| Titanium alloy has the advantages of high specific strength,excellent mechanical property,and superior corrosion resistance.It has been widely used in aerospace,biomedical industry and other fields.With the development of additive manufacturing(AM),Ti-6Al-4V has also become one of the major materials of AM process.However,the temperature of molten pool during the AM process is beyond 1600?,and Ti-6Al-4V reacts actively with the oxygen in air,consequently,a shielding gas environment is needed during the AM process.The main solution now to prevent atmospheric contamination is an inert gas chamber,which provides a large inert gas environment to protect the titanium AM processes.Inert gas chamber can prevent the oxidation of Ti-6Al-4V,but the chamber itself will restrict the size of the products produced.Since all the mobile and processing equipment must be put into the chamber,it becomes difficult to realize as the size of the products to be formed or repaired increases.Moreover,plenty of time is needed to make the chamber be filled with inert gas,such as argon,and the cost becomes prohibitively high.This paper presents a local shielding gas nozzle based on the inside-beam powder feeding technology.The laser cladding forming of titanium alloy directly in open environment was conducted with the designed coaxial local shielding gas nozzle.The flow channel and CFD simulation models of the nozzle were established based on the structural characteristics of the inside-beam powder feeding nozzle.Through comparison of the mole fraction distribution cloud diagrams in the simulation results,the inner protection nozzle of the vertical inlet and cambered surface flow channel was determined.Gas flow characteristics of the outlet airflow were analyzed with the Fluent 18.0 software,and the atmosphere with the oxygen content less than 2000 ppm was defined as the effective protection range and length of the nozzle.Through the effective protection length of the airflow at different nozzle inlet angles and numbers,the final structure of the inner protection nozzle with the number of inlets of 4 and the inlet angle of 45° were finally determined.The single-layer cladding experiment of titanium alloy in the open environment was carried out using the inner protection nozzle.Oxidation discoloration trend of the single clads under different shielding gas flows was consistent with the change in effective protective length at the corresponding flow rate in the CFD simulation.In order to solve the oxidation problem of the inner protection nozzle with the height deposited increases,the outer protection nozzle was designed to protect the high temperature area behind the molten pool during the AM process.Through the comparison of the effective protection length and depth in the CFD simulation,a small gap and straight flow was determined as the outer nozzle structure.The effective protection lengths of the double-deck local shielding gas nozzle of different flow rates under the flat bound state were analyzed.At the same time,the infrared camera was used to monitor the high temperature area during the cladding process,and the judgment criteria for effective protection of the nozzle was defined.That is,1/2 of the effective protection length is larger than the length of the high temperature area behind the center of the molten pool.A silver-white deposited part was obtained using the double-deck local shielding gas nozzle.In order to quantitatively characterize the amount of protective gas required under different processing parameters,a model for minimum quantity of shielding gas needs to be established.Firstly,the optimal distribution relationship between the inner and outer nozzles within the total flow rate of 18?50L/min was determined by Fluent software,and the mathematical model between effective protection length and gas flow was established.Then a response model between the high temperature length behind the molten pool and the process parameters was established with apply of the surface response analysis method.Finally,the minimum quantity shielding gas model of the single clad in open environment was established based on the judgement criteria,which can guide the choose of gas flow rate according to the processing parameters of the laser cladding forming of titanium alloy.Laser cladding forming of titanium alloy in open environment was conducted with the coaxial double-deck local shielding gas nozzle designed in this article.The microstructure of the formed part is transformed into a stripe and needle-like trend from the equiaxial with the gas flow rate increasing.When the shielding gas flow rate is larger than that in the minimum quantity model,surface of the formed part appeared bright silver-white,and the internal oxygen content was less than 2000ppm.The micro hardness level of the formed part was increased by 20?50HV on the rolled parts.The formed part has a lower elongation but higher tensile strength than the substrate,indicating that the plasticity of the formed part is reduced and the strength is increased.After simulation analysis and experimental verification,the local shielding gas nozzle designed and implemented here can protect the laser cladding forming process of titanium alloy in the atmospheric environment.It has the characteristics of good real-time performance,easy movement,and unlimited size of processed parts.It provides technical support for the forming and repair of the large titanium structural parts. |
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