- In this study, fast welding of an aluminum alloy AlSi12 and a structural steel S355 was investigated using a single mode fiber laser system delivering a maximum laser power of 10 kW with a laser beam quality of M2 = 2. The laser beam was focused with a welding head OL YW50 F500 HQ from Precitec GmbH & Co. KG resulting in a spot size of 70 μm, thus delivering a maximum intensity up to several 108 W/cm2 to the workpiece surface. Therefore, narrow weld seams and large penetration depths could be achieved. However, detrimental effects like humping, spiking, spatter tendency, pore formation, and hot cracking, reported by Li et al. [J. Mater. Process. Technol. 214, 565–570 (2014)] and Zhang et al. [Optic Laser Technol. 98, 97–105 (2018)], have been observed at some parameter settings. The welding experiments performed on AlSi12 with a laser beam power of 10 kW, focus position on the material surface, and welding speeds from 2 to 10 m/min reveal penetration depths of about 12 mm independently from the distinct welding speed. Therefore, the weld bead width was 5.5 mm for 2 m/min, thus decreasing to 1.1 mm at the highest welding speed of 10 m/min. By shifting the focus position inside the material, the penetration depth rises significantly and reached a maximum of 17.5 mm for a welding speed of 2.5 m/min and a focus position of 6 mm underneath the sample surface. For this, the weld cross section width, measured at half of the penetration depth, was 2.2 mm. For the structural steel S355, the maximum penetration depth was determined to be 19.5 mm, here with 10 kW laser power, focus position on the material surface, and a comparable welding speed of 2.4 m/min. Therefore, a weld bead width of about 0.75 mm was measured, while the weld cross section width was about 0.55 mm. Finally, first butt welds of 19.5 mm thick S355 samples were generated characterized by smooth welds with a bead width of about 2.5 mm and a weld cross section width of 0.5 mm, respectively.