الفهرس 
Heat transfer coefficientOnly 14 pages are availabe for public view
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Abstract
The cast structure parameters, as grain morphology (columnar, equiaxed or mixed) and size, have a
significant influence on casting mechanical properties. These parameters are affected by
solidification conditions. The cast structure determines the degree of chemical inhomogeneity in
the cast material, which affects the mechanical properties. Therefore, the study of the factors
affecting the as-cast structure is important for controlling the mechanical properties.
The aims of the present work contribute to the understanding of the columnar and equiaxed structure
formation and the influence of structure on mechanical properties.
The unidirectional solidification technique was used to produce samples with different columnar and
equiaxed zones under controlled solidification condition.
As the transition from columnar to equiaxed morphology (CET) is not yet completely understood, a
one dimensional heat flow model combined with solidification kinetics was developed to explain this
phenomena. The model was used to define the conditions at which the transition occurs. Another
two-dimensional heat flow model considering heat transfer across specimen in addition to the heat
transfer along the specimen was also developed to obtain more accurate cooling curves and to define
the solidification parameters at different positions through the specimens. This has helped to
explain the cast structure obtained.
The materials used in the investigation were aluminium and aluminium copper alloys with Cu wt %I, 3
and 4.5, prepared using a commercial purity aluminium 99.85%, and a high purity copper 99.99%. Two
groups of experiments were conducted to provide the details needed
for the two models. In both cases, the cooling curves were accurately monitored using a
data acquisition system.
The samples of the first group, designed for studying cooling curve analysis and mechanical
behaviour, were directionally solidified against copper chill to form cylindrical specimens of 50
mm in diameter and 150 to 200 mm long using specially constructed experimental set-up. The melt
superheat varied between 50 to 200 k.
The second group of directional solidification experiments, designed for studying CET, produced
cylindrical specimens with 12.5 mm m diameter and 200 mm long. The process variables were also
different.
Morphological measurements were made on longitudinal sections to define the extension of the
columnar and equiaxed zones. The grain size and the volume fraction of the eutectic phase
surrounding primary phase have been measured.
The parameters estimated from the analysis of the cooling curves and the models calculations were
correlated with the columnar and equiaxed measurements.
from the present work, data on the heat transfer coefficient and its variation with the
solidification condition are obtained. These data are of utmost importance for the practical
solidification studies. A contribution to the description of columnar and equiaxed structures is
also obtained .
. A compression test was performed in order to determine a formability index for each structure
type. The effect of the different parameters affecting the structure on the formability index was
discussed.
from the results, it is concluded that the heat transfer coefficient is affected strongly by the
air gap formation and the transition from columnar to equiaxed structure occurs at a critical
temperature gradient,
(G) equals 6 K/mm and growth rate, ( R) that ranges from 0.6 to 0.9 rn!sec.
Also, the volume of fraction of eutectic phase increases with increasing cooling rate. The columnar
structure in the longitudinal direction has the highest values of strength coefficient (K) and
strain hardening exponent (n) followed by the equiaxed structure. The columnar structure in the
transverse direction has the lowest values.