A Dynamic Study On The Compressible Vapor Inside Transient Keyhole During The Deep Penetration Laser Welding

The vapor generated during the laser welding plays an important role in the welding process stability and the weld quality. The vapor inside keyhole is high temperature, transient and invisible by naked eye. Besides, the weld pool, keyhole and metallic vapor dynamics have different timescales during the laser welding. Due to the absence of effective investigated method, the dynamic behavior and mechanism of metallic vapor inside keyhole is not clear currently.In this paper, the modeling for the compressible metallic vapor inside transient keyhole is developed. A novel highly efficient multiple time scale method combing a dual-time stepping with incompressible and compressible fluid solvers is proposed. The calculation efficiency by this method is significantly improved and can almost reach tens of times than that of theoretical method.The vapor inside transient keyhole during typical laser welding process for a 304 steel has been simulated. It is found that the time dependent distributions of vapor plume characteristics, including temperature, pressure, velocity, density and Mach number, inside transient keyhole induced by laser welding can be reasonably predicated. The results indicate that the highest temperature of metallic vapor is about 4000 K, and the averaged pressure and density of metallic vapor inside keyhole are respectively 100~113kPa and 0.15~0.3kg/m3. The simulation results are good agreement with the experimental results and literature data.In this paper, by comparing temperature, pressure, velocity, Mach number and density distributions in the different time, it is found that metallic vapor inside keyhole has uneven distributions and high transient characteristic. The transient Mach number can easily reach as large as 0.8, even exceed 1.0, hence the metallic vapor can be identified as violent compressibility. Some typical phenomena during the laser welding including the local evaporation on the keyhole surface and the swing of the metallic vapor at the keyhole opening, in accordance with the experimental observation and literature results, can also be easily found in our simulation results. According to the variations of vapor characteristic parameters in different laser process, it is found that the heat input is larger, the vapor temperature, pressure and Mach number are higher, and the oscillation degree of the velocity and inclination of the metallic vapor at the keyhole opening obviously increase.