Abstract
High-power fiber laser welding is an efficient and effective way to produce heavy section structures. However, there is a significant challenge in producing the welds with free of imperfections such as nail-head-shaped welds, spatters, and root sagging. This is partially due to a lack of understanding of the welding mechanism of high-power fiber laser. In this paper, we were especially interested in the mechanism to improve the appearance of welds, and we focused on the autogenous laser welding on thick stainless steel plates by a 10 kW fiber laser. To look into the relations of process parameters and the quality of welds, a high-speed imaging system was applied to observe the molten pool flow and vapor plume during the welding process. The appearances of welds subjected to different welding conditions were analyzed. The results showed that (1) nail-head-shaped welds were suppressed by using a gas jet during laser welding process. (2) In the forward welding, a gentle upwelling molten metal flow on the rear keyhole wall, a deeper weld pool and a weaker vapor plume resulted in no spatter. (3) The gravity affected the formation of underfills and root sagging significantly during autogenous laser welding of thick plates. (4) When the workpiece was placed vertically in the transverse position, the welding process was stable without an aggregation of molten melt at the back surface. Moreover, the mechanisms of forming root sagging and humps were different at the top surface.
Highlights
As the 10 kW level high-power laser became commercially since 2005 [1], it has made it feasible to perform autogenous welding on thick-plates with single pass [2]
Rai et al [9] found that possible causes of a nail-head shaped weld were (1) the thermal radiation of the ejected vapor plume on the weld pool surface and (2) the upward flow of the molten metal driven by the Marangoni convection and the evaporation recoil pressure
They carried out numerical simulation to verify that the Marangoni convection drove the melt flow from center to side of the weld pool, this caused the formation of a nail-head-shaped weld
Summary
As the 10 kW level high-power laser became commercially since 2005 [1], it has made it feasible to perform autogenous welding on thick-plates with single pass [2]. Zacharia et al [10] argued that the Marangoni convection by the surface tension was the main reason to widen the upper pool of weld. They carried out numerical simulation to verify that the Marangoni convection drove the melt flow from center to side of the weld pool, this caused the formation of a nail-head-shaped weld. To address this issue, Bachmann et al [11]
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