| As a new welding technique with high quality, laser+arc hybrid welding process has received significant attentions all over the world for its particular advantages, such as the large welding penetration, high welding speed, less deformations and good bridging ability for relatively large gaps, etc. In recent years, with the development of high-power laser apparatus, high-power laser beam are increasingly applied in hybrid welding applications, in which high-power laser plays the principal role with the assistance of arc. Due to the low electric-photo transformation efficiency of laser apparatus and the absorption and defocusing effects of arc plasma on laser beam, which intensifies with the increase of laser power, the utilization ratio of laser energy decreases significantly. Therefore, the applications of high-power laser beam will induce serious waste of energy. Based on the development status of laser apparatus in our country, it is significative to develop low-power YAG laser+TIG arc hybrid welding technique with high welding efficiency, low energy consumption and proprietary intellectual property rights, and it is benefit for the application and development of hybrid welding technique in industries. Therefore, low-power YAG laser (less than 500W) hybrid with TIG arc is used to weld and the characteristics of lower-power YAG laser+TIG arc welding process is studied. Low-power YAG laser+TIG hybrid welding technique with filler wire is developed. The variety of arc plasma behaviors and spectra in hybrid welding process is analyzed and the interaction between low-power YAG laser and TIG arc is discussed. Based on these, the phase matching mode between laser pulse and alternating current (AC) arc in low-power YAG laser+TIG arc hybrid welding process is controlled and the hybrid welding quality is improved. The main research contents are as follows:1. The characteristics of low-power YAG laser+TIG arc hybrid welding process (with or without filler wire) are investigated and the hybrid welding parameters of magnesium alloy and titanium alloy structures are obtained. The results indicate that the hybrid welding seam formation is good by single-side welding with both sides formation and the welding speed is much higher than TIG welding. The addition of filler wire avoids the defects of collapse and undercut. The hybrid welding penetration with back-feeding filler wire is two times deeper than that with front-feeding filler wire. The heat-affected-zone (HAZ) in hybrid welding joint with back-feeding filler wire is narrow and the grain size hardly grows, so the tensile strength of hybrid welding joint reaches 95% of base metal. Hybrid welding technique with filler wire supplies the feasibility to weld thick plates by low-power YAG laser+TIG arc hybrid welding process.2. The welding plasma behaviors and spectra are acquired and the effect of low-power YAG laser on welding plasma is studied. When laser beam act on the arc plasma and target metal, magnesium alloy is vaporized and ionized rapidly. The welding plasma intensifies and expands in a large extent. Especially in the negative wave of AC arc, the stability of arc plasma is improved significantly. The plasma spectra shows that there are only spectral lines of Mg I in laser welding and the intensities are weak. The spectra of TIG welding and hybrid welding processes consists of Mg I, Mg II, Ar I and Ar II lines. When laser beam is added, the intensities of spectral lines of Mg are much stronger than that in TIG welding, while the intensities of spectral lines of Ar are a little weaker than that in TIG welding. Spectral lines of Mg I are used to estimate the electron temperature and density of welding plasma and the results indicate that the electron temperature decreases and the electron density increases under the action of laser beam. The effects of laser beam on plasma behaviors, electron temperature and density in hybrid welding process with filler wire are weaker than that without filler wire. The threshold value of laser power which can intensify the TIG arc plasma is 50W-60W in hybrid welding process without filler wire and about 70W in hybrid welding process with filler wire.3. The weld seam formation, plasma behaviors, plasma spectra and arc voltage are analyzed and the interaction between low-power YAG laser and TIG arc plasma is studied. The results indicate that the interaction between low-power YAG laser and TIG arc includes the assistance effect of arc plasma on low-power YAG laser and the intensifying effect of YAG laser on arc plasma. On one hand, the action of TIG arc changes the surface condition of target metal and the initial condition of keyhole formation changes from the stage of heating base metal to the stage of plasma formation, which accelerates the process of keyhole formation in hybrid welding process. Meanwhile, the arc plasma and the Ar gas make the laser-induced plasma spray away from the workpiece with a gradient from the incident direction of laser beam, which weakens the absorption and defocusing effects of laser induced-plasma on laser beam. On the other hand, low-power YAG improves the stability of TIG arc plasma and expands the arc plasma with the concentration of the electric conducting channel with high temperature, which increases the arc voltage and arc energy density. The cooperation of both effects increases the welding penetration significantly.4. The effect of pulse laser wave and the phase matching between pulsed laser and AC arc on hybrid welding process is investigated. The phase matching controlling system of pulsed laser and AC arc is designed to control and improve the hybrid welding process. The results indicate that with the laser pulse energy as a constant, a high power density pulse with short duration is set at the beginning of pulse wave and the laser pulse duration is increased properly, which can accelerate the forming process of laser keyhole, intensify the impact effect of laser pulse on molten pool and increase the effecting time of laser keyhole on arc plasma, then the hybrid welding penetration increases with the percents of 15%～20%. The experimental results of phase matching mode between laser pulse and AC arc show that hybrid welding joints with high quality can be obtained whether the laser pulse acts on the positive wave or the negative wave. However, in hybrid welding process, it is helpful to improve the weld formation when laser pulse acts on the negative wave of AC arc. Meanwhile, both the power input and the spot force to base metal in the negative wave are higher than those in the positive wave, so the hybrid welding penetration with the phase matching mode that laser pulse acts on the negative wave of AC arc is 5%～10% higher than that of the positive wave.
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