Research On Technology And Stability Of 304 Stainless Steel Electron Beam Freeform Fabrication

In this paper, the research of electron beam freeform fabrication(EBF3) was conducted. The coupling between single layer deposit process and forming coefficient was analysed as well as the microstructure of the single layer deposit, base on that, the microstructure of multi-layer deposits was studied and the effect of deposits layers on the microstructure of the multi-layer deposits was mainly analyzed. Simulation method was used to analyze the influence of the state of the wire on the stability of multi-layer deposits process. The force model of the liquid-gas boundary of the molten pool was established, and the dynamic equilibrium mathematical equation of the liquid-gas boundary was obtained. The distribution of each force was analysed, and the dynamic equilibrium process and instability process was discussed. The stability of the liquid-gas boundary of the molten pool was of great significance for the stability of deposit process and the forming and quality of the deposits.Coupling effect between parameters and forming coefficient showed that a change in one of the parameters would cause the change of the degree of the influence of the effect between the other parameters and forming. Temperature distribution showed that the layer of the deposits increase, the maximum temperature of each layer increase, and ultimately tended to a constant. For thin wall deposits of 304 stainless steel less than two layers high, cooling is dominated by a broad path through the substrate. For higher deposits, cooling is restricted to the path through the previous layers, resulting in lower cooling rates and higher temperatures in the molten pool. The change of cooling path made the white stripe appeared in the first two layers disappear gradually, and the elongated austenite structure which through the multi-layer deposits was turned to appear. The continuous deposit time had little influence on the heat deformation of the wire, but the effect of the angle of wire feed was the largest, the wire extension was long and the angle of wire feed was small led to the heat deformation of the wire easily, which resulted in the break of the deposit process.In the process of EBF3, the liquid-gas boundary of the molten pool was mainly drove by metal surface tension pressure(Pσ), vapor recoil pressure(Pg) and hydrostatic pressure(Ph) in the normal direction. Pσ and Pg maintained the stability of the molten pool while Ph prompted the molten pool instable. Pσ and the hydrostatic pressure caused by keyhole inside wall surface vapor recoil pressure(Ph-V) were similar in magnitude and large in value, and it was the main force that maintaining the dynamic balance of the liquid-gas boundary. Under the each role of the normal force, liquid-gas boundary of the molten pool was in dynamic balance state. The value of electron beam increase made the hydrostatic pressure increase, if the change of Pσ and Pg would counteract the increase of Ph, then the liquid-gas boundary would be restored to the state of dynamic balance, otherwise it would cause the liquid-gas boundary instable and the liquid metal flowing.