Role of MHz-scale dynamic laser beam shaping with coherent beam combining in the suppression of Fe-Al intermetallics in 22MnB5 steel welds

Laser welding of Al-Si coated 22MnB5 remains challenging due to aluminium (Al) enrichment from the coating, which promotes the formation of α-ferrite and Fe(Al,Si) intermetallics in the fusion zone, significantly reducing weld strength. Current mitigation strategies based on beam shaping have mainly been limited to static or low-frequency (kHz-scale) modulation, while the influence of higher-frequency beam dynamics on melt pool mixing and solidification remains insufficiently explored. This study investigates dynamic laser beam shaping using a high-power coherent beam combining (CBC) laser system capable of spatial and temporal modulation of energy delivery. Butt welds were produced using three selected shapes, namely single-click, forward-triangle and spiral. Temporal characteristics, including shape cycle time, dwell time and the dimensionless number of shape cycles per interaction time were quantified and related to the melt pool and weld response. The static single-click beam produced limited melt pool mixing, resulting in the highest Al accumulation (∼2.2 wt%), coarse martensite, and the lowest tensile strength (764 ± 30 MPa, ∼49.3% weld efficiency). In contrast, the high-frequency (1428 kHz) forward-triangle beam operating in the time-average quasi-static regime enhanced melt pool homogenisation, reduced Al segregation to ∼1.3 wt%, suppressed α-ferrite/Fe(Al,Si) formation, and achieved the highest tensile strength (1217 ± 26 MPa, ∼78.5% weld efficiency). The findings reveal that weld improvement in Al-Si coated 22MnB5 steel is depended on the combined influence of number of shape cycles, directional spatial intensity distribution and dwell time to suppress localised thermal superheating.