Study On One-side Welding With Back Formation Of Medium Or Heavy Plates By The MAG-TIG Twin Arc Welding

As the products in shipbuilding,ocean engineering and heavy machinery industries become much larger and heavier in recently years,butt welding of thick plates is becoming more and more popular.The efficiency of root welding in thick plate is very important for the whole welding process.One side welding with back formation is an efficient welding process for medium or thick plate welding.The method can avoid the auxiliary processes such as turnover,back gouging or forced forming method for sticking back of the plate in traditional welding process.It has the advantages of improving the working conditions of welders and improving the manufacturing efficiency.At present,the one-side welding with back formation process is mostly achieved by swing single-arc welding.The quality stability of the weld joint and the reliability of the weld formation are poor,and the welding speed is slow.Therefore,the manufacturing efficiency is greatly limited.In this study,a pulsed MAG arc is combined with a DC TIG arc,the MAG-TIG double arc vibration heat source is formed by utilizing the electromagnetic repulsion between the two arcs.The MAG-TIG double arc heat source significantly improves the weld bead penetration and formation control of the backing weld bead for the medium or thick plates.The main conclusions and contents of this paper are as follows:(1)The MAG-TIG double arc heat source achieves sound fusion of the backing welded joint with lower welding line energy(89%of single MAG arc welding).The back penetration is uniform,continuous and stable.The root reinforcement of the one side welding with back formation is moderate.The backing welded joints have good mechanical properties.The fracture of the tensile specimens occurs on the base metal(Q235-B)far away from the welded joint and its heat affected zone.The elongation ratio after breaking of the sample is 21.8%.The average tensile strength of the specimen is 463 Mpa.The hardness distribution is in a smooth transition and there are no areas of excessive hardness.At the same time,the backing weld joint has good bending performance and impact toughness.Under the experimental conditions,for 24 mm thickness plates,the heat input is 798 J/mm,the height of the single weld with filled metal can reach about 7.0 mm.The MAG-TIG double arc heat source root welding process can eliminate the turnover and back gouging in traditional manufacturing,and greatly improves the quality stability of welded joints and the manufacturing efficiency of the root welding for the medium and thick plates.(2)In the MAG-TIG twin-arc welding process,the electromagnetic repulsion between the two arcs is reasonably used to form the double arc vibration pattern,the front MAG arc offset vibration and the back TIG arc vibration.At the same time,the discharge stability of the MAG arc plasma is improved.The electromagnetic interaction between MAG arc and TIG arc is analyzed by electromagnetic principle.The maximum and minimum deviation of the pulse MAG arc at the base and peak state and different wire-electrode distances,MAG welding currents,TIG currents and MAG arc voltages are calculated.Based on the arc behaviour and its stability under the corresponding parameters,the stable parameter range of the MAG-TIG double arc oscillating heat source welding were that MAG welding current of 270-300 A.TIG welding current of 50-150 A,the arc spacing of 20-25 mm and MAG arc voltage of 30.0-31.6 V.(3)When MAG-TIG twin-arc welding is applied,precise control of the back formation size during the one side welding with back formation process can be achieved by adjusting the offset area ratio of the front molten pool.Due to the electromagnetic repulsion effect of the TIG arc on the front MAG arc,the front MAG arc generates an offset vibration at the front of the molten pool.As a result,the distance from the molten pool front to the axis of the welding wire(displacement distance of the weld pool front)increases.When the ratio k of the front offset area of the molten pool(S1)to that of the MAG arc heating circular area(S)is between 24.2%and 33.4%.the reinforcement height of the backing weld seam dimension is 2.0±0.5 mm.Under the condition of constant welding heat input,the size of the root reinforcement of one side welding with back formation welding is proportional to the vibration ratio of the front molten pool.When the deviation percentage of the front molten pool is small(k<24.2%),the height of the root reinforcement is small(<1.5 mm).When the deviation percentage of the front MAG molten pool is large(k>36.9%);the height of the root reinforcement is large(>3.0 mm).According to this regularity,it can be realized the adjustment of the forming dimension of the backing weld in the one side welding with back formation of the thick plate.(4)Compared with single MAG arc welding,the MAG-TIG twin-arc welding molten pool and the backside forming is more stable in the one side welding with back formation process for the medium or thick plates.The mechanism analysis for the stable formation of one side welding with back formation of medium or thick plates by the MAG-TIG twin arc welding is as follow:First,owing to the electromagnetic repulsion effect between two arc electromagnetic fields,the two arcs are swing deviation respectively.Therefore,the force of arc plasma on molten pool in vertical direction can be reduced.Second,due to the reheating of the rear TIG arc,the temperature distribution of the molten pool is changed.Under the combined action of the shear force by arc plasma on the molten pool and Marangoni convection effect,the liquid metal flows to the rear of the weld pool in the MAG-TIG twin arc swing welding.The influence of gravity on the flow of the molten pool is reduced.Thirdly,high frequency vibration of the front MAG arc promotes convection of liquid metal.The liquid metal temperature gradient decreases at the front of the molten pool.At the same time,the temperature of liquid metal is low.The surface tension of liquid metal is larger.It is favourable for controlling the stable back formation in the root welding.