| Compared with the traditional underwater welding reparing technology,laser metal deposition technology has the advantages of high precision,short cycle and strong controllability.It can achieve the "additive formation" of damaged parts and the rapid in-situ repair of metal components with complex shape and high-quality performance.However,the effect of water environment on beam propagation,mass transfer process and metallurgical behavior limits the development and application of the underwater laser metal deposition technology.Based on designing and manufacturing the underwater laser additive manufacturing nozzle,the underwater local dry wire-based laser metal deposition additive manufacturing system was built and the underwater laser metal deposition experiments were carried out for TC4 titanium alloy.On this basis,the interaction mechanism between laser,water and molten pool during the process of underwater laser additive was explored,and the mass transfer modes as well as deposited appearance and formation were revealed.Meanwhile this paper investigated the evolution behaviours of grain and microstructure in different regions of the underwater deposited layer and the corresponding mechanical properties.By designing and developing a double-layer gas curtain underwater nozzle the underwater local dry wire-based laser metal deposition additive manufacturing system was built.Through observing the dynamic change of water environment,metal transfer and fluid behaviour of molten pool,the interaction mechanism between laser,metal and water during the underwater laser metal deposition process was studied.At the lower gas flow rate,the absorption,refraction and scattering on the laser beam by the residual water on the substrate surface and aerosol particles not only interfere the laser beam transmission,resulting in unstable mass transfer process and poor deposition layer formation,but also decrease the laser power density and then reduce the heat input,which accelerates the cooling rate and promotes the formation of martensite α′.With increasing the gas flow rate,the laser propagation and mass transfer process is stable as the residual water on the substrate surface gradually disappears.Meanwhile the uniform appearance of deposited layer without defects such as pores and cracks is obtained.However,there is still a large amount of martensite inside the deposited layer,attributed to the laser power density weakened by aerosol particles and the cooling effect of water environment.During the underwater laser metal deposition process of the multi-track deposition layer,the hydrogen and oxygen elements could diffuse into the interior of deposited layer through crack source and reduce the toughness of deposited metal.During the cooling process,the uneven cooling rate and martensitic transformation cause the residual stress inside the deposited metal and form a stress concentration at the crack source.The crack extends into the deposited metal and forms a macro crack across the whole deposited layer.By increasing the heat input and adjusting the gas flow rate,the oxidation layer on the deposited layer surface is eliminated and the martensite content reduces,eliminating the cracks and obtaining the uniform underwater deposited layer with multi tracks without defects such as cracks and spatters.Based on the in-situ X-ray observation system,the metal transfer behaviour and forming control mechanism during the underwater laser metal deposition process were investigated.The mass transfer mode of wire-based underwater laser metal deposition mainly includes droplet mode,liquid bridge mode and spreading mode.Within the liquid bridge mode,the mass transfer process is continuous and the flow of molten pool is stable.In addition,no spatter and wire adhesion was observed during the liquid bridge mode while the forming accuracy of deposition layer is high.For multi-layer deposition layer,the effect mechanism of additive manufacturing route,layer increment and cooling interval on mass transfer mode and the control of threedimensional forming accuracy was explored.The mass transfer mode depends on the relative distance between wire end and molten pool.When the process parameters and interlayer variables are matched reasonably,the molten metal can contact with the molten pool surface before droplets produce,and then transfer to the molten pool through the liquid bridge,forming a liquid bridge transition.In addition,during the additive manufacturing process of wall,the spacing between laser and wire is stable,and the mass transfer keeps the liquid bridge mode.Consequently,the transition of molten metal is stable and the forming accuracy of deposited layer is high.The deposition appearance,grain characteristic,microstructure and mechanical properties of the deposited layer were analyzed.By establishing the finite element model of underwater laser melting deposition temperature field,the grain,microstructure transformation and property control mechanism of underwater multi-layer walled were studied.In the bottom region of the wall,the heat during the laser metal deposition process flow through base metal to the water environment.The large temperature gradient and high cooling rate promotes the grain grow epitaxially,forming the columnar β grains.Although the cooling rate of decreases from 606 ℃/s to 320 ℃/s during thermal cycles,the higher cooling rate inhibits the diffusion transformation,and the microstructure is mainly composed of acicular martensite α′.With the progress of underwater laser metal deposition,the change of heat dissipation conditions in the deposition zone and the heat accumulation,the peak temperature of the molten pool increases while the temperature gradient decreases,causing the grain growth mode changes from columnar mode to mixed mode.The equaxied grains form in the upper region of the wall.With the decrease of cooling rate,the microstructure of the deposited layer gradually transits to lamellar and strip shape along the positive building direction,and the α lath is gradually coarsened and form the basket-weave structure.Due to the formation of lamellar α/α′,the hardness of the deposition area is higher than that of the base metal,and fluctuates in the range of 348-411 HV accompanied by the microstructure transformation.The formation of columnar grain gives rise to the anisotropy mechanical properties of the wall.The tensile strength of thin-walled parts along the building direction is 830.4 MPa,which is about 93% of that of base metal,and the average elongation is 7.9%,which is sl ightly lower than8.2% of base metal.The mechanical properties along the deposition direction are related to the position of the deposition layer.The average tensile strength is about1024.1 MPa,but the elongation is lower with an average value of 5.0%. |
PDF Full Text Request