The article discusses the simulation of forming process of cold isostatic pressing of the selective laser-sintered components. A cylindrical specimen and a turbine component made of stainless steel powder were fabricated by this forming route. In order to achieve the purpose of the near net shape, the simulations of cold isostatic pressure were carried out. The simulations were based on the Drucker-Prager-Cap constitutive model. The properties of the metallic powder were measured by the experiments. The effect of a rubber bag on cold isostatic pressing is discussed. The results show that a rubber bag has little influence on both shape and size of the specimen. The results of the simulations show good agreement between the experimental results and the calculated results. The simulations can give a useful direction to the design of dimensions of the combined forming process of selective laser sintering and cold isostatic pressing. IntroductionSelective laser sintering (SLS) is a technology with the most potential among rapid prototyping (RP) technology that could manufacture complex parts of any type rapidly [1] and need not use tools and dies. Therefore, SLS has the advantages of short manufacturing cycle, low cost, and high efficiency, which are major advantages over traditional manufacturing technologies for metal parts. The polymer materials of low melting point such as polystyrene (PS) and polyamide (PA) are the main materials for SLS and the high level of porosity exists in the as-processed components. So the SLSed parts usually are infiltrated by low melting point metal or alloy, such as Cu, bronze, and brass, to fill the pores and improve the intensity. The forming of metallic powder materials by PR technology is becoming an important research field due to the enlarged application domain and high value of metal parts. Although metal parts of some alloy materials could be formed via selective laser melting (SLM), the laser power that SLM uses (that is, 100-200 W) is 10-20 times larger than that of SLS. And the laser scan rate of SLM is lower than 200 mm/s (usually the laser scan rate of SLS is 1,500-2,500 mm/s). These factors have led to high costs of SLM. And SLM is limited to the manufacture of small size and thin-walled workpieces. Forming material is also limited to Ti6Al4V, tool steel, ferroalloy, stainless steel, and TiC [2]. Cold isostatic pressing (CIP) can densify powder or porous parts. But when CIP is used alone, it cannot manufacture the complex parts because of the difficulties in manufacturing the bags.
Selective laser sintering (SLS) could manufacture complex parts rapidly which, however, have high porosity and low intensity. While the parts made by cold isostatic pressing have advantages of uniform structure without composition segregation, high-dimension precision and high density. However, it could not form high complex parts because of the difficulties in manufacturing bag. A combination of SLS and cold isostatic pressing is expected to use the advantages of the two methods and is an efficient way to make complicated parts rapidly. After SLS and cold isostatic pressing, dimensions of parts decrease and relative density increases. To predict final dimensions and density, the finite element simulations are performed for cold isostatic pressing. The results show the parts made from ball shape powder contract symmetrically. The simulation results agree with the achieved geometries within 4%. Comparisons are made with that parts made from irregular powder. The SEM pictures after SLS are also showed. This has an important indication to process of SLS and cold isostatic pressing forming.
Selective Laser Sintering could manufacture high complex metal parts in short time but with high porosity and low strength. The components from Cold Isostatic Pressing have excellent performance with uniform organizational structure, high size precision, and high density. It, however, could not form high complex parts because of the difficulties of bag manufacture. So it will be a good method to combine Selective Laser Sintering and Cold Isostatic Pressing to make complicated metal parts. In this paper, the specimens of stainless steel were made by the combined Selective Laser Sintering and Cold Isostatic Pressing forming route. And the simulation of Cold Isostatic Pressing was carried out by finite element method and Drucker-Prager-Cap constitutive model in ABAQUS/Explicit computer program. The property of metal powder was measured by experiments. The effects of bag on Cold Isostatic Pressing have been discussed. It is different from the Cold Isostatic Pressing of metal powder that the bag has little influence on both shape and size of the specimen. The results of simulation show a good agreement between the experimental results and the calculated results. The simulation can give a useful direction to dimension and shape designs of the combined forming of Selective Laser Sintering and Cold Isostatic Pressing.
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