Research On Welding Stability And Inhomogeneity Of Thick-plate Narrow-gap Laser Welding

Thick plate welding is a critical and essential process of manufacturing large-scale equipment in many fields,such as rail traffic,shipbuilding,aerospace and so on.The service performance and life of the large-scale equipment was directly determined by the welding quality.The traditional arc welding technology was hard to meet the exacting manufacturing and service requirements owing to the low welding efficiency,high heat input,large deformation and residual stress caused by the wide groove size,slow welding speed and shallow penetration of single weld.The unique advantages including high efficiency and quality make the laser welding technology become one of the important research area of thick plate welding.In order to satisfy the actual demand of rail traffic,shipbuilding,aerospace and so on,the butt joint and T-joint of low-alloy high-strength steel with 20 mm thickness were joined by combining the high power laser welding process and the narrow-gap hybrid laserarc welding process.The welding process,formation,microstructure,residual stress and performance were in-depth studied,which provided theoretical basis and engineering experience for popularization and application of laser welding technology to the field of thick plate welding.The main results obtain in this study were summarized as follows:(1)Through the study of narrow gap high power laser backing welding process,it was found that behavior of the metallic vapor and formation of the backing weld were deeply influenced by the gap width.Matching of welding process and root size was the necessary condition to obtain adequate weld appearance under the proper gap width.On this basis,the process window of high power laser backing welding was put forward.(2)Through the study of narrow-gap hybrid laser-arc welding process,it was found that the laser power,welding current,laser-arc distance and welding speed were determinant factors of arc behavior in narrow-gap groove.The unstable arc behavior including arc deflection and burning up was closely related to the unstable behavior of the molten metal existed at the end of the wire.The unstable arc behavior caused uneven heating of the groove,which was considered to be the basic reason for the formation of lack of fusion defects.(3)Through the study of microstructure and residual stress of the multi-layer welded joint combined with numerical simulation,it was found that the inhomogeneity of microstructure and residual stress was resulted by the uneven multiple thermal cycles during the multi-layer welding process.The inhomogeneous microstructure and residual stress between different layer was caused by discrepant welding processes while the inhomogeneous microstructure and residual stress at the interlaminar zone was caused by the remelting and heat treatment of the upper layer on the lower layer.Besides,base metal which possessed more complex phase composition,lower and wider phase-transition temperature were considered to have higher tendency to form the inhomogeneous microstructure and residual stress.(4)Through the study of the performance of the multi-layer welded joint,it was found that performance of the multi-layer welded joint at different locations was significantly different owing to the inhomogeneous microstructure and residual stress existed along the thickness direction.Uneven phase composition and grain sizes lead to the decreasing of harness and tensile properties from lower layer to upper layer and the fluctuation of hardness at the interlaminar zone.The small grain size of lower weld increased the difficulty of crack propagation,leading to better fatigue property of lower weld than that of upper weld.In addition,inhomogeneous microstructure and residual stress at interlaminar zone caused uneven force and deformation,resulting in large number of second crack and lead to poorer fatigue property of interlaminar zone than that of other weld locations.The inhomogeneous microstructure at the interlaminar zone lead to galvanic reactions and accelerated the corrosion process,meanwhile,graded distribution of the residual stress deeply induced the corrosion process and further weaken the corrosion resistance of the interlaminar zone.(5)The interlaminar thermal cycle was effectively controlled by induction preheating process,the inhomogeneity of microstructure was notable restrained,the fluctuation of hardness within the interlaminar zone was minished,the fatigue property and the corrosion resistance of the interlaminar zone were improved.