Three-dimensional Finite Element Simulation On Selective Laser Sintering And Isostatic Pressing Of Pure Tungsten Powder

In this paper,finite element method(FEM)and multi particle finite element method(MPFEM)are used to simulate the selective laser sintering(SLS)and hot isostatic pressing(HIP)of tungsten powder from continuous scale and particle scale in three-dimensions,respectively.The variation of temperature field and stress field in the forming process of tungsten powder under different laser sintering process conditions and the formation mechanisms are considered systematically and comprehensively.The evolution mechanism of pores is analyzed,and the optimal process parameters for preparing high-density tungsten materials are obtained.On this basis,hot isostatic pressing method was used to investigate the post-treatment of three kinds of sintered parts to obtain pure tungsten parts with high relative density.Not only the influences of different pressing pressure on the relative density of the three kinds of sintered parts were studied,but also the pore filling mechanism and densification mechanism of the sintered parts were explored,and the stress in the compact was analyzed to optimize the post-treatment.The main innovative results are as follows.In the process of single layer and single pass selective laser sintering,the larger the laser power,the more serious the deformation degree of the part,and the greater the influence on its performance.By analyzing the thermal stress field of single layer and single pass,it can be found that the laser forming process can be divided into two regions,i.e.,the heat action region and the heat affected region.The stress concentration in the heat affected region is obvious,which is also the main region prone to edge warping.The main reason for this kind of problem lies in the stress concentration phenomenon caused by the uneven distribution of the stresses and the sharp change of stress in a short time.In order to eliminate the stress concentration,actions can be taken by adjusting scanning powder to control the sintering temperature of the component in large scale,or by adjusting the scanning speed to control the sintering temperature in small scale.From the single-layer multi-pass and multi-layer single pass laser sintering process,it can be seen that the quality of the parts is improved with the superposition of the sintering layers.At the same time,the quality of the first layer directly affects the subsequent forming process.After the simulation analysis,it is found that the quality of the sintered part is better when the scanning power is 200～250W and the scanning speed is 50～100 mm/s.The stress concentration can be alleviated with the increase of sintering layers.In the simulation process of multi particle finite element selective laser sintering,the densification degree of 50 mm/s sintering part is the highest,followed by 100 mm/s,and the densification degree of 150 mm/s sintering workpiece is the lowest,which can be ascribed by the faster laser speed inducing the shorter laser radiation and heating time.Therefore,the range of 50～100 mm/s can be selected for laser sintering.It can also be found that the densification trend of the three kinds of sintered parts is similar in the process of hot isostatic pressing,and the final compaction density can reach about 98%.The pore filling between particles mainly depends on the particle deformation.At the initial stage of compaction,the stress difference between particles is obvious.With the continuous compacting,when the pressure reaches 1000 MPa,the equivalent von mises stress between particles is almost the same,and the residual stress is eliminated.