Technology And Numerical Simulations On Combined Process Of Powder Laser Rapid Forming And Isostatic Pressing

Powder laser rapid forming technology mainly includes Selective Laser Sintering and Selective Laser Melting which both have advantages such as short forming cycle, without use of mold, and being able to form high complex shape parts, but at the same time they also have relatively low density and intensity. It is difficult for rapid forming to form large size metal parts. Isostatic Pressing technology can improve products density and has uniform microstructure without composition segregation. But for the parts with complex shape, the making of metal bag is difficult. The technology of rapid forming combining with Isostatic Pressing is a new and rapid manufacturing method of complex metal parts. In recent years, reports about the technology of rapid forming combining with Isostatic Pressing had appeared internationally. Because the density of parts made from Selective Laser Sintering were low (about 38%), they needed to some post-processions such as degreasing, Cold Isostatic Pressing, sintering and Hot Isostatic Pressing etc which were more tedious process. While direct Hot Isostatic Pressing results of the parts from Selective Laser Sintering were not ideal. Although the parts density from Selective Laser Melting was high (about 90%), direct Hot Isostatic Pressing process of the part required that surface had no connected pores. So research about the process of powder laser rapid forming combined Isostatic Pressing is still in its initial stage. Because the change of product size in Isostatic Pressing forming process is big, in order to achieve the purpose of the net-shape forming(the process that need little machining), reduce the test times and costs, it is necessary to simulate the process of Isotatic Pressing. The reports on simulations are rare, and results of numerical simulations were not ideal.According to the research status, in order to make the full dense metal of complex shape by rapid forming methods, this paper conducted further researches on different post-processing methods and densification mechanism, including Selective Laser Sintering\Cold Isostatic Pressing\Hot Isostatic Pressing, direct Hot Isostatic Pressing of Selective Laser Sintering parts and Hot Isostatic Pressing process of Selective Laser Melting parts. And these processes were simulated. Powder laser rapid forming technology could be further developed, and its application field could be broadened. Therefore, this research has the quite big theoretical and practical significance.Firstly, Selective Laser Sintering\Cold Isostatic Pressing\Hot Isostatic Pressing process was studied. Because of low relative density of Selective Laser Sintering parts, the mechanisms were mainly rearrangement and plastic deformation of particles. The big pores inside significantly reduced after Cold Isostatic Pressing and particle boundary could distinguish still. The density of parts before Hot Isostatic Pressing had been high (more than 90%), so the densification was based on plastic deformation and diffusion-creep mechanisms. Internal pores had turned into the ball-shape after Hot Isostatic Pressing. By the experiments of Cold Isostatic Pressing the pressure-density curve of stainless steel powder under tap density p=0.398exp(ρ/0.105) and the pressure-volumetric plastic strain curve p=18.86exp(6.41εvp)were achieved. They provided important material parameters for numerical simulations of Cold Isostatic Pressing. The results showed that the initial density difference had little effect on the the pressure-density curve. This paper simulated the Cold Isostatic Pressing process of the Selective Laser Sintering parts using Cam-Clay model and the Drucker-Prager-Cap model in ABAQUS software. Two models had a good accordance with experimental results, which had obvious smaller error than literature. This was because shrinkage of the Selective Laser Sintering parts was uniform and material parameters from experiments were used. SEM micro-morphology graphs of powder had significant effection on the properties of material. The irregular powder particles existed orientation, so the shrinkage in vertical direction was different with the other two directions in powder bed. The parts manufactured from spherical powder had more uniform shrinkage in all directions. Because the stiffness of rubber bag was smaller, elasticity was excellent, while parts of Selective Laser Sintering were harder than rubber bag, the shape of bag would change with parts of Selective Laser Sintering. So the bag had little influence on the shape and dimension of parts. It was the ratio of stiffnes of parts and bag that controlled the deformations in the press process. Although the increasement of friction coefficient led to bigger diameter and smaller hight of the column part, the influence on the results was little. When friction coefficient was not zero, the effect on the results had no obvious trend except for fluctuation. From the results of orthogonal experiments the stress-strain curves had the greatest impact on the simulations than the other parameters. Cold Isostatic Pressing of complex parts was simulated. The parts had only uniform contraction without apparent shape distortion which was similar to the simple parts. The error of gear numerical simulation results were much less than that of literature, because material hardening parameters from the experiments were more accurate. The initial size of turbine parts was designed. Firstly the initial sizes were achieved by hardening curve. Secondly the initial sizes were corrected for several times through the numerical simulations. Finallly final initial shape close to the design size was obtained. Because Selective Laser Sintering\Cold Isostatic Pressing\Hot Isostatic Pressing process was tedious, this study improved the route of Selective Laser Sintering/Hot Isostatic Pressing process by use of glass medium. Gear parts with a relative density of up to about 90% were created. Parts volume had obvious contraction. The internal pores were mutual disconnected spherical and had no obvious grain boundary. Hot Isostatic Pressing process of Selective Laser Sintering parts was simulated for the first time by ABAQUS sequence coupled thermal-stress analysis. The hyperbolic sine of creep equation of porous materials creep behavior programmed by ABAQUS creep subroutine was used for simulations. Simulation results showed that due to product volume was relatively small and heating and cooling speeds was slow, temperature difference on the whole parts in the process of forming was small, so parts shrinked more uniform. According to the density time curve, due to the initial density was low, particle rearrangement was main densification mechanism of the initial stage. Density increased rapidly which led to the rapid change in curve. When the relative density reached about 0.7, densification rate became slow where densification mechanism was mainly plastic deformation. When the relative density reached about 0.9, as the relative density was higher, densification mechanism was mainly diffusion-creep. Densification rate became much slower. Because Selective Laser Melting parts don't need to get rid of polymer adhesive, its post-processing steps are the most smallest. In directly Hot Isostatic Pressing of the parts, even if initial relative density of the parts was high (about 95%), there was no guarantee that completely densification could be achieved after Hot Isostatic Pressing. Whether parts could be fully densified was related to internal pore size, distribution, whether they are connected to each other and impurities existence.The results showed that because of the long forming time (about 6 hours) and slow heating rate, the temperature field was very uniform at all times in Hot Isostatic Pressing process, which could be approximated as a quasi-Thermal equilibrium (or quasi-steady state). Since there was no temperature gradient throughout the specimen, thermal stress was small. There was only volume shrinkage of the samples and little the shape change under Isostatic Pressing conditions. For traditional Hot Isostatic Pressing process with the powder plus bag the deformation of part was relatively large because of complex part shape coupled with the impact of bag. There was not only the volume contraction but also the shape change. This was mainly because bag and cores existed in traditional method with powder plus bag that were made up of dense material, internal powder materials were densified through the deformation of bag under pressure. Due to good liquidity of internal powder materials, its shape would change with that of metal bag. So the main reason for the uniform contraction of Selective Laser Melting parts in Hot Isostatic Pressing was that the bag was not used. The size and density of the final parts can be predicted by simulations that could provide useful guidance to the design of the initial size and shape of the combined forming process of Isostatic Pressing and rapid forming.