Welding Procedure Research In Underwater Laser Welding Of TC4 Titanium Alloy

There are more and more restrictions to the conventional underwater welding methods as the scope of deep sea resources development extends to deeper,wider ocean areas and large use of new materials.In addition,water environment with high pressure and high radiation in nuclear power plants also puts forward higher requirements for the underwater welding techniques.Compared with the underwater arc welding,underwater laser welding has the advantages of smaller influence by hydraulic pressure,wider welding material,smaller heat affected zone and lower residual stress.What's more,TC4 titanium alloy is widely used because of its good corrosion resistance and fatigue resistance.However,the underwater laser welding technology of China is still in a very backward position.Among them,the key issues such as how to realize underwater laser welding of titanium alloy and welding procedure research in TC4 titanium alloy are to be further explored.Therefore,this paper focuses on the welding method and process test scheme that can achieve underwater laser welding of TC4 titanium alloy.Firstly,the feasibility of direct underwater laser welding of TC4 titanium alloy is studied in this paper.The effect of water environment on the laser welding quality of TC4 titanium alloy was analyzed based on evaluation criteria such as weld formation,mechanical properties,fracture surface morphology,and phase composition.It is found that the weld obtained by direct underwater laser welding has poor continuity,stability and formation,severe weld oxidation.What's more,there are a large number of cracks,pores and intermetallic compounds in the weld seam with low mechanical properties.The weld does not meet the requirements of welding experiments and practical applications.Therefore,direct underwater laser welding of TC4 titanium alloy is not feasible.Based on the above,double layer gas-assisted draining water device in underwater laser welding is designed and processed in order to achieve local dry welding and eliminate the negative effects of water on welding quality.And the air intake method,the type and flow rate of the incoming gas are studied.The result is as follows:the outer layer is fed with compressed air and the inflation mode is side tangent direction inflation.The inner layer is passed in argon gas and the inflation mode is oblique tangent direction inflation.What's more,the inner and outer gas flow rate is selected to be 50 L/min,and the gas flow is continued for 10 s before welding.In addition,through the Fluent software simulation,it is found that the double layer gas-assisted draining water device can form a stable local drying space under the above conditions.Finally,the welding procedure research in local dry underwater laser welding is performed.The result is that the optimal process parameters for local dry underwater laser welding of 2 mm thick TC4 titanium alloy are:laser power 2.5 kW,welding speed 20 mm/s,defocusing distance 0 mm.Then by comparing the regular laser welding weld with the same parameters,the feasibility of local dry underwater laser welding of TC4 titanium alloy is demonstrated.The results show that there is little difference between weld formation and joint mechanical properties with regular laser weld,and the underwater weld formation is well,the weld width is uniform,and there are no defects such as spatter,undercuts and surface cracks.Tensile strength and impact toughness reach 843.53 MPa and 38.044 J/mm~2,respectively 94.22%and 76.32%of the base metal,and the welding quality meets the requirements.