Investigations Of CO₂ Laser Beam Welding Technologies For The Long Deck Of Full-Aluminum Structure Vessels

The main application backgrounds of this paper are the long deck of full-aluminum structure high speed vessels, and its investigated subjects are long distance laser welding of aluminum alloys in ship building industry. Against the problems existed in the present welding technology for the long deck of full-aluminum structure vessels, basic CO₂ laser welding technologies and long distance CO₂ laser welding technologies of 4.0 mm and 6.0mm thickness 5083 aluminum alloy and 6.0 mm thickness 6061 aluminum alloy have been investigated on the base of analyzing the characteristics of CO₂ laser welding system in ship industry and calculating the optical parameters of long distance welding system. The properties of welded joints are also evaluated. The application feasibility in ship building industries of CO₂ laser welding the long deck of full-aluminum structure vessels has been discussed. The behavior of CO₂ laser induced plasma in laser welding of aluminum alloys is investigated through high speed camera and spectroscopic technology.The penetration welding threshold of 5083 aluminum alloy is lower than that of 6061 aluminum alloy, because Magnesium is easily volatilized. When the diameter of shielding gas nozzle is 5 mm, it can not only offer good laminar flow length, but also offer enough shielding gas for diluting the density of plasma. The optimized flow rates of shielding gas for laser welding of 5083 aluminum alloy are 20 l/min Helium gas and 2 l/min Argon gas, and those are 30 l/min Helium gas and 2 l/min Argon gas for laser welding of 6061 aluminum alloy. The match of laser power, welding speed and wire feeding speed plays a great role for the formation of welds. For the three kinds of aluminum plates, long distance CO₂ laser welding of aluminum alloys in ship industry has been completed through laser scanning welding and wire feed laser welding method, and the weld with good appearance and no defects is acquired. At laser, eight programs of welding technology have been obtained.Based on the acquires of laser welding optical system of long full-aluminum structure vessels decks, the influence of laser beam quality, focal length and the required maximum radius of focal spots for welded materials on optical parameters of flying optical system is calculated and analyzed based on the theory of Gauss-beam propagation and focus. The differences of the DC035 and RS200 CO₂ lasers used in flying optical processing system are compared. In theory, tow kinds of CO₂ lasers both can use for long distance processing optical system, but in practice, DC035 CO₂ laser with good beam quality would better use for long distance CO₂ laser welding of aluminum alloys in ship industry than RS200 CO₂ laser.The thinner the aluminum alloy plate in ship industries is, the more the allowable variation of focal spot radius (ωivar) and positions ( ) for producing full penetration welds in long distance CO₂ laser welding is. Under the condition of experiments, full penetration welds of 4mm thickness 5083 aluminum alloy with good appearance could be obtained when sivarωivar and were smaller than 30μm and 4.0 mm; full penetration welds of 6mm thickness 5083 aluminum alloy with good appearance could be obtained when sivarωivar and were smaller than 20μm and 3.0 mm; full penetration welds of 6mm thickness 6061 aluminum alloy with good appearance could be obtained when sivarωivar and were smaller than 15μm and 2.0 mm. For the groove of I-type, the allowable gaps of laser scanning welding and wire feed laser welding are 0.5mm and 0.7mm. The requirements of grooves preparation and clamps accuracy in laser welding can not be achieved in actual production process and that becomes the biggest barrier in the application of laser welding technologies for the long deck of full-aluminum structure vessels.The tensile strength values of 5083 welded joints with filler wire and double-pass welded joints are up to 95 percent of base mental, and the yield strength are equivalent to those of base metal. The tensile fracture of laser beam welded specimens takes place in the area close to the weld bond. The tensile strength values of 6061 welded joints with filler wire and double-pass welded joints are up to 82 percent of base mental, and the yield strength are lower than those of base metal. The tensile fracture of laser beam welded specimens takes place along the 45°diagonal line of welds. Laser welded joints of 5083 aluminum alloy would not crack after 150°three-point bending tests, but laser welded joints of 6061 aluminum alloy would crack along weld bond after 90°three-point bending tests. The impacting toughness of 5083 aluminum welded joints is much higher than that of base mental, and the impacting toughness of 6061 aluminum welded joints is lower than that of base mental.Under the experimental conditions, the increment of laser power played little role for laser-induced plasma temperature when the laser power extended the threshold power of penetration welding. The plasma temperature was lower when laser focal spot was near the surface of the plate than far from it. The effect rule of welding speeds on laser-induced plasma temperature likes inverted U curve. Helium provided good cooling effects on the plasma temperature. For a given diameter of nozzle, laser-induced plasma temperature had been increasing before decreasing when the flow rate of shielding gas was increasing. That would also increase when adding Argon in Helium and increasing Argon in Helium. The weld appearance was good when the temperature of laser-induced plasma was in the range of 6000±200K. The plasma above the plates has no influence on the weld depth and weld width.Investigations of laser welding technologies for the long deck of full-aluminum structure vessels have great values in the engineering application and also in the theory study. On one hand, the achievement of investigations can offer basic technologies for demonstrating the feasibility of CO₂ laser welding the long deck of full-aluminum structure vessels, and it also plays an important role in determining the industrial laser solution of the long deck of full-aluminum structure vessels. On the other hand, high power laser welding technologies of aluminum alloy, the analysis for behaviors of the keyhole and plasma in high power laser penetration welding have instructive significance for the laser welding technology of aluminum alloys in automobiles, aircrafts and aerospace industries.