signal reproducibility making it suitable for proxy metric of plasma plume activity provides a robust in-situ monitoring in laser DED. SS316L depositions were performed using laser zoom optics at a fixed working distance of 13 mm to evaluate the effect of laser spot diameter on clad morphology. Spot diameter was varied (0.8, 1.3, 2.1, and 3.5 mm) while input parameter such as laser power (800 W), scan speed (14.6 mm/s), powder feed rate (11 g/min), shielding gas flow rate (15 L/min), and carrier gas flow rate (4 L/min) were kept constant. After deposition, clads were sectioned, metallographically prepared, etched with Beraha II reagent for 30 s, and examined using a Leica DM750 metallurgical microscope in bright-field mode. 3. Results 3.1. SS 316L powder analysis Figure 2 presents the SEM image and particle size distribution of the gas-atomized SS316L powder used in this study. The SEM micrograph (Fig. 2 a) shows predominantly spherical particles with occasional satellite formations, typical of gas-atomized powders. The particles exhibit generally smooth surfaces with minor irregularities, indicating good powder quality and flowability. The particle size distribution (Fig. 2 b) spans 45–135 µm and follows a near-Gaussian profile with a peak at approximately 75–85 µm. This uniform size distribution meets SS316L powder specifications for DED and supports stable powder flow and repeatable clad morphology [ 28 ]. XRF analysis has been carried out to determine the elemental composition of the SS316L powder. The average weight percentages of the constituent elements are listed in Table 1 . The results are in close agreement with standard SS316L specifications, indicating that the powder meets the compositional requirements for use in the DED process [ 29 ]. Table 1 Chemical composition of SS316L powder measured by XRF analysis. Element Fe Cr Ni Mn P S Si Concentration (wt. %) Balance 19.3 12.1 0.8 0.25 0.7 0.5 3.2. Temporal evolution of plasma plume morphology captured via CMOS camera during DED of SS316L at different laser spot diameters The CMOS camera enabled two-dimensional visualization of plasma plume morphology during SS316L deposition, with representative images provided in the supplementary data (Appendix A, Fig. S1 ). Sequential images acquired over 0–2800 ms were converted to 8-bit grayscale and binarized using a global threshold in ImageJ to enable plasma plume morphology analysis (Fig. 3 ). At a spot diameter of 0.8 mm (Fig. 3 a), the plasma plume exhibited pronounced vertical elongation and strong temporal oscillations, characterized by periodic expansion and contraction. The plasma plume area varied between 4.0 to 11.7 mm² over the observation period, indicating highly unstable plasma plume dynamics. In contrast, deposition at a 1.3 mm spot diameter (Fig. 3 b) resulted in a comparatively stable plasma plume morphology, with the plasma plume area fluctuating within a narrower range of 5.3 to 9.2 mm² throughout the deposition duration, reflecting steady melt pool behaviour. Increasing the spot diameter to 2.1 mm (Fig. 3 c) led to renewed instability, with more frequent and pronounced plasma plume area fluctuations ranging from approximately 2.0 to 7.5 mm². This condition exhibited intermittent plasma plume expansion and contraction, suggesting reduced melt pool stability. At the largest spot diameter of 3.5 mm (Fig. 3 d), plasma plume formation was strongly suppressed and remained virtually absent over the entire observation window. Binary images revealed inadequate melting, powder dispersion along the deposition track, and incomplete fusion. Overall, the plasma plume morphology exhibited a non-monotonic dependence on laser spot diameter. Strong instabilities were observed at small (0.8 mm) and intermediate (2.1 mm) spot diameters, whereas the 1.3 mm condition produced the most plasma stable plume. While CMOS imaging captures spatial plasma plume dynamics and qualitative stability trends, it does not resolve the underlying excitation and vaporization processes. Therefore, time-resolved OES is employed in the following section to provide quantitative insight into plasma plume intensity and fluctuation behaviour overcoming the qualitative nature and computational burden associated with image-based plume analysis. 3.3. Plasma plume intensity and fluctuation response during SS316L laser cladding at different laser spot diameters Figure 4 shows the variation of plasma plume intensity measured at the Fe I emission wavelength of 520.79 nm during SS316L clad deposition for different laser spot diameters. At a spot diameter of 0.8 mm, the emission intensity ranged from 550 to 5800 a.u., with an average value of 1621 a.u. over the 0–2800 ms acquisition period. Increasing the spot diameter to 1.3 mm reduced the intensity range to 550–3000 a.u., with a corresponding average of 986 a.u. A further increase to 2.1 mm resulted in intensities between 450 and 3140 a.u., with an average value of 655 a.u. At the largest spot diameter of 3.5 mm, the emission intensity was strongly suppressed, ranging from approximately 450 to 1280 a.u., with an average of 506 a.u. over the deposition duration. The monotonic decrease in emission intensity with increasing spot diameter reflects a progressive reduction in the population of excited Fe atoms contributing to the 520.79 nm transition. A quantitative estimation of the excited-state number density facilitates an atomistic-level insights of the diminishing plasma plume emission response as a function of increasing laser spot diameter. This evaluation is performed using the formalism described in Eq. ( 1 ), which enables the extraction of excited state population. $$\:{N}_{m}\:=\frac{{I}_{mn}}{{A}_{mn}.h.\nu\:}$$ 1 Where N m represents population of the upper state (m − 3 ), I mn indicates intensity of an emission line (a.u.), A mn for the transition probability (s − 1 ), h denotes Planck constant (J·s), and ν represents frequency of photon (s − 1 ). For calculating N m at various laser spot diameters, a transition probability of 320000 s − 1 at 520.79nm has been referenced from the NIST atomic spectra database. The computed values of the excited state number density derived from the emission line at 520.79 nm exhibit a clear and consistent correlation with the corresponding intensity signal measured at varying laser spot diameters. At a spot diameter of 0.8 mm, where the highest emission intensity has been recorded, the population of atoms in the upper excited state has been estimated to be 14×10 24 ± 5.6 ×10 24 m − 3 . This trend is well-aligned with the elevated plasma plume intensity signal observed in this condition. A systematic reduction in both the emission intensity and the excited state population has been observed with increasing spot size: for 1.3 mm, 8×10 24 ± 1.6 ×10 24 m −3 for 2.1 mm, 7×10 24 ± 2.3 ×10 24 m −3 ; and for 3.5 mm, a minimum value of 4.5×10 24 ± 0.63 ×10 24 m −3 has been obtained. The mutual consistency observed in the temporal intensity profiles and the corresponding excited atom densities highlights the influence of upper-level population dynamics in governing the plasma emission behaviour. Upon rigorous examination of the intensity signal profile (Fig. 4 ), distinct temporal variations were observed throughout 0 to 2800 ms, characterized by variations in plasma plume intensity magnitude across different spot diameter conditions, as shown in Fig. 5 . These variations, characterized by a periodic peaks and troughs, reflect dynamic variations in energy absorption might be happening within the melt pool. Notably, the fluctuation patterns strongly depended on the laser spot diameter, suggesting that energy distribution correlates with plasma plume stability. The intensity signal fluctuation was calculated using Eq. ( 2 ), based on data from three repeated experiments conducted at each spot diameter to quantify these temporal instabilities. $$\:Fluctuation\:intensity\:percentage\:\left(\%\right)\:=\frac{\sqrt{\frac{1}{n}{\sum\:}_{t=1}^{n}(I\left(t\right)-{I}_{Ave}{)}^{2}}}{\left({I}_{Ave}\right)}\times\:100$$ 2 Where I Ave represents mean plasma plume intensity signal (a.u.), I(t) denote plasma plume intensity signal at particular point (a.u.), n denote number of time points. The temporal fluctuation of the plasma plume intensity exhibited a clear non-monotonic dependence on laser spot diameter, contrasting with the monotonic decrease observed in both average plume intensity and excited-state population. At a spot diameter of 0.8 mm, the fluctuation was highest at 42.5 ± 2.1%. Increasing the spot diameter to 1.3 mm reduced the fluctuation to 21.5 ± 4.1%, indicating improved plume stability. However, a further increase to 2.1 mm led to a resurgence of fluctuation to 29 ± 3.2%, deviating from the declining intensity trend. At the largest spot diameter of 3.5 mm, the fluctuation dropped sharply to a minimum of 4.0 ± 1%. This non-monotonic variation in OES signal with laser spot diameter (Fig. 5 ) suggests corresponding changes in melt pool stability during deposition. To assess how these process-level variations are reflected in the deposited material, the following section presents the microstructural characteristics of SS316L clads produced at different laser spot diameters. 3.4. Microstructural evolution of SS316L clads with varying laser spot diameters Laser spot diameter strongly influences the microstructural characteristics of SS316L clads, as shown in Fig. 6 , which exhibits a characteristic fish-scale morphology associated with a top-hat laser energy distribution. For the 0.8 mm spot diameter (Fig. 6 a), two distinct microstructural zones were identified. Zone 1 (Z1), occupying approximately 72% of the clad area, consisted predominantly of long and medium-length columnar grains growing directionally from the fusion boundary toward the clad centre, consistent with previous observations in SS316L clads [ 32 , 33 ]. High-magnification images revealed cellular and columnar cellular substructures, particularly near the fusion boundary. Zone 2 (Z2), accounting for the remaining 28% of the clad, exhibited a higher fraction of equiaxed grains interspersed with medium-length columnar grains. Grain refinement was more pronounced toward the clad top, where fine cellular substructures dominated, in agreement with earlier reports [ 34 , 35 ]. Increasing the spot diameter to 1.3 mm (Fig. 6 b) resulted in an expansion of Z1 to approximately 78% of the clad area, accompanied by a reduction of Z2 to 22%. Z1 was characterized by a dense population of directionally solidified columnar grains driven by steep thermal gradients and unidirectional heat flow toward the substrate. Well-defined cellular substructures were observed within both long and medium-length columnar grains, particularly near the fusion boundary. In contrast to the 0.8 mm condition, the columnar-to-equiaxed transition in Z2 was less pronounced, indicating enhanced microstructural stability. At a spot diameter of 2.1 mm (Fig. 6 c), the columnar-grain-dominated Z1 region further expanded to approximately 88% of the clad area, while Z2 was confined to a narrow region near the clad top. Z2 primarily consisted of fine equiaxed grains with occasional medium-length columnar grains. Cellular substructures were consistently observed across both zones, similar to those detected at smaller spot diameters. 4. Discussion To interpret the experimental results, a multi-scale analysis is employed to correlate plasma plume behaviour, its temporal fluctuations and clad morphology its microstructure with variations in laser spot diameter. Building on observations from CMOS imaging (Fig. 3 ), OES-based intensity and fluctuation analysis (Fig. 5 ), and microstructural characterization (Fig. 6 ), this section elucidates the underlying physical mechanisms governing these trends. The discussion first examines the influence of spot diameter on surface energy density and laser–material interaction, followed by a thermophysical force balance explaining plasma plume fluctuations and melt pool behaviour, and finally addresses the role of thermal gradients and solidification parameters in microstructural evolution. 4.1. Correlating surface energy density and plasma plume emission dynamics with clad morphology in SS316L laser deposition For in-depth understanding of laser spot diameter regulates clad morphology and plasma plume characteristics as a process indicator, the effect of spot diameter on surface energy density (E f ) is analysed using the following relationship using Eq. ( 3 ): $$\:{E}_{f}=\frac{P}{V\:X\:f\:}$$ 3 Where E f is surface energy density (J/mm 2 ), P is laser power (W), V is the scan speed used for clad deposition (mm/s) and f is the laser spot diameter (mm). Calculated surface energy densities for SS316L deposition at constant 800 W and 14.6 mm/s are: 68.4 J/mm² (0.8 mm), 42.1 J/mm² (1.3 mm), 26.0 J/mm² (2.1 mm), and 15.6 J/mm² (3.5 mm). These values underscore the substantial decrease in localized thermal input with increasing spot diameter. As shown in Fig. 7 , laser spot diameter systematically altered heat distribution, surface energy density, and clad geometry, which in turn governed plasma plume behaviour. At a spot diameter of 0.8 mm, the highly concentrated top-hat heat input produced the highest surface energy density (68.4 J/mm²) and a depth aspect ratio of 0.37, characteristic of a transition regime between conduction and keyhole modes [ 36 , 37 ]. The deep melt pool geometry and elevated vaporization enhanced laser energy coupling through vapor-phase absorption and internal reflections, resulting in high plasma plume intensity and an elevated excited-state Fe I population at 520.79 nm (Fig. 4 ). Excessive vaporization, however, partially attenuated beam penetration, contributing to plume instability (Fig. 5 ) [ 39 – 41 ]. Increasing the spot diameter to 1.3 mm reduced the surface energy density to 42.1 J/mm² and the depth aspect ratio to 0.27, indicative of stable conduction-mode melting (Fig. 7 ) [ 36 , 37 ]. The broader, shallower melt pool limited vaporization, leading to reduced plasma plume activity (Figs. 3 , and 5 ). This clear correspondence between depth aspect ratio and plume emission underscores surface energy density as a key governing parameter. At a spot diameter of 2.1 mm, the surface energy density decreased further to 26.0 J/mm², yielding a depth aspect ratio of 0.16 and signalling a transition toward lack-of-fusion behaviour [ 36 , 38 ]. The reduced thermal input suppressed melt pool stability and metal vaporization, resulting in weaker plasma plume formation and lower spectral intensity at 520.79 nm (Figs. 3 and 5 ), consistent with low-energy deposition regimes [ 42 ]. At the largest spot diameter of 3.5 mm, the surface energy density dropped to 15.6 J/mm², producing a broad, shallow heat profile insufficient for stable metallurgical bonding (Fig. 7 ). This condition corresponded to a lack-of-fusion regime, characterized by incomplete melting, poor clad integrity, and negligible plasma plume activity, as evidenced by minimal emission intensity and excited-state population (Figs. 3 and 5 ) [ 36 , 38 ]. These observations are consistent with reported critical energy density thresholds below which fusion quality degrades sharply [ 43 ]. Overall, the strong correspondence between surface energy density, plasma plume intensity and morphology, and clad geometry demonstrates that plasma plume diagnostics, particularly OES-derived intensity and fluctuation metrics provide a sensitive, in-situ indicator of spot-diameter-dependent deposition behaviour at high data acquisition speed (1ms) in SS316L DED. Nevertheless, the cause of non-monotonic plasma plume fluctuations, remains complex and may involve melt pool hydrodynamics controlled by competing thermophysical forces, discussed in the following section. 4.2. Thermophysical force interactions governing plasma plume fluctuations and melt pool dynamics in SS316L laser deposition To elucidate the mechanisms underlying the spot-diameter-dependent plasma plume fluctuations observed in Fig. 5 , a thermophysical force-balance analysis of the melt pool is undertaken. This analysis considers the key contributions of recoil pressure, Marangoni convection driven by surface tension gradients, buoyancy, and hydrostatic forces. These forces, schematically illustrated in Fig. 8 , are governed by the measured melt pool surface temperatures and the calculated surface tensions, which vary with laser spot diameter. Understanding the interplay of these forces is vital for interpreting melt pool instabilities and their manifestation through plasma plume behaviour, which ultimately impacts clad morphology in DED process. At a spot diameter of 0.8 mm, the measured average melt pool surface temperature is 2715 ± 234°C, as obtained using a two-colour pyrometer (Fig. 9 ). This corresponds to a surface energy density of 68.4 J/mm², producing deep clad penetration and intense vaporization. The surface tension of the SS316L under these conditions has been calculated using thermophysical property data reported by Chen et al. [ 44 ]. The Eq. ( 4 ) has been derived based on temperature and surface tension data as mentioned in a temperature range from 1400 to 3100°C: $$\:\sigma\:\:=-0.4319\text{*}\text{T}+2435.6$$ 4 Where σ is surface tension in mN/m, T is melt pool surface temperature in 0 C. Using this equation, the surface tension is approximately 1262 ± 100 mN/m. As expected, surface tension decreases with increasing temperature due to reduced cohesive forces between molten metal atoms as shown in Fig. 9 . This high-temperature regime, approaching the boiling point of SS316L, drives significant metal vaporization and hence, strong recoil pressure [ 47 ] as described by Eq. ( 5 ): $$\:{P}_{r}={P}_{o\:}exp\{-\frac{{{m}_{mol}h}_{vap}}{R}(\frac{1}{{T}_{S}}-\frac{1}{{T}_{vap}}\left)\right\}$$ 5 Where P r is recoil pressure (Pa), P 0 is reference vapour pressure (Pa), h vap is enthalpy of vaporisation of SS316L (J/kg), m mol is molar mass (g/mol), R is universal gas constant (8.314 J/mol·K), T s is surface temperature (K). Since the pyrometer measures an average surface temperature, localized temperatures at the laser focal region are expected to be higher, promoting atomic excitation and partial ionization. Consistent with this, the dominance of Fe I emission observed in Section 3.3 confirms that the plasma plume primarily consists of excited neutral species. The deep melt pool geometry supports multiple internal laser reflections, enhancing local energy absorption and reinforcing vaporization and recoil pressure [ 45 , 46 ]. Simultaneously, a strong radial temperature gradient develops across the melt pool surface. Owing to the negative temperature coefficient of surface tension for SS316L, this gradient induces outward Marangoni convection from the hotter centre toward the cooler periphery (Fig. 8 ). While hydrostatic pressure and buoyancy-driven convection are present, their influence is minor due to the small melt pool volume. Consequently, the melt pool dynamics are dominated by the interaction between recoil pressure and Marangoni convection, leading to oscillatory instabilities in melt pool geometry and vapor ejection. These instabilities manifest as strong plasma plume intensity fluctuations, reaching 42.5% at 0.8 mm spot diameter (Fig. 5 ). Thus, high-frequency plume oscillations can be directly attributed to the imbalance between recoil pressure and thermocapillary flow as evidenced by the coupled plume morphology and intensity fluctuations shown in Figs. 3 and 5 [ 45 , 48 ]. At a spot diameter of 1.3 mm, the average melt pool surface temperature decreased to 2545 ± 195°C, resulting in an estimated surface tension of 1336 ± 84 mN/m (Fig. 9 ). The lower thermal input reduced metal vaporization and recoil pressure, producing a shallower and more stable clad (Fig. 7 ) characteristic of conduction-mode melting. Under these conditions, recoil pressure decreased more rapidly than Marangoni forces, allowing surface-tension-driven flow to dominate melt pool dynamics. This stabilized outward flow promoted a smoother liquid–vapour interface and more uniform vapor ejection, consistent with previous numerical and experimental studies [ 49 , 50 ]. Buoyancy effects were further suppressed due to the reduced aspect ratio. As a result, plasma plume fluctuations decreased substantially to 21.5% (Fig. 5 ), indicating a quasi-steady vaporization regime. Further increasing the spot diameter to 2.1 mm significantly reduced the thermal energy input, yielding an average surface temperature of 1685 ± 60°C and a corresponding surface tension of 1707 ± 26 mN/m (Fig. 9 ). Under these conditions, recoil pressure and Marangoni convection were both substantially weakened, resulting in a shallow and less dynamic melt pool (Fig. 7 ). Reduced thermal gradients and increased melt viscosity limited fluid flow and wetting behaviour, leading to poorer dilution and non-uniform clad geometry [ 51 , 52 ]. Buoyancy-driven convection and hydrostatic effects became negligible due to the shallow melt pool. Although overall vaporization and emission intensity decreased (Fig. 5 ), plasma plume fluctuation increased to 29%, reflecting unsteady and localized vaporization events arising from non-uniform heating and melt pool instability. At the largest spot diameter of 3.5 mm, the surface energy density dropped to 15.6 J/mm², and the average surface temperature declined to 1469 ± 44°C, corresponding to a surface tension of 1800 ± 23 mN/m (Fig. 9 ). At this low thermal input, metal vaporization and recoil pressure were negligible, suppressing both melt pool depression and plasma plume formation. The weak thermal gradients limited Marangoni convection, while elevated surface tension further restricted melt pool spreading and fluidity. Consequently, clad formation was dominated by lack-of-fusion defects and poor metallurgical bonding, consistent with reported critical energy density thresholds for effective deposition [ 53 ]. Plasma plume activity was absent, with negligible emission intensity and the lowest recorded plume fluctuation of 4% (Figs. 3 and 6 ), confirming an inefficient deposition regime. 4.3. Influence of laser spot diameter on solidification dynamics and microstructural evolution The spot-diameter-dependent variations in melt pool stability and thermal conditions directly govern solidification behaviour during deposition. Changes in the temperature gradient (G) and solidification rate (R), through the G/R and G×R parameters, grain morphology and microstructural zoning in SS316L clads. Accordingly, this section examines how the plasma plume–inferred thermal conditions manifest in the observed microstructural evolution with increasing laser spot diameter. At a spot diameter of 0.8 mm (Fig. 6 a), the high surface energy density (68.4 J/mm²) generated a deep melt pool with steep thermal gradients and strong recoil pressure. The resulting high G/R ratio at the melt pool boundary favored elongated columnar grains in Z1, while the large G×R product promoted cellular and columnar cellular substructures near the fusion boundary. Local reductions in G/R toward the clad top, arising from heat accumulation and reduced thermal extraction, enabled equiaxed grain formation, leading to the development of Z2. Increasing the spot diameter to 1.3 mm (Fig. 6 b) reduced the energy density to 42.1 J/mm² and lowered the melt pool surface temperature from 2715 ± 234°C to 2545 ± 195°C. This produced a shallower and more stable melt pool, expanding Z1 to ~ 78% of the clad area and suppressing the columnar-to-equiaxed transition. The higher effective G/R ratio inhibited constitutional undercooling, promoting sustained directional solidification. The concurrent reduction in plasma plume intensity fluctuation (from 42.5% to 21.5%) reflects improved melt pool stability. At a spot diameter of 2.1 mm (Fig. 6 c), further reduction in energy density (26 J/mm²) and surface temperature (1685 ± 60°C) expanded Z1 to ~ 88% of the clad, confining Z2 to a narrow region near the top. Although G/R decreased, the G×R product remained sufficient to sustain complex cellular and dendritic substructures. The dominance of columnar grains is consistent with reports of suppressed equiaxed solidification under reduced energy input [ 54 ]. Overall, the observed microstructural evolution follows classical solidification theory, with G/R governing columnar-to-equiaxed transitions and G×R controlling substructure fineness [ 55 ]. In SS316L DED, modest variations in G and R induced by spot diameter adjustment significantly alter the solidification pathway [ 56 ]. The strong correspondence between plasma plume characteristics and microstructural features reinforces the utility of plume-based diagnostics as an in-situ indicator of melt pool stability and solidification conditions. 4.4. Consolidated plasma plume–based interpretation of spot-diameter-induced deposition behaviour Figure 10 consolidates the influence of laser spot diameter on surface energy density, plasma plume behaviour, and clad geometry, providing a unified mechanistic view of the deposition process. Reducing the spot diameter from 3.5 mm to 0.8 mm increases the surface energy density from approximately 15.65 J/mm² to 68.4 J/mm², leading to intensified vaporization and a corresponding rise in plasma plume intensity from ~ 500 a.u. to > 1600 a.u. However, this elevated energy concentration also amplifies melt pool instabilities (depth aspect ratio-0.37), manifested as a high plasma plume fluctuation level of ~ 42%. At an intermediate spot diameter of 1.3 mm, a balanced energy input (42.14 J/mm²) produces a marked reduction in plume fluctuation (21.56%) and yields more uniform clad morphology (depth aspect ratio-0.27) with stable depth-to-height aspect ratios. Further increasing the spot diameter to 3.5 mm lowers the surface energy density below 15.65 J/mm², suppressing vaporization and plasma plume activity, resulting in minimal plume fluctuation (4.05%) with lack of fusion. This integrated framework demonstrates that plasma plume intensity and its temporal fluctuation as OES signal serve as quantitative, real-time indicators of spot-diameter-induced transitions in melt pool stability and deposition morphology. Accordingly, spot diameter emerges as a critical design parameter, and its optimization guided by in-situ plasma plume diagnostics offers a robust pathway for achieving stable thermal–fluid behaviour and consistent deposition quality in laser DED. 5. Conclusions This study establishes optical emission spectroscopy (OES), together with CMOS-based plasma plume imaging and pyrometry, as a physics-based in-situ diagnostic framework for resolving the mechanistic influence of laser spot diameter on melt pool stability during SS316L directed energy deposition (DED). The key findings are as follows: Laser spot diameter is a critical control parameter governing surface energy density and deposition regime. A small spot diameter (0.8 mm) induces strong vaporization and large plasma plume fluctuations with highly unstable CMOS-observed plume morphology, whereas a large spot diameter (3.5 mm) suppresses plume activity and leads to lack-of-fusion. An intermediate spot diameter (1.3 mm) provides a balanced thermal regime, exhibiting a stable and symmetric plasma plume signal and resulting in uniform clad formation. Time-resolved OES measurements of plasma plume intensity and fluctuation directly capture spot-diameter-dependent melt pool behaviour. The Fe I 520.79 nm emission line systematically tracks changes in plasma plume signal and excited-state population, while plume fluctuation uniquely reflects melt pool instability arising from force imbalance. Plasma plume fluctuation exhibits a non-monotonic dependence on spot diameter, driven by the competing effects of recoil pressure and thermocapillary (Marangoni) flow. This behaviour serves as a sensitive indicator of melt pool stability. Microstructural evolution follows solidification theory, with G/R and G×R governing grain morphology and substructure scale. Smaller spot diameters promote finer cellular substructures, while increasing spot diameter favours columnar grain dominance. Overall, this work demonstrates that OES-derived plasma plume intensity and fluctuation provide quantitative, low-overhead, real-time indicators of spot-diameter-induced melt pool dynamics, elevating OES from a qualitative monitoring tool to a mechanistic diagnostic. The results provide a foundation for spot-diameter-aware monitoring and future closed-loop control strategies in laser directed energy deposition. Declarations Competing Interests Declaration of competing interests☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Ravi K R reports equipment, drugs, or supplies was provided by India Ministry of Science & Technology Department of Science and Technology. Ravi K R reports equipment, drugs, or supplies was provided by Science and Engineering Research Board. none If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contribution Mohit Singh: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – original draft.Misba Amin: Data curation, Investigation.Manoj J: Data curation, Investigation.Ravi K. R.: Conceptualization, Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review and editing. Acknowledgement The authors express their gratitude to the Department of Science and Technology, Government of India [Grant No: DST/TDT/AMT/2017/225 (G)]; the Science and Engineering Research Board (SERB), Government of India [Grant No: CRG/2021/002636]; and the Prime Minister’s Research Fellowship (PMRF), Government of India for supporting the work. References Vafadar A, Guzzomi F, Rassau A, Hayward K (2021) Advances in metal additive manufacturing: a review of common processes, industrial applications, and current challenges. 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Declaration of competing interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Ravi K R reports equipment, drugs, or supplies was provided by India Ministry of Science & Technology Department of Science and Technology. Ravi K R reports equipment, drugs, or supplies was provided by Science and Engineering Research Board. none If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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10:26:31","extension":"xml","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":141581,"visible":true,"origin":"","legend":"","description":"","filename":"1a9f6c3a3dd04f7b9a29f8f6803b6f181structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/e1dcc876a04296d1261a0872.xml"},{"id":100876686,"identity":"24eb863c-c642-4af8-b91e-3ec1c8b846b5","added_by":"auto","created_at":"2026-01-22 10:26:29","extension":"html","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":154499,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/c366b61f6d856d73ecc35b37.html"},{"id":101202728,"identity":"c2d052fa-b7f9-445e-bd48-cda3fc588da2","added_by":"auto","created_at":"2026-01-27 09:37:21","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":198833,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic of the laser direct energy deposition (DED) setup with in-situ monitoring sensors (b) Mechanism of plasma plume formation and atomic emission during laser interaction with SS316L, showing indexed spectra and identification of the Fe I 520.79 nm diagnostic emission line.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/7261ab326176a1831bd6a26e.jpg"},{"id":100950175,"identity":"aaa9ea69-0720-4931-8a74-854e217f59fe","added_by":"auto","created_at":"2026-01-23 07:07:06","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":126337,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM image and (b) Particle size distribution of the feedstock SS316L powder.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/489629016d8d9e0c19afec02.jpg"},{"id":100876666,"identity":"ce15768d-2aec-4803-a0a1-45c573db52a4","added_by":"auto","created_at":"2026-01-22 10:26:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":101171,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSuperimposed binary plasma plume images and plume area during SS316L clad deposition at laser spot diameters: (a) 0.8 mm, (b) 1.3 mm, (c) 2.1 mm, (d) 3.5 mm.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/7d620eacb072c52a6812205e.png"},{"id":100950135,"identity":"035409d9-f948-4a58-acda-cbd39c5d3750","added_by":"auto","created_at":"2026-01-23 07:06:57","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":89362,"visible":true,"origin":"","legend":"\u003cp\u003eOES intensity signal at 520.79nm captured during SS316L clad deposition at various laser spot diameters.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/0f48444dab3a65cc871b177b.jpg"},{"id":100950048,"identity":"3e961e15-0b92-4c5f-9bb0-6718a0a80e3e","added_by":"auto","created_at":"2026-01-23 07:06:45","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":35020,"visible":true,"origin":"","legend":"\u003cp\u003eStacked plot showing the variation of plasma plume intensity, excited-state atom population, and signal fluctuation percentage as a function of laser spot diameter during SS316L clad deposition.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/e82e840fdfaf7efe012b1553.jpg"},{"id":101296731,"identity":"9c3224cd-1c97-48a4-90eb-66b48c5055d8","added_by":"auto","created_at":"2026-01-28 09:19:33","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":182631,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructural evolution in SS316L clads morphology with varying spot diameter: (a) 0.8 mm, (b) 1.3 mm, (c) 2.1 mm. (LL-CG – Large Lengthy Columnar Grains, ML-CG – Medium-Length Columnar Grains, EG – Equiaxed Grains, CSS – Cellular Substructures).\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/f1cf482a1f8cd0ae3e951ac0.jpg"},{"id":100876749,"identity":"2e959054-d689-45df-894c-ad385667f82c","added_by":"auto","created_at":"2026-01-22 10:26:30","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":77103,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of laser spot diameter on surface energy density, and clad morphology during AM of SS316L.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/be135727383a703e2998b846.jpg"},{"id":100876753,"identity":"b80d6fc9-ed96-49d7-83cc-d8f624f38e28","added_by":"auto","created_at":"2026-01-22 10:26:31","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":33260,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of key force interactions governing melt pool behaviour during laser-based AM of SS316L.\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/39f6eb4a1b4fda3425d328de.jpg"},{"id":100876661,"identity":"55fee0f5-cc42-4044-b90d-96d267a00a98","added_by":"auto","created_at":"2026-01-22 10:26:29","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":30427,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of varying laser spot diameter on surface temperature and surface tension in SS316L laser AM.\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/4e298c103f8241d7aab4e42a.jpg"},{"id":101203210,"identity":"4b80bcff-204f-47e3-ba1b-e8b7b787b2b3","added_by":"auto","created_at":"2026-01-27 09:39:04","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":50384,"visible":true,"origin":"","legend":"\u003cp\u003eConsolidated correlation between laser spot diameter, surface energy density, plasma plume response, and clad geometry in SS316L laser DED.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/f7b42a213329483e0ab5fd9e.jpg"},{"id":106809085,"identity":"6c5b9281-dff4-4699-b7dd-f7853de24cae","added_by":"auto","created_at":"2026-04-13 16:06:15","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1832187,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/7b2dd2da-7400-4a47-b283-cf5d19c08be1.pdf"},{"id":100876660,"identity":"18b86a5a-4ce5-47e3-990f-5828166db1dc","added_by":"auto","created_at":"2026-01-22 10:26:29","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":668928,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryFile.docx","url":"https://assets-eu.researchsquare.com/files/rs-8623620/v1/585978ceba7df5d1333ebc9a.docx"}],"financialInterests":"Competing interest reported. Declaration of competing interests\n \n☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\n \n☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:\n\nRavi K R reports equipment, drugs, or supplies was provided by India Ministry of Science \u0026 Technology Department of Science and Technology. Ravi K R reports equipment, drugs, or supplies was provided by Science and Engineering Research Board. none If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.","formattedTitle":"Mechanistic influence of laser spot diameter on SS316L directed energy deposition revealed by in-situ plasma plume diagnostics","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAdditive manufacturing (AM) enables the fabrication of complex metallic components with high design flexibility and localized control over material deposition [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Among the various AM technologies, direct energy deposition (DED) has emerged over the past decade as a powerful method for manufacturing and repairing metallic components with high geometric adaptability [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, the reliability and quality of DED components are governed by melt pool stability, which is highly sensitive to the spatial distribution of laser energy at the laser material interface [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Extensive research has established that process parameters such as laser power, scan speed, and powder feed rate strongly influence melt pool dynamics, defect formation, and microstructural evolution across steels, aluminium, titanium, and nickel-based alloys. These studies have primarily focused on defining stable deposition conditions under fixed laser beam configurations [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], leaving limited insight into how variations in beam spot size influence melt pool behaviour.\u003c/p\u003e \u003cp\u003eBeyond conventional process parameters, recent studies have specifically investigated laser spot diameter as a means to tailor melt pool behaviour and deposition outcomes. Ma et al. [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] showed that variations in spot size directly influence microstructural development, and mechanical performance in SS316L. Similarly, Kies et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] reported that reducing the spot diameter to 0.6 mm during DED of high-manganese steel refined dendritic microstructures, suppressed ε-martensite formation, and reduced elemental segregation due to increased solidification rates and reduced melt pool volume. Antony et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] further identified an optimal spot diameter range (300\u0026ndash;500 \u0026micro;m) for achieving process stability and improved mechanical strength. Kong et al. have reported that smaller spot diameters promote grain refinement, whereas larger diameters (\u0026asymp;\u0026thinsp;1.3\u0026ndash;3 mm) enhance powder catchment efficiency captured using high speed camera. Despite these advances, existing studies predominantly rely on post-deposition characterization of geometry, microstructure, or mechanical properties. As a result, the underlying thermophysical mechanisms by which laser spot diameter through its direct control of spatial energy density governs vaporization onset, melt pool instability, and deposition behaviour remain insufficiently resolved. This limitation is especially critical when laser spot diameter is varied within a single build to balance geometric resolution and deposition efficiency, as shown by Arias et al. via post-process correlation with clad morphologies [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In such strategies, smaller spot diameters enable the fabrication of thin-walled features and fine geometries, whereas larger spot diameters increase powder catchment efficiency and deposition rate, thereby reducing overall build time, as demonstrated by Fillingim et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. In the absence of in-situ, physics-based indicators capturing spot-diameter-induced melt pool process conditions, reliable implementation of such adaptive strategies remains challenging.\u003c/p\u003e \u003cp\u003eTo overcome the limitations of post-deposition analysis and enable in-situ insight into melt pool stability, a range of in-situ monitoring techniques have been explored in laser-based AM [\u003cspan additionalcitationids=\"CR16 CR17 CR18\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. High-speed imaging has been widely used to visualize melt pool and spatter dynamics with high temporal resolution, providing qualitative insight into transient instabilities. However, the large data volume requirements limit its applicability for in-situ feedback and control [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Alternatively, CMOS-based imaging offers a more compact and data-efficient approach and has been employed to track plasma plume morphology and oscillatory behaviour during deposition [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Thermal sensing techniques, such as single- and two-colour pyrometry, have further been used to estimate melt pool surface temperature and cooling trends, yet they provide limited information on vaporization-driven dynamics. Importantly, the laser spot diameter directly alters surface energy density and vaporization intensity, diagnostics capable of probing plasma plume behaviour are uniquely suited to reveal the underlying thermophysical mechanisms linking spot diameter to melt pool stability.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eAs plasma plume emissions predominantly occur in the UV\u0026ndash;VIS spectral range, spectroscopic sensing techniques are particularly well suited for their detection and characterization. Optical emission spectrometer (OES) has proven particularly effective in assessing AM quality, enabling the detection of process anomalies, estimation of excitation temperature, and indirect assessment of melt pool conditions based on spectral emissions [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Despite its demonstrated utility, most prior OES studies have focused on conventional process parameters such as laser power or scan speed, leaving the role of spot diameter in in-situ process monitoring and control underexplored. Prior studies have employed single-wavelength monitoring, line-to-continuum ratios, and element-specific intensity ratios to detect process anomalies, identify lack-of-fusion defects, and estimate plasma excitation temperature [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. These approaches demonstrate the capability of OES to capture transient plasma plume dynamics and provide spectrally resolved insights into vaporization and energy transfer during AM [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Although OES is commonly used for process monitoring, its potential to mechanistically interpret spot-diameter effects on melt pool behaviour via plasma plume analysis remains underexplored.\u003c/p\u003e\u003cp\u003eThe present study introduces a multi-sensor, in-situ diagnostic framework to resolve the mechanistic influence of laser spot diameter on melt pool behaviour during SS316L directed energy deposition. The integrated setup employs synchronized OES, CMOS imaging, and a two-colour pyrometer to monitor plasma plume dynamics, excited-state atom populations, and melt pool surface temperatures at various spot diameters ranging from 0.8 mm to 3.5 mm. The study establishes a dynamic understanding of the role of spot diameter on melt pool behaviour and resulting clad morphology by correlating emission intensity, thermal field evolution, and surface energy density. This approach lays the foundation for in-situ process optimization and the implementation of data-driven, closed-loop control strategies in laser-based AM.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eThe SS316L powder used in this study was characterized for morphology, particle size distribution, and elemental composition before experimentation. Powder morphology was examined using scanning electron microscopy (SEM; Carl Zeiss EVO 18), while elemental composition was determined by X-ray fluorescence spectroscopy (XRF; Malvern Panalytical Epsilon 4).\u003c/p\u003e \u003cp\u003eThe experiments were carried out using a custom laser-based DED system equipped with a 2 kW diode laser (Laserline) operating in the 900\u0026ndash;1070 nm wavelength range. The laser delivered a uniform top-hat beam profile to maintain consistent spatial energy distribution at the laser\u0026ndash;substrate interface. A schematic of the experimental setup and integrated in-situ diagnostics is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. During deposition, the optically emissive plasma plume was imaged using a CMOS camera (Basler acA1920-40gc, 75 mm lens) at 42 fps, while time-resolved spectroscopic diagnostics were obtained using an OES (Avantes ULS4096CL-EVO) operating over 482\u0026ndash;571 nm with 0.05 nm resolution at a 300 \u0026micro;s integration time and 700 \u0026micro;s acquisition interval. A probe-guard-protected collimating lens (Indian Patent Application No. 202411021830; Design Registration No. 438737-001) enhanced signal collection, while a discrete powder splitter (Design Registration No. 451912-001) [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] stabilized powder flow and minimized background noise for reliable three-dimensional spectral acquisition. Melt pool surface temperature was measured using a two-colour pyrometer (Micro-Epsilon CTRM-2H1SF100-C3) to support thermal and surface-tension-related analysis, with a temperature range of 550\u0026ndash;3000\u0026deg;C and a sampling rate of 1 kHz.\u003c/p\u003e \u003cp\u003eThe physical basis of plasma plume formation and atomic emission during laser interaction with SS316L, along with the spectroscopic analysis approach adopted is schematically illustrates in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. Plasma plume 2D morphology is captured using a CMOS camera, while time-resolved OES records wavelength- and intensity-resolved plume dynamics during DED. Three-dimensional OES spectra were reconstructed from continuously acquired frames over the 510\u0026ndash;550nm range, enabling tracking of transient excitation behaviour. Indexed emission peaks correspond to Fe, Cr, and Mo species, identified using the National Institute of Standards and Technology (NIST) atomic spectra database and literature [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Following prior systematic validation (Singh et al. [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]), the Fe I 520.79 nm line is employed as a single-wavelength proxy due to its high signal-to-noise ratio, spectral stability, high transition probability, negligible self-absorption, clear separation from nearby Cr I lines (with 0.05 nm spectrometer resolution) and signal reproducibility making it suitable for proxy metric of plasma plume activity provides a robust in-situ monitoring in laser DED.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSS316L depositions were performed using laser zoom optics at a fixed working distance of 13 mm to evaluate the effect of laser spot diameter on clad morphology. Spot diameter was varied (0.8, 1.3, 2.1, and 3.5 mm) while input parameter such as laser power (800 W), scan speed (14.6 mm/s), powder feed rate (11 g/min), shielding gas flow rate (15 L/min), and carrier gas flow rate (4 L/min) were kept constant. After deposition, clads were sectioned, metallographically prepared, etched with Beraha II reagent for 30 s, and examined using a Leica DM750 metallurgical microscope in bright-field mode.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. SS 316L powder analysis\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the SEM image and particle size distribution of the gas-atomized SS316L powder used in this study. The SEM micrograph (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) shows predominantly spherical particles with occasional satellite formations, typical of gas-atomized powders. The particles exhibit generally smooth surfaces with minor irregularities, indicating good powder quality and flowability. The particle size distribution (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) spans 45\u0026ndash;135 \u0026micro;m and follows a near-Gaussian profile with a peak at approximately 75\u0026ndash;85 \u0026micro;m. This uniform size distribution meets SS316L powder specifications for DED and supports stable powder flow and repeatable clad morphology [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eXRF analysis has been carried out to determine the elemental composition of the SS316L powder. The average weight percentages of the constituent elements are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The results are in close agreement with standard SS316L specifications, indicating that the powder meets the compositional requirements for use in the DED process [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical composition of SS316L powder measured by XRF analysis.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMn\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eConcentration (wt. %)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBalance\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e12.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003e3.2. Temporal evolution of plasma plume morphology captured via CMOS camera during DED of SS316L at different laser spot diameters\u003c/em\u003e \u003c/p\u003e \u003cp\u003eThe CMOS camera enabled two-dimensional visualization of plasma plume morphology during SS316L deposition, with representative images provided in the supplementary data (Appendix A, Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Sequential images acquired over 0\u0026ndash;2800 ms were converted to 8-bit grayscale and binarized using a global threshold in ImageJ to enable plasma plume morphology analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAt a spot diameter of 0.8 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), the plasma plume exhibited pronounced vertical elongation and strong temporal oscillations, characterized by periodic expansion and contraction. The plasma plume area varied between 4.0 to 11.7 mm\u0026sup2; over the observation period, indicating highly unstable plasma plume dynamics. In contrast, deposition at a 1.3 mm spot diameter (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) resulted in a comparatively stable plasma plume morphology, with the plasma plume area fluctuating within a narrower range of 5.3 to 9.2 mm\u0026sup2; throughout the deposition duration, reflecting steady melt pool behaviour. Increasing the spot diameter to 2.1 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) led to renewed instability, with more frequent and pronounced plasma plume area fluctuations ranging from approximately 2.0 to 7.5 mm\u0026sup2;. This condition exhibited intermittent plasma plume expansion and contraction, suggesting reduced melt pool stability. At the largest spot diameter of 3.5 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), plasma plume formation was strongly suppressed and remained virtually absent over the entire observation window. Binary images revealed inadequate melting, powder dispersion along the deposition track, and incomplete fusion. Overall, the plasma plume morphology exhibited a non-monotonic dependence on laser spot diameter. Strong instabilities were observed at small (0.8 mm) and intermediate (2.1 mm) spot diameters, whereas the 1.3 mm condition produced the most plasma stable plume. While CMOS imaging captures spatial plasma plume dynamics and qualitative stability trends, it does not resolve the underlying excitation and vaporization processes. Therefore, time-resolved OES is employed in the following section to provide quantitative insight into plasma plume intensity and fluctuation behaviour overcoming the qualitative nature and computational burden associated with image-based plume analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Plasma plume intensity and fluctuation response during SS316L laser cladding at different laser spot diameters\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the variation of plasma plume intensity measured at the Fe I emission wavelength of 520.79 nm during SS316L clad deposition for different laser spot diameters. At a spot diameter of 0.8 mm, the emission intensity ranged from 550 to 5800 a.u., with an average value of 1621 a.u. over the 0\u0026ndash;2800 ms acquisition period. Increasing the spot diameter to 1.3 mm reduced the intensity range to 550\u0026ndash;3000 a.u., with a corresponding average of 986 a.u. A further increase to 2.1 mm resulted in intensities between 450 and 3140 a.u., with an average value of 655 a.u. At the largest spot diameter of 3.5 mm, the emission intensity was strongly suppressed, ranging from approximately 450 to 1280 a.u., with an average of 506 a.u. over the deposition duration. The monotonic decrease in emission intensity with increasing spot diameter reflects a progressive reduction in the population of excited Fe atoms contributing to the 520.79 nm transition.\u003c/p\u003e \u003cp\u003eA quantitative estimation of the excited-state number density facilitates an atomistic-level insights of the diminishing plasma plume emission response as a function of increasing laser spot diameter. This evaluation is performed using the formalism described in Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), which enables the extraction of excited state population.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{N}_{m}\\:=\\frac{{I}_{mn}}{{A}_{mn}.h.\\nu\\:}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere N\u003csub\u003em\u003c/sub\u003e represents population of the upper state (m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), I\u003csub\u003emn\u003c/sub\u003e indicates intensity of an emission line (a.u.), A\u003csub\u003emn\u003c/sub\u003e for the transition probability (s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), h denotes Planck constant (J\u0026middot;s), and ν represents frequency of photon (s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). For calculating N\u003csub\u003em\u003c/sub\u003e at various laser spot diameters, a transition probability of 320000 s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 520.79nm has been referenced from the NIST atomic spectra database. The computed values of the excited state number density derived from the emission line at 520.79 nm exhibit a clear and consistent correlation with the corresponding intensity signal measured at varying laser spot diameters. At a spot diameter of 0.8 mm, where the highest emission intensity has been recorded, the population of atoms in the upper excited state has been estimated to be 14\u0026times;10\u003csup\u003e24\u003c/sup\u003e \u0026plusmn; 5.6 \u0026times;10\u003csup\u003e24\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e. This trend is well-aligned with the elevated plasma plume intensity signal observed in this condition. A systematic reduction in both the emission intensity and the excited state population has been observed with increasing spot size: for 1.3 mm, 8\u0026times;10\u003csup\u003e24\u003c/sup\u003e \u0026plusmn; 1.6 \u0026times;10\u003csup\u003e24\u003c/sup\u003em\u003csup\u003e\u0026minus;3\u003c/sup\u003e for 2.1 mm, 7\u0026times;10\u003csup\u003e24\u003c/sup\u003e \u0026plusmn; 2.3 \u0026times;10\u003csup\u003e24\u003c/sup\u003em\u003csup\u003e\u0026minus;3\u003c/sup\u003e; and for 3.5 mm, a minimum value of 4.5\u0026times;10\u003csup\u003e24\u003c/sup\u003e \u0026plusmn; 0.63 \u0026times;10\u003csup\u003e24\u003c/sup\u003em\u003csup\u003e\u0026minus;3\u003c/sup\u003e has been obtained. The mutual consistency observed in the temporal intensity profiles and the corresponding excited atom densities highlights the influence of upper-level population dynamics in governing the plasma emission behaviour.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUpon rigorous examination of the intensity signal profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), distinct temporal variations were observed throughout 0 to 2800 ms, characterized by variations in plasma plume intensity magnitude across different spot diameter conditions, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. These variations, characterized by a periodic peaks and troughs, reflect dynamic variations in energy absorption might be happening within the melt pool. Notably, the fluctuation patterns strongly depended on the laser spot diameter, suggesting that energy distribution correlates with plasma plume stability. The intensity signal fluctuation was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), based on data from three repeated experiments conducted at each spot diameter to quantify these temporal instabilities.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:Fluctuation\\:intensity\\:percentage\\:\\left(\\%\\right)\\:=\\frac{\\sqrt{\\frac{1}{n}{\\sum\\:}_{t=1}^{n}(I\\left(t\\right)-{I}_{Ave}{)}^{2}}}{\\left({I}_{Ave}\\right)}\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere I\u003csub\u003eAve\u003c/sub\u003e represents mean plasma plume intensity signal (a.u.), I(t) denote plasma plume intensity signal at particular point (a.u.), n denote number of time points.\u003c/p\u003e \u003cp\u003eThe temporal fluctuation of the plasma plume intensity exhibited a clear non-monotonic dependence on laser spot diameter, contrasting with the monotonic decrease observed in both average plume intensity and excited-state population. At a spot diameter of 0.8 mm, the fluctuation was highest at 42.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1%. Increasing the spot diameter to 1.3 mm reduced the fluctuation to 21.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.1%, indicating improved plume stability. However, a further increase to 2.1 mm led to a resurgence of fluctuation to 29\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2%, deviating from the declining intensity trend. At the largest spot diameter of 3.5 mm, the fluctuation dropped sharply to a minimum of 4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1%. This non-monotonic variation in OES signal with laser spot diameter (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) suggests corresponding changes in melt pool stability during deposition. To assess how these process-level variations are reflected in the deposited material, the following section presents the microstructural characteristics of SS316L clads produced at different laser spot diameters.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Microstructural evolution of SS316L clads with varying laser spot diameters\u003c/h2\u003e \u003cp\u003eLaser spot diameter strongly influences the microstructural characteristics of SS316L clads, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, which exhibits a characteristic fish-scale morphology associated with a top-hat laser energy distribution. For the 0.8 mm spot diameter (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea), two distinct microstructural zones were identified. Zone 1 (Z1), occupying approximately 72% of the clad area, consisted predominantly of long and medium-length columnar grains growing directionally from the fusion boundary toward the clad centre, consistent with previous observations in SS316L clads [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. High-magnification images revealed cellular and columnar cellular substructures, particularly near the fusion boundary. Zone 2 (Z2), accounting for the remaining 28% of the clad, exhibited a higher fraction of equiaxed grains interspersed with medium-length columnar grains. Grain refinement was more pronounced toward the clad top, where fine cellular substructures dominated, in agreement with earlier reports [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIncreasing the spot diameter to 1.3 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb) resulted in an expansion of Z1 to approximately 78% of the clad area, accompanied by a reduction of Z2 to 22%. Z1 was characterized by a dense population of directionally solidified columnar grains driven by steep thermal gradients and unidirectional heat flow toward the substrate. Well-defined cellular substructures were observed within both long and medium-length columnar grains, particularly near the fusion boundary. In contrast to the 0.8 mm condition, the columnar-to-equiaxed transition in Z2 was less pronounced, indicating enhanced microstructural stability.\u003c/p\u003e \u003cp\u003eAt a spot diameter of 2.1 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), the columnar-grain-dominated Z1 region further expanded to approximately 88% of the clad area, while Z2 was confined to a narrow region near the clad top. Z2 primarily consisted of fine equiaxed grains with occasional medium-length columnar grains. Cellular substructures were consistently observed across both zones, similar to those detected at smaller spot diameters.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eTo interpret the experimental results, a multi-scale analysis is employed to correlate plasma plume behaviour, its temporal fluctuations and clad morphology its microstructure with variations in laser spot diameter. Building on observations from CMOS imaging (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), OES-based intensity and fluctuation analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), and microstructural characterization (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), this section elucidates the underlying physical mechanisms governing these trends. The discussion first examines the influence of spot diameter on surface energy density and laser\u0026ndash;material interaction, followed by a thermophysical force balance explaining plasma plume fluctuations and melt pool behaviour, and finally addresses the role of thermal gradients and solidification parameters in microstructural evolution.\u003c/p\u003e \u003cp\u003e \u003cem\u003e4.1. Correlating surface energy density and plasma plume emission dynamics with clad morphology in SS316L laser deposition\u003c/em\u003e \u003c/p\u003e \u003cp\u003eFor in-depth understanding of laser spot diameter regulates clad morphology and plasma plume characteristics as a process indicator, the effect of spot diameter on surface energy density (E\u003csub\u003ef\u003c/sub\u003e) is analysed using the following relationship using Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e):\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{E}_{f}=\\frac{P}{V\\:X\\:f\\:}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere E\u003csub\u003ef\u003c/sub\u003e is surface energy density (J/mm\u003csup\u003e2\u003c/sup\u003e), P is laser power (W), V is the scan speed used for clad deposition (mm/s) and f is the laser spot diameter (mm). Calculated surface energy densities for SS316L deposition at constant 800 W and 14.6 mm/s are: 68.4 J/mm\u0026sup2; (0.8 mm), 42.1 J/mm\u0026sup2; (1.3 mm), 26.0 J/mm\u0026sup2; (2.1 mm), and 15.6 J/mm\u0026sup2; (3.5 mm). These values underscore the substantial decrease in localized thermal input with increasing spot diameter.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, laser spot diameter systematically altered heat distribution, surface energy density, and clad geometry, which in turn governed plasma plume behaviour. At a spot diameter of 0.8 mm, the highly concentrated top-hat heat input produced the highest surface energy density (68.4 J/mm\u0026sup2;) and a depth aspect ratio of 0.37, characteristic of a transition regime between conduction and keyhole modes [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The deep melt pool geometry and elevated vaporization enhanced laser energy coupling through vapor-phase absorption and internal reflections, resulting in high plasma plume intensity and an elevated excited-state Fe I population at 520.79 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Excessive vaporization, however, partially attenuated beam penetration, contributing to plume instability (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) [\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Increasing the spot diameter to 1.3 mm reduced the surface energy density to 42.1 J/mm\u0026sup2; and the depth aspect ratio to 0.27, indicative of stable conduction-mode melting (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The broader, shallower melt pool limited vaporization, leading to reduced plasma plume activity (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). This clear correspondence between depth aspect ratio and plume emission underscores surface energy density as a key governing parameter. At a spot diameter of 2.1 mm, the surface energy density decreased further to 26.0 J/mm\u0026sup2;, yielding a depth aspect ratio of 0.16 and signalling a transition toward lack-of-fusion behaviour [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The reduced thermal input suppressed melt pool stability and metal vaporization, resulting in weaker plasma plume formation and lower spectral intensity at 520.79 nm (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), consistent with low-energy deposition regimes [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. At the largest spot diameter of 3.5 mm, the surface energy density dropped to 15.6 J/mm\u0026sup2;, producing a broad, shallow heat profile insufficient for stable metallurgical bonding (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). This condition corresponded to a lack-of-fusion regime, characterized by incomplete melting, poor clad integrity, and negligible plasma plume activity, as evidenced by minimal emission intensity and excited-state population (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. These observations are consistent with reported critical energy density thresholds below which fusion quality degrades sharply [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Overall, the strong correspondence between surface energy density, plasma plume intensity and morphology, and clad geometry demonstrates that plasma plume diagnostics, particularly OES-derived intensity and fluctuation metrics provide a sensitive, in-situ indicator of spot-diameter-dependent deposition behaviour at high data acquisition speed (1ms) in SS316L DED. Nevertheless, the cause of non-monotonic plasma plume fluctuations, remains complex and may involve melt pool hydrodynamics controlled by competing thermophysical forces, discussed in the following section.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Thermophysical force interactions governing plasma plume fluctuations and melt pool dynamics in SS316L laser deposition\u003c/h2\u003e \u003cp\u003eTo elucidate the mechanisms underlying the spot-diameter-dependent plasma plume fluctuations observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, a thermophysical force-balance analysis of the melt pool is undertaken. This analysis considers the key contributions of recoil pressure, Marangoni convection driven by surface tension gradients, buoyancy, and hydrostatic forces. These forces, schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, are governed by the measured melt pool surface temperatures and the calculated surface tensions, which vary with laser spot diameter. Understanding the interplay of these forces is vital for interpreting melt pool instabilities and their manifestation through plasma plume behaviour, which ultimately impacts clad morphology in DED process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt a spot diameter of 0.8 mm, the measured average melt pool surface temperature is 2715\u0026thinsp;\u0026plusmn;\u0026thinsp;234\u0026deg;C, as obtained using a two-colour pyrometer (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). This corresponds to a surface energy density of 68.4 J/mm\u0026sup2;, producing deep clad penetration and intense vaporization. The surface tension of the SS316L under these conditions has been calculated using thermophysical property data reported by Chen et al. [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The Eq.\u0026nbsp;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) has been derived based on temperature and surface tension data as mentioned in a temperature range from 1400 to 3100\u0026deg;C:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\sigma\\:\\:=-0.4319\\text{*}\\text{T}+2435.6$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere σ is surface tension in mN/m, T is melt pool surface temperature in \u003csup\u003e0\u003c/sup\u003eC. Using this equation, the surface tension is approximately 1262\u0026thinsp;\u0026plusmn;\u0026thinsp;100 mN/m. As expected, surface tension decreases with increasing temperature due to reduced cohesive forces between molten metal atoms as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThis high-temperature regime, approaching the boiling point of SS316L, drives significant metal vaporization and hence, strong recoil pressure [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] as described by Eq.\u0026nbsp;(\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e):\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:{P}_{r}={P}_{o\\:}exp\\{-\\frac{{{m}_{mol}h}_{vap}}{R}(\\frac{1}{{T}_{S}}-\\frac{1}{{T}_{vap}}\\left)\\right\\}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere P\u003csub\u003er\u003c/sub\u003e is recoil pressure (Pa), P\u003csub\u003e0\u003c/sub\u003e is reference vapour pressure (Pa), h\u003csub\u003evap\u003c/sub\u003e is enthalpy of vaporisation of SS316L (J/kg), m\u003csub\u003emol\u003c/sub\u003e is molar mass (g/mol), R is universal gas constant (8.314 J/mol\u0026middot;K), T\u003csub\u003es\u003c/sub\u003e is surface temperature (K). Since the pyrometer measures an average surface temperature, localized temperatures at the laser focal region are expected to be higher, promoting atomic excitation and partial ionization. Consistent with this, the dominance of Fe I emission observed in Section \u003cspan refid=\"Sec5\" class=\"InternalRef\"\u003e3.3\u003c/span\u003e confirms that the plasma plume primarily consists of excited neutral species.\u003c/p\u003e \u003cp\u003eThe deep melt pool geometry supports multiple internal laser reflections, enhancing local energy absorption and reinforcing vaporization and recoil pressure [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Simultaneously, a strong radial temperature gradient develops across the melt pool surface. Owing to the negative temperature coefficient of surface tension for SS316L, this gradient induces outward Marangoni convection from the hotter centre toward the cooler periphery (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). While hydrostatic pressure and buoyancy-driven convection are present, their influence is minor due to the small melt pool volume. Consequently, the melt pool dynamics are dominated by the interaction between recoil pressure and Marangoni convection, leading to oscillatory instabilities in melt pool geometry and vapor ejection. These instabilities manifest as strong plasma plume intensity fluctuations, reaching 42.5% at 0.8 mm spot diameter (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Thus, high-frequency plume oscillations can be directly attributed to the imbalance between recoil pressure and thermocapillary flow as evidenced by the coupled plume morphology and intensity fluctuations shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt a spot diameter of 1.3 mm, the average melt pool surface temperature decreased to 2545\u0026thinsp;\u0026plusmn;\u0026thinsp;195\u0026deg;C, resulting in an estimated surface tension of 1336\u0026thinsp;\u0026plusmn;\u0026thinsp;84 mN/m (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The lower thermal input reduced metal vaporization and recoil pressure, producing a shallower and more stable clad (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) characteristic of conduction-mode melting. Under these conditions, recoil pressure decreased more rapidly than Marangoni forces, allowing surface-tension-driven flow to dominate melt pool dynamics. This stabilized outward flow promoted a smoother liquid\u0026ndash;vapour interface and more uniform vapor ejection, consistent with previous numerical and experimental studies [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. Buoyancy effects were further suppressed due to the reduced aspect ratio. As a result, plasma plume fluctuations decreased substantially to 21.5% (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), indicating a quasi-steady vaporization regime.\u003c/p\u003e \u003cp\u003eFurther increasing the spot diameter to 2.1 mm significantly reduced the thermal energy input, yielding an average surface temperature of 1685\u0026thinsp;\u0026plusmn;\u0026thinsp;60\u0026deg;C and a corresponding surface tension of 1707\u0026thinsp;\u0026plusmn;\u0026thinsp;26 mN/m (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Under these conditions, recoil pressure and Marangoni convection were both substantially weakened, resulting in a shallow and less dynamic melt pool (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Reduced thermal gradients and increased melt viscosity limited fluid flow and wetting behaviour, leading to poorer dilution and non-uniform clad geometry [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. Buoyancy-driven convection and hydrostatic effects became negligible due to the shallow melt pool. Although overall vaporization and emission intensity decreased (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), plasma plume fluctuation increased to 29%, reflecting unsteady and localized vaporization events arising from non-uniform heating and melt pool instability.\u003c/p\u003e \u003cp\u003eAt the largest spot diameter of 3.5 mm, the surface energy density dropped to 15.6 J/mm\u0026sup2;, and the average surface temperature declined to 1469\u0026thinsp;\u0026plusmn;\u0026thinsp;44\u0026deg;C, corresponding to a surface tension of 1800\u0026thinsp;\u0026plusmn;\u0026thinsp;23 mN/m (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). At this low thermal input, metal vaporization and recoil pressure were negligible, suppressing both melt pool depression and plasma plume formation. The weak thermal gradients limited Marangoni convection, while elevated surface tension further restricted melt pool spreading and fluidity. Consequently, clad formation was dominated by lack-of-fusion defects and poor metallurgical bonding, consistent with reported critical energy density thresholds for effective deposition [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. Plasma plume activity was absent, with negligible emission intensity and the lowest recorded plume fluctuation of 4% (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), confirming an inefficient deposition regime.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Influence of laser spot diameter on solidification dynamics and microstructural evolution\u003c/h2\u003e \u003cp\u003eThe spot-diameter-dependent variations in melt pool stability and thermal conditions directly govern solidification behaviour during deposition. Changes in the temperature gradient (G) and solidification rate (R), through the G/R and G\u0026times;R parameters, grain morphology and microstructural zoning in SS316L clads. Accordingly, this section examines how the plasma plume\u0026ndash;inferred thermal conditions manifest in the observed microstructural evolution with increasing laser spot diameter.\u003c/p\u003e \u003cp\u003eAt a spot diameter of 0.8 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea), the high surface energy density (68.4 J/mm\u0026sup2;) generated a deep melt pool with steep thermal gradients and strong recoil pressure. The resulting high G/R ratio at the melt pool boundary favored elongated columnar grains in Z1, while the large G\u0026times;R product promoted cellular and columnar cellular substructures near the fusion boundary. Local reductions in G/R toward the clad top, arising from heat accumulation and reduced thermal extraction, enabled equiaxed grain formation, leading to the development of Z2. Increasing the spot diameter to 1.3 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb) reduced the energy density to 42.1 J/mm\u0026sup2; and lowered the melt pool surface temperature from 2715\u0026thinsp;\u0026plusmn;\u0026thinsp;234\u0026deg;C to 2545\u0026thinsp;\u0026plusmn;\u0026thinsp;195\u0026deg;C. This produced a shallower and more stable melt pool, expanding Z1 to ~\u0026thinsp;78% of the clad area and suppressing the columnar-to-equiaxed transition. The higher effective G/R ratio inhibited constitutional undercooling, promoting sustained directional solidification. The concurrent reduction in plasma plume intensity fluctuation (from 42.5% to 21.5%) reflects improved melt pool stability. At a spot diameter of 2.1 mm (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), further reduction in energy density (26 J/mm\u0026sup2;) and surface temperature (1685\u0026thinsp;\u0026plusmn;\u0026thinsp;60\u0026deg;C) expanded Z1 to ~\u0026thinsp;88% of the clad, confining Z2 to a narrow region near the top. Although G/R decreased, the G\u0026times;R product remained sufficient to sustain complex cellular and dendritic substructures. The dominance of columnar grains is consistent with reports of suppressed equiaxed solidification under reduced energy input [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOverall, the observed microstructural evolution follows classical solidification theory, with G/R governing columnar-to-equiaxed transitions and G\u0026times;R controlling substructure fineness [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. In SS316L DED, modest variations in G and R induced by spot diameter adjustment significantly alter the solidification pathway [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. The strong correspondence between plasma plume characteristics and microstructural features reinforces the utility of plume-based diagnostics as an in-situ indicator of melt pool stability and solidification conditions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e4.4. \u003cem\u003eConsolidated plasma plume\u0026ndash;based interpretation of spot-diameter-induced deposition behaviour\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e consolidates the influence of laser spot diameter on surface energy density, plasma plume behaviour, and clad geometry, providing a unified mechanistic view of the deposition process. Reducing the spot diameter from 3.5 mm to 0.8 mm increases the surface energy density from approximately 15.65 J/mm\u0026sup2; to 68.4 J/mm\u0026sup2;, leading to intensified vaporization and a corresponding rise in plasma plume intensity from ~\u0026thinsp;500 a.u. to \u0026gt;\u0026thinsp;1600 a.u. However, this elevated energy concentration also amplifies melt pool instabilities (depth aspect ratio-0.37), manifested as a high plasma plume fluctuation level of ~\u0026thinsp;42%. At an intermediate spot diameter of 1.3 mm, a balanced energy input (42.14 J/mm\u0026sup2;) produces a marked reduction in plume fluctuation (21.56%) and yields more uniform clad morphology (depth aspect ratio-0.27) with stable depth-to-height aspect ratios. Further increasing the spot diameter to 3.5 mm lowers the surface energy density below 15.65 J/mm\u0026sup2;, suppressing vaporization and plasma plume activity, resulting in minimal plume fluctuation (4.05%) with lack of fusion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis integrated framework demonstrates that plasma plume intensity and its temporal fluctuation as OES signal serve as quantitative, real-time indicators of spot-diameter-induced transitions in melt pool stability and deposition morphology. Accordingly, spot diameter emerges as a critical design parameter, and its optimization guided by in-situ plasma plume diagnostics offers a robust pathway for achieving stable thermal\u0026ndash;fluid behaviour and consistent deposition quality in laser DED.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis study establishes optical emission spectroscopy (OES), together with CMOS-based plasma plume imaging and pyrometry, as a physics-based in-situ diagnostic framework for resolving the mechanistic influence of laser spot diameter on melt pool stability during SS316L directed energy deposition (DED). The key findings are as follows:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eLaser spot diameter is a critical control parameter governing surface energy density and deposition regime. A small spot diameter (0.8 mm) induces strong vaporization and large plasma plume fluctuations with highly unstable CMOS-observed plume morphology, whereas a large spot diameter (3.5 mm) suppresses plume activity and leads to lack-of-fusion. An intermediate spot diameter (1.3 mm) provides a balanced thermal regime, exhibiting a stable and symmetric plasma plume signal and resulting in uniform clad formation.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eTime-resolved OES measurements of plasma plume intensity and fluctuation directly capture spot-diameter-dependent melt pool behaviour. The Fe I 520.79 nm emission line systematically tracks changes in plasma plume signal and excited-state population, while plume fluctuation uniquely reflects melt pool instability arising from force imbalance.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003ePlasma plume fluctuation exhibits a non-monotonic dependence on spot diameter, driven by the competing effects of recoil pressure and thermocapillary (Marangoni) flow. This behaviour serves as a sensitive indicator of melt pool stability.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eMicrostructural evolution follows solidification theory, with G/R and G\u0026times;R governing grain morphology and substructure scale. Smaller spot diameters promote finer cellular substructures, while increasing spot diameter favours columnar grain dominance.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eOverall, this work demonstrates that OES-derived plasma plume intensity and fluctuation provide quantitative, low-overhead, real-time indicators of spot-diameter-induced melt pool dynamics, elevating OES from a qualitative monitoring tool to a mechanistic diagnostic. The results provide a foundation for spot-diameter-aware monitoring and future closed-loop control strategies in laser directed energy deposition.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003cp\u003eDeclaration of competing interests☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Ravi K R reports equipment, drugs, or supplies was provided by India Ministry of Science \u0026amp; Technology Department of Science and Technology. Ravi K R reports equipment, drugs, or supplies was provided by Science and Engineering Research Board. none If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eMohit Singh: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing \u0026ndash; original draft.Misba Amin: Data curation, Investigation.Manoj J: Data curation, Investigation.Ravi K. R.: Conceptualization, Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing \u0026ndash; review and editing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors express their gratitude to the Department of Science and Technology, Government of India [Grant No: DST/TDT/AMT/2017/225 (G)]; the Science and Engineering Research Board (SERB), Government of India [Grant No: CRG/2021/002636]; and the Prime Minister\u0026rsquo;s Research Fellowship (PMRF), Government of India for supporting the work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eVafadar A, Guzzomi F, Rassau A, Hayward K (2021) Advances in metal additive manufacturing: a review of common processes, industrial applications, and current challenges. 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J Mater Eng Perform 30:6996\u0026ndash;7006. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11665-021-06101-8\u003c/span\u003e\u003cspan address=\"10.1007/s11665-021-06101-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Laser spot diameter, Directed energy deposition, In-situ diagnostics, Plasma plume dynamics, Optical emission spectrometer, CMOS imaging","lastPublishedDoi":"10.21203/rs.3.rs-8623620/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8623620/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn 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 (~\u0026thinsp;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 (~\u0026thinsp;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.\u003c/p\u003e","manuscriptTitle":"Mechanistic influence of laser spot diameter on SS316L directed energy deposition revealed by in-situ plasma plume diagnostics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-22 10:26:19","doi":"10.21203/rs.3.rs-8623620/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a11ec539-22ea-4c71-b921-db22ff96c99e","owner":[],"postedDate":"January 22nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T16:02:49+00:00","versionOfRecord":{"articleIdentity":"rs-8623620","link":"https://doi.org/10.1007/s40964-026-01665-0","journal":{"identity":"progress-in-additive-manufacturing","isVorOnly":false,"title":"Progress in Additive Manufacturing"},"publishedOn":"2026-04-09 15:57:16","publishedOnDateReadable":"April 9th, 2026"},"versionCreatedAt":"2026-01-22 10:26:19","video":"","vorDoi":"10.1007/s40964-026-01665-0","vorDoiUrl":"https://doi.org/10.1007/s40964-026-01665-0","workflowStages":[]},"version":"v1","identity":"rs-8623620","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8623620","identity":"rs-8623620","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}