| Titanium and titanium alloy materials have properties advantages such as high specific strength,good fatigue strength and good biocompatibility,and have been widely used in aerospace,military and medical fields.Compared with traditional manufacturing technology,metal additive manufacturing(3D printing)technology can form parts with complex structures and excellent properties.TC4 alloy powder is the current mainstream metal additive manufacturing technology raw material.The properties of the powder will affect the defects and properties of the formed parts.In order to ensure the quality of the additively manufactured titanium alloy forming parts,the TC4 alloy powder needs to meet the characteristics such as high sphericity,good morphology and small particle size.At this stage,the Electrode Induction Melting Gas Atomization(EIGA)method is the mainstream preparation technology of TC4 alloy powder for metal additive manufacturing,and the smelting process is one of the core processes of the powdering technology.The difference in induction coil structure and melting process parameters will affect the properties of the titanium melt entering the atomization zone,thereby affecting the properties of the TC4 alloy powder.Therefore,the research on the smelting process and related experiments of EIGA method has positive theoretical and practical significance.In this paper,COMSOL Multiphysics software is used to simulate the melting process of EIGA method,and the influence of induction coil structure parameters and melting process parameters on the heating and melting effect of TC4 alloy bars are analyzed.According to the results of numerical simulation analysis,the improved induction coil was wound and the melting process parameters were designed,the preparation and property analysis of the titanium powder were carried out,and the influence of the structure parameters of the induction coil and the melting process parameters on the properties of the titanium powder was studied.Finally,the selective laser melting(SLM)forming properties of the powder with the best properties is studied.The main findings of this paper are as follows:(1)According to the actual working conditions of melting process of EIGA method,a two-dimensional axisymmetric model was established,and the electromagnetic field,temperature field,phase field and melting-related data of the TC4 alloy bar during heating and melting were analyzed by COMSOL Multiphysics software.Increasing the number of coil turns can increase the magnetic field distribution area and uniformity,make the induced magnetic field and temperature distribution in the TC4 alloy bar more uniform,and can also increase the power and melting rate on the TC4 alloy bar.But after the 3 turns are increased to 4 turns,the improvement of the melting effect is not obvious.So choose an induction coil with 3 turns.Increasing the diameter of the copper tube will make the smelting effect of the TC4 alloy bar worse,while reducing the diameter of the copper tube can increase the current density on the coil,and the magnetic field generated by the coil is stronger,which is beneficial to improve the smelting effect of the TC4 alloy bar.Through analysis,it is concluded that after reducing the diameter of the copper tube,the current density has a greater effect on the smelting effect of the TC4 alloy than the coupling degree of the coil and the rod.Therefore,the diameter of the copper tube is 8 mm.Reducing the coil spacing can compress and enhance the magnetic field.By drawing the magnetic field distribution curve at a distance of 1 mm from the inner side of the coil,it can be seen that reducing the coil spacing can make the magnetic field distribution more compact and increase the magnetic induction intensity,which is beneficial to improve the smelting effect of TC4 alloy bars.So choose an induction coil with a coil spacing of 4 mm.(2)The simulation research on the melting process parameters shows that increasing the melting power will increase the power input to the TC4 alloy bar,and with the increase of the melting power,the higher the thermal efficiency of the bar,the melting rate will increase with the increase of the melting power.Although the reduction of the feed speed will reduce the production efficiency,it can increase the heating time of the bar and make the bar obtain higher Joule heat.Therefore,appropriately reducing the feed speed is beneficial to the smelting of the bar.(3)Based on the numerical simulation results,the melting process parameters were designed and the improved induction coil was used to prepare the TC4 alloy powder.In the case of the same powder preparing parameters,the original two-turn coil and the improved three kinds of coils were subjected to powder preparing experiments,and the properties was tested after airflow crushing.The results show that the properties of titanium powder prepared by the S4 coil with 3 turns,8 mm copper tube diameter and 4 mm coil spacing is better.Based on the S4 coil,the effect of melting power on the properties of TC4 alloy powder was studied.The results show that: the higher the melting power,the higher the temperature of the titanium melt,the better the match between the spheroidization time and the solidification time of the droplets,and the smaller the contact angle of the interface with the airflow,the better the crushing effect.The titanium powder prepared under 28 k W melting power has high sphericity and fluidity,and the yield of fine powder is improved.The lower the feed speed,the longer the heating time of the TC4 alloy bar.At a feed speed of 40 mm/min,the bar can be melted to form a higher temperature and lower mass flow rate,which is beneficial to improve the crushing effect.Therefore,the properties of TC4 alloy powder with a lower feed rate is better.In the end,a better smelting plan was obtained: the number of coil turns is 3,the diameter of the copper tube is 8 mm,the coil spacing is 4 mm,the melting power is 28 k W,the feed rate is 40 mm/min,the atomization pressure is 35 bar,and the width of the spray disc is 0.6 mm.(4)The TC4 alloy powder was prepared by the smelting scheme obtained by the research and the properties of SLM forming was studied.The three parameters of laser power,scanning speed and scanning distance were designed respectively.Through orthogonal experiment analysis,surface SEM analysis and mechanical properties analysis.It is concluded that the TC4 alloy sample formed with a laser melting of 280 W,a scanning speed of 1200 mm/s and a scanning distance of 0.14 mm has the lowest porosity and the best surface morphology.Through the annealing treatment research,the mechanical properties of the TC4 alloy samples are better after the 800?/2h/FC annealing treatment.The TC4 alloy sample has a maximum elongation of 14.89 %,a maximum yield strength of 964.91 MPa,and a maximum tensile strength of 1036.27 MPa,which proves that this batch of TC4 alloy powder can be applied to the SLM process. |
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