Numerical Simulation Study On Transient Behaviors Of Molten Pool And Keyhole During Laser Welding Of Stainless Steel

Laser welding is an advanced material connection processing technology,which has many advantages such as high efficiency,high precision,easy automation,welding of refractory materials and dissimilar materials,small weld heat zone and high aspect ratio of weld.Now laser welding technology is developing rapidly,and it has been spread and applied in many fields such as lithium battery industry and shipbuilding industry.So the requirements for laser welding quality are getting higher and higher.The research shows that the transient behaviors of the molten pool and keyhole affect the quality of laser welding.In order to fully understand the laser welding process and improve the quality of laser welding,the paper mainly investigates the transient behaviors of the molten pool and keyhole during the laser welding process of 304 stainless steel under different penetration statuses and different laser incident angles.In this paper,the numerical simulation method is used to show the evolution of the molten pool and keyhole in 304 stainless steel laser welding under six conditions.Based on the results of numerical simulation,combined with relevant knowledge such as surface tension and Marangoni force,the characteristics of the shape of molten pool,the flow field of the molten pool,and the behavior of the keyhole are analyzed,providing some theoretical references for obtaining a good weldment.Firstly,three types of weldment samples,including non-penetration,insufficient penetration and full penetration,are trial-produced using different laser welding process parameters.At the same time,different laser incident angles are used for welding tests.In addition,some weldments are selected for cutting,grinding,polishing and corroding to obtain the cross-sectional topography of the weld,providing a basis for verifying the simulation results.Then,based on the basic equations of fluid mechanics,a three-dimensional transient model is established to simulate the coupling between the molten pool and the keyhole during the laser welding process.In the proposed model,a ray-tracing heat source is used to describe the distribution of laser energy,and many factors such as thermal radiation,thermal convection,heat of evaporation,Marangoni force,recoil force and surface tension are considered.Secondly,based on the established model,the FLOW3 D fluid simulation software is used to simulate the three types of laser welding processes,namely,non-penetration,insufficient penetration and full penetration.Comparisons of simulation and test results show that the established model can represent the laser welding process.Based on the analysis of the simulation results,it is found that the molten pool and the keyhole have their own characteristics in each penetration welding state.The keyhole fluctuates the most during full penetration welding.The flow inside the molten pool is relatively stable in non-penetration welding,and the flow inside the molten pool is relatively complicated during insufficient penetration and full penetration welding.In full penetration welding,the energy absorbed by the weldment varies greatly.The relationship between the welding process parameters and the penetration status of the weldment is obtained from the analysis of the test results.Finally,based on the established laser welding mathematical model,the FLOW 3D software is used to simulate the welding process with laser incidence angles of 30°,0°,and-30°,respectively.And the reliability of the simulation results is verified by combining the test results.The variations of temperature field,flow field in molten pool and keyhole shape are analyzed at different laser incident angles.The results show that the flow in the molten pool is affected by the laser incident angle.Analyzing the recoil force on the rear keyhole wall,it can be found that the positive incident angle welding is beneficial to suppress the formation of spatters,while the negative promotes the generation of spatters.When the laser incident angle is positive,the stability of keyhole increases,but when it is negative,the stability of keyhole decreases,and the collapse of keyhole increases which raises the probability of bubbles generation.In addition,the analysis of test results show that as the incident angle of the laser increases,the penetration depth of the weldment decreases,and the welding quality can be improved by using an appropriate laser incident angle.