This paper investigated the weld bead formation in welding of thick section stainless steel at 0.2m/min-2m/min welding speeds with a 10kW fiber laser. The influence of shielding conditions, beam spot size and defocused distance on weld bead shape was studied at low welding speeds. The result showed that meandering bead occurring at the laser power of 10kW and the welding speed of 0.3m/min was due to the severe oxidization of the molten pool. This bead meandering could be prevented by adjusting shielding conditions. Beam spot size had a significant influence on the penetration depth, and the penetration depth at 0.1mm beam spot size was smaller than that at 0.2mm beam spot size. The defocused distance where the maximal penetration could be obtained at 10kW laser power and 0.3m/min was about -7mm. In the final, the instability of penetration depth induced by thermal lens effect was also discussed.

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