Densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing

Densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing was investigated. Hot isostatic pressing was carried out for a bundle of copper-coated tungsten fibers in copper powder. Due to tungsten-fibers and copper coating layers, the densification rate of a tungsten-fiber-reinforced copper powder compact was slower than that of pure copper powder. The constitutive equations by McMeeking and co-workers and by Abouaf and co-workers were implemented into a finite element program (ABAQUS) to analyze densification behavior of tungsten-fiber-reinforced copper powder compacts under hot isostatic pressing. Finite element calculations were compared with experimental results for the variation of relative density with time for copper powder compacts during hot isostatic pressing. Density distributions in copper powder compacts were also investigated by comparing experimental results with finite element calculations.     

Creep densification of copper powder compact

Densification of metal powder under high temperature processing was investigated. Experimental data were obtained for copper powder under hot isostatic pressing, hot pressing and uniaxial creep compression. Theoretical calculations from the constitutive models by McMeeking and co-workers were compared with the experimental data. The agreements between experimental data and theoretical calculations are reasonably good when hydrostatic stress is dominant, but not as good when deviatoric stress increases.     

Near-net-shape forming of 316L stainless steel powder under hot isostatic pressing

Near-net-shape forming of 316L stainless steel powder is investigated under hot isostatic pressing (HIPing). A stainless steel powder compact and an insert were encapsulated by a stainless steel container and hot isostatically pressed to produce an axisymmetric near-net-shape part. To simulate densification and deformation of a powder compact in the container during HIPing, the constitutive model of Abouaf et al., and that of McMeeking and co-workers were implemented into a finite element analysis. The thickness effect of the container on densification was also studied for the axisymmetric part during HIPing. Densification of a three-dimensional asymmetric part during HIPing was also investigated by comparing finite element calculations with experimental data by Eisen et al.     

A densification model for mixed metal powder under cold compaction

Densification behavior of mixed copper and tool steel powder under cold compaction was investigated. By mixing the yield functions proposed by Fleck et al. and by Gurson for pure powder in terms of volume fractions of Cu powder and the fraction of contact, a new mixed yield function was employed for densification of powder composites under cold compaction. The constitutive equations were implemented into a finite element program (ABAQUS) to compare with experimental data and with results from the model of Kim et al. for densification of mixed powder under cold isostatic pressing and die compaction. Finite element calculations by using the yield functions mixed by the fraction of contact agreed better than those by volume fractions of Cu powder with experimental data.     

Near-net-shape forming of alumina powder under hot pressing and hot isostatic pressing

Densification and deformation of alumina powder under hot pressing and hot isostatic pressing were investigated. Finite element calculations were performed by implementing constitutive equations for grain growth, power law creep and diffusional creep in the user defined subroutine CREEP of ABAQUS. An alumina compact of valve head shape was produced under hot pressing and its forming process was predicted by the finite element calculation. Densification behavior of an alumina powder compact encapsulated by a stainless steel container was also investigated under hot isostatic pressing. Inhomogeneous deformations of an alumina powder compact due to the shield effect of a container during hot isostatic pressing were observed experimentally and predicted by the finite element analysis.     

Effect of friction between powder and a mandrel on densification of iron powder during cold isostatic pressing

The effects of friction between the powder and the mandrel on densification behavior of metal powder were investigated under cold isostatic pressing. The friction coefficients between the powder and the mandrels with different surface roughness were determined from the relationship between the compaction pressure and the ejection pressure of the mandrel from powder compacts. The elastoplastic constitutive equations based on the yield function of Shima and Oyane were implemented into a finite element program (ABAQUS) to simulate compaction responses of metal powders during cold isostatic pressing. Finite element results were compared with experimental data for pure iron powder under cold isostatic pressing.     

Near net shape processing of a sintered alumina component: adjustment of pressing parameters through finite element simulation

Near net shape forming of alumina powder by cold die pressing and pressureless sintering was investigated. From experimental data of triaxial compression test of alumina powder, a hyperbolic cap model with a critical state line was proposed for densification of alumina powder at room temperature. For pressureless sintering, the phenomenological model for densification and viscous behavior of alumina powder proposed by Kim and co-workers was used. The constitutive models were implemented into a finite element program (ABAQUS) to simulate densification of alumina powder under cold die pressing and pressureless sintering. Finite element results were compared with experimental data for density distribution and deformation of an alumina powder compact under cold die pressing and pressureless sintering. New conditions of compaction were then proposed to reduce the distortion of the sintered part.     

Densification behavior of aluminum alloy powder under cold compaction

Densification behavior of aluminum alloy (Al6061) powder was investigated under cold compaction. Experimental data were obtained under triaxial compression with various loading conditions. A special form of the Cap model was proposed from experimental data of Al6061 powder under triaxial compression. The proposed yield function and several other yield functions in the literature were implemented into a finite element program (ABAQUS) to compare with experimental data for densification behavior of Al6061 powder under cold isostatic pressing and die compaction. The agreement between finite element calculations from the proposed yield function and experimental data is very good under cold isostatic pressing and die compaction.     

Numerical simulation and experimental investigation on densification,shape deformation,and stress distribution of Ti6Al4V compacts during hot isostatic pressing

Hot isostatic pressing of metal powders involves a complex thermal and mechanical coupling process. A constitutive model based on Perzyna’s elastic-viscoplasticity equation was proposed, and a Lagrangian finite element method was applied to analyze the large deformation, nonlinear friction, powder flow, and densification behavior during hot isostatic pressing. The mechanical behavior of the powders was analyzed in terms of stress distribution. For comparison to the simulation results, the density, shape deformation, and residual stress of the specimens were evaluated using Archimedes’ principle, 3D measuring technology, and Empyrean X-ray diffractometer.     

Experimental based finite element simulation of cold isostatic pressing of metal powders

The present work addresses the various ingredients required for reliable finite element simulations of cold isostatic pressing (CIP) of metal powders. A plastic constitutive model for finite deformation is presented and implemented into an explicit finite element (FE) code. The FE implementation is verified so that numerical errors (both temporal and spatial errors) are kept under control. Thereafter, uniaxial die compaction experiments are performed required for determining the material parameters in the constitutive model. Subsequently they are applied for the simulation of a “complex” CIP process. The experimental observations of the complex CIP process were used to validate the overall method by comparing the FE results (final dimensions and average relative density) to the experimental observations. The numerical results (final dimensions and relative density) are in good agreement with the experimental observations.     

Deformation processing of metal powders: Part I—Cold isostatic pressing

Analytical and numerical techniques are used to analyze in detail the stage I Cold Isostatic Pressing (CIPing) of metal powders. Plastic yielding is considered to be the only densification mechanism and the constitutive model developed recently by Fleck, Kuhn and McMeeking [(J. Mech. Phys. Solids40, 1139 (1992)] is used to describe the elastoplastic behavior of the metal powder. It is shown that, for the case of powder consolidation in a long cylindrical tube, a change in length is not possible, unless it is accompanied by some distortion (“shape change”) of the CIPed specimen. The numerical implementation of the constitutive elastoplastic equations in a finite element program is discussed. The finite element method is used to analyze the CIPing of titanium powder. The predictions of the finite element solution agree well with available experimental data.     

Model for compaction of metal powders

A new yield criterion for metal powder compaction based on continuum mechanics has been proposed. It includes three parameters to characterize the geometric hardening of powder compact and strain hardening of incompressible metal matrix. The elasto-plastic finite element method to describe compaction of metal powders has been formulated using the new yield criterion. The values of parameters in the yield criterion could be determined through cold isostatic pressing (CIP). The finite-element method was used to simulate compaction behaviour of copper powders of different shape and mean particle size.     

Genetic algorithm-based numerical optimization of powder compaction process with temperature-dependent cap plasticity model

In this paper, a shape optimization technique is presented for the cold and hot isostatic pressing of metal powders based on the genetic algorithm (GA) approach. The GA technique is used to obtain the desired optimal compacted component by changing the boundaries of component and verifying the prescribed constraints. The coupled thermomechanical analysis of hot isostatic pressing is employed for metal powders during densification process. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, temperature-dependent cap plasticity model, and frictional contact algorithm. The modified cap plasticity takes the temperature effects into the numerical simulation of highly nonlinear behavior of metal powder. Finally, numerical examples are analyzed to demonstrate the feasibility of proposed optimization algorithm for designing powder components in the cold- and hot-forming processes of powder compaction.     

冷等静压工艺对纳米WC-8Co硬质合金性能的研究

以纳米WC-8Co粉末和VC、Cr3C2、TaC晶粒长大抑制剂作为YG8硬质合金原料,采用金属模压预成型+冷等静压处理获得压坯,后经真空烧结制备WC-8Co硬质合金.研究冷等静压工艺对硬质合金收缩率、致密度、硬度的影响,结果表明经冷等静压处理的生坯烧结后的硬质合金具有低收缩率、高致密度和硬度的特点.     

数值模拟在粉末冶金中的应用概况

简介数值模拟应用在粉末冶金中的一些基本模型和方法以及近来在粉末压制、注射成形、热等静压成形等领域的研究应用情况     

Ti6Al4V合金整体零件的两步热等静压近净成形工艺探究

针对热等静压整体成形高温合金零件容易产生部分区域致密度较低、整体性能不均一的问题,提出了热等静压两步成形方法,在较低的温度和压力作用下成形为不带连通孔隙的原始零件,去除控形模具后用合适的温度压力作用使不致密区域致密,提高零件均一性。以Ti6Al4V粉末材料为例,使用有限元模拟和实验测试相结合的方法,确定了两步成形法的工艺参数,并成形了叶盘零件。SEM结果显示：热等静压两步法成形的零件组织由板条状α+β相组成,原始颗粒边界消失,不连通孔隙闭合。断口形貌显示:在合适的两步成形工艺参数加载下,粉末颗粒冶金结合牢固, 不再成为裂纹起始处,拉伸强度提高。两步法拉伸性能略优于常规热等静压拉伸性能,性能达到同规模锻件水平。     

The analysis of shape change during isostatic pressing

To improve the design of optimum consolidation cycles for powder processing, it is important to be able to predict the final shape of the product. When components are solidified from metal powders by isostatic pressing, substantial shape changes can occur due to the constraint of the can. This effect can be modelled in terms of a new constitutive law for the plastic yielding of powder aggregates. It is found that in the early stages of consolidation, the shape is controlled by the container and the powder has no influence. The container deforms into shapes it would achieve if it were empty. This suggests that the initial design of a container can be carried out with an empty can under external pressure.     

Wear resistance of in situ MMC produced by supersolidus liquid phase sintering (SLPS)

The resistance to abrasive wear can be effectively raised by hard particles embedded in a metal matrix. Powder metallurgy allows to admix larger particles than those precipitated from the matrix. To save costs ferrotitanium (FeTi) particles are admixed to a tool steel powder and are transformed in situ into titanium carbonitrides during compaction by supersolidus liquid phase sintering (SLPS) instead of hot isostatic pressing. Pin-on-abrasive paper-tests as well as scratch tests reveal an increase in wear resistance of in situ SLPS–metal-matrix-composites (MMC) over hot isostatic pressing (HIP)–MMC, which is linked to the large size of precipitated hard particles in the former. The application of these laboratory results is seen in reinforcing inserts of wear parts.     

Wear resistance of high-speed cutting powder steels with molybdenum disulfide additives

The effect of molybdenum disulphide additives on the tribological properties of high-speed powder steels produced by hot isostatic pressing has been investigated. The firction coefficients are determined at sliding velocities from 0.5 to 2.0 m/s and under loads from 1 to 15 Mpa, and the wear resistance of the experimental steel is determined. It is shown that the molybdenum disulphide additives reduce the friction coefficient and promote the wear resistance of the steel.     

Wear and fracture toughness of partially stabilized zirconia ceramics under dry friction against steel

The wear of two ceramic materials containing partially stabilized zirconia is studied under unlubricated friction against steel. The first material, with ZrO₂ and 3 mol % Y₂O₃, was obtained by cold isostatic pressing and sintering. The second material, comprising ZrO₂ and 4 mol % Y₂O₃, was fabricated by additional hot isostatic pressing. The samples of both materials were fabricated with high and low values of fracture toughness. The samples with high fracture toughness are found to wear more intensively. This fact can be explained by surface micro-cracking during braking as a result of phase transformations.