Melt flow regularity and hump formation process during laser deep penetration welding

•The relationship between molten pool flow and hump formation in laser deep penetration welding is investigated.•The flow velocity of molten pool in non-penetration welding is around 0.2 m/s.•The full-penetration welding can change flow direction and velocity of molten pool and inhibit formation of hump.•The eruption speed of laser-induced vapor on keyhole front wall can exceed 30 m/s.•The dynamic behaviors of keyhole wall are closely related to molten pool flow and hump formation.