Densification of Metal Powders During Sintering

241 Pages·1995·5.422 MB·English

by  V. A. Ivensen (auth.)

DENSIFICATION
OF METAL POWDERS
DURING SINTERING
STUDIES IN SOVIET SCIENCE
DENSIFICATION
OF METAL POWDERS
DURING SINTERING
v.
A.lvensen
All-Union Scientific Research Institute for Hard Alloys
Moscow, USSR
Translated from Russian by
Eric Renner
®
CONSULTANTS BUREAU -NEW YORK - LONDON-1973
Vladislav Aleksandrovich Ivensen was born in Moscow in 1908. In 1931 he was
graduated from the Institute of Fine Chemical Technology as a specialist in the
technology of rare metals. His first work centered on the alumino-thermal
method of obtaining carbon-free metals. Since 1936 his work has been con
cerned with the technology of hard alloys. He has been the director of one of
the laboratories of the All-Union Scientific Research Institute of Hard Alloys
since 1947. In the same year he received the degree of Candidate of Technical
Sciences for his work on the theory of sintering.
The original Russian text, published for Metallurgiya Press in Moscow in 1971,
has been corrected by the author for the present edition. This translation is
published under an agreement with Mezhdunarodnaya Kniga, the Soviet book
export agency.
KINETIKA UPLOTNENIYA METALLICHESKIKH POROSHKOV PRI SPEKANII
V. A. Ivensen
KHHETHKA ynnOTHEHHR METAnnH'IECKHX nOPOWKOB nPH CnEKAHHH
I;1BEHCEH BnaAI1CnaB AneKcaHAPoBI1'1
Library 01 Congress Catalog Card Number 72-94822
ISBN 978-1-4757-0108-1 ISBN 978-1-4757-0106-7 (eBook)
DOl 10.1007/978-1-4757-0106-7
© 1973 Consultants Bureau, New York
Softcover reprint of the hardcover 1st edition 1973
A Division of Plenum Publishing Corporation
227 West 17th Street, New York, N. Y. 10011
United Kingdom edition~published by Consultants Bureau, London
A Division of Plenum Publishing Company, Ltd.
Davis House (4th Floor), 8 Scrubs Lane, Harlesden, NW10 6SE, London, England
All rights reserved
No part of this publication may be reproduced in any form without written
permission from the publisher
Preface
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.
Many of the published reports deal with "models" of sinter
ed bodies (beads or wires sintered to each other or to a flat
plate, and also solid bodies with fine drilled holes). Such exper
iments have certain advantages (the possibility of exact quanti-
v
vi PREFACE
tative calculation of the change in the area of contact or change
in the diameter of pores}, but do not make it possible to evaluate
the basic processes that occur in sintering of real powders, the
densification of which is closely connected to the nonequilibrium
condition of the crystal lattice of the metal. On the other hand,
the investigations of the sintering of compacts made so far do not
give a very clear idea of the kinetics of densification, which would
result if the process could be isolated from the effect of outside
factors (the gas pressure in closed pores, for example). In some
cases densification occurs in a relatively short length of time,
which has frequently led to erroneous or exceedingly rough esti
mates of densification as a function of time (as will be shown later).
The empirical formulas that have been published satisfactorily
describe the course of densification during sintering for a long
period but not in the beginning of sintering.
In this author's opinion, the formulation of hypotheses and
theories of sintering based on strict physical concepts must be
preceded by a fairly complete phenomenological study of the pro
cess. The schematic and often unpersuasive nature of the theo
retical concepts is due precisely to the fact that the theoretical
formulations are based on an incomplete phenomenological de
scription of the process.
The author believes that the present condition of the theory
of sintering justifies a return to phenomenological investigations
of the process in order to reveal the laws and, if possible, a quan
titative description of densification during sintering. In solving
theoretical problems in sintering one cannot bypass the empirical
laws if they are fairly reliable, encompass a wide range of original
materials and sintering conditions, and are expressed by simple
mathematical functions. For this reason the examination of the
kinetics of densification during sintering will begin with a de
scription of the basic, most general laws of densification of powder
compacts during sintering.
Let us mention some characteristic features of the ter
minology used in this work. Although the terms "reduction in the
volume of pores" and "densification" are not synonymous, the
author sometimes substitutes one for the other. Both processes
are the same in essence, and the substitution of "densification"
for "reduction in the volume of pores" makes it possible to shorten
PREFACE vii
the text without interfering with the clarity of the presentation.
The author takes this liberty in order to avoid tedious repetition
of "reduction in the volume of pores."
For easier reference, the notation that is used in several
chapters is presented at the beginning of the book. For conve
nience, the equations are numbered by the following system: The first
number is the number of the chapter; the second number is the
numerical order of the equation in the chapter.
Where no other source is given, the experimental data used
in this work were obtained at the All-Union Scientific-Research
Institute of Hard Alloys by N. V. Baranova and L. P. Usol'tseva
under the direction of the author. A number of calculations, in
cluding the value of function FT in Table 27, were made by V. A.
Fal 'kovskii. The electron microscopic studies (Chapter VII) were
made by N. V. Baranova, N. P. Vasil'eva, N. F. Koval'skaya, and
T. A. Sultanyan.
Contents
Notation ~ 1
0 • 0 • 0 •••• 0 " " 0 0 0 0 0 0 0 0 0 II •••• 0 • 0 0 •• 0 0 D
Chapter I
Laws Governing the Relationship between the Initial and
Final Densities of Sintered Bodies • • • • • • • • • . • • • • .• 3
Chapter II
Conditions for Observing Densification Process in Pure
Form. 9
0 •••• 0 • 0 •• 0 • " • 0 •••••••• 0 •• 0 0 • 0 0 • 0 0 0
Chapter III
Volume of Pores in Relation to Isothermal Sintering Time •• 23
Chapter IV
Phenomenological Importance of the Constants of the Kinetic
Equation and Their Dependence on Temperature ....... 45
Chapter V
Basic Differences in the Densification Process in Crys-
talline and Amorphous Bodies . . • • • . • • . • • • • . • • • • • 57
Chapter VI
Change in the Surface and Volume of Pores under
Various Sintering Conditions • • • • • • • • • • • • • • • • • • • • 69
Chapter VII
The Flow of Metal under the Influence of Surface Tension
at Room Temperature ••.•••••..••••••••••••••• 85
Chapter VIII
Phenomenology of Sintering and Modern Theoretical
Concepts. .101
0 0 0 0 •• 0 0 II •• 0 0 •• 0 ••• 0 •••• 0 •• " •• 0
ix
x CONTENTS
Chapter IX
Quantitative Estimate of the Effect of the Geometric
Factor 113
0 • 0 • • 0 0 0 • 0 • 0 0 • • • • • • 0 • • • 0 • • • • • • • • • ••
Chapter X
Quantitative Estimate of the Effect of the Substructural
Factor. . . . . . . . . . o. 123
0 • 0 • 0 • • • • • • • • 0 • • • • • • 0 • •
Chapter XI
Phenomenological Theory of Sintering •• 135
0 0 •••• 0 • • • • • ••
Chapter XII
Calculating Densification from the Kinetic Constants of
the Powder. . . . . . . . . . . . 167
0 0 • 0 • 0 • 0 • • • • • • • • • • ••
Chapter XIII
Clarification of the Nature of Phenomenologically Ele-
mentary Processes and Unresolved Problems of Theory.. 195
Chapter XIV
Phenomenological Generalizations and Sintering Practice o. 213
0
Appendix
Method of Determining the Kinetic Constants of the Powder
and Calculating the Reduction in Volume of Pores and
Other Densification Characteristics from the Constants
of the Powder 225
0 0 • • • 0 0 0 • • • • 0 • 0 • • • • • 0 • • • • • • • ••
Literature Cited 237
0 • 0 •• 0 0 •••• 0 • 0 ••••• 0 ••• 0 • • • • ••
Notation
l linear dimension of the body
tl.l change in dimension during sintering
vp volume of pores in compact before sintering, cm3/g
vs volume of pores after sintering, cm3/g
v relative volume of pores, expressed as Vs /vp
relative volume of pores at beginning of isothermal sintering
(i.e., after constant sintering temperature is reached)
relative volume of pores at nominal beginning of isothermal
sintering (at time TO)
volume of porous body in absolute units, cm3
volume of porous body before sintering
volume of porous body after sintering
porosity expressed as volume percent of porous body
porosity before sintering
porosity after sintering
density of porous body, g/cm3
density of compact before sintering
density after sintering
density of solid (compact) substance (without pores)
relative density
relative density before sintering
relative density after sintering
surface of interconnected pores per unit mass of porous
body, cm3/g
surface of pores before sintering
surface of pores after sintering
gas permeability (volume of air reduced to 760 mm Hg pass
ing through a porous body with a cross section of 1 cm2
2 NOTATION
and thickness of 1 cm in 1 min at some constant difference
in pressure, given in the text)
Gp gas permeability of compact
Gs gas permeability after sintering
N concentration of defects (this value is used together with
factors a and b and without them; it has no dimensions)
aN the kinetic characteristic of the concentration of defects
determining the flow of an imperfect crystal, h-1
aNin the relative concentration (or kinetic characteristic of con
centration) of defects at the time isothermal sintering ac
tually begins
aNo the kinetic characteristic of the concentration of defects in
the original powder
Ea the activation energy of the elimination of defects, ca1jg-atom
Eb the activation energy of flow associated with lattice defects,
ca1jg-atom
the average rate of increase in temperature to the beginning
QI
of isothermal sintering, deg/h
T absolute sintering temperature
Tm absolute melting point
T isothermal sintering time, h
Tin the time from the beginning of heating to the actual beginning
of isothermal sintering
TO the time from the beginning of heating to the nominal begin-
ning of isothermal sintering (corresponds to the time co
ordinate of the point of intersection of the sloping and hori
zontal lines of the idealized temperature graph, Fig. 40)