Hybrid welding combinng YAG laser or fiber laser with MIG arc was carried out on aluminum alloy, and the effects of setup conditions of a laser beam and a wire, and the laser beam parameters on wire melting phenomena and gap tolerance in butt joint were investigated. It was found that, in order to obtain a deeper penetration, the laser beam should be separated from the wire with a distance at which there was no direct interaction between a laser beam and a droplet during its transfer. On the contrary, in order to get a wider gap tolerance for a butt joint, for example, it was better for the laser beam to be set to cross with wire over 2mm so that the laser beam could directly irradiate on the wire surface to melt it. As a result, the arc current could be decreased so efficiently that the molten pool size formed by MIG arc decreased and the gap tolerance increased. In the case of the utilization of a fiber laser, it was found that the wire melting phenomena were affected by the laser beam parameters such as the beam diameter and the defocused conditions when welding was not done at a focal position. It was clarified that even at the same laser beam diameter on the work surface the wire melting phenomena could be different under the different defocused conditions. In the defocused conditions that the focal position was over the work surface, the molten droplet on wire tip evaporated more easily than the case that the focal position was within the work surface. From the viewpoint of laser absorption by the wire or the molten droplet, according to an arc phenomenon approoach, it was found that about 10% of laser energy was absorbed during hybrid welding when the laser beam was directly irradiated on the wire surface.

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Dissimilar metals joints of galvannealed steel (GA-steel) and commercially available pure aluminum(A1050 sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. In this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metals sheets. In addition, the laser beam was scanned at various frequencies and patterns though the　ƒθ lens using two dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined. The compound layers in the weld interface were observed by optical microscope, and the layer thicknesses were measured. The thicknesses were in the range of 7 to 20 μm . The mechanical properties of welded joinnts were evaluated by the tensile shear test and the peel test. In the tensile shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base alumimum sheet. In the peel test of the specimens subjected to the laser beam of 1200 to 1400 W power under the roller pressure of 2.94kN, the specimen fracture took place in the place in the base aluminum sheet. Even if the compound layer was thick. high joint strength was obtained. In order to know the reason for such high strength of joints with thick compound layer and the joining mechanism, the compound layer was observed by the HR-TEM. The TEM observation results revealed that the mein phase in the compound layer was the solid solution of Al + Zn. Moreover , the intermetallic compound was identified as FeAl, Fe₂Al₅, Fe₄Al₁₃ and Fe₂Al₅Zn_0.4 phase by electron diffraction. The Fe₃Zn₁₀(Γphase) of Fe-Zn intermetallic compound was confirmed on a Fe base meaterial. It is guessed that the joining areas were heated in a range of 782℃ more than 665℃, a melting point of the Al, by laser irradiation because the　δ_1K phase aspect was not confirmed. Because the surface of A1050 and Zn plated layer were meltedd thinly, hte layer was over 10μm thicker. The reason for the production of high strength joints with the relatively thick intermetallic compound layer was attributed to the formation of (Al + Zn) phase with finely dispersed intermetallic compounds.

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The objectives of this research are to investigate the effects of various welding conditions on penetration and defect formation, to clarify their welding phenomena and to develop the procedure of reduction of the defect. Fiber laser bead-on-plate welding was performed on several aluminum alloys, in particular A5083, at the power of 6kW or 10kW and several power densities from 0.4kW/mm² to 0.9MW/mm². It was found that the weld beads were narrower and deeper with an increase in the laser power density. For ezample, fully penetrated weld beads in 10mm thick plates were produced at the laser power density of 640kW/mm² and the welding speed of 10m/min. However, convex-concave bead surfaces were formed. Moreover, in the case of the high power density, no porosity and many pores were present at high and low welding speeds, respectively. On the other hand, in the case of the ultra-high power density, few pores were generated in high speed welding. These reasons were interpreted by observing keyhole behavior, bubbles formation and the molten pool geometry during high power fiber laser welding with a high-speed video camera and microfocused X-ray transmission in-situ observation method. Moreover, the porosity in the weld bead was reduced and prevented by the utilization of nitrogen gas instead of Ar gas, or the forward inclination angle of 40° (50° from the right angle) in Ar shielding gas.

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Laser welding of aluminum alloys is difficult because of their low laser coupling, easy formation of welding defects, etc., and thus the establishment of in-process monitoring technique is expected in various industries to obtain highly reliable laser welds. In this study, therefore, both the reflected laser beam and radiation light from the molten pool were investigated as monitoring signals during YAG laser welding of A5052 and A5182 aluminum alloy to confirm validity and usefulness of these signals for monitoring. At the same time, laser-welding phenomena were observed through a high-speed video camera to better interpret the monitored signals. Two signals were detected by utilizing photo sensors and band pass or cut off filters coaxially against a laser beam and from the above-back direction. In this paper, experimental setup, and monitoring and obsevation results were presented. The correlation between monitoring signals and welding phenomena was clarified when the welding defects such as underfilling and through-holes were formed.

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