| High-Nb TiAl alloy has the advantages of low density,high strength,high melting point,and excellent high temperature oxidation resistance and creep resistance.It shows important application value and development potential in many fields such as aerospace and industrial automobiles.However,the poor room temperature plasticity of High-Nb TiAl alloy increases the difficulty of traditional processing and becomes a major obstacle restricting its engineering application.With the rapid development of Additive Manufacturing technology,selective laser melting forming technology has provided a new solution for the formation of High-Nb TiAl alloy.Due to the increase in research efforts,the technology has increasingly demanded spherical metal powders.However,due to the monopoly of milling technology and the high price of milling equipment,the price of powder used in SLM remains high,which has hindered the research of SLM forming High-Nb TiAl alloy parts to a certain extent.The purpose of this paper is to study the effect of nozzle structure on the gas flow field and the change of the melting efficiency of the induction coil in combination with numerical simulation and atomization experiment,so as to optimize the nozzle structure and induction melting system,and improve the yield of gas atomization and pulverization.At the same time,the self-made High-Nb TiAl alloy spherical powder was characterized,and the relationship between the particle size and structure of the powder was analyzed.Based on this,in view of the SLM formability of homemade High-Nb TiAl alloy powder,the influence of different processes on its forming properties was explored.The main results are as follows:(1)Optimization of atomizing nozzle structure:Through the combination of numerical simulation and experiments,the influence of the nozzle structure on the gas flow field is studied,thereby improving the nozzle with a low yield of the original powder,which makes the atomizing powder yield greatly improve.It is found that when the atomizing pressure is 4MPa,the nozzle structure with s=0.5mm and?=36ohas the best atomizing performance,and the yield of fine powder(-250/?M,d<60?m)is 37.4%.(2)Optimization of induction melting system:When the distance between the coil and the raw material rod is L=15 mm,and the coil turns h=14 mm,the faster the induction system melts the raw material rod(2 min/589 g),The temperature of falling melt is as high as 1695?,and the yield of atomizing experiment fine powder(-300/?M,d<50?m)is as high as 32.1%.(3)Phase transitions in High-Nb TiAl alloy powders were discovered:XRD was used to study the structure of powders with different particle sizes,and the results showed that there was a size-driven???2structural transition.The thermodynamic properties of the phase transitions of powders with different particle diameters were measured.It was found from the DSC curve that at a temperature of 600°C,powders smaller than 25?m undergo a phase transition of?2??.When the temperature further increases to 1150?,the?phase endothermic transforms into?+?/?2phase.(4)The process parameters of selective laser melting molding were optimized:the influence of laser power P,scanning gap h,and scanning speed v on the hardness and density of the formed block was studied.When the laser process parameter combination is explored and obtained as P=170 W,h=85?m,and v=1100 mm/s,the density of the molded part is the best,which can reach 99.01%.In addition,the influence of the energy density E on the compressive properties of the molded part was studied.When the energy density E=61.38J/mm3,the compressive strength of the molded part was 1463.2 MPa and the elastic modulus was 45.73 GPa.The optimized printing parameters are used for SLM molding of the shaped parts,and the molded parts are printed completely without cracks and warps on the surface. |
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