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1973 | Buch

Notation Kapitel lesen Erstes Kapitel lesen

Densification of Metal Powders During Sintering

verfasst von: V. A. Ivensen

Verlag: Springer US

Buchreihe : Studies in Soviet Science

Enthalten in: Professional Book Archive

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Über dieses Buch

Sintering of powder metal compacts is one of the basic oper­ ations in powder metallurgy. The useful properties of a machine part are obtained after considerable densification of the sintered material. Although the mechanical properties of the part depend on other structural factors besides porosity, porosity is the main factor. Usually, the practical problem in sintering is to obtain a part with the desired or permissible porosity. Thus, knowledge of the laws governing densification and its final result is neces­ sary to control this process in the production of powder metal parts. The laws governing densification are also important for a more exact physical theory of sintering, which is still in the initial stages of its development. Such processes as the change in the density of lattice defects and the flow of crystalline substances during sintering have not yet received a complete physical inter­ pretation. Analysis of the laws of sintering may provide addition­ al material for more complete phenomenological characteristics of these processes that will be useful for further development of theoretical concepts of the flow of imperfect crystals under small loads. Although a substantial amount of experimental material has been accumulated, generalizations are still difficult.

Inhaltsverzeichnis

Frontmatter
Notation
V. A. Ivensen
Chapter I. Laws Governing the Relationship between the Initial and Final Densities of Sintered Bodies
Abstract
A review of the laws governing densification during sintering should begin with the constant specific reduction in the volume of pores, which was established twenty years ago [1]. When porous bodies compacted from a given powder at different pressures, and differing in initial porosity, are sintered under the same conditions, the volume of pores is the same in all bodies in most cases. If Vp and Vs are the volumes of pores before and after sintering, then Vg/Vp = const (on condition that densification is not disturbed by associated processes that affect the volume of the sintered body).
V. A. Ivensen
Chapter II. Conditions for Observing Densification Process in Pure Form
Abstract
In plots of vs/vp vs. dp one observes both random and systematic deviations from constant values of vs/vp. The first may be due to nonidentical sintering conditions of bodies with different initial porosities and other accidental circumstances. Systematic deviations that are consistently observed in sintering of bodies from a given powder are usually closely connected with the structural characteristics. In most cases the reason for the deviation from a constant value of the specific reduction in volume of pores can be determined.
V. A. Ivensen
Chapter III. Volume of Pores in Relation to Isothermal Sintering Time
Abstract
An important stage in studying the mechanism of a complex physical process is the investigation of its kinetic characteristics by observing the changes with time of the parameters characterizing the development of the process. Although measuring the reduction of linear dimensions or the volume of a sintered body with time presents no experimental difficulties, the investigation of the kinetics of densification of powders is far from complete. Most investigations have been limited to qualitative or semiquantitative descriptions of the process, and only in recent years have empirical formulas been presented with constants that have no clear physical or phenomenological interpretation.
V. A. Ivensen
Chapter IV. Phenomenological Importance of the Constants of the Kinetic Equation and Their Dependence on Temperature
Abstract
Replacing v in Eq, (III-4) with vin (volume of pores at the beginning of isothermal sintering) or τ = 0 in Eq. (III-14), we find that q = -dv/dτ • It follows that constant q is the rate of reduction in the volume of pores at the beginning of isothermal sintering» Its dimension is h-1.
V. A. Ivensen
Chapter V. Basic Differences in the Densification Process in Crystalline and Amorphous Bodies
Abstract
The general rule observed in sintering of metal powders — the proportionality of the rate of reduction in the volume of pores to the mth degree of reduction in the volume of pores attained — obviously reflects several characteristic features of the densification mechanism. Additional information on the characteristics of densification in crystalline powders may permit comparison of the laws governing densification of amorphous and crystalline bodies.
V. A. Ivensen
Chapter VI. Change in the Surface and Volume of Pores under Various Sintering Conditions
Abstract
In the preceding chapter we examined the phenomenological manifestations of the two elementary processes — bulk flow and elimination of imperfections in crystals — that are primarily responsible for the course of densification. During sintering there are signs of the development of still another elementary process — migration of the substance along the surface of pores (self-diffusion surface). The outward manifestation of this process is a substantial increase in the gas permeability of porous bodies after sintering at low temperature, which is due to the smoothing out of the surface relief and the simplification of the shape of interconnected pores.
V. A. Ivensen
Chapter VII. The Flow of Metal under the Influence of Surface Tension at Room Temperature
Abstract
Of considerable interest among the various manifestations of the dependence of the flow rate of crystalline materials on the concentration of imperfections is the high flow rate of imperfect crystals of metals obtained by electrolysis of salt solutions at a high current density. The dendrite-like crystals obtained under these conditions undergo a change in shape under the influence of surface tension at room temperature. Observations of the changes in the shape of dendrite-like crystals and the gaps (fissures) between them can yield important additional information on the nature of elementary processes developing during sintering.
V. A. Ivensen
Chapter VIII. Phenomenology of Sintering and Modern Theoretical Concepts
Abstract
For a quantitative estimate of the effect of the elementary processes on densification it is first necessary to describe them quantitatively as independent processes, after which one can attempt to establish (quantitatively) their interdependence and their effect on densification. The problem would be completely solved if it were possible to describe these processes on the basis of modern physical concepts of lattice imperfections and their part in the flow of a crystalline substance. However, as will be indicated below, the attempts made up to the present time have not been successful — at best they give some qualitative explanation, but no quantitative description.
V. A. Ivensen
Chapter IX. Quantitative Estimate of the Effect of the Geometric Factor
Abstract
The characteristics of densification of porous crystalline substances previously considered reflect mainly the effect on the flow rate of lattice defects in crystalline substances (substructural factor). The densification rate also varies with the distribution and magnitude of capillary pressures associated with the shape, size, and relative concentration of pores. The effect of the geometric factor (geometric characteristic of the pores) is reflected in the variation of the densification rate with the dispersity of the powder.
V. A. Ivensen
Chapter X. Quantitative Estimate of the Effect of the Substructural Factor
Abstract
The quantitative characteristic of the change of flow with time during sintering due to the influence of the substructural factor can be found by means of comparing the general densification law expressed by Eq. (III-14), reflecting the combined effect of the geometric and substructural factors on the densification process, with Eq. (IX-4), characterizing the effect of the geometric factor alonC. This method made it possible to propose a mathematical description of the change of flow with time about 15 years ago [56].
V. A. Ivensen
Chapter XI. Phenomenological Theory of Sintering
Abstract
Mathematical formulations accounting for the effect of the geometric and substructural factors reflect the kinetics of elementary processes determining the course of densification. From these data it is possible to work toward a more complete description of the densification process, including its temperature dependence.
V. A. Ivensen
Chapter XII. Calculating Densification from the Kinetic Constants of the Powder
Abstract
During isothermal sintering the reduction in volume of pores, as was shown in the preceding chapter, is described by Eq. (XT-6), reflecting the basic characteristics of the kinetics of densification. However, densification during isothermal sintering constitutes only part of the total densification attained in sintering. A substantial portion of the total densification occurs in the period of rising temperature. The rate of reduction in volume of pores at the beginning of isothermal sintering, and the general course of subsequent densification along with it, depends not only on the temperature but also to a considerable extent on the conditions of heating to isothermal sintering temperature. Therefore, for wider use of the kinetic equation (not only for phenomenological analysis of sintering but also for practical purposes) it is necessary to determine quantitatively the changes that occur in the sintered body before isothermal sintering begins.
V. A. Ivensen
Chapter XIII. Clarification of the Nature of Phenomenologically Elementary Processes and Unresolved Problems of Theory
Abstract
The phenomenological analysis of elementary processes that occur during sintering leads to a number of conclusions on the lattice defects responsible for accelerated flow of a crystalline substance:
1.
These defects constitute a small fraction of the defects in real crystals.
 
2.
The defects responsible for accelerated flow may differ substantially in their energetic characteristics for different powders. The activation energy of the elimination of defects may vary within wide limits, depending on the conditions in which the powder was obtained.
 
3.
The activation energy of the elimination of defects is independent of the concentration of defects. It is a constant for a powder obtained by a given method.
 
4.
The process of the elimination of defects responsible for accelerated flow obeys second-order kinetics.
 
V. A. Ivensen
Chapter XIV. Phenomenological Generalizations and Sintering Practice
Abstract
Densification during sintering of metal powders, if its development is not disturbed by other processes or external influences, obeys the simple laws described above. These laws also extend to powders of carbides, nitrides, and borides of refractory metals.
V. A. Ivensen
Backmatter
Metadaten
Titel
Densification of Metal Powders During Sintering
verfasst von
V. A. Ivensen
Copyright-Jahr
1973
Verlag
Springer US
Electronic ISBN
978-1-4757-0106-7
Print ISBN
978-1-4757-0108-1
DOI
https://doi.org/10.1007/978-1-4757-0106-7