Numerical Simulation And Experiment Study Of The Nail-head Weld Shape With High Power Laser Welding

As a band-new material joining technology, laser welding possess lots ofadvantages which cannot be matched by any traditional welding methods. In recentyears, laser welding of thick plate with single pass is possible with the advent ofhigh power and high beam quality fiber laser. However, the laser energy densitywhich irradiate on the surface material is up to106-108W/cm2with high laser power.As a consequence, the dynamic behavior of keyhole, weld pool and metallic vaporplume is extremely unstable during laser welding process, which is easy to causewelding spatter, undercut, hump, nail-head weld. These defects make the weldformation quality cannot be ensured during laser welding of thick plate. So, it is veryimportant to study the formation mechanisms and suppression of weld defects.SUS304stainless steel is a type of austenitic stainless steel with good corrosionresistance, no-magnetic, high tenacity and plasticity, and the application of SUS304stainless steel is becoming wider and wider in many field s, such as aerospace,shipping industry, nuke industry, chemical and biological engineering. Therefore,SUS304stainless steel was used to research the numerical simulation and technologytest of laser welding, and aiming to reveal the formation mechanism and suppressionof “nail-head” weld defects. In this paper, the contents of the research are as follows:Firstly, a three-dimensional steady-state model was developed to investigate theheat transfer and fluid dynamics with the finite element software COMSOLMultiphysics during laser welding process. Three-dimensional heat transfer, fluiddynamics in weld pool driven by the combination of Marangoni force, buoyancyforce and latent heat were taken into account in this model. The solid-liquid phasechange behavior is calculated by an additional source term, and the latent heat forsolid-liquid phase change is solved using equivalent specific heat method. Asegregated solver was used to calculate different physics separately thus reducingthe problem size. So, direct solver and iterative solvers was used to solve for thetemperature, velocity and pressure, respectively. The effect of with or withoutMarangoni convection and different Marangoni coefficient on weld pool oftemperature field, fluid field and weld pool shape are analyzed. The results showthat Marangoni convection at the upper surface lead to the hot liquid metal flowingfrom the center of weld pool to near the boundaries of the weld pool, resulting in"nail head" shape of the weld cross section. Furthermore, the problem of “nail-head” weld shape become more and more prominent with the Marangoni coefficientincrease from0N (m· K) to-1e-3N (m·K). Experimental results show that thecomputed weld pool shapes are in good agreement with the experimental results.Secondly, based on a modified “sandwich” novel method, and high-speed videocamera was used to directly observed the phenomenon during laser welding ofSUS304stainless steel, the relationship between the dynamic behavior of keyhole,metallic vapor plume, weld pool and the formation mechanisms of “nail-head” welddefect were analyzed. The results show that the liquid metal upward-directed aroundthe keyhole was driven by upward friction due to the high-speed ejection of metallicvapor plume. This effect lead to more and more liquid metal gathered in the uppersurface. Then the hot liquid metal flow from regions near the keyhole to regions nearthe boundary of weld pool duo to Marangoni convection caused by the surfacetension gradient, and result in increasing the width of weld pool at upper surface.Two mechanisms lead to a distinct “nail-head” weld defect.Finally, a simple variable method was used to analyze the effect of weldingspeed, defocused distance, laser power, blowing type, nozzle size on weldappearance during laser welding of SUS304stainless steel, the aim is to achieve theeffectively process parameters to control the nail-head weld defect. Then laserwelding with rust powder was carried out in order to study the influence of with orwithout rust powder on weld appearance. The results reveal that the biggest welddepth-to-width ratio and smallest angle of fusion line weld cross section has beenachieved when the welding speed is25mm/s and defocused distance is-3mm atcoaxial blowing, and the “nail-head” weld defect was better control. The results alsoshow that the weld quality significant improve with powder, the weld depthincreased, weld width decreased, the “nail-head” weld defect was effectivelysuppressed, and changed to “V” shape of weld cross section, which can effectivelydecrease deformations and residual stress of the workpiece after cooling d own. So,optimizing welding parameters and adding active powder were two valid means tocontrol “nail-head” weld defect, and also can provide theoretical and Technicalguidance for improving the welding quality and decreasing the welding distortion.