Study On Heat And Mass Transfer And Keyhole Instability Behavior Of Laser Melt Pool With The Adjustable Ring Mode Laser

The keyhole mode adopted by laser deep penetration welding can obtain a larger penetration depth,but keyhole instability also leads to the instability of molten pool,resulting in porosity and spatter.The adjustable laser center power of the adjustable ring mode laser helps to achieve sufficient depth penetration,while the ring power ensures good temperature distribution,which plays an important role in keyhole stability and welding efficiency.Therefore,the heat and mass transfer behavior and keyhole stability of t the adjustable ring mode laser molten pool under different beam characteristics were systematically studied in this study,and the mechanism of beam characteristics increasing the stability of the molten pool was clarified,thus providing theoretical guidance for the further development and engineering application of the adjustable ring mode laser technology.Firstly,a heat and mass transfer model of molten pool was established by fluid volume method,taking into account the vapor recoil pressure,Marangoni convection,surface tension and buoyancy.Considering the heat source attenuation in three aspects:the shield of metal vapor plume,the absorption of liquid metal and laser defocusing,the superposition of ring laser and the center laser,the adjustable ring mode laser heat source model was established.According to the solidification structure theory and dendrite growth dynamics,the cellular automata method was used to establish the solidification structure calculation model.Secondly,the heat and mass transfer model of molten pool was used to simulate the formation process of deep fusion welding keyhole and the collapse and instability process of keyhole wall,and the heat and mass transfer mechanism in molten pool was analyzed.The simulation results of molten pool cross-section profile are in good agreement with the experimental results under the same conditions,and the error of molten pool depth is less than 6%.By comparing the morphology of the molten pool and keyhole with different beam characteristics and the keyhole stability,it is concluded that the high energy density of the center is conducive to the increase of penetration depth.The ring laser makes the adjustable ring mode laser obtain more stable keyhole,and the adjustable ring mode laser has higher welding efficiency under the same penetration depth.When the scanning speed increases from 10 mm/s to 60mm/s,the stability of the laser keyhole with the adjustable ring mode laser is reduced by 5.97%,but the gauss laser is 1.7 times more stable than the adjustable ring mode laser keyhole.Finally,the solidification conditions of the molten pool under different beam characteristics are analyzed.It is concluded that simply changing the beam characteristics does not have obvious influence on the solidification conditions of the molten pool boundary.The solidification conditions of the adjustable ring mode laser melt pool under different process parameters were studied.The results showed that with the laser scanning speed increasing from 10 mm/s to 60 mm/s,the temperature gradient and solidification rate increased,and the temperature gradient（G）/solidification rate（R）decreased.By analyzing the effects of different process parameters on the microstructure,it is concluded that the morphology of the columnar crystal is affected by the growth of synzygous crystals and dendrite competition.When the nucleation rate in the molten pool increases from 3.0×10⁷ m^-2 to 4.5×10⁷ m^-2,the number of nucleation in the molten pool increases and the epitaxial grains are easily blocked.As the scanning speed increases from 10 mm/s to 30 mm/s,the transformation from columnar to equiaxed crystals（CET）occurs in the weld,and a large number of equiaxed crystals are formed at the top of the weld,refining the grain size.