Study On Process Optimization Of Laser-MAG Hybrid Horizontal Welding

In recent years,with the continuous development of technology in exploring offshore oil and gas,offshore oil and gas resources have become an important incremental source in the world.The South China Sea is rich in oil and gas resources,and most of then are located in deep water.In order to develop oil and gas resources in deep water area,a large number of subsea pipelines need to be laid.Among them,J-laying is a kind of laying method,which puts the pipeline close to the vertical shape on the seabed,and it is a main method of laying subsea pipelines in deep water.For the connection of vertical pipes,due to the limitation of operation space,generally only one welding workstation is equipped,and the welding method of horizontal welding position is used.At present,the welding methods mainly include flux cored wire semi-automatic welding and GMAW,which have problems such as low welding efficiency,low penetration of root welding,large amount of weld filling and defects after welding.As a new welding method,laser-MAG hybrid welding has many advantages,such as deep penetration,high welding efficiency and strong process adaptability.In order to explore the adaptability of new welding methods in J-shaped subsea pipeline laying and improve the shortcomings of existing methods,this paper taked the backing welding of X80 pipeline steel as the finally research object,carried out the technology research of laser-MAG hybrid horizontal welding,and explored the feasibility of using laser-MAG hybrid horizontal welding in J-shaped subsea pipeline laying system.Firstly,the optical fiber laser-MAG side axis hybrid welding method was used in the experiment to carry out the research of butt welding in the position of horizontal welding.The effects of laser power P_L,welding current I,welding speed V,defocusing amount??and distance between laser and wire D_LAon the weld formation of laser-MAG hybrid welding were studied.The optimized process parameters were obtained:laser power is between 3000W and 3750W,defocusing amount is between-1and-2mm,distance between laser and wire is between 1mm and2mm,welding current is between 160A and 200A,welding speed is between 800 mm/min and 1200 mm/min.With the limits of above parameters,the weld was formed well,the welding efficiency was high and the penetration was large.Then,the adaptability research of laser-MAG hybrid horizontal welding technology was carried out.The results show that for 4mm butt welding,one side welding and two sides forming can be realized,the maximum applicable gap was 1.5mm,and the misalignment was 1.5mm.It has good adaptability to the natural groove process,which can adapt to 10^°groove angle,and can achieve good forming in the range of 1mm gap and misalignment.In the experiment of unequal thickness plates,it is found that for the butt joint experiment of 4mm+6mm and 5mm+8mm unequal thickness plates,one sided welding and two sided forming can be realized,and the weld joint is formed well.4mm+6mm unequal thickness plates can adapt to 1mm gap and 1.5mm misalignment.Finally,three kinds of test plates with different groove of 30^°+20^°,25^°+25^°,50^°+0^°and three kinds of blunt edges of 2mm、3mm and 4mm were designed to carry out the research on the technology of backing welding of laser-MAG hybrid horizontal welding.The results show that the groove angles have a certain influence on the downward flow of the weld pool in the hybrid horizontal welding.Under the same groove angle,the weld with smaller angle of lower plate is better than the weld with the same angle of upper and lower plate.The best groove angle is upper plate 30^°+lower plate 20^°,which realized the one sided welding and two sided forming of 4mm blunt edge backing welding.In the experimental study of laser-MAG hybrid welding process,Analysator Hannover was used to study the stability of hybrid welding.The results show that after adding the laser,the arc is more stable,the voltage fluctuation is reduced,and the probability density distribution is more concentrated.Further analysis of the horizontal welded joint shows that the microstructure of the horizontal welded joint has the characteristics of asymmetry and non-uniformity.Under the effect of gravity,the width of the weld area below the weld center is smaller than above the center,and the width of the heat affected zone of the upper plate is larger than that of the lower plate.Due to the rapid solidification of the weld pool after welding,and the heat dissipation is the fastest in the direction perpendicular to the fusion line.At this time,the columnar crystal formed by the heat conduction grows along the conduction direction,and the columnar crystal density of the lower plate is greater than that of the upper plate.According to the requirements of API 5L standard,tensile,bending,impact and microhardness tests were carried out on the welded joint of X80 pipeline steel.The weld joint has good impact toughness and hardness,which tensile strength can reach 711MPa,the test piece breaked in the base metal and the front bending and back bending were bent to150^°.The highest hardness area of laser-arc zone was heat affected zone,of which in laser zone was weld center.Therefore laser-MAG hybrid horizontal welding can meet the requirements of high penetration and performance under the condition of high speed welding.