Research On Welding Characteristics Of CO₂ Laser-GMA Hybrid Welding For Ultra-Low Carbon Bainitic Steel

Ultra-low carbon bainitic steel with optimum mechanical properties that combine high strength and toughness achieves the extensive attention in military and civilian areas. The traditional welding methods used in welding these materials have lots of disadvantages, such as high heat input, welding deformation, low efficiency and so on. In this paper, 12 mm ultra-low carbon bainitic steel has been welded by using CO2 laser-GMA hybrid process and multipass technique in order to disclosure the relatingship of welding parameters and defects, welding parameters, microstructures, mechanical properties.The experiment results indicate that the effect of shielding gas composition, groove geometry, focal point position, wire feeding rate and heat input on weld shape are significant. Argon, helium and CO2 gas are used. Helium is added to avoid the plasma formation by the CO2 laser. CO2 is added to make junction smoother. In order to use the laser energy completely, the groove geometry with rootface of 3mm is used in the butt joint. Focal point position is very important for the first filling pass. It should be in the scale of the rootface. For the other filling passes the focal point position could be changed in a large scale. Because the wire can obtain energy from laser indirectly, the high quality welds and the stability of the welding process could be received by using the higher wire feeding rate. Because the change of the heat input on each pass is proper, the high quality welds and the more efficient process could be received with three passes welding.The experiment results indicate that there are many types of defects in the joint, such as poor adhesion, crack, porosity, inclusion and so on. And the main reasons which generate defects are welding parameters and experiment conditions. If thermal source couldn't heat the groove completely, poor adhesion happens. The lower width/depth ratio and the faster cooling rate result in the generation of the crack. The porosity and the inclusion usually happen when the welding process and droplet transfer process are unstable. The tendency of porosity reduces, and other defects could be eliminated in this experiment. Microstructural analyse show the weld is consists of acicular ferrite and granular bainite. Mechanical property tests show that there are no microstructures which lead the mechanical properties to fall obviously in the weld HAZ. The tensile strength of the joint is 829.4 MPa when the specimens of weld were failed in base metal. When the specimens were failed in welds, the defects lead the specimens to fracture. The specimens always have ductile fracture surface. The Charpy V-notch impact tests at room temperature and -40℃were taken. The impact properties of the welds are higher than the base metal.