Page 272 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

molten metal flows into the thicker regions Section 10.6 Defects 25 I

Cash 1
that have not yet solidified. Porous regions S d

may develop at their centers because of con-
traction as the surfaces of the thicker region (b)
begin to solidify first. Microporosity also can Sand
develop when the liquid metal solidifies and 'ii
shrinks between dendrites and between den- (al
drite branches.

Porosity is detrimental to the ductility Chill K” _`
xg ?
of a casting and its surface finish, making the Porosny
casting permeable and thus affecting the Q
pressure tightness of a cast pressure vessel.
Porosity caused by shrinkage can be reduced /
or eliminated by various means, including the

° Adequate liquid metal should be pro- BOSS
vided to avoid cavities caused by Chill
shrinkage.
Internal or external chills, as those used (C)
in sand casting (Fig. 1O.14), also are an
FIGURE |0.l4 Various types of (a) internal and (b) external chills
effective means of reducing shrinkage
(dark areas at corners) used in castings to eliminate porosity caused by
porosity. The function of chills is to in-
shrinkage. Chills are placed in regions where there is a larger volume of
crease the rate of solidification in critical
metal, as shown in (c).
regions. Internal chills usually are made
of the same material as the casting and are left in the casting. However, problems
may arise that involve proper fusion of the internal chills with the casting; thus,
foundries generally avoid the use of internal chills for this reason. External chills
may be made of the same material as the casting or may be iron, copper, or
graphite.
With alloys, porosity can be reduced or eliminated by making the temperature
gradient steep. For example, mold materials that have higher thermal conductiv-
ity may be used.
Subjecting the casting to hot isostatic pressing is another method of reducing
porosity (see Section l7.3.2).

Because gases are more soluble in liquid metals than solid metals _6
ig 10.l5), when a metal begins to solidify, the dissolved gases are expelled >` \;\0.“\
from the solution. Gases also may result from reactions of the molten metal E
with the mold materials. Gases either accumulate in regions of existing poros- 3
ity (such as in interdendritic regions) or cause microporosity in the casting, 8
particularly in cast iron, aluminum, and copper. Dissolved gases may be re- fig) Fusion
moved from the molten metal by flushing or purging with an inert gas or by g
melting and pouring the metal in a vacuum. If the dissolved gas is oxygen, the I?
molten metal can be deoxidized. Steel usually is deoxidized with aluminum, SOM
silicon, copper-based alloys with phosphorus, copper, titanium, or zirconium-
bearing materials. Melting point
Whether microporosity is a result of shrinkage or is caused by gases may Temperature_,
be difficult to determine. If the porosity is spherical and has smooth walls
(similar to the shiny holes in Swiss cheese), it is generally from gases. If the FIGURE l0.|5 Solubility of hy-
walls are rough and angular, porosity is likely from shrinkage between den- drogen in aluminum. Note the
drites. Gross porosity is from shrinkage and usually is called a shrinkage sharp decrease in solubility as the
cavity. molten metal begins to solidify.