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Yan Y., Yang G., Mao Z.
Thus it is common in investment castings of
steels and nickel-based alloys, especially when these
are cast into hot moulds at temperatures near
1000C, and even more when these moulds are
backed by insulating material, all at this high
(b) Sr modified
(c) Na modified
Figure 5.42 Portions of the growth front of the AI-Si eutectic in (a) the unmodified condition, and modified with
(b) strontium and (c) sodium (based partly on Flood and Hunt 1981).
by the filling of a boss in the cope will often also
cause some penetration in the drag surface too.
The point discontinuity shown in Figure 2.27 will
be a likely site for metal penetration into the mould.
If the casting is thin-walled, the p
zone the percentage of air increases, until it reaches
100 per cent air in the external zone.
It i s found that similar evaporation a n d
condensation zones are present for other volatiles
in the greensand mould mixture. Marek and Keskar
Figure 2.10 Fracture surfaces showing ( a ) a fairly new thin film on an A1-7Si4.4Mg alloy; (h) a film on
an A1-5Mg alloy (courtesy Divandari 2000).
Figure 2.11 Multiply ,foldedfilni on the ,fructure surfuce of an AI-7Si-0.4Mg alloy
and 1 mm respectively, y = 1.9 Nm-'? and p = 5714
kgm-3, and the height of the sprue in
of serious defects. (The unconstrained filling of
moulds without risk can only be achieved by
Both of these kinds of streams exhibit surfaces
that are effectively stationary, and thus grow a thick
oxide. When the rising melt
out using chemically bonded sand moulds to make
test castings as small slabs with overall dimensions
approximately 50 mm high, 40 mm wide and 15 mm
thick. The parallel faces of the slabs allowed X-ray
examination without further preparation.
However, for investment casting the ceramic shell
allows a complete range of temperatures to be chosen
without difficulty. From Equation 3.3 it is seen
that the freezing time is proportional to the difference
between the freezing point of the
Figure 6 1 Thin slice of an AI.1
7Si-O.4Mg alloy casting taken
from around a phenolic urethane
bonded sand core: (a) a general
view, showing the sand-cast
surface made by the core and
several subsurface pores; and
6.1 Nucleation of gas porosity
Although the problem of the nucleation of cavities
and bubbles is in principle similar to that of the
nucleation of dense phases such as inclusions, there
are differences that make it worthwhile to loo
Hydrogen content (mVl00 g)
because this part of the bifilm will open first, having
the largest area to gain gas from solutio
126.96.36.199 Entrained bubbles
When the liquid metal tumbles into the mould in a
ragged fashion, it necessarily entrains air or other
mould gases. These will attempt to rise and separate
from the metal if they have time and if they are not
No back-wall echo could be seen amid the fog of
scattered reflections. However, in the early days of
the Cosworth process, with long settling time of
the liquid metal, and quiescent transfer into the
mould, suddenly back-wall echoes could be
Table 7.1 Solidification shrinkage for some metals
create a pore by nucleation in the interior liquid. In
this case there is clearly no connection to the outside
surface of the casting, as illustrated in the larger
section shown in Figure 7.18. After nucleation,
further solidification will pr
-c crack length
.-. . .
to be expected on any new casting. The reverse
camber was then constructed into the mould to
give a straight casting. Although Longdens
nomogram is probably somewhat specific to his
type of machine tool bases, it is presented in Figure
the sand grains that confers the improved smoothness
to the cast surface. The action is that shown in
Figure 2.2 for all film-forming alloys.
History now appears to have turned full circle
because some resin binders for sands have recently
Figure 5.24 Computer simulated
macrostructure of growth inwards
from the sides of an ingot for
progressively increasing casting
temperature (a) to (d). Reprinted
with permission from J. Materials
Science, Chapman & Hall, London.
Liquid silver is analogous to copper in that it
dissolves oxygen. In terms of the Ellingham diagram
(Figure 1.5) it is seen that its oxide, Ag20, is just
stable at room temperature, causing silver to tarnish
(together with some help from the p
It is instructive to compare the fluidity observed
for semi-solid (strictly partly solid) and metal
matrix composite (MMC) cast materials. Such
materials have a viscosity that may be up to 10 to
50 times that of the parent liquid. However, the