Research On Dynamic Behavior Of Aluminum Alloy Keyhole And Molten Pool In Laser Welding With Premelting Liquid Filler

At present,the laser beam welding with filler wire(LWFW)technology has been widely applied in the fields of aviation,aerospace,car manufacturing and so on.The(LWFW)technology is able to improve the gap adaptability,change the compo sition of weld,and control the microstructure and properties of the joints.Usually,the weld surface of LWFW is more smooth and uniform than that of autogenous laser welding.However,the requirement for alignment accuracy and direction of the laser beam and filler wire is more stringent,since the laser spot size is very small.In the process of wire feeding,the reflection of incident laser from the filler wire has a negative effect on the utilization of laser energy,causing fluctuations of the laser e nergy in the welding process.And the negative effect is particularly serious w hen welding the aluminum alloys with high reflectivity.Therefore,in this dissertation,aiming at the LWFW technology for aluminum alloy,the influence of filler metal on the keyhole and molten pool dynamic behavior were studied.A new filler feeding method was developed to improve the stability of the welding process.The influence of filler metal on the dynamic behavior of the molten pool and keyhole in conventional LWFW process were studied systematically.Furthermore,a new method of laser welding with pre-melting liquid filler was proposed.The effects of liquid filling method on the stability of laser welding process for aluminum alloy were expounded and verified in terms of physical properties and welding performance of the molten pool and keyhole surface.At the same time,based on the finite element simulation method,the influence of filler meltal with different feeding positions on the shape of keyhole and the flow behavior of molten pool were clarified.Based on the advanced X-ray transmission monitoring system for dynamic behavior of molten pool,the influence of filling metal on the flow behavior of the molten pool and shape of keyhole in conventional LWFW process was first investigated.It was proved that filling metal had a large impact on the keyhole when it filled along the keyhole edge.It could hinder the rapid escape of the internal metal vapor and caused the phenomenon of necking and closure in the lower part o f the keyhole,inducing,more bubbles at the bottom of the keyhole.The internal flow of molten pool was also complex.Two vortexes were produced in the back of the keyhole.At high speed condition,the fluctuations at bottom of the keyhole and the number of bubbles were reduced.The flow path of the molten pool became more simple,and the vortexes behind the molten pool vanished.The liquid metal in the rear of the weld pool had clear upward flowing trend,thereby the porosity rate of the weld joints were reduced.To solve this problem in conventional LWFW process,the new method of laser welding with pre-melting liquid filler was achieved: the filling metal was melted by arc energy and fed into molten pool in the form of liquid,meanwhile,the molten pool and keyhole were mainly formed only by laser energy.According to the state of filling metal melted by arc,two filling modes were found in laser welding with liquid filler: partial melting state and full melting state.The results showed that the molten p ool size,the distance between filling metal and the keyhole were increased by the laser welding with pre-melting liquid filler process,which avoided the direct impact of the filling metal on the keyhole and reduced the porosity rate of the aluminum alloy weld joints.Even at the high speed welding of 15 m/min,the filling metal could be stable melted and a good weld shape was obtained by laser welding with pre-melting liquid filler process.And the maximum laser and wire offset could be up to 1 mm.In order to further reveal the influence mechanism of filling metal to keyhole and molten pool flow characteristics,the hot-flow coupling model for threedimensional transient process of the laser welding with filling metal was established.The influence of liquid filling metal on the keyhole and molten pool flow characteristics were clarified.Keyhole front wall maximum projection angle was prposed to characterize for the stability of keyhole.Increasing the distance between filling metal and the keyhole and welding speed could reduce the maximum projection angle of keyhole front wall,thereby improving the keyhole stability.