The structures of bogies are complex and the welds distributed densely and the numbers are lot for the high speed trains. The medium plate welding of frames with ultra narrow gap wire filling laser welding technology can solve the problem of multiple passes

large deformation and residual stress of traditional MAG welding

and it can also solve the problem of the sensitivity of groove clearance of laser welding. This paper is focused on the molten drop transition of ultra narrow gap wire filling laser welding. The results showed that the transition mode of molten drop was first unfold transition and then liquid bridge transition and finally particle transition when the laser wire spacing was different. The transition mode of molten drop changed from unfold transition under lower power to liquid bridge transition under higher power. The welding speed didn’t affect the transition mode and still the liquid bridge transition. The feeding speed would affect the molten period of the wire and then produce different molten transition mode. It should choose negative defocus when welding thick plate and should choose positive defocus when welding thin plate.