Research On Metallurgical Kinetics And Formability Optimization Of Back Reflection Laser Welding For Titanium Alloy Sheet

Back reflection laser welding is a new method of high efficient laser welding for sheet,which can achieve the X-shaped welding forming under the process parameters that conventional laser welding can only obtain the V-shaped weld;however,the theoretical research of this high efficient laser welding method for sheet type is still relatively weak.The article put TC4 as the research object and combined the numerical simulation analysis and experimental research method,afterwards,metallurgical dynamics behaviors such as heat transfer and flow and solidification of molten pool were simulated and analyzed under back reflection efficiency laser welding,aiming at revealing the forming mechanism of back reflection synergistic laser welding from the perspective of metallurgy kinetics;At the same time,the optimization method that auxiliary electric field was introduced to realize back reflection laser welding was put forward based on the back reflection synergistic effect and combined with the studies on pluripotent field mechanism of action,than,exploratory experimental study was performed,which provide technical support for further expanding application field of high quality and efficient welding methods for sheet.The main work done and the results accomplished are as follows:(1)"Double gauss heat source model" of three-dimensional transient temperature field was established based on FLUENT in the process of Titanium alloy sheet back reflection efficiency laser welding,than,the distribution of temperature field was simulated in the process of welding,the influence of main laser parameters to shape of molten pool and distribution of temperature gradient were also simulated.The results showed that the heat input state of "double heat source" was the root cause that temperature field distribution of molten pool was from the "V" shape to the "X" shape into "V" shape;Width and depth of molten pool increased gradually with the increasing of power or reducing of the scanning speed,and temperature gradient was proportional to the power and inversely proportional to the speed;The model established can involve the temperature field distribution of molten pool in the back reflection laser welding by experimental verification.(2)An unified model of gas phase area,liquid phase area and solid phase area during themolten pool flow process of back reflection induced synergistic laser welding was established;Three-phase change of keyhole in the molten pool and distribution of flow field was obtained.The results showed that the distribution of temperature field caused by heat input was the direct impetus of phase change;Width and depth of molten pool increased gradually with the increasing of power or reducing of the scanning speed;The coupling effect of plasma & vapor cloud caused by keyhole and back plume induced by back subplate is the main reason for the formation of X-type weld,and surface tension is the main driving force of the "oval reflux ring" formation.(3)Refering to the simulation results of the temperature field and flow field,dendrite growth and solute distribution of molten pool was simulated and analyzed by using the CA method in the process of back reflection efficiency laser welding;The shape of molten pool formed by back reflection laser welding was used to contrast with simulated topography.The results showed that the impact of temperature gradient to the rate of crystallization was significant,and both presented inverse proportion;The effect of melt flow direction to characteristics of isometric crystal was significantly,which led the phenomenon that the left side was longer than right side,side of top and bottom was basic equal;The phenomenon that closed or semi-closed area was formed by secondary dendrite formation was the cause of solute accumulation;The consistency of morphology tested and topography simulated was high,which indicates that this model can better simulate the grain growth in the process of back reflection welding(4)The optimization method that auxiliary electric field was introduced to realize back reflection laser welding was put forward based on the back reflection synergistic effect and combined with the studies on pluripotent field mechanism of action,than,exploratory experimental study was performed and organization and mechanical properties were tested.The results showed that auxiliary electric field had no effect on weld positive morphology and molten pool morphology,but it had significant effect on the back of weld;Auxiliary electric field can refine grain to a certain extent,meanwhile,auxiliary electric field had a direct link with "black spots or patches" in the weld microstructure;Besides,auxiliary electric field had almost no effecton on the tensile properties and hardness of the weld joint.