Study On Behavior Of Melting Wire Transfer And Copper/steel Welding In Electron Beam Welding With Filler Wire

In the electron beam welding with filler wire process, the behavior of melting wire transfer was studied. The mechanisms of wire transfer and weld formation were revealed. The weld appearance, microstructure and properties of the electron beam welded copper/steel with filler wire joints were studied. The process adaptation of electron beam welding with filler wire in dissimilar metal welding was verified.The wire feed system was built firstly, and it was coupled with the electron beam welding equipment. In order to reveal the behavior of melting wire transfer in electron beam welding with filler wire, charge-coupled device visual system was set up to observe the welding process.Compared with autogenous electron beam welding, the electron beam welding with filler wire is more complicated due to the addition of the wire. Therefore, we firstly studied the effects of new addition variables such as wire feed rate, wire feeding angle and wire feed position on the welding process. The results showed that the welding process could be more stable with precise feed wire when the front wire feed position was adopted. In addition, the depth of the weld increased with the increase of wire feeding angle. The ideal wire feeding angle is 60° with the condition of our platform in this research. And the choice of wire feed rate should be matched with the welding speed and beam current. The results also showed that spatial relationship between beam and wire could obviously influence the weld appearance. Moreover, we studied how the spatial relationship between beam and wire influenced the weld appearance. The measured results of weld depth from large to small under different spatial relationships between beam and wire could be concluded as follows: autogenous, separated, partially intersecting, completely intersecting and non-contacting. Meanwhile, in the aspect of weld width, the value of non-contacting was seen to be smallest among all the different relationships. Additionally, the reinforcement from high to low could be indicated as below: non-contacting, completely intersecting, separated, partially intersecting and autogenous.The melting wire transfer behaviors with same process parameters under different spatial relationships between beam and wire were investigated by charge-coupled device visual system to further reveal the effects of spatial relationships between beam and wire on weld appearance. The results showed that the melting wire transfer behavior was influenced by all the spatial relationship between beam and wire, transition height and wire feed rate. A total of six typical transfer modes including metal transfer, insertion, molten-metal-bridge, small-droplet, big-droplet and mixed modes are observed in the experiments. The metal-vapor jet force can greatly affect the behavior and stability of melting wire transfer and weld appearance. Therefore, reducing the transition height to decrease the influence of metal-vapor jet force on the melting wire and guaranteeing the match of wire feed rate and melting rate are essential to stabilize the transfer. Finally, molten-metal-bridge transfer can be proved to be the best transfer mode in electron beam welding with filler wire due to its wide range of process parameters and good weld stability. The optimized parameters for steel wire were: beam current 25 m A, wire feed rate in [1.87 m/min, 2.60 m/min], and with partially intersecting relationship. And the optimized parameters for copper wire were: beam current 30 m A, wire feed rate in [0.6 m/min, 1.80 m/min], and with partially intersecting relationship. Meanwhile, the weld homogenization could be obtained by the wire feed process.Because autogenous electron beam welded 304 stainless steel and QCr0.8 copper alloy with same thickness joint always showed surface collapse defect, electron beam welding with copper filler wire of copper/steel was carried out to diminish the surface collapse defect and verify the feasibility of the process. The molten-metal-bridge transfer mode was adopted in the electron beam welding of copper/steel with filler wire. The results showed that the feed wire could supplement alloy element loss in the molten pool timely. Electron beam welding of copper/steel can obtain good weld appearance. In our experimental condition, the optimum process parameters of weld appearance are consistent with the orthogonal test results of tensile strength. The optimum process parameters are as follows: beam current 30 m A, welding speed 100 mm/min, wire feed rate 1 m/min and beam offset-0.3 mm. The weld is mainly composed of dendritic α phase with little globular ε phase. The tensile strength of the optimum joint is up to 276 MPa, which is approximately 76% of that of copper base material. Higher microhardness value stays in the zone rich in Fe. And lower microhardness values are in the zone rich in Cu. The microhardness distribution of the top is more unstable than that of the bottom. The main factor for fracture is that the grain grows coarse seriously in the heat-affected zone near copper side. Subsequently, the adaptation of the gap in wire feed process was studied. The results showed that the electron beam welding with filler wire can once obtain good weld appearance with the plate thickness of 2.7 mm, with no base plate and the gap of 1.6 mm.Due to the mismatch of thermo-physical properties from copper and steel, the high residual stress often exists in the joint. Finally, the effects of filler wire on residual stress in electron beam welded QCr0.8 copper alloy to 304 stainless steel joints were studied. The results showed that the addition of copper wire having excellent plasticity into the gap could reduce the residual stress, especially the most harmful longitudinal stress by 53 MPa, compared with that of autogenous welding.