Research On Pre-melted Electron Beam Additive Manufacturing Method And Deposition Mechanism Of TC4

The emergence of additive manufacturing provides a new approach for design of lightweight structures and functionally graded materials,increases the design freedom,and can also realize the near net shape of complex hollow structures that cannot be manufactured by traditional manufacturing methods.However,there are some problems in additive manufacturing,such as coarse grain,uneven element distribution,powder oxidation and difficulties in process control.In view of the above problems,this paper put forward pre-melted electron beam additive manufacturing（PEBAM）method,based on the traditional electron beam freeform fabrication（EBF）,to avoid the problems of excessive heat input caused by the formation of molten pool on the base metal and uneven element distribution caused by the violent stirring in the molten pool by electron beam,so as to achieve the purpose of refining the internal grains and improving the mechanical properties of parts.This paper studied the proposed PEBAM method firstly,analyzed the influence of the shape and material of the diversion nozzle on the morphology and microstructure of the deposit,and determined the selection principle of the diversion nozzle.Then,the effects of deposition parameters,such as slot width,angle,height of the diversion nozzle,electron beam power,wire feeding rate on liquid metal transition and deposit formation were analyzed by numerical simulation method.Finally,the microstructure and mechanical properties of the multi-layer deposit obtained by PEBAM were analyzed,and the influence mechanism of thermal cycle on the internal microstructure and stress distribution of the deposit was researched in detail.The research results showed that the critical droplet radius of TC4 wire with diameter of 1.0mm is r=（3r_wσ_wsinθ/2ρg）^1/3,and it is 2.3mm at 2200℃.When the liquid metal temperature was low,there was no carbon related phase in the deposit by using the graphite diversion nozzle.When the liquid metal temperature was high,the graphite diversion nozzle would decompose and carbon elements would enter the deposit to form carbides.However,when the liquid metal temperature was high,there was no tungsten related phase in the deposit by using pure tungsten diversion nozzle.Therefore,graphite diversion nozzle is suitable for metal deposition with low melting point,while pure tungsten diversion nozzle is suitable for metal deposition with high melting point.When the slotting width of the diversion nozzle was 4mm,the angle was 30°and the height was 0.1mm,the appearance of the deposit was well formed.Once the liquid bridge was generated,it would not disappear until the end of the deposition because there would be a steady stream of liquid metals to maintain the existence of the liquid bridge in the subsequent deposition process.The increase of the slotted width of the diversion nozzle will weaken the drainage and buffering effect of the diversion nozzle.The increase of the angle and height of the diversion nozzle will increase the impact force during droplet transfer.The increase of the height of the diversion nozzle will also reduce the continuity between the liquid metal at the slotted part of the diversion nozzle and the liquid metal that has been transferred to the substrate during the deposition process,reducing the stability of the deposition process.In the process of PEBAM,the influence of wire feeding rate on the width and height of deposit was less than that of electron beam power.Raising the electron beam power would not only increase the area of high-temperature liquid metal,but also reduce the height and increase the width of deposit,resulting in much process difficulty of deposition process.There was no spatter or wire sticking during the deposition process,which avoided the control difficulty in the EBF process.TC4 multilayer deposit was studied by using pure tungsten diversion nozzle,the height of the five-layers deposit prepared by PEBAM was higher than that of by EBF.The interface of the deposit prepared by PEBAM was‘arch bridge’shape,while that was‘concave’shape by EBF.The formation mechanism of the‘arch bridge’interlayer interface was that the base metal did not melt after the liquid metal was transferred to the base metal through the drainage effect of the diversion nozzle,and the liquid metal was only wetted and spread on the upper surface of the base metal.The average cooling rate of each layer in PEBAM process was higher than that of EBF process in the range of 500-1670℃.The difference in cooling rate between the second and fifth layer of the former process was 80℃/s,while which was 39℃/s of the latter process.The maximum number range ofαphase width in PEBAM deposit was 1/4-1/5 of the EBF deposit.There was no obvious element segregation in the deposit,which avoided the problems of coarse grain and uneven element distribution in the EBF method.In the process of five-layers deposition,the PEBAM method would not cause theα-βsolid-state phase transition of other layers,while the EBF method would melt the adjacent layer and cause theα-βsolid-state phase transitions of all previous layers.The tensile strength of TC4 deposit prepared by EBF process was 784 MPa and the fracture strain was 0.22,while they were 840 MPa and 0.34 by PEBAM method.The fracture path of the PEBAM tensile specimen intersected with the interlayer interface,indicating excellent interlayer bonding.The residual stress on the side of the deposit manufactured by PEBAM was almost in the range of 570-700 MPa,and the maximum deformation of the substrate was 2.61mm.The stress at the longitudinal middle position in the upper surface of the first layer changed periodically with multiple deposition processes,and the variation range of the maximum stress was less than 50 MPa,which was stable at 500 MPa finally.The deformation of the deposit hardly decreased in the whole deposition process.Each deposition process would increase the deformation at the end of the first layer,but the increment of the deformation decreased gradually.The variation range of the longitudinal height in the upper surface of the deposit was 0.2 mm,which had little effect on the subsequent deposition process.