| Titanium alloy has the advantages of high melting point,high tensile strength,high specific strength,high specific stiffness,strong biocompatible phase,no magnetism and strong corrosion resistance.Therefore,it is widely used in aerospace,military industry,shipbuilding industry,automobile industry,Petrochemical and biomedical fields.However,titanium alloy has poor thermal conductivity,low plasticity and high hardness,which results in large deformation resistance,large flow stress,high deformation temperature,difficult machining and long processing cycle in hot working.The use of additive manufacturing technology can solve the problem of titanium alloy forming and processing,and further expand the application of titanium alloy.High quality titanium powder is the basis for the manufacture of titanium alloys.Because titanium and titanium alloys are extremely active under high temperature conditions of gold,they are easily reacted with most of the simple substances and compounds,and are highly polluted.Therefore,the preparation of high purity and high quality titanium powder is very difficult.At present,the preparation process of titanium powder mainly has problems oflow yield of fine powder,high ratio of satellite balls,low production efficiency,and easy contamination of titanium powder.This study draws on the advantages of titanium powder preparation technology at home and abroad.Using high frequency induction principle,fluid mechanics principle,combined with theoretical research,simulation research and experimental research,the Wire Induction heating Gas Atomization(WIGA)technology was deeply studied systematically.This study lays the theoretical and technical foundation for the industrial application of WIGA technology.The work completed in this study and the main results are as follows:The COMSOL melting model for 2-5 mm titanium wires of different diameters was established.Through numerical simulation and optimization design,the angle of the induction coil of the melter is 90° and the optimal diameter of the coil is 8mm.The melting temperature fields of titanium wires of different diameters were analyzed by simulation and experimental studies.The result was a wire feed speed of 450 kHz,a diameter of 2 mm,3 mm5 4 mm and 5 mm of titanium wire forming a length of 15 mm flow with a critical wire feed speed of 65 mm/s,58 mm/s,50 mm/s and 43 mm/s.Under these conditions,in the case of heating to produce a liquid flow having a superheat of 350± 50?,the critical output power of the power source is 54 kW,43 kW,38 kW,and 36 kW,respectively.The experimental results and the simulation results prove each other,and the engineering application basis of the melting of titanium wire is obtained.Combining the advantages of both confined and free-fall atomizers,an unconstrained tightly coupled atomizer was developed.The design of the titanium wire melter is located in the lower part of the atomizer,which subverts the characteristics of the atomization of the upper part of the traditional gas atomization milling technology,avoiding the melting of the crucible and the diversion system to the titanium alloy in the traditional tight coupling gas atomization method.Contamination of the melt.The high-frequency induction melting device is tightly coupled with the high-efficiency atomizer,which finitely shortens the distance between the atomizing gas flow from the atomizer to the metal liquid,and effectively reduces the atomic gas flow energy loss.The melting zone and the atomization zone of the titanium wire are all placed inside the high-frequency induction coil,so that the melting point of the titanium wire is infinitely close to the atomization point of the titanium liquid,thereby effectively solving the problem that the atomic gas flow energy loss is large and the titanium liquid superheat degree is significantly reduced.The superheat of the titanium liquid from the melting zone to the atomization zone 1s ensured to achieve high superheat atomization.The "gas-liquid" two-phase flow model of atomization of titanium alloy solution by argon gas with Free fall-Close coupled WIGA technology was established by Fluent 18.0 finite element software,and the structural parameters of the atomizer were studied.The optimal structural parameters of the atomizer are as follows:the angle of the atomizing airflow 1s 30°,the width of the nozzle ring 1s 0.8 mm,and the diameter of the ring slit is 17 mm.Combined with the flow field velocity and temperature curve,the cooling rate of the titanium liquid atomization process is calculated to be 1.2×106 ?/s.This study is an in-depth study of the atomization mechanism of titanium powder.The atomization process of titanium powder 1s divided into three parts:one atomization,two atomization and powder cooling.The atomized liquid flow crushing process is divided into four stages:the stage of entering the atomization area,the fluctuation of the liquid flow,the intermal fracture stage,the severe crushing stage and the stable atomization stage.After the stable atomization,the liquid flow crushing process mainly occurs in the atomization triangle zone,and the gas flow breaks the solution by shearing.The cooling rate of the molten metal in the atomization zone is 1.2×106 ?/s.The secondary atomization and cooling phase of the liquid flow originates from the interaction between the gas and the liquid.As the atomization pressure increases,the droplet disturbance increases,the opening angle of the"atomizing umbrella,increases,and the powder particle size becomes fine.The powder particle size distribution showed double peaks at pressures of 2.5 MPa and 3.5 MPa.When the pressure is 4.0MPa,the particle size refinement is the most obvious,which is the optimal atormization pressure.The Free fall-Close coupled WIGA atomization experimental equipment was established based on the design principle of WIGA,combined with titanium wire melting simulation and experimental research,and titanium melt atomization simulation results.Perform functional debugging of various parts of the device and overall operation of the device to provide a platform for atomization experiments.The optimum parameters for atomization of titanium wire with diameters of 2 mm,3 mm,4 mm and 5 mm were obtained by a single variable test,with atomization pressures of 3.0,4.0,4.0 and 5.0 MPa and melt superheat of 350 ?,respectively.When the feeding speed of the titanium wire is 75,60,55,45 mm/s,the powder D50 is 38.2?m-45.7?m,respectively,and both are less than 50?m,which proves that the fine powder yield is high.The TA1 and TC4 powders prepared by the experiment have smooth surface and good sphericity,and there are basically no "satellite ball" particles.The powder is dense inside,no pores and inclusions,and all technical indicators meet the requirements for additive manufacturing.The TA1 powder with a diameter of 15-53?m has a cooling rate in the atomization process ranging from 6.2×105 to 5.4×106?/s,which is similar to the cooling rate obtained in the atomizatioi simulation study of 1.2×106 ?/s,which verifies the accuracy of the simulation study.The function of the cooling rate of the powder as a function of the particle size distribution and the functional relationship between the particle size and the secondary dendrite spacing are obtained by fitting calculation.The application of Selective Laser Melting(SLM)to TA1 powder prepared by WIGA technology was studied.When the forming laser power is 120 W,the scanning speed is 600 mm/s to 1600 mm/s,the energy density is between 75-200 J/mm3,and the relative density of the workpiece is 97.4-99.7%.When the laser power is 120 W and the scanning speed is 800 mm/s to 1200 mm/s,the condition that the forming power density is 125 J/mm 3-100 J/mm3 is the optimum forming condition of the titanium powder.The microstructure of the formed part is a fine needle-shaped martensite structure.Its impact and tensile fractures are evenly distributed equiaxed dimples with better mechanical properties than castings and forgings.This study demonstrates that titanium powder prepared by WIGA technology can meet the needs of additive manufacturing. |
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