Pressing Of Metal Powders Research Articles

Direct multiscale modeling of cold pressing of metal powders

A new numerical approach is proposed to model powder pressing. It is called direct multiscale modeling. In direct multiscale modeling, each macroscopic finite element has a corresponding microstructure regarded as a powder representative cell. On the assumption that the strain rate of the macroscopic finite element and average strain rates of the representative cells are equal, the kinetics of macroscopic deformation and microscopic distortions of the representative cells is determined at every instant. The problem dealing with the effect of the initial spheroidization of intraagglomerate pores on the compaction of agglomerated powders is considered as an example. The new approach permits assessing the effect of the initial intraagglomerate pore shape on the closure kinetics of interagglomerate pores in pressing.

Constitutive Modeling for Hot Isostatic Pressing of Metal Powders

Constitutive Modeling for Hot Isostatic Pressing of Metal Powders

Modelling of Hot Isostatic Pressing of Metal Powders

Modelling of Hot Isostatic Pressing of Metal Powders

New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.The precipitation of β-FeC2O4.2H2O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.Acicular shaping of the β-FeC2O4.2H2O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.

Industrial Use of the High-Rate (Impact) Pressing of Metallic Powders

Features of the high-rate (impact) pressing of metal powders are discussed. It is shown that impact pressing compacts powders to a relative density which is 97% of the theoretical value after a single loading. The pressure which exists during high-rate pressing is usually higher than the static pressure for the same density of the pressed powder. The force which presses the products out of the die is 1.5-2.5 times lower than in static pressing. Preliminary pressing of the powder reduces friction losses and ensures a more uniform distribution of density over the height of the product. Recommendations are given on the loading rate in high-rate pressing and on the use of this operation in relation to the shape of the product. An example is given to illustrate the use of high-rate pressing for making large-diameter gaskets from copper powder.

Hot Isostatic Pressing of Metal Powders

Hot Isostatic Pressing of Metal Powders

Tungsten-free die steels with increased thermal stability

1. An austenitic dispersion-hardened chromium manganese die steel type 5Kh10G15SM2F2R has been developed whose thermal stability is 160–190°C higher than for the martensitic class steels 4Kh4VMFS and 5Kh2MNF used currently, for similar purposes. 2. Strengthening of steel 5Kh10G15SM2F2R occurs as a result of precipitation of carbide phases type VC, Cr23C6, and Mo2C during aging. 3. Steel type 5Kh10G15SM2F2R is recommended for hot-forming tools for alloys that are difficult to work, liquid forming of copper alloys, and for hot pressing of metal powders whose operating temperature does not exceed 830–850°C at pressures up to 700–800 MPa.

Compactibility and strength of rolled strip from powders (review)

M. B. Shtern, G. G. Serdyuk, L. A. Maksimenko, et al., Phenomenological Theories of Powder Pressing [in Russian], Naukova Dumka, Kiev (1982). G. M. Zhdanovich, Theory of Pressing of Metal Powders [in Russian], Metallurgiya, Moscow (1969). G. Ya. Gun, Theoretical Principles of Plastic Working of Metals [in Russian], Metallurgiya, Moscow (1980). A. V. Stepanenko, L. A. Isaevich, A. A. Veremeichik, and T. A. Medvedeva, "Theoretical investigation of the process of cold extrusion of rods from unplasticized metal powders. I. Distribution of porosity and stresses in the conical seat of plastic deformation," Poroshk. Metall., No. 5, 6-10 (1987). A. V. Stepanenko, L. A. Isaevich, and A. A. Veremeichik, "Theoretical investigation of the process of cold extrusion of rods from unplasticized metal powders. II. Strain hardening of the material of the powder," Poroshk. Metall., No. 6, 12-15 (1987). M. V. Storozhev and E. A. Popov, Theory of Plastic Working of Metals [in Russian], Mashinostroenie, Moscow (1977). E. Thomsen, C. Young, and S. Kobayashi, Mechanics of Plastic Strains in the Working of Metals [Russian translation], Mashinostroenie, Moscow (1969). G. M. Zhdanovich, Some Problems in the Theory of the Process of Pressing of Metal Powders and Their Mixtures [in Russian], Belorussk. Politekh. Inst., Minsk (1960). V. B. Shatilov and Yu. S. Volkov, "Determination of the porosity of P/M parts," Zavod. Lab., 38, No. 6, 719-720 (1972).

An empirical model for hot isostatic pressing of metal powders

A new mathematical model is proposed to describe time dependent pressurization and densification of metal powders at constant temperature. It is a first order ordinary differential equation in the pressure and a densification measure, which incorporates instantaneous response, equilibrium response, and creep response. The model is based, to some extent, on micromechanical (hollow sphere) analysis. However, the development is mostly empirical, based on previously published experimental data on hot isostatic pressing of lead and tin powders. These data show that an initial rapid change in pressure (30 seconds rise time) is accompanied by equally rapid change in relative density. This suggests that a creep law may not be adequate to describe the response to rapid change of pressure. Accordingly, the pressure rate is included in the proposed model. The data support the assumed linear instantaneous and equilibrium response laws and the exponential creep response law. Agreement between theoretical creep curves and experimental data is very good.

Experimental investigation of strains in the triaxial pressing of metal powders by the dividing grating method

The dividing grating method makes it possible to study the distribution of strains and compressibility of materials in the course of pressing of metal powders when the cell size is chosen in accordance with Eq. (2). Investigations with 0.5-mm-base gratings of pressing in different densification steps have revealed zones of uneven microdeformation, whose volume decreases with rise in the mean level of specimen strain. The fact that in the triaxial pressing of metal powders a strain distribution close (on a macrolevel) to homogeneous is obtained confirms the possibility of producing homogeneous blanks of uniform density by this method. Use of dividing gratings with different bases enables strains experienced by powder materials in the course of shaping to be investigated at different structural levels. An investigation, conducted with the aid of a grating with a 0.1-mm base, into the pressing of PMS-2 copper powder revealed the presence in local volumes of substantial angular distortions attributable to asymmetry of the microstress tensor. The maximum of the parameter of quasihomogeneity of PMS-2 copper powder was found to attain 0.876 mm. In the course of densification it decreased to 0.185 mm when the porosity of compacts did not exceed 0.02.

Stresses in the punch in the impact pressing of metal powders

The most heavily stressed element of the pressing tool, operating under the most severe conditions, is the upper punch. A dominant influence on the operating life of the punch is exerted by the stress set up in it during impact. In the most heavily stressed, upper part of the punch the compressive stress is a minimum when the striker and punch diameters are equal. The stress diminishes with increasing punch length. Use of a striker with a spherical end face produces a substantial stress concentration in the contact zone, which may lead to the fracture of the punch. Decreasing the radius of the sphere significantly reduces this stress. With increasing impact velocity, the stress in the punch grows. In each specific case the maximum permissible impact velocity is limited by the maximum stress that the punch can withstand.

Energy expenditure in the hot isostatic pressing of metal powders

Energy expenditure in the hot isostatic pressing of metal powders

Apparatus for the hot isostatic pressing of metal powders

Apparatus for hot isostatic pressing has been classified according to nature of the pressure-generating medium, design features, and method of pressure generation. A procedure is described for designing HHP mechanisms in which the blank is in direct contact with the working medium. An expression is derived for determining the maximum pressure in the chamber and the container wall thickness with allowance for the temperature field. Calculations are made of the stresses set up in the walls of the heated container subjected to the action of external and internal pressures.

Apparatus for studying the process of triaxial pressing of metal powders

Apparatus for studying the process of triaxial pressing of metal powders

Plasticity criteria for porous metals

Use of the theory of plasticity for calculations of the pressing of metal powders and of the forging of sintered blanks in powder metallurgy opens up many possibilities [i]. Developments in this field may be said to have commenced in the late1960s and early 1970s, for although the working of porous media such as soils and rocks had been studied long before that period, it was only then that this approach began to be adopted in investigations into the working of porous metals.

Energy and force conditions of hydrostatic pressing of blanks from powder materials

An examination is made of the hydrostatic pressing of metal powders. The total work done in this process is made up of six components. Their determination and methods of calculating them are described. Equations are proposed for calculating the work done in overcoming friction in the gland of the hydrostatic pressure vessel plunger. The equations allow for the variation of the coefficient of friction in the gland, which is a function of pressure in the vessel. Energy expenditure in hydrostatic pressing determined by calculation is in accord with experimental data. This shows that the equations proposed can be employed for choosing plant in the development of new technological processes for the production of parts from metal powders by hydrostatic pressing.

Calculated coefficient of external friction in the pressing of metal powders

On the basis of the molecular-mechanical theory of friction of solids a relationship has been obtained which makes it possible to utilize the known frictional parameters of dense metals and determine by calculation coefficients of external friction in the pressing of metal powders. A check revealed good agreement between data yielded by the theoretical relationship between the coefficient of friction and pressing pressure and experimental results for PO2 tin and PMS-2 copper powders.

Major advances made in the field of hot pressing of metal powders

Major advances made in the field of hot pressing of metal powders

Coefficient of external friction in the pressing of metal powders

Experimental data on the external friction in the pressing of copper and iron powders have been processed graphicoanalytically, using Deryagin's binomial law, and the mechanical and adhesional components of the coefficients of friction in the pressing of these powders have been determined. A rise in pressing pressure decreases the static coefficient of friction by reducing its adhesional component.

Some features of the vacuum pressing of metal powders

Some features of the vacuum pressing of metal powders