Numerical Analysis Of Thermal Process And Fluid Flow In Weld Pool During Oscillating Laser-GMAW Hybrid Welding Of Lap Joints

To meet the challenge for environment protection and resource conservation,light weight design has gained more attention in automobile industry.The ultra-high strength steel 22MnB5 is a candidate material for manufacturing the lightweight automobile parts due to its high strength,relative low density and low cost.A novel oscillating laser-GMAW hybrid welding process has been developed to deal with 22MnB5 sheets joining of lap joints and it proves to be promising.However,it alsohas some problems,such as the unclear process mechanism and the sensitivity to weldbead defects.These problems put a restriction on the development and application of this new hybrid welding process.Therefore,the numerical analysis of the thermal process and fluid flow in weld pool is essential and helpful to uncover the formation of those weld bead defects and further make a deep insight into this process.Experiment is conducted for the oscillating laser-GMAW hybrid welding process of 22MnB5 sheets of lap joints.The PyroCam and high speed camera are employed to observe the weld pool temperature field and fluid flow evolution,respectively.Three welding parameters are investigated,such as lateral distance of hybrid heat source,welding speed and laser oscillating.Their effects on weld bead appearance,geometry on transverse cross-section and temperature field on joint surface are analyzed.These experimental results provide data support for the model development and numerical analysis of this hybrid welding process.A numerical model is then developed for this process.The model uses an asymmetric distribution of arc heat and a cylindrical volumetric distribution of laser heat to describe the heat transfer from the combined heat source to the workpieces under lap joint configuration.A method is developed to transform the surface heat flux on the phase interface traced by VOF.The droplet transfer is treated as a molten metal fluid flow into the calculation domain from a velocity inlet boundary.The effects of arc pressure,electromagnetic force,surface tension,Marangoni effect and buoyancy are also considered.The heat transfer and fluid flow in weld pool is calculated and this result is helpful to get a deep insight into the hybrid welding process and to direct the optimization of process parameters.There are several weld bead defects occurred in this hybrid welding process,such as non-uniform width weld bead,discontinuity and lack of fusion at top sheet.Their formation mechanisms are investigated experimentally and numerically,respectively.In terms of the experiment,the arc behavior,metal transfer and weld pool fluid flow are examined according to the high speed images and the formation mechanisms corresponding to these defects are suggested.As to the simulation,the accuracy of the model developed before is improved by integrating experimentally measured parameters,such as the droplet velocity,transient arc power and fusion area on sheets at transverse cross-section.The model has been verified by the temperature measurements along the weld pool centerline and the weld geometry on transverse cross-section.The effects of lateral distance of hybrid heat source on heat transfer and fluid flow in weld pool are numerically simulated.Several characteristic variables are defined to describe the flow patterns in weld pool.It is found that the lateral distance of hybrid heat source determines the droplet impingement position and in turns the fluid flow in weld pool.With more offset of the hybrid heat source to top sheet,a stronger lateral flow occurs,which leads to a larger depression and a weak backwards flow in weld pool.The larger depression does harm to the stability of arc and metal transfer and thus results in non-uniform width or even discontinuous weld bead defects.The weak backwards flow is insufficient to fill the pit in the weld pool and thus lack of fusion occurs.When the laser power increases or the welding speed decreases,the keyhole has a larger size and its effect on weld pool temperature field and fluid flow becomes significant.In this condition,the keyhole should be taken into consideration.As the first step of this work,however,a model just covering the oscillating laser welding process is established.The oscillating laser heat source is the combination of two Gaussian heat source,one describing the uniform effect of the oscillating on heat distribution and the other denoting the weakened keyhole effect.A ray tracing method is employed to calculate the laser heat deposition on keyhole wall.The keyhole dynamic behavior,heat transfer and fluid flow in weld pool are calculated.It is found that the multiple reflections improve the laser absorption on the lower part of keyhole.The keyhole wall has a non-uniform temperature distribution and only some spots reach boiling point.The local boiling phenomenon causes the keyhole instability.The oscillating laser drives the keyhole and weld pool to move periodically and contributes to the mixing in the weld pool.These results lay a foundation on further study of the oscillating laser-GMAW hybrid welding process.