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Full text of "Modern Mechanical Engineering Vol-I"


CASTINGS  MADE  IN  METALLIC  MOULDS         143
selection of alloys is essential because of the multifarious uses to which the
castings are applied. In some cases the expansion of an alloy under high
temperatures would preclude its use. In many cases steam, oil, alkaline
liquids, sea water, and corrosive fluids would disintegrate some alloys, while
having no effect on others. Some alloys are too brittle for certain services,
others are too soft, while in some, an element will sweat out from the mass.
These facts indicate the difficulties which have to be surmounted by the die-
caster.
In the zinc-base alloys this metal may be used in a range of from 50 per
cent to 80 per cent, tin from 5 per cent to 30 per cent, and copper and alu-
minium from a mere trace to about 5 per cent. Antimony may be present
from i per cent to 5 per cent, its function being to reduce shrinkage and
impart hardness. Only a small quantity is necessary, since zinc is hard,
and does not shrink so much as tin or lead. These alloys melt at from 800
to 850 F. They are affected by alkaline and salt waters. They are the
easiest to cast, and the strongest castings are obtained by keeping the tin
and copper at, say, from 2 per cent to 5 per cent. Tin is liable to sweat
out below the temperature of fusion.
The tin-base alloys contain from 60 per cent to 90 per cent of the metal
with from 3 per cent to 7 per cent copper, and about the same proportion of
antimony. These alloys are excellent. They are softer than those with
a zinc base, and produce castings of good finish, but the price of tin makes
them expensive. Babbitt is composed of tin 89 per cent, copper 3-7 per cent,
antimony 7-3 per cent with a trace of bismuth. These alloys melt at from
200 to 300 F. lower than those having a zinc base.
In the lead-base alloys the proportions of that metal are high, but their
uses are almost confined to the bearing metals. Lead may range from 60
per cent to 90 per cent, tin from 2 per cent to 20 per cent, antimony from
4 per cent to 25 per cent. The alloys lack strength, and are heavy. The
tin increases the tenacity and toughness of the lead, while the latter renders
the tin more malleable and ductile. The higher the percentage of tin, the
better is the surface of the castings, being smoother and brighter. Shrinkage
is reduced. Antimony increases fluidity and imparts hardness. The
maximum hardness is imparted with 17 per cent of antimony. Up to 13
per cent it expands the lead. Lead will not alloy with zinc, because segrega-
tion occurs during cooling.
When aluminium-base alloys were required for some parts of machine-
guns, pistols, grenades, binoculars, &c., difficulties were encountered because
of the higher melting-point. The temperature of any alloy must not be
higher than would prevent it being melted in an iron pot. Aluminium exerts
a solvent effect on iron, and a small percentage is found in the castings. An
excess over 3 per cent renders the aluminium alloy useless, causing it to
become viscous by the raising of the melting-point. A standard mixture is,
aluminium 92 per cent, copper 8 per cent. Small quantities of zinc, nickel,
and manganese may be included.
Attempts to die-cast brass and bronze have not been crowned with