Chemical specifications for zinc casting alloys are given in ASTM B86 for No. 2, No. 3, No. 5 and No. 7 alloys and ASTM B791 for ZA-8, ZA-12 and ZA-27.
Alloys covered by B86 are hypo-eutectic (i.e. they contain less aluminum than the eutectic chemistry of 5% Al) with a composition close to 4% Al. Alloys included in B791 are hypereutectic with an aluminum content greater than the eutectic chemistry. All of the zinc casting alloys have dendritic/eutectic microstructures, however, the hypoeutectic alloys solidify with zinc-rich dendrites, whereas hypereutectic alloys solidify with aluminum-rich dendrites.
Chemical specifications for zinc casting alloys are given in ASTM B86 for
No. 2, No. 3, No. 5 and No. 7 alloys and ASTM B791 for ZA-8, ZA-12 and
Alloys covered by B86 are hypo-eutectic (i.e. they contain less aluminum
than the eutectic chemistry of 5% Al) with a composition close to 4% Al.
Alloys included in B791 are hypereutectic with an aluminum content greater
than the eutectic chemistry. All of the zinc casting alloys have
dendritic/eutectic microstructures, however, the hypoeutectic alloys
solidify with zinc-rich dendrites, whereas hypereutectic alloys solidify
with aluminum-rich dendrites.
The mechanical and physical properties of the castings, and to a lesser
extent their corrosion properties, are closely linked to the specific alloy
type, the casting process and quality of the castings produced, the amount
of ageing or service life of a component casting and the level of
impurities, amongst others.
Both Cu and Mg increase strength properties, reduce
ductility and inhibit intergranular corrosion. Iron is present as small
FeAl3 particles and does not influence mechanical properties unless it
exceeds 0,1%. When limited to the specified amounts shown in ASTM B86 and
B791 Pb, Sn and Cd do not cause intergranular corrosion
or the lowering of physical and mechanical properties. Other impurity
elements such as Cr, Ni, Mn, Ti, Sb,
In, As, Bi and Hg do not normally occur in
sufficient quantities in zinc casting alloys to be of concern.
Gravity Casting Alloys
A recent alloy development is generation of a family of zinc foundry alloys
suitable for sand, permanent mold, plaster mold, shell mold and investment
casting. Many of these alloys also can be die cast in cold chamber machines
where strength and/or hardness beyond the properties of AG41A are required.
Currently, two alloys are finding increasing application, a 12%-Al alloy
(ILZRO 12) and a 27%-Al alloy (Zn-27Al). These two gravity casting zinc
alloys have been designed for structural applications and should not be
confused with die casting and slush casting alloys, which often are used
for decorative gravity cast parts.
The mechanical properties of these zinc alloys make them attractive
substitutes for cast iron and copper alloys in many structural and
pressure-tight applications. Because zinc is less costly than copper, these
zinc alloys have a distinct cost advantage over copper-base alloys. The ease
of machining of zinc and its inherent corrosion resistance give it
advantages over cast iron.
Zinc gravity casting alloys have attractive foundry properties. Due to their
low melting temperatures (below 540°C) and casting temperatures, energy
requirements are low. They are readily cast in thin sections - less than
2,5 mm in sand molds. Melting and casting of these alloys are virtually
pollution free. No fluxing or degassing is required, and because of the low
casting temperatures minimal pollution from the sand mold results.
The 12%- Al alloy is preferred for heavy sections and is suitable for
permanent mold casting in both metal and graphite molds. Its permanent mold
casting characteristics are similar to those of aluminum permanent mold
alloys. The 27%-Al alloy should be specified when higher mechanical
properties are required in thin section sand castings. Care should be taken
to prevent hot spots in the mold, which contribute to underside shrinkage.
Zinc gravity casting alloys can be used for general industrial applications
where strength, hardness, wear resistance or good pressure tightness is
required. Zinc alloys often are employed to replace cast iron because of
their similar properties and higher machinability ratings. The good bearing
and wear characteristics of zinc alloys permit them to be used for bearing
bushings and flanges. Other applications in which zinc alloys have been
successfully substituted for cast iron or copper alloys include
fuel-handling components, pulleys, electrical fittings and hardware
Wrought Zinc and Zinc Alloys
Wrought zinc and zinc alloys may be obtained as rolled strip, sheet and
foil; extruded rod and shapes; and drawn rod and wire. These metals exhibit
good resistance to corrosion in many types of service, and because the
corrosion products that may form on them are white, other materials are not
stained by them.
Wrought zinc has chemical characteristics particularly adapted to certain
uses, such as dry batteries and photoengraver`s plate, and offers
combinations of desirable physical and mechanical properties at relatively
low cost. In common with many other metals and alloys, wrought zinc creeps
under constant loads that are substantially less than its ultimate strength;
that is, wrought zinc does not have clearly defined elastic module, and
hence creep data from service tests must be used in designing for strength
and rigidity under conditions of continuos stress.
All severe fabrication of wrought zinc should be done at temperatures above
20°C. Rolled zinc of the proper grade is readily drawn into a great variety
of articles such as batter cups, eyelets, meter cases, novelties, flashlight
reflectors and fruit-jar caps. Suitable grades of rolled zinc also are
readily rolled, press formed, stamped or spun into items such as plates for
addressing machines, buckles, ferrules, ornaments, nameplates, gaskets,
weather-stripping and lamp parts.
The ordinary grades of wrought zinc can be soldered easily by conventional
methods. The usual precautions should be observed regarding proper cleaning
and fluxing. The metal must not be overheated to the point where it melts.
Pulsed-arc welding may be used for joining; gas welding of zinc is used only
for repair work.
Wrought zinc is easily machined using standard methods and tools. However,
if it is necessary to machine zinc containing exceedingly coarse grains,
the metal should be heated to a temperature between 70 and 100°C in order
to avoid cleavage of crystals.
In general the same methods are used for finishing wrought zinc and zinc
alloys as are used for finishing zinc-base die castings. However, in
bake-enameling of wrought zinc, greater caution should be exercised in
avoiding temperatures high enough to impair mechanical properties. For
this reason air-dried finishes are preferable.