Laser-induced breakdown spectroscopy (LIBS) of LPBF-fabricated alloy steel: effect of surface roughness and the laser–material interaction mechanism

Abstract

Laser-induced breakdown spectroscopy (LIBS) is an ideal method for the online elemental analysis of Laser Powder Bed Fusion (LPBF) components due to its in situ and rapid characteristics. However, the inherent surface roughness of LPBF-fabricated parts causes fluctuations in LIBS spectral signals, consequently affecting the accuracy and stability of quantitative analysis. The underlying physical mechanism by which roughness influences spectral signals remains unclear, which restricts the development of relevant spectral correction algorithms. To address this, we prepared alloy steel samples with surface roughness (S_a) ranging from 4.52 to 12.82 µm by adjusting LPBF process parameters and performed LIBS analysis. By analyzing spectral intensity, signal-to-noise ratio (SNR), and relative standard deviation (RSD), we found that the characteristic spectral line intensities of Fe, Cr, Mn, and Ni initially increased and then decreased with increasing roughness, reaching a peak at S_a = 5.61 µm. This peak intensity was 46.4% higher than that of the roughest sample (S_a = 12.82 µm). Plasma temperature and electron density also reached their maximum values at S_a = 5.61 µm (15 790 K and 1.84 × 10¹⁷ cm⁻³ respectively). The observation results of the volume morphology of the ablation plume and the ablation pit both indicate that roughness affects the LIBS signal through a triple coupling effect: low roughness (S_a = 4.52 µm) leads to energy loss due to high reflectivity, while high roughness (S_a ≥ 7.15 µm) weakens ablation efficiency due to an increased ablation threshold and non-uniform energy distribution caused by microstructures. The sample with S_a = 5.61 µm represents an optimal balance between reflectivity and ablation threshold, exhibiting the largest integral of plume area and time (IPAT) and largest volume of ablation pits uniform ablation, which generates stable plasma and, consequently, high-quality spectral signals. This study elucidates the physical mechanism by which roughness influences LIBS spectral signals through a chained pathway of “laser ablation-plasma evolution-spectral response,” laying a theoretical foundation for the development of spectral correction algorithms tailored for complex LPBF surfaces.