F. Weinberg International Symposium on Solidification Processing. Proceedings of the F. Weinberg International Symposium on Solidification Processing, Hamilton, Ontario, August 27–29, 1990

401 Pages·1990·13.938 MB·English

by  J. E. Lait and I. V. Samarasekera (Eds.)

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PROCEEDINGS OF THE F. WEINBERG INTERNATIONAL
SYMPOSIUM ON SOLIDIFICATION PROCESSING,
HAMILTON, ONTARIO, AUGUST 27-29, 1990
F. Weinberg
International Symposium
on Solidification Processing
Editors
J.E. Lait
Research and Development
Stelco Steel
Hamilton, Ontario
I.V. Samarasekera
The Centre for Metallurgical
Process Engineering
The University of British Columbia
Vancouver, British Columbia
Symposium organized by the Basic Sciences Section of
The Metallurgical Society of CIM
29th ANNUAL CONFERENCE OF METALLURGISTS OF CIM
29e CONFÉRENCE ANNUELLE DES MÉTALLURGISTES DE UICM
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Foreword
This international symposium, to be held August 27-29, 1990 in Hamilton, Ontario, is in
honour of Prof. Fred Weinberg, who retired from The University of British Columbia in
June 1990, following a distinguished career. The symposium is an integral part of the 29th
Annual Conference of Metallurgists of The Metallurgical Society of The Canadian Institute
of Mining and Metallurgy.
Professor Fred Weinberg is a pioneer in research on solidification processing and has made
seminal contributions to the fundamentals of solidification. He has gained international recog-
nition for his work on continuous casting of steel, casting of non-ferrous metals and cast
iron, semiconductor and optoelectronic crystal growth, as well as for his research on the
high-temperature low ductility of steels, grain boundary studies and interdendritic fluid flow,
to mention but a few. He was a lecturer of the Institute of Metals and recipient of the Robert
H. Mehl medal of the AIME in 1975. For his outstanding career contributions he has also
received the Alean Award of The Metallurgical Society of CIM in 1988, and was made a
Fellow of the Metallurgical Society of the AIME in 1988, and a Fellow of The Canadian
Institute of Mining and Metallurgy in 1990. Professor Weinberg has published over 100 papers
and has received recognition for his work through several best paper awards, including the
Charles H. Herty Award of the AIME in 1974, Robert W. Hunt Award of the ISS-AIME
in 1980 and the Howe Medal of the AIME in 1979.
Professor Weinberg graduated from the University of Toronto in 1947 with a degree in en-
gineering physics, and was awarded an M.Sc. in 1948. He received a Ph.D. in 1951 for his
work on "Dendritic Growth of Metals" with the late Prof. Bruce Chalmers who was renowned
for his reserarch on solidification. Following his Ph.D., Dr. Weinberg joined the Physical
Metallurgy Research Laboratory of the Department of Energy, Mines and Resources in Ottawa
and was head of the Metal Physics Section between 1961 and 1967. He has been a professor
with the Department of Metals and Materials Engineering at The University of British Colum-
bia since 1967, and was head from 1980 to 1985. Professor Weinberg's dedication to research
and commitment to excellence has been a source of inspiration to all who have worked with
him. He has challenged and guided many graduate students contributing immeasurably to
their professional growth and has mentored numerous others. This symposium is a tribute
to Professor Weinberg—an outstanding educator and researcher.
The symposium consists of six sessions — Fundamentals of Solidification, Non-ferrous Cast-
ing, Continuous Casting of Steel, Static Casting of Cast Iron, Novel Solidification Studies,
and Semiconductor and Optoelectronic Crystal Growth — each a reflection of Professor
Weinberg's research interests. This world-class symposium owes its success to the quality
of the contributions, and we wish to express our appreciation to all the authors especially
to the invited speakers, Dr. N. Bryson, Prof. M. Flemings, Prof. H. Fredricksson, Prof.
I. Minkoff and Prof. R. Brown for their contributions. This distinguished group will enhance
the event with their knowledge of the subject and outstanding work. Thanks are also due
to the session chairmen, Prof. M.E. Glicksman, Prof. J.E. Gruzleski, Prof. J.K. Brimacombe,
Dr. H. Biloni, Prof. D. Apelian and Dr. G. Elliot for their efforts in soliciting papers and
organizing each session. Without their commitment this symposium would not have come
to fruition.
The financial sponsorship of The Metallurgical Society of CIM which enabled promotion
of the symposium is gratefully acknowledged. The sponsorship of the Basic Sciences Sec-
tion of The Metallurgical Society is also recognized. We would further like to express our
gratitude to Prof. G.A. Irons, Technical Program Chairman for the 29th Conference of
Metallurgists, for his efforts in integrating the symposium with the conference program, to
Prof. D.A.R. Kay, Conference Chairman, and to the Board of The Metallurgical Society
who encouraged and supported our endeavour to honour an eminent Canadian metallurgist.
J.E. Lait
Research and Development
Stelco Steel
I.V. Samarasekera
The Centre for Metallurgical Process Engineering
The University of British Columbia
June 1990
3
Recent research in solidification (Invited Paper)
F. Weinberg
Metals and Materials Engineering Department, The University of British Columbia,
Vancouver, British Columbia, Canada, V6T 1W5
Abstract
A number of areas in the general field of solidification have been recently
examined by the present author and his associates. These investigations
will be discussed with particular reference to the solidification problems
they present and, in some cases, possible solutions. The areas include the
columnar to equiaxed transition, large spangles in galvanized sheet,
solidification and segregation of nodular iron, magnetic fields in
semiconductor crystal growth, Cu in GaAs, corrosion at bicrystal grain
boundaries in AlCu, and foamed metals.
The present author and his associates have recently been investigating
a range of topics in solidification. In this presentation the topics will
be considered in turn. The basic problems in solidification encountered
will be outlined, and the problems discussed.
Columnar to Equiaxed Transition
The columnar to equiaxed transition is a classic problem, important in
both ferrous and nonferrous solidification. The transition involves many
of the basic features of solidification - nucleation, dendritic growth,
solute segregation, heat flow, fluid flow and others. If the columnar to
equiaxed transition can be made precise and reproducible, and a model
developed which quantitatively predicts when the transition occurs, using
known parameters, then experimental measurements can be made to
quantitatively verify the overall model and the constituent mechanisms.
Models of the columnar to equiaxed transition have been proposed, (Hunt
(1)) which predict when the transition occurs. However, they have not been
verified experimentally in castings, and they require input parameters
which can only be estimated. In a series of experiments the present author
and his associates have examined the columnar to equiaxed transition in
Sn-Pb (2), Al-Cu (3) and Pb-Sn (4).
In the Sn-Pb alloys, for directional solidification from a water cooled
copper chill, columnar to equiaxed transitions were observed which occurred
sharply and reproducibly, as shown in Figure 1. The position of the
transition could be moved toward the chill by reducing the rate of heat
extraction from the chill. Let us assume that nuclei are present in the
melt during solidification, and that the transition occurs when a nucleus
is ahead of a dendrite. This requires the liquid ahead of the dendrite tip
to be thermally or constitutionally supercooled. Since pure metals do not
exhibit a transition, and thermal supercooling is not generally possible
4 F. WEINBERG INTERNATIONAL SYMPOSIUM ON SOLIDIFICATION PROCESSING
where a positive thermal gradient exits, the supercooling can be attributed
to constitutional supercooling ahead of the dendrite tip. Such
supercooling has been measured by Burden and Hunt (5) in Al-Cu alloys, the
amount of supercooling being in the order of several degrees C for shallow
thermal gradients and normal freezing rates. As the thermal gradient ahead
of a dendrite tip decreases, the width of the supercooled region in which a
nucleus can grow increases. At some point there could be a high
probability that a nucleus is present in the supercooled region which grows
and results in the transition occurring.
To determine whether this concept could account for the position of the
transition, at least as a first step, the temperature gradient ahead of the
dendrite tip in the Sn-Pb alloys, determined by interpolation of measured
temperatures, was plotted as a function of distance from the chill and
related to the transition position. The results are shown in Figure 2.
The transition occurs when the temperature gradient in the melt at the
liquidus temperature is 0.11°C/mm for Sn-10 wt pet Pb and 0.10 and
0.13°C/mm for 5 and 15 wt pet. Pb respectively (2). In Al-3 wt pet Cu the
transition occurs near 0.66°C/mm (3). When TiB is added to Al-3 wt pet Cu
2
a fine grained structure is obtained with gradients much larger than
0.06°C/mm. The amount of TiB« required is small, about 171 ppm, below
which columnar growth occurs. It is proposed that a very large density of
nuclei are produced by the TiB« in the melt. The probability of nuclei
being present in the supercooled layer ahead of an advancing tip is high.
These nuclei grow blocking further dendritic growth. This continues as the
new grains start to grow as columnar grains. Accordingly the critical
parameters when a nucleant is added to a melt are the density of nuclei,
and their ability to grow at small supercoolings.
Since the temperature and solute gradients being dealt with are small,
a heat transfer mathematical model of the solidification system offers a
means of establishing accurate temperatures, temperature gradients, and
growth velocities in the non-steady state conditions being considered.
This was done in the Sn-Pb and Al-Cu systems but the measurements could not
be fitted to the model predictions. Further modelling and experiments have
been done with care in the Pb-Sn system aiming for an accuracy of 0.2°C in
predicting temperatures during solidification. It was found that the
boundary conditions related to heat flux across the alloy surfaces could
not be defined well enough to produce the accuracy required.
If the assumption is made that the columnar to equiaxed transition occurs
when a critical small gradient is reached ahead of the dendrite tip, and
heterogeneous nuclei are present in the melt, what is the effect of fluid
flow produced by electromagnetic stirring or buoyancy flow on the
transition? Increasing fluid flow would tend to decrease the thermal
gradients in the liquid and produce a transition sooner in a given thermal
field, as is observed. This differs from the normal assumption that fluid
flow generates nuclei by a dendritic secondary branch remelting process,
producing the transition.
Hot Dipped Galvanized Steel Sheet
Although hot dipped galvanizing has been used industrially for many years,
the solidification of the galvanized layer - particularly the growth of
large spangles - is not clearly understood.
F. WEINBERG INTERNATIONAL SYMPOSIUM ON SOLIDIFICATION PROCESSING 5
Fig.l. Etched vertical sections of ingots showing Fig.2. Temperature gradient vs distance from the chill
the columnar to equiaxed transition with 3 in Sn 10 pet Pb with different heat transfer coefficients.
superheats: (a) 19°C, (b) 31°C, (c) 36°C. Sample Vertical lines are CET positions for tests 7,12,10 & 14.
width 35 mm.
Spangles are very large, faceted, dendritic grains. An example of a spangle of 13mm diameter
is shown in Figure 3. Much larger spangles can be obtained in commercially galvanized
sheets. In an investigation by the present author and his associates (6) a number of features
concerning spangle growth were observed.
1) Spangle growth is dependent on bath composition. Spangles from when at least 0.04
wt pet of Pb, Bi or Sb is added to the galvanizing bath. They do not form when the
same amount of Mg, Sn or Cd are added.
2) The growth of the large dendrites producing the spangle is not due to an increase in
melt supercooling. Melt supercooling is small - less than 1°C generally.
3) Dendrites in hexagonal zinc grow in the basal plane. If dendritic growth in the spangle
is confined to the basal plane, the spangle surface will have to be closely aligned to the
basal plane since the galvanized layer is about 40 μπι thick. This is not the case. The
basal planes of spangles are tilted at large angles with respect to the spangle surface.
4) When a galvanizing bath which produces spangles on a galvanized sheet is solidified in
bulk, spangles do not form.
One can speculate on the mechanism of spangle formation.
The size of the spangle is determined by the velocity of the primary dendrite branches forming
the skeleton of the grain. Large spangles result from high dendrite velocities. Adding a small
amount of Pb, Bi or Sb to the melt increases the dendrite velocity by decreasing the dendrite
tip radius. Each of the added elements have a very small segregation coefficient in Zn (less
and 0.01) which would result in a high solute concentration ahead of the dendrite tip. The
decrease in dendrite tip radius results from the relatively low surface tension of the additions.
The relationship between grain diameter and surface tension of the six additions considered is
shown in Figure 4.
The large title angle of the basal plane with respect to the spangle surface, combined with the
sharply linear primary dendrite spikes in the basal spangle, indicate growth occurs in
crystallographic planes but not in crystallographic directions. The directions are confined by
6 F. WEINBERG INTERNATIONAL SYMPOSIUM ON SOLIDIFICATION PROCESSING
the melt surface. The mechanism by which this occurs is unclear.
The observation that large spangles form in the galvanized layer, and not
in the bulk, for a given melt could be accounted for by assuming dendrites
have higher velocities along melt surfaces. This was observed to be the
case for water (7) where the dendrite velocity increased from 0.3 cm/s in
free growth to 1.2 cm/s on a glass rod and 3.0 cm/s on a brass plate.
Whether a similar effect occurs for Zn dendrites growing at the melt
surface is not clear. We note that with the basal plane tilted with
respect to the melt surface, some primary dendrites will grow along the
melt/air interface, others along the melt/steel interface.
Nodular Cast Iron
A. The nucleation and growth mechanism for nodules in cast iron is still
not clearly defined. The role of magnesium added to the melt could be to
act as a getter for elements which contribute to the flake form of the
graphite in grey cast iron, or itself could directly affect the growing
graphite particle. If it is the latter, then one might anticipate that Mg
would be distributed throughout a nodule since it would be continually
acting on the nodule surface.
Experiments using secondary ion mass spectrometry (SIMS) have shown that
Mg, at low levels, is uniformly distributed throughout the nodule (8)
indicating the prime role of Mg is not gettering. This does not clarify
how Mg atoms modify the growth morphology of graphite to produce spheres
instead of flakes. The effect of impurity elements present in the melt on
growth morphologies in metals is not clearly understood.
B. The segregation of alloying elements in nodular iron has been
examined, primarily using microprobe analysis. The results show a number
of interesting features (9).
1) Defining an effective segregation coefficient as the ratio of solute
concentration at the centre of an initial primary dendrite branch, to
that of the solute melt concentration in which the dendrite is growing,
the effective segregation coefficients differ markedly from the
equilibrium values taken from Fe based binary diagrams. Cu, Ni and Si
have equilibrium coefficients of 0.62, 0.75 and 0.87 which increase to
F. WEINBERG INTERNATIONAL SYMPOSIUM ON SOLIDIFICATION PROCESSING 7
effective coefficients of 1.37, 1.23 and 1.09 respectively. On the
other hand Cr and Mo with equilibrium coefficients of 0.99 and 0.53
decrease to effective coefficients of 0.60 and 0.26 respectively. Mn
shows little change, from 0.64 to 0.70 for equilibrium to effective
segregation coefficient. The results clearly indicate that solutes
cannot be considered individually in a melt component system when
considering segregation during solidification. It is not clear how the
various solutes interact, or how to determine the interaction from first
principles for such a complex system.
2) Using the effective segregation coefficients, and the Scheil equation, a
reasonable correlation could be obtained for the solute distribution of
each element, measured by microproble analysis and the calculated
distribution. It thus appears segregation can be determined on an
individual basis.
Crystal Growth of GaAs
A) LEC GaAs has a high dislocation density, 10* to 105/cm^ being typical
values. Much effort is being directed towards reducing the dislocation
density by varying the growth conditions.
In metals, the dislocation density can be markedly reduced by high
temperature annealing, or by annealing under an applied stress. This does
not appear to be the case for GaAs.
An investigation was undertaken (10) to determine to what extent as-grown
dislocations moved and annihilated in GaAs as a result of annealing and
stress/annealing. The dislocations were observed by cathodoluminescence.
The results showed the following:
1) At annealing temperatures of 1000°C (0.81 of the melting temperature)
the as-grown dislocations did not move.
2) With a small stress, both static and cyclic, the as-grown dislocations
did not move.
3) The applied stress results in slip occurring in the GaAs producing slip
lines on the surface consistent with normal slip in an F.C.C. system.
The as-grown dislocations disappeared when local slip occurred.
4) The inability of the as-grown dislocations to move cannot be readily
attributed to impurity locking, since the residual impurity level in the
GaAs wafers is very low. It is postulated that movement is inhibited
because dislocation climb is inhibited by the structure. Edge
dislocations terminate on a Ga or an As plane. As a result climb would
require the edge to move through alternate Ga and As planes which
appears unlikely.
B) Crystal Growth - Magnetic Fields
Efforts are continuing by producers of LEC GaAs crystals to improve crystal
yield, and crystal quality. This is being done by modifying the growth
conditions. Because there are many subtle growth variables contributing to
crystal quality, and growth is done under high pressure, it is difficult
and expensive to conduct extensive experimental programmes. As a result
much effort has been directed to mathematical modelling of the process,
both related to the growth process, and the generation of dislocations
during growth (11). The modelling of dislocation generation indicates
where and when dislocations are generated during growth, and how this
varies with the most important growth variables.