Study On Welding Characteristics And Molten Pool Behaviors Of Dual-spot Fiber Laser Welding

Compared to the other fusion welding techniques,laser welding has significant advantages on welding precision and efficiency,which has been widely used in automotive and aerospace industries.However,during the actual application processes,it was found that there still existed some limitations for conventional single-spot laser welding.On the one hand,due to the small spot diameter and high power density,keyhole collapse and fierce molten pool fluctuation were easily generated in laser keyhole welding process,which would deteriorate the weld quality.On the other hand,it needed precise fit-up requirement before welding,which led to bad welding adaptability.Previous studies indicated that a dual-spot laser welding method could solve these problems effectively.But up to now,the researches about dual-spot laser welding characteristics were not systematic,and the corresponding welding mechanisms were still not fully clear,which made it difficult to guide the applications of dual-spot laser welding.Aiming at the above-mentioned problems,in this disseration the characteristics of power density distribution(power field)in dual-spot fiber laser welding were analyzed firstly.The dynamic characteristics of molten pool surface with different power field modes and their influences on weld formation were systematically studied based on the visual sensing method.The keyhole and melt flow behaviors of the intenal molten pool were calculated and analyzed synchronously by finite element method.The influence mechnism of complex power field on molten pool behaviors in dual-spot laser welding was revealed according to the above studies.Further,the wire melting characteristics of dual-spot laser welding and weld morphologies control of unconventional cross-type joint were studied,respectively,and the corresponding optimal power field design were also obtained followed by good weld.Firstly,the characteristics of power density distribution with different beam distance and energy ratio in dual-spot laser welding were measured and calculated.The critical power density of laser deep penetration welding for stainless steel was obtained.Combining the researaches of physical characteristics of molten pool surface and weld formation with different power fiels modes in dual-spot laser welding,the internal relations between power field,molten pool behavior and weld formation were revealed.The experimental results indicated that increasing the beam distance,using equal power density and side-by-side beam configuration would decrease the melting efficiency.When two separated keyholes with tandem configfuration were created in molten pool,an enhanced melt flow was generated along the direction of molten pool width.The power field characteristics would influence the flow direction and velocity of the enhanced flow by changing the keyhole number,position and energy efficiencies,which then affected the final weld morphologies.When the beam distance increased,the weld depth and width both decreased.The increased energy ratio led to that the weld depth firstly decreased and then increased,while the weld width showed the opposite variation tendency.The minimun weld depth and maximum weld width were obtained at the energy ratio of 50/50.The weld depth with side-by-side configuration was smaller than that of tandem configuration,but the relative size of weld width was controlled by welding speed.In order to elucidate the melt flow behaviors with different keyhole modes in dual-spot laser welding,a three-dimensional transient mathematical model of thermal-fluid coupling was developed,and the effects of melt flow behaviors on weld formation and alloying element distribution were then studied using this model.The research results showed that when a big keyhole was created in dual-spot laser welding,the vortex existed in the rear keyhole disappeared,which weakened the impact effect on keyhole and then improved the keyhole stability.With two keyholes in tandem configuration,the melt between the two keyholes flowed upward with a high velocity due to the effects of the metal vapor recoil pressure;the melt velocity along the direction of the molten pool width also increased following the increase of the molten pool width.Increasing beam distance or using unequal power density would enlarge the acting zone and change the action direction of the metal vapor recoil pressure,which would decrease the melt velocity and molten pool width.The flow direction of the whole molten pool was uniform with side-by-side beam configuration,which led to better molten pool stability.A vortex with high flow velocity was generated when two keyholes were with tandem configuration,which was beneficial to uniform distribution of alloying element in molten pool.According to the unstability of wire melting in conventional single-spot laser welding with filler wire,the improvement effects of power field modes on wire melting behaviors were studied based on the energy distribution analyse of the dual-spot laser,and the optimal power field mode was also obtained.The results showed that side-by-side configuration could improve wire melting stability significantly.In liquid bridge mode,side-by-side configuration could improve the weld morphology further compared to conventional laser welding with filler wire;for droplet mode which could not be applied by conventional laser welding with filler wire,side-by-side configuration still led to stable wire melting and transfer following the significant improvement of welding stability.On the basis of the aforementioned studies on heat and mass transfer of dual-spot laser welding,and in view of the fact that the joining zone of unconventional cross-typw joint was difficult to fuse,the dual-spot laser welding technique was successfully applied to fabricate the unconventional cross-type joint through the welding scheme that two keyholes with tandem configuration were produced in molten pool,which was get through the flexibility power field design in dual-spot laser welding.A joint with good weld morphology and satisfied mechanical property was also obtained.The feasibility and advancement of dual-spot laser welding in industry application were proved.