Design And Optimization Of Additive Manufacturing Process Of TC4 Component In Ultra-Low Temperature Application Environment

The selective laser melting(SLM)in the additive manufacturing technology can make the compact metal components,which are suitable for the aerospace parts such as the engine liquid hydrogen impeller,such as the engine liquid hydrogen impeller,compared to the traditional processing process,it can make complex shape parts and do not need molds.However,the presence of large column grains and texture in microstructure of the manufacturing components can lead to the anisotropy of the organization and the mechanical properties,while the SLM components also face residual stress problems,which often reduce the mechanical performance and service performance of the components,and in the ultra-low temperature environment,the effect on the service performance of the component can be more serious.At present,optimization of process parameters is an effective solution,but the relationship between process parameters and microtissue anisotropic and residual stress has not been clearly revealed.Based on the above reasons,this paper takes Ti6Al4V(TC4)liquid hydrogen impeller applied in ultra-low temperature environment as an example to explore the microstructure and stress evolution characteristics of TC4 component in the SLM process by numerical simulation method,analyzes the influence of process parameters on them,and optimize the SLM process parameters of the TC4 impeller with the results of the simulation and experiment.The sequential coupling method was used to simulate the SLM process on the local region of the component with several models.The temperature field,microstructure and residual stress between grains of the representative region were analyzed on the microscopic scale.The effects of laser power,scanning speed and scanning strategy on the SLM process were discussed respectively.The results show that as the laser power increases or the scanning speed decreases,the molten pool size increases,the temperature peak value of the thermal cycle curve in the forming region increases and the solidification rate decreases,the size of the generated columnar crystals increases,and the average level of residual stress on the representative region increases.When the scanning direction of each layer is rotated by 90°,compared with the scanning strategy with the same scanning direction of all layers,the temperature gradient of the center point of the scanning trajectory undergoing thermal cycling in the subsequent scanning can be reduced,the cross-layer growth of columnar crystals can be reduced,the residual stress in the scanning direction can be reduced,and the gap between the residual stress in the scanning direction and the transverse direction can be reduced.According to the simulation results,the relative optimal process parameters are preliminarily determined as laser power of 200 W,scanning speed of1.0m/s,and interlayer scanning direction rotation of 90°.TC4 member was prepared by optimizing SLM process parameters,and the plate TC4 member was successfully prepared without obvious warping deformation.The quasi-static tensile test of TC4 component was carried out at room temperature and low temperature(liquid nitrogen temperature 77K),and the microhardness of the component was characterized.The results showed that: The yield strength and tensile strength of SLM TC4 at room temperature are as high as 1214 MPa and 1255 MPa respectively,which is more than 20% higher than traditional TC4,and has better work hardening ability.The yield strength and tensile strength of SLM TC4 at low temperature are further improved.The microhardness of the component is evenly distributed in a small local area.The experimental results show that the mechanical properties of the component are good,and the rationality of the process parameters optimized by local SLM simulation is verified.A three-dimensional model was established according to the actual TC4 liquid hydrogen impeller,and the SLM process of the component was simulated by thermal-mechanical coupling finite element model.The influence of process parameters on the macroscopic residual stress of the component was analyzed,and the rationality of the optimized process parameters was verified again.The results show that the residual stress on the SLM impeller component increases with the increase of laser power or the decrease of scanning speed.The residual stress of 90° interlayer scanning is lower than that of all layers in the same direction.The optimization process parameters selected by the SLM simulation results of the impeller components are consistent with the optimization process parameters selected by the local simulation results.The rationality of the optimization process parameters is verified again,and the relative optimal SLM process parameters of the TC4 liquid hydrogen impeller component are determined.