The Efficiency of Twist Extrusion for Compaction of Powder Materials

Powder Metallurgy and Metal Ceramics - Within the theory of plasticity for porous bodies, the compaction of titanium powders by twist extrusion and uniaxial pressing in rigid dies was qualitatively...     

Features of Compaction Kinetics for Powder Materials under Nonisothermal Conditions. Part 1. Compaction Kinetics in Regular and Fiber Regimes

New features of nonisothermal compaction kinetics are established: presence of an induction period during which compaction hardly occurs and the possible “explosive” nature of compaction. Numerical calculations make it possible to refine the picture of compaction in a wave regime and to separate two autonomous processes: propagation of a compression zone within the space from layer to layer and material compaction with time in the compression zone. It is shown that with an increase in the height of a compressed specimen there is a continuous transition from a regular regime to a wave regime of compaction.     

Features of Compaction Kinetics for Powder Materials under Nonisothermal Conditions. Part 1. Compaction Kinetics in Regular and Fiber Regimes

New features of nonisothermal compaction kinetics are established: presence of an induction period during which compaction hardly occurs and the possible “explosive” nature of compaction. Numerical calculations make it possible to refine the picture of compaction in a wave regime and to separate two autonomous processes: propagation of a compression zone within the space from layer to layer and material compaction with time in the compression zone. It is shown that with an increase in the height of a compressed specimen there is a continuous transition from a regular regime to a wave regime of compaction.

     

Numerical Modelling of the Compaction of Powder Articles of Complex Shape in Rigid Dies: Effect of Compaction Scheme on Density Distribution. Part 2. Modelling Procedure and Analysis of Forming Schemes

A modified finite element method is proposed using a previously formulated version of porous body plasticity theory, which is intended for modelling compaction of powder materials. A number of compaction schemes are considered for articles with changes with respect to height. The effect of external friction and compaction scheme on the occurrence of compaction is analyzed. A compaction scheme whose use provides preparation of powder articles with uniform density distribution is studied separately. Theoretical results obtained are quite close to experimental data.     

Compressibility Moduli for Porous Materials Incorporating Volume Fraction

Traditional deformation parameters such as drained, unjacketed, and pseudo-bulk compressibilities are developed for a saturated porous medium, assuming they can be uniquely determined by measuring the liquid pressure, confining pressure, and changes in volume. Physically, however, the liquid volume fraction (porosity for a saturated porous medium) plays an important role, yet it is not directly measurable. In this paper the compressibilities are defined in terms of the experiments used to evaluate them and then mathematically related to the compressibilities of the liquid and solid phases and the volume fraction. The results can then be used to determine the effects of the volume fraction and compressibilities of each phase on the deformation properties of the porous medium. This theory is then shown to be a generalization of previous work, and comparisons are made with previously derived relationships of Zimmerman, Biot, and Gassmann.     

Numerical Modeling of the Compaction of Powder Articles of Complex Shape in Rigid Dies: Effect of Pressing Method on Density Distribution. 1. Mechanical Model of Powder Densification

A theory of plasticity of a porous body was formulated taking into account the specifics of powder behavior under pressing. The proposed model of the material under compression is one-parameter with all functions depending on the current density. In order to determine the parameters of the model, use was made of the equilibrium density attained during the compression of an unbonded powder body, that value of density beyond which further deformation is not accompanied by volume change. Methods for determining the material parameters of the model are described.     

为温压工艺设计的混合粉

在工业生产用压机的压制条件下，对为温压设计的经粘结剂处理的材料的性能进行了研究，定量地探讨了粉末温度对压制压力的一致性、稳定性和对零件的生坯与烧结件性能的影响。评价了混合粉对温度波动和生产中断的承受能力。     

Fabric Study of Granular Materials after Compaction

Numerous micromechanical models have been developed based on assemblies of spherical particles with certain fabric distributions. Most of these distributions are hypothetical, and only very few of them can be determined experimentally. This paper presents a study to provide some useful fabric information for granular material. The discrete element method is used to study the microscopic information for granular materials after compaction. Specimens with 520 identical ellipsoidal elements are generated and compressed under different conditions. Up to six different aspect ratios are used to study their effect on the compression process. Two different compression methods and five different microfrictions between particles are used. The fabric of the specimens after compaction, including the total number of contacts, the distribution of particle orientations, the distribution of branch vectors, the distribution of the length of branch vectors, and the spatial distribution of a similar length of branch vector, is presented. The relations between these fabrics and particle shape, microfriction, and the compression process are also developed.     

纳米硬质合金粉的桥接团粒对粉末成形影响

采用汞压仪和AMT-2400孔径分析仪,研究纳米硬质合金粉末\     

铝粉末压制过程有限元模拟研究

根据粉末材料的塑性理论及Shima-Oyane屈服准则,利用体积可压缩有限元法,采用MARC有限元分析软件对铝粉的单向压制和双向压制过程进行了数值模拟。得到了粉末材料在压制过程中的变形特征、相对密度分布及粉末质点的流动规律,得到单向压制和双向压制的密度分布图,并对其差异进行了分析。模拟结果对揭示粉末压制过程机理,制订压制工艺都有显著意义。     

Electrical Conductivity Changes During Compaction of Pure Iron Powder

Abstract

Comparisons are made between the use of traditional powder metallurgy and the newer forms of spray deposition over a range of applications. Overall, it is concluded that with spray deposition being more suited to the production of some components and established PM methods preferable for others, the technologies are complementary rather than competitive. PM/0265     

Compaction and Sintering Behavior of Niobium Powder for Making Consumable Electrodes

Niobium consumable electrodes of 10?mm dia?×?50?mm length were prepared by cold isostaic pressing with compaction pressure of 125?C250?MPa. The compacts were sintered under vacuum at different temperatures in the range of 1000?C1800?oC. The CIPed and sintered electrodes were characterized with respect to chemical analysis, density, microstructure and bend strength. The results showed that there is a marginal improvement in density at a sintering temperature in the range of 1000?C1400?oC, while a significant improvement in density was observed at sintering temperature in the range of 1600?C1800?oC. The bend strength of sintered electrode was found to increase with increasing sintering temperature and that attains a highest value at a sintering temperature in the range of 1600?C1800?oC. Sintering at a temperature higher than 1400?oC leads to decrease in oxygen content of the electrodes. The oxygen content was decreased from 2000 to 500?ppm when electrode was sintered at 1800?oC.     

Features of Compaction Kinetics for Powder Materials under Nonisothermal Conditions. Part 2. Existence of Critical Specimen Dimensions during the Preparation of Highly Compacted Materials

On the basis of a numerical solution of the problem for nonisothermal compaction and cooling of viscous compressible materials within a cylindrical chamber the effect of thermal factors and specimen geometrical dimensions on the characteristic times for two autonomous processes is analyzed: spatial propagation of a compression zone through a specimen from layer to layer and temporary additional compaction of material within the compression zone itself. It was revealed that the effect of retarding compaction (induction period) increases with distance of a specimen cross section from the moving piston as conditions for thermal insulation of the specimen ends worsen. The previously known experimental result of two critical specimen dimensions that govern conditions for preparing pore-free materials is confirmed and substantiated.     

粉末冶金温压成形技术的研究现状

粉末冶金温压成形,在得到较高致密度零件的同时,可以较铸造和锻造显著的降低原料成本,缩短零件的研制周期,具有重要的研究价值。本文概述了国内外温压成形技术的应用、发展现状及温压成形工艺的关键技术,指出了粉末冶金温压成形技术的发展方向。