Study On Technology, Mechanism And Quality Controlling Of CO₂ Laser-Arc Hybrid Welding

Laser-arc hybrid welding which couples laser beam and arc into one process avoids the disadvantages of individual process and has its own particular advantages, such as deeper welding penetration, more stable welding arc, higher welding speed, less deformability and stronger ability to bridge large gaps, etc. For these advantages, the laser-arc hybrid welding technologies have become more and more attractive in recent years and have strong industrial application prospect in many fields, such as aerospace, automotive, off-road vehicle, shipbuilding, oil and pressure vessel industries, etc. However the researches on this process are very scattered and the useful information is relatively scarce. The studies on laser-arc interaction mechanism are also very few, and further systemic investigations are needed.In this dissertation, the basic processing which considers different processing parameters such as shielding gas, laser-arc interaction, weld microstructure and mechanical performance with the CO2 laser-TIG (tungsten inert gas) and MIG (metal inert gas) hybrid welding are studied in detail by the use of the mild steel and stainless steel,. The following are the main results:A system of laser-arc hybrid welding was developed, which include a CO2 laser, a TIG and a MIG arc welder and CNC controller, etc. Some parts were also optimized to fit the process. Using this system, the welding parameters can be adjusted conveniently and the laser-arc synergetic effects can be obtained. Both the laser-TIG and laser-MIG hybrid welding were investigated in detail and the optimal parameter range was obtained. The experimental results demonstrate that there exists strong laser-arc interaction through the electrical channel between laser induced plasma and arc column observed in the experiment. Compared to the individual laser welding, the weld penetration depth and the welding speed of laser-MIG hybrid welding increase by 1.6 and 2.7 times respectively, and the weld penetration depth of laser-TIG hybrid welding also increases by 1.44 times.The shielding gas parameters of laser-arc hybrid welding were studied systemically, which demonstrate the evident effects of shielding gas parameters on the weld penetration depth. It was found that the optimal gas shielding method is the hybrid protecting method coupling the torch and coaxial nozzle, which can achieve efficient synergetic effects and produce the full penetration weld under the considerably wide parameter range. It is also testified that the mixed gas of helium and argon is more suitable for laser-arc hybrid welding. The shielding gas parameters affect the weld penetration and the process stability by influencing the plasma shape (efficient plasma height interacting with laser), which is achieved by two ways: laser-arc plasma interaction and gas flowing direction and velocity.A more comprehensive quantitative analysis method was originally developed firstly to study the heat sources interaction extent between the laser and arc during the hybrid welding process. A dimensionless parameter,ψ, was introduced to indicate the changes of heat sources interaction: the biggerψdenotes the stronger heat sources interaction. The agreement of the computed results by this method with the phenomenon observed in hybrid welding indicates this method can comprehensively reflect the laser-arc interaction of hybrid welding fast and accurately. By this method, it can be observed that, for different arc type, a significant difference of the heat sources of laser-arc hybrid welding from the analytical results. This difference is mainly caused by the effect of electrode polarity on the characteristic of laser induced plasma and the difference of arc characteristic.It was found by the microscopic observations that in the weld region of laser-MIG hybrid welded mild steel, the microstructure of the arc zone is coarse columnar dendrite perpendicularly growing from the melting pool wall to the center; that of laser zone is relatively fine columnar dendrites perpendicularly growing from the melting pool wall to the center and fine equiaxed dendrite homogeneously nucleating in the center zone, which was not reported eslswhere. The aser zone has higher microhardness of the weld region and narrower heat affected zone. Moreover, the effects of laser-arc energy ratio, groove type and shielding gas on the microstructure and mechanical performance of the weld produced by hybrid welding are obtained by experiments, which is instructive for the choose of welding parameters.Laser-TIG hybrid welding was used to weld superfine grain steel in this dissertation. The experiments showed that the joint strength of laser-TIG hybrid welded superfine grain steel is higher than that of base metal, and the microhardness is also lower than that of laser welded one, which can avoids the decrease of the weld toughness resulting from the overhigh microhardness in the weld region. Moreover, laser-TIG hybrid welding can obtain narrower HAZ to restrain the grain coarsening and avoid the appearance of softened one because of the higher speed. These indicate the great prospect of hybrid welding for superfine grain steel.Finally the weld defects, welding adaptability and critical welding speed of laser-arc hybrid welding were investigated in detail. Many useful conclusions, such as the experiential formula of critical undercut speed, some methods for restraining weld defects, the limit of gap bridging and weldable misalignment, and the range of critical welding speed were firstly obtained. Based on the experimental results and discussions, a scheme for choosing the best hybrid processing parameters to control the bead quality was developed firstly, which is highly helpful to the equipment developing of laser-arc hybrid welding.