Research On The Droplets Transfer Behaviors Of Laser-CMT Hybrid Welding

In order to solve the problem of poor welding formation and large deformation of engine shell welded joint,especially for thin-wall butt joint and fillet joint,laser-CMT hybrid welding technology may be one of the preferred methods.Therefore,the characteristics of droplet transfer in laser-CMT hybrid welding were investigated by using a high-speed video camera,and the influence rules of parameters on the droplet transfer behavior during laser-CMT hybrid welding were analyzed.Based on this,the welding experiment of ultra-high strength steel is carried out,and a hybrid method suitable for laser-CMT hybrid welding is proposed,which further reduces the heat input and weld deformation.The reasons for the increase of welding efficiency,the change of the droplet transfer frequency and droplet shape by the addition of laser were analyzed.And,the welding stability of laser-CMT and laser-MAG was also compared.It is considered that the molten pool was spread out after adding laser,but at the same time,the thermal effect of the laser plasma would lead to the welding wire melting continuously.Both of them determine the duration of the liquid bridge.The laser plasma forms a stable conductive channel,which changes the direction of the arc plasma.Therefore,the electromagnetic force and plasma fluid force on the droplet are changed accordingly.At the same time,the arc plasma induces the laser plasma to erupt.However,the arc disappears when the CMT is short circuiting,and the plasma on the keyhole surface is inhibited by the contacting of welding wire and molten pool.The spatter and droplet in laser-MAG welding may affect the stability of keyhole,which causes defect such as porosity.Although the plasma is inhibited when the droplet falls into the molten pool,but the result is worse than CMT.The influence of the parameters on the droplet transfer behavior was investigated,the main factor is the distance between laser and wire,which has a decisive influence on the coupling between laser and arc.The defocusing amount and the shielding gas flux affect the transfer behavior by change the influence of laser energy.When the laser power is increased,the wire melting accelerates and the weld pool is enlarged obviously,which helps to increase the transfer frequency.The arc becomes more stable when the welding speed is high,and the transfer frequency is greatly improved by avoiding wire conglutinating.When the laser with different wavelengths is used,the plasma temperature increases with the laser power increasing.And,the temperature of the CO2 laser plasma exceeds that of the fiber laser 1000 K when the power equals 2500 W.The CO2 laser plasma erupts violently,while the main composition of the fiber laser keyhole is the metal vapor,which has little influence on the volume of the droplets but make the molten pool spread out.Because of the lack of plasma attraction,fiber laser has little influence on the CMT arc.The welding of butt joint of 2mm ultra-high strength steel was carried out.The calculation result shows that the efficiency coefficient of CMT arc heat input is 0.3-0.39.In order to control the distance between laser and wire better,reduce the difficulty of matching and the arc current,a hybrid method for controlling droplet transfer in laserCMT was proposed.With this method,the welds with good welding formation and no porosity were obtained.After the welding,the roundness of the thin-wall component changes little,the tensile property can be reached 752 MPa,and the fracture position of the specimen is located in the base metal.