Metal Powders Densification Under Hot Isostatic Pressing: Numerical Simulation And Experiment

Hot isostatic pressing (HIP) of metal powders, as one of the most often used technology in powder metallurgy, is a process involving high-pressure gas isostatically applied to a container at an elevated temperature. Metal powders in containers perform densification during HIP process. There are many advantages of compacts, such as full dense, excellent mechanical performance, near-net shapes and so on. It has much practical utility in modern industry. It has been an academic vanguard and focus of powders densification behavior, prediction the deformation of containers and powders during HIP process by numerical simulation. However, there is still no universal model to accurately present the deformation of powders. Moreover, mechanical performance for differenct material parts after HIP also needs to intensive study. Therefore, systematically investigations have been carried out.The main research contents and results are as follows:(1) The Perzyna viscoplastic model, complying with ellipsolid yield criterion for porous metal materials was investigated. Through the study on ellipticity of many ellipsolid yield criterions, the uniaxial stress model agrees very well with empirical models, and can become the uniform stress parameters for ellipsolid yield criterion. The geometric hardening parameter is determined by material properties. Combining the yield criterion with Perzyna viscoplastic model, it can be used to simulate the viscoplastic deformation behavior of metal powders during HIP process.(2) To investigate into the densification behavior of an AISI 316L stainless steel powder during HIP process, a three dimensional thermo-mechanical coupling case was carried out by the finite element analysis. The distrubitons of relative density, temperature field, equivalent Cauchy stress for the compact were studied. The densification history for some typical positions was also discussed. The results show that there is a slight decrease in relative density for the compact due to thermal expansion at the early stage of the HIP. With the rise of temperature and pressure, the improvement of relative desnisty is rapid. Once the job prolongates to the holding stage, the improvement of relative desnisty is slow. Within the decent stage, it is also helpful to improve the relative density of the compact. Due to deformation and temperature gradient of the compact, the relative density of the compact is not uniform. By comparing with experimental results, the deformation of the container and the relative density distribution of the compact are in reasonable agreement with predict results. So, the Perzyna viscoplastic model is suitable for the simulation of metal powders during HIP process. The mechanical properties of the HIPed parts were also researched. The results show that the property balance of the HIPed samples is very good. The average data of mechanical properties exceed the standard for forgings of American Society for Testing and Materials (ASTM). The yield strength and the tensile strength are 64% and 35% higher than that in the ASTM, respectively.(3) The mechanical properties of the HIPed Inconel 625 superalloy powder were investigated. Through a series of high-temperature compression tests, room-temperature and high-temperature tensile tests, the results present that the activation energy of this superalloy is about 503.5kJ/mol. The hyperbolic sine (Arrhenius type) constitutive model can be expressed asε=7.389×1017sinh(0.0057σ)3.51 exp(-503.5×103/(RT)). During 200-800℃, the yield strength of compression is within 500-400MPa. But, while the temperature is higher than 800℃, the yield strength of compression decrease dramatically. The elongation is 48% lower than that in the ASTM. The prior particle poundary (PPB) in compacts by HIP of the Inconel 625 powder is in evidence. The PPB seriously decreases the plasticity of compacts. The simulation for the HIP of the supperalloy powder with a complex shape was studied. The results reveal that the densification behavior is very similar with it on the AISI 316L stainless steel powder. The relative density distribution in the compact agrees very well with the experimental data. But the lowest relative density has much underprediction.According to the researches of constitutive models for metal powders, mechanical properties for Inconel 625 and so on, it is helpful to further study the densification behavior of metal powders during HIP process in the futher.