| Laser + GMAW-P hybrid welding(HLAW-P)is an increasingly accepted joining technology for a variety of industrial sectors. The main advantages are the wider gap tolerance, higher welding speed, and controlling the bead humping defect,improving the weld metal microstructure by using filler materials. In order to achieve a stable and repeatable welding process, it is essential to ensure the stability of the arc and the consistency of the droplet transfer mod.Dring HLAW-P process, the laser-induced plasma changes the melting characteristics and forces acting on the droplet, and so it destroys the force balance and thermal equilibrium of GMAW-P. Therefore, the primary objective of this investigation is to compare the effect of laser-induced plasma on the metal transfer in Laser + GMAW-P hybrid welding, and find the appropriate sets of pulse parameters to achieve the transfer mode of one-droplet-per-pulse. Another problem is that GMAW-P usually employs the pulse waveform containing different stages(peak current time, droplet detachment time and base current time), thereby the understanding of the interaction between arc plasma and laser radiation during different stages can be an improvement for the combined processes.In this paper, a 650 nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. An optical emission spectroscopy system was employed to estimate plasma temperature, electron number densities distribution. A data acquisition system was employed to measure the current and voltage. We focused on the voltage-ample characteristics, characteristics of HLAW-P plasma, electrode melting and the droplet formation, detachment, transfer and impingement onto the workpiece. We also showed a result of the plasma temperature and electron number density distribution during different welding prameters.The experimental results showed as follows:1).By using the Fourier transform expansion, we deconvolved the Gaussian and Lorentzian constituents which compose the spectra. By using the stark broadening method, we caculated the electron number density of HLAW-P plasma.The thermodynamic state was analysised by using the critical electron density LTE criterion. The results showed that due to the melt of the wire and the metal vapor from the keyhole the HAW-Pd welding plasma satisfied local thermodynamic equilibrium(LTE) conditions. Based on the Boltzmann theory, the present paper calculated the temperature distribution of the plasma and analyzed the principle of lines selection by real time scanning the space of the HLAW-P plasma measurements.2).The coupling mechanism of laser-induced plasma and arc plasma was studied in this paper. The change of partical nubber density and the forces acting on the particals was analyzed to explain laser-induced plasma formation and the arc blow phenomenon.When the current transition from the base time to pulse time, the ends of electrode wire produced a large amount of metal vapor, in this case, electromagnetic force is not so strong and the inverse-bremsstrahlung absorption effect make the laser-induce plasma heightest during the pulse period. When the welding current is raised to the pulse time, the electromagnetic force gets stronger and push the laser-inced plasma away from the keyhole, the laser-induce plasma become lowest during the pulse period. When the current is at a low level(base current), plasma jet is not so strong and the electrons natural path effect points the arc to the laser-induce plasma. When the elding current is raised to a high level(peak current), the plasma jet effect gets stronger and points the arc straight down to the plate; the plasma jet effect prevails over the electrons natural path effect through all the arc length.3). Combined with the results of high-speed photograpy and spectroscopy signal, the distribution characteristics of Ar and iron spectrum of HAW-P in peak current time were researched. It is found that the spectral lines intensity in the arc center is much stronger than in the laser center. The differences of Fe I is much stronger than Ar I between this two regions.4).The laser energy transfer efficiency was calculated by using the Beer-Lambert law. It is found that the elctron number density of HLAW-P welding is about 3 times the laser welding. The laser transmission efficiency is reduced from 94.16% in CO2 laser welding to 85.84% in HLAW-P welding.5). A forces mode was proposed to explain the metal transfer behavior, the effect of different forces on the metal transfer was analysied. The vapor jet force from the kehole acted to tetain the liquid drop on the electrode. The laser-induced plasma changed the arc plasma shape and the electromagnetic force on the droplet.The axial electromagnetic force at the bottom of the droplet becomes an attaching force, due to arc contraction, which sustains the droplet. Based on the experimental result, a metal transfer control method was given. In order to obtain the droplet transfer mod of one droplet per pulse in hybrid welding progress. On the one hand, the heat input to the droplet should be controlled, the arc voltage need to be reduced for decreasing the arc heating.The changed current distribution in hybrid welding leads to an upward and inward electromagnetic force near the bottom of the droplet and the increase of droplet formation time, so the pulse duration should be increased.6). The effect of the welding direction on the plasma and metal transfer behavior of CO2 Laser + GMAW-P hybrid welding processes was investaged. It is found that the laser-leading mode expand and increase the plasma cross-sectional area. This reduce the electrical resistance of the arc and hence reduce the require voltage, which gave a much more stable welding progress. The arc-leading mode may lead to insufficient ionization of gas, hence, shrinkage of hot plasma arc, which lead to a longer time to generate a drop. For the same pulse duration, the laser leading arrangement metal transfer mode is apparently the spray type. The total weld pool of arc leading mode could be depicted as ―gourd-shaped‖ while the laser leading is like a ―bullet‖. The temperature and electron density distribution show bimodal behavior in case of arc leading, while this phenomenon does not exist in laser leading mode, so the double elliptic planar distributionwhich conventional simulation process used is not applicable in the laser leading mode.7).The effect of gas composition influence on the plasma behavior and droplet transfer in CO2 laser GMAW-P hybrid welding was studyed. The helium content not only affected the laser-induced plasma during laser welding, but also affected the plasma shape during CO2 laser-GMAW-P hybrid welding. To obtain a relatively ideal welding process, 50% helium content should be more appropriate. With the increase of helium content, the arc length became shorter. The plasma energy is more concentrated. Plasma temperature distribution was more shrinkage and current density distribution was more concentrated. The voltage of phase pulse time was reduced. In contrast, the voltage of phase pulse off time was increased. The increase of helium content in the shielding gas increased droplet size and droplet formation time, and eventually made an off-axis droplet phenomenon which consequently increased spatter generation.8). The effect of heat source separation distance and laser power influence on the plasma behavior and droplet transfer in CO2 laser GMAW-P hybrid welding was studyed.Heat source separation distance and laser power significantly affect arc voltage distribution. High laser power and short DLA reduces the voltage of phase PB, in contrast, increases the voltage of phase PD and PP. The laser-induced plasma significantly affects the forces acting on the droplet that determine the droplet transfer mode. It changes the current distribution in hybrid welding and leads to an upward and inward electromagnetic force near the bottom of the droplet. The changed electromagnetic force increased droplet size and droplet formation time, and eventually makes an off-axis droplet phenomenon. The vapor jet force induced by the the keyhole plasma acts on the droplet as a retention force, this force decreases when the DLA becomes larger and increase when the laser power becomes higher. |
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