Numerical simulation of full penetration laser welding of thick steel plate with high power high brightness laser

• Lower surface of melt pool is longer and more unstable than upper surface.
• A part of laser beam directly escapes from lower exit without any contact with metal.
• Transient behavior of molten pool and keyhole has periodic feature.
• One typical cycle of molten pool fluctuation takes about 8 ms.
• Flow field is decided by the competition between surface tension and recoil pressure.

Full penetration laser welding was carried out on a 10 mm steel plate using a 16 kW maximum power continuous wave thin disk laser. Upper surface and lower surface of molten pool were observed synchronously with two high speed CCD cameras during the welding process. The lower surface was much longer and more unstable than the upper one. A three dimensional laser deep penetration welding model in which volume of fluid (VOF) method was combined with a ray-tracing algorithm was used to simulate the dynamic coupling between keyhole and molten pool in laser full penetration welding. The calculated weld cross-section morphology and molten pool length on both upper side and lower side agree well with experimental results. Evolution of molten pool in lower side during full penetration laser welding was analyzed, periodical features of energy coupling, molten pool behavior and keyhole dynamics in laser full penetration welding were identified and discussed.