| 800 MPa grade low alloy high strength steel Q690 has excellent strength and toughness,which is widely applied in engineering,port machinery,marine equipment,and other fields.However,there are still some problems in the traditional welding process,such as high heat input,large welding deformation,low welding efficiency and so on.The laser-arc hybrid welding process can make full use of the laser and arc,presenting the advantages of large penetration,strong bridging ability,fast welding speed,small welding deformation and so on.In this paper,the 16 mm thickness Q690D steel was selected as the research material.Utilizing the flux cored filler wire with better welding performance,the experiment of laser-arc hybrid welding was carried out.The effect of process parameters on welding performance in laser-flux cored arc(FCA)hybrid welding was studied systematically.The microstructure and properties of the hybrid welding joint and the laser welding joint of Q690D were compared and analyzed.Firstly,the effect of the introduction of laser and the leading mode on droplet transfer behavior and welding process stability during laser-FCA hybrid welding was studied.The results showed that three basic droplet transfer modes of short-circuiting transfer,globular transfer,and projected transfer were observed with the increase of welding current during flux cored arc welding(FCAW)and laser-FCA hybrid welding.In hybrid welding,the attraction and compression of laser to the arc led to the change of electromagnetic force and plasma drag force acting on the droplet,which led to the change of droplet transfer behavior and process stability.The droplet transfer behavior and process stability of hybrid welding with different leading modes were different in the three droplet transfer modes,among which the hybrid welding with arc leading in the projected transfer mode had the highest welding process stability.Subsequently,the effect of the process parameters in laser-FCA hybrid welding on weld geometry and the interaction between process parameters were studied systematically.It was found that hybrid welding could obtain greater weld penetration than laser welding at the same laser power,and larger weld width with more stable weld bead than arc welding at the same arc power.With the increase of welding current and the decrease of welding speed,the promoting effect of arc on laser was more obvious,which was of positive significance to improve the weld penetration of hybrid welding.The distance between laser and arc affected the weld penetration by affecting the actual position of the laser and the coupling effect of the two heat sources in hybrid welding.The larger weld penetration with smaller weld width could be obtained in arc leading mode,which was beneficial to give full play to the potential of the laser heat source.By applying flux pad,the full penetration one-side single-pass welding with back formation of non-grooved 16 mm thickness Q690D steel butt joint was realized under the optimized process parameters of hybrid welding.The welded joint was in good formation,without sidewall unfused,cracks,and other defects.However,the full penetration one-side welding with back formation could not be achieved well by using single-pass laser welding process.Hybrid welding presented more significant process advantages.Further,the microstructure of the hybrid welding joint and laser welding joint of Q690D were characterized.It was found that the content of hardened structure in the weld zone and heat affected zone of the hybrid welding joint was lower.The grain sizes of the two kinds of welded joints were:arc area of hybrid welding>laser area of hybrid welding>laser welding.The microstructure size in the welded joint was related to the heat input.The larger the power parameters used,the more significant the coarsening of austenite grains,and the secondary structure formed after solid-state phase transformation would also be coarser.Finally,the mechanical properties and corrosion resistance of the hybrid welding joint and laser welding joint of Q690D were tested.The results showed that the hybrid welding joint had a significantly lower microhardness distribution.The tensile specimens of the two welding processes fractured from the base metal in the tensile test,and the use of "low strength matching" flux cored filler wire in hybrid welding did not reduce the joint strength.The impact absorbing energy at low temperature(-20℃)of the two kinds of welded joints was higher than the critical value specified in the standard.The impact toughness of different areas of welded joints was:base metal>heat affected zone>weld zone,and the differences in metallurgical process and microstructure accounted for the variances in impact toughness.The corrosion resistance of different areas of hybrid welding joint in 3.5 wt.%NaCl solution was weld zone>base metal>heat affected zone,while the corrosion resistance of pure laser welding joint was weaker than that of base metal due to no wire filling.The differences in alloy elements and microstructure were the main reasons for the variances in corrosion resistance of different areas.In this paper,the process,microstructure and properties of laser-FCA hybrid welding for Q690D high strength steel were studied.The comprehensive welding performance of laser-FCA hybrid welding for Q690D high strength steel was objectively evaluated.The research results could provide necessary experimental methods and theoretical basis for laser-arc hybrid welding of quenched and tempered high strength steel of medium thickness plate,expecting to promote the further application of laser-arc hybrid welding technology. |
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