Study On Bypass-current Wire-heating Plasma Arc Welding Of Marine Titanium Alloy

Titanium alloys were widely used in high-tech shipbuilding,such as nuclear submarine,deep water HOY and polar ship.Bypass-current wire-heating plasma arc welding was proposed to improve wire melting efficiency.The technology combines the characteristics of controllable heat input,high wire melting efficiency and strong penetration ability to realize efficient welding of titanium alloy.The contents of this paper are as follows:The optimum parameters were determined and the single-side welding and double-side forming of 8mm thick TC4 titanium alloy was realized.The weld seam was bright silver,beautiful and without any welding defects.The effects of main current,wire-heating current,welding speed,wire feeding speed and protective gas flow on the weld formation were analyzed.The high-speed camera was used to analyze the effects of nozzle height,wire feeding speed,main current and wire-heating current on droplet transfer.The influence of wire-heating current on the heat input of base metal and the melting efficiency of welding were deduced.The results showed that wire-heating current could improve the melting efficiency of wire without increasing the heat input of.The effect of wire-heating current on the melting efficiency of wire was verified.The results showed that the ultimate wire feeding speed increases with the wire-heating current.The metallographic analysis showed that the grains size in the weld zone was the largest.The grain size in the upper part of the weld zone was long strip and developed towards the are center.The grain size in the middle and lower part of the weld zone was relatively smaller and irregular.The grain size of the coarse grain zone was smaller than that of the lower part of the weld zone.The grain size of the coarse grain zone is relatively consistent with that of the weld zone.The grain size in the fine grain zone was smaller,and most of them were fine equiaxed grains with regular shape.The size of equiaxed crystals in the transition zone decreaseed gradually.The average tensile strength of the joints made by bypass-current wire-heating plasma are welding was 964MPa.The fracture occurred at heat-affected zone,and the fracture form was ductile fracture.By comparison,the average tensile strength of the joints made by traditional filling wire plasma arc welding was 928MPa.The fracture occurred at nision line zone,with the poor strength and hardness.The heat-affected softening range of the joints made by bypass-current wire-heating plasma arc welding was relatively small.In addition,the mechanical properties of deposited metal made by bypass-current wire-heating plasma are welding were tested.The results showed that the transverse tensile strength of the additive manufacturing structure was 977MPa,and the fracture occurred at the tail.The vertical tensile strength was 936MPa,and the fracture occurred at the upper part.The fracture forms were ductile fracture.A small range of softening zone appeared in the heat-affected zone at the bottom.