| Laser welding has the advantages of small heat affected zone,low residual stress,high energy density,precise and controllable welding process,which is widely used in automobile and aviation industry.Laser welding is a very complex process,in which the weld pool transmission mechanism and keyhole behavior determine the weld morphology and the quality of the welded joint.However,the physical phenomena behind these welds are not fully understood yet.Therefore,this paper uses the software Fluent to establish a dissimilar metal laser welding and aluminum alloy dual-beam welding three-dimensional transient coupling heat and flow model to study the laser welding process melt pool flow and heat transfer,keyhole evolution and the mechanism of mass transfer between dissimilar metals.Furthermore,the melt pool morphology and porosity formation law of dual-beam welding was also investigated.Steel/aluminum laser lap welding experiments were carried out with DP600 steel and 6016 aluminum alloy,which are commonly applicate for bodywork,to study the forming pattern of the weld under different laser parameters in this paper.SEM and EDS were used to test the welded specimens.It can conclude that the welded joint is tightly bonded,the surface morphology of the weld is smooth,but there are porosity at the welding interface of aluminum material.There are some Fe-Al intermetallic compounds in the middle and sides of the weld section.A three-dimensional transient numerical model for laser deep penetration welding of dissimilar metals was established.The numerical simulation is used the basic equations of fluid mechanics,multiphase flow VOF equation,porous media equation,etc.for solving the calculation.A three-dimensional adaptive cone Gaussian heat source model is used to simulate the laser beam,the mass transport follows Fick’s law,and the forces such as surface tension,Marangoni force and recoil pressure are mainly considered in the melt pool.The User–Defined Functions(UDFs)were implemented by using the C programming language,which include the phase interface adjustment function,source term equation and some boundary conditions,etc.The relative importance of convection and conduction and the effects of various driving forces on the convection in the melt pool were analyzed in terms of dimensionless numbers.Conclusion indicate that maximum melt pool temperature and liquid flow rate at the peak has slightly wave.The dynamic balance of recoil pressure and surface tension determines keyhole stability.Convection,diffusion and keyhole behavior all have significant impact to metal transfer.The conclusion between numerical simulation and experimentation has better match.To further research the 6016 aluminum alloy melt pool morphology and porosity formation law,take advantage of the keyhole coupling melt pool dynamic mechanism,the establishment of aluminum alloy dual-beam welding three-dimensional transient model.The three-dimensional cone heat source model is improved by considering the influence of beam distribution position and welding parameters on the coupling energy effect of two beams.The VOF method was used to calculate the keyhole free surface of keyholes,and the enthalpy pore medium method was used to treat the solid-liquid phase transition.It can be derived that in dual-beam laser welding,the coupling effect between keyholes enhances the melt flow in Y direction,and the weld pool surface changes from ellipse to circle.When the spot spacing is 0.6 mm,there was a periodic process of separation and fusion in addition to the common stages of growth,maintenance,breakup and shrinkage.In the case of dual-beam welding,the anti-deformation ability of keyhole is stronger,and the fracture frequency and fluctuation amplitude of keyhole depth are smaller than those of single beam welding. |
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