Numerical analysis of the effect of stationary laser beam properties on Al-Si coating mixing in 22MnB5 steel

• CFD study of Al-Si coating mixing in stationary laser-processed 22MnB5 steel. • Reliable Al contamination analysis with mesh size matching 30 µm coating thickness. • Positive defocusing with large beam waist radius produces a Y-shaped melt pool. • Y-shaped melt pool configuration lowers Al contamination in the fused region. This study presents a numerical investigation of molten pool dynamics and coating material mixing under stationary laser irradiation, using a computational fluid dynamics (CFD) model to analyse contamination effects and the resulting melt pool geometry with a fixed laser beam relative to the workpiece. The model considers a 1.6 mm-thick 22MnB5 steel coated on both sides with a 30 µm-thick Al-Si coating. A grid independency analysis, performed with mesh sizes from 15 µm to 120 µm, showed that mesh sizes at least matching the Al-Si coating thickness are required for accurate representation of fluid dynamics and contamination effects in terms of average Al content and volume percentage of the fused zone contaminated by the coating material. A numerical investigation was carried out to evaluate how variations in laser beam waist radius (0.225, 0.3, and 0.375 mm) and defocusing distance (-0.8 mm, 0, and +0.8 mm) influence the melt pool geometry and the Al-Si coating contamination in 22MnB5 steel. Results indicated that, under positive defocusing conditions, a larger beam waist radius, that is the radius of the laser beam at the focal point, favoured the formation of a Y-shaped keyhole geometry and contributed to lower Al-Si contamination levels in the melt pool. Higher contamination levels were observed in the simulations with a beam waist radius of 0.225 mm, compared to the cases with 0.3 mm and 0.375 mm, indicating an inverse relationship between beam waist radius and coating contamination.