Objective Because of its high power, flexibility, compact size, and low operating cost, high-power fibre lasers have sparked widespread interest in laser welding. However, because of the high energy density of high power fibre lasers, producing a large number of spatter particles during welding is easy, which has a serious negative impact on welding. On the one hand, spatters will pollute the focusing mirror or be located in the laser beam transmission path, resulting in a change in the focusing characteristics of the laser beam and a loss of laser energy transmission, which will seriously affect the welding process’s stability; on the other hand, spatters will cause weld metal loss, resulting in welding defects such as depression and incomplete weld. As a result, studying spatter generation is crucial for understanding the physical process of high power fibre laser welding and optimising welding technology.

Methods In this paper, a high-speed camera was used to observe the formation of spatters and the fluctuation behaviour of the molten pool during high-power fibre laser welding. The dimension and quantity of spatter were measured after welding using a scanning electron microscope, and the plate mass loss before and after welding was calculated. The power density of the fibre laser was altered by adjusting the defocus, and the number of spatters and plate mass loss were investigated. Finally, the effect of defocus on mass loss and spatter formation is investigated using the experimental results.

Results and Discussions The formation of typical spatter particles in fibre laser deep penetration welding was observed using a high-speed camera (Fig.2). The spatter particles collected from the glass cylinder after welding were measured using a scanning electron microscope. The results show that the spatter particles were mostly distributed in the >50--100 μm range (Fig.3). The relationship between plate mass loss and spatter number was then established, and the variation of spatter number and plate mass loss with defocus was demonstrated (Fig.6). Finally, the spatter was related to the inclination angle of the front keyhole wall, the laser-induced vapour on the front keyhole wall, and the surface tension of the molten pool; the reason why the number of spatters increased with defocus was summarised.

Conclusions The spatter behaviour and effect of defocus in high power fibre laser deep penetration welding are investigated in this paper through in-situ optical observation of the molten pool and measurement of plate mass loss. The results show that the formation of spatter in fibre laser molten pool can be divided into three steps: molten pool bulges on the edge of keyhole→ the bulge is elongated to form a liquid column→ the liquid column breaks up to overcome the surface tension and form spatter particles. The spatter particles are mainly distributed between 50 and 100 μm. With the increase in laser defocus, the spatter number, plate mass loss and weld width gradually increase, whereas the penetration gradually decreases. Further investigation reveals that the eruption of spatters is related to the inclination angle of the front keyhole wall, impact force of laser-induced evaporation of the front keyhole wall, and molten pool surface tension. Increasing the defocus reduces the inclination angle of the front keyhole wall and the surface tension of the molten pool, which increases the impact force of the evaporation vapour from the front wall to the upper edge of the rear keyhole wall, resulting in more spatters.