Research On Controllability Technology Of TC4 Titanium Alloy Underwater Laser Wire Filling Welding

Underwater laser welding technology is a relatively efficient in-situ repair technology,which has broad application prospects in underwater industry.It can greatly simplify the complex process of underwater facilities maintenance and disassembly on land.As for the laser welding technology of titanium alloy,because of its fast welding speed and good quality of weld,and high degree of automation,it is favored by today’s society,and its application scope is more and more extensive.For this reason,this paper focuses on the experimental method and technology of TC4 titanium alloy underwater laser wire filling welding.In order to better match the underwater torch with laser system and eliminate the influence of external factors,based on previous studies in the laboratory,the effects of gas flow and defocusing on weld formation,microstructure and mechanical properties of weld joints were discussed.It was found that too little shielding gas flow could not protect the weld from oxidation,while too much shielding gas flow would make the weld morphology worse,and at the same time,a good weld could be obtained under appropriate defocusing.Therefore,the optimum ranges of the two are as follows: the back air flow rate is 35 L/min and the defocusing rate is 0 mm.On this basis,the effects of laser power,welding speed and wire feeding speed on the weld morphology of TC4 titanium alloy underwater laser filler welding were discussed.It was found that the changes of laser power and welding speed mainly changed the heat input of laser welding,and then affected the weld morphology.Too fast or too slow wire feeding speed also had a great impact on the weld morphology.Secondly,the effects of laser power,welding speed and wire feeding speed on the microstructures and mechanical properties of TC4 titanium alloy underwater laser wire filling welding were studied.It is found that the weld exists in the form of columnar crystal,and the grain size varies from the base metal to the center of the weld.The mechanical properties of welded joints also vary with the parameters.Combined with the influence of different parameters on weld morphology,the optimum parameters of underwater laser wire filling weldin g of 2 mm thick TC4 titanium alloy can be obtained as follows: laser power 3.0 k W,welding speed 20 mm/s,wire feeding speed 60 mm/s,defocusing rate 0 mm,back protective gas flow 35 L/min.At this time,the weld resistance to tension is improved.The str ength can reach 852.81 MPa.Finally,the underwater laser wire-filling welding and onshore laser wire-filling welding of TC4 titanium alloy were carried out under each group of optimum technological parameters.The underwater laser wire-filling welding experiment was demonstrated from the aspects of weld formation,microstructure and mechanical properties of weld joint.The results show that there is no significant difference between underwater weld joint and onshore weld joint in forming,microstructure and mechanical properties.There are no surface cracks and welding pore defects in underwater and onshore welds,and the welds are well formed.The microstructures at the center of the joint are mainly composed of columnar crystals,and the inner columnar crystals are acicular martensite.In addition,the microhardness in the weld center and heat-affected zone is much higher than that in the base metal,and the microhardness in the weld center reaches the highest value,and gradually decreases from the weld center to the heat-affected zone.The impact toughness of onshore welded joint is 38.99 J/cm2,and that of underwater welded joint is 39.07 J/cm2,which reaches 78.38% of the base metal impact toughness.The welded joint meets the requirements.It lays an important foundation for the application of underwater laser wire-filling welding technology in practical production.