Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant

•Laser welding with filler wire was applied in multi-layer narrow-gap welding in vertical positions.•The process stability and weld defects were studied and compared in laser vertical-up and vertical-down welding.•The droplet transfer behavior and fluidity of molten pool were investigated for laser vertical welding.•Laser welding with beam oscillation could increase laser irradiated area and homogenize temperature in molten pool.•Weld formation was improved and 20-mm vertical welded joint was obtained without defects by laser welding.