Zinc-Aluminum Foundry Alloys 8, 12 and 27 comprise a new family of zinc
casting alloys that have proven themselves in a wide variety of demanding
applications. They are engineering materials well suited to applications
requiring high as-cast strength, hardness and wear resistance. These three
alloys offer designers and casting specifiers viable, cost-effective
alternatives for their component requirements.
The alloys are designated 8, 12 and 27 because of their approximate aluminum
content. Each also contains copper and magnesium to provide an optimum
combination of properties, stability and castability. Widespread commercial
acceptance of these alloys has resulted in the issuing of national and
international standards, notably, ASTM B 669 and ISO/DIS 301.
Zinc-Aluminum Foundry Alloys have several advantages over other commonly
used casting alloys based on iron, aluminum or copper.
Compared to cast iron, the alloys have better machinability, can be cast
to closer tolerances and have a superior as-cast surface finish. They also
are generally better suited for short production runs and less likely to
require protective finishes. These advantages have resulted in substantially
reduced production costs for numerous applications.
Compared to aluminum, the zinc alloys are harder and stronger, machine more
easily, have superior pressure tightness, and have substantially better
wear and bearing characteristics. Also, alloys 8 and 12 are not subject
to incendive sparking. The alloys become viable choices when aluminum is
inadequate in one of these areas. Although they are more expensive on a
unit volume basis, castings are potentially cost competitive when aluminum
castings require heat treatment, hard anodizing, epoxy impregnation, or
bronze bushing inserts.
Compared to copper, the most expensive of the common foundry alloys, the
lower inherent cost of the zinc alloys combined with their lower densities
can result in a material cost saving of up to 60 per cent. They also have
higher as-cast strength and hardness, and equivalent or superior
machinability and wear resistance.
The economic benefits and inherent properties of Zinc-Aluminum Foundry
Alloys account for their use in a rapidly growing list of industrial
- Industrial fittings and hardware
- Pressure tight housings
- Sleeve bearings, thrust washers and wear plates
- Electrical switchgear and hardware
- Hose couplings and connectors
- Fire fighting hardware
- Pneumatic and hydraulic cylinder components
- Industrial machine hardware
- Electrical conduit fittings
- Door hardware and lock components
- Pulleys and sheaves
- Non-sparking mine hardware
- Decorative hardware
- Electronic instrument chassis, hardware and covers.
Advantages to the foundry:
- Low melting costs
- Extended foundry equipment life
- Clean foundry environment
- No fluxing or degassing
- Excellent mould filling characteristics
- Few casting rejects
- Low melt losses
- Excellent as-cast strength
Advantages to the casting specifier:
- High tensile strength and hardness
- Excellent machinability
- Superior pressure tightness
- Good bearing and wear characteristics
- Easily cast in thin sections
- Wide choice of casting methods
- Existing patterns and match plates normally usable
Zinc-Aluminum Foundry Alloys are readily melted in
refractory-lined or non-metallic crucible furnaces similar to those
used for other non-ferrous foundry alloys. In general, it is recommended
that a separate crucible be reserved for melting because of the low
impurity limits specified for the alloys. While crucibles which have
previously held aluminum alloys can be used if thoroughly cleaned,
those that have held lead or tin-containing copper alloys must be avoided.
The zinc alloys melt in less time and do not require fluxing or degassing
as is common with aluminum alloys. Energy requirements for melting are
about 1, 1 and 3 those of iron, bronze and aluminum, respectively, which
results in substantial energy savings. Melting the alloys produces no fumes
and the relatively low casting temperatures, 450-600°C help to extend the
service life of foundry equipment. The normal foundry practice of blending
foundry returns with fresh ingots is recommended.
Casting. The zinc alloys have excellent mould filling characteristics
and low casting temperatures compared to most other foundry alloys. These
inherent properties account for fewer casting rejects, reduced metal
losses, and the casting versatility of the alloys. They can be cast
using all the traditional processes including sand, permanent mould,
pressure die, shell and investment casting.
Sand Casting. Zinc alloys 12 and 27 are generally selected over
alloy 8 for sand casting. They can be poured in virtually any of the
non-ferrous sand systems - synthetic or natural. Both alloys are relatively
insensitive to variations in mould hardness, permeability and moisture
The alloys are tolerant of most foundry gating and feeding systems. Castings
can be produced using match plates designed for aluminum, bronze or cast
iron with little or no modification.
Permanent Mould Casting. Alloys 8 and 12 are recommended for
permanent mould casting, with alloy 8 offering faster cycle times and a
better surface for applying decorative plated finishes. Compared to alloy 8,
alloy 12 castings have superior strength, hardness, wear resistance and
dimensional stability. Both alloys have very good fluidity which permits
casting of thin, intricate sections without misruns.
In general, ferrous permanent moulds designed for aluminum are suitable for
casting zinc alloys. Permanent moulds also can be made from either bronze,
aluminum, rubber or graphite. Thick-walled castings in alloy 12 may require
increased feeding because of its wide freezing range.
The commercial availability of alloy 12 led lo the development of a new
casting technology based on graphite permanent moulds. The low casting
temperatures of the alloys make the use of graphite moulds feasible for
medium-volume production requirements. Mould life is typically in excess
of 25,000 cycles and in most instances is significantly higher. Major
benefits of graphite permanent moulding are low tooling costs, excellent
castings tolerances, and the ability to produce castings with exceptionally
good surface finish.
Pressure Die Casting. When die cast, alloys 8, 12 and 27 provide
substantial property improvements over conventional zinc and aluminum die
casting alloys. The improved strength and wear characteristics of these
alloys allow this highly economical process to be selected for applications
where the traditional die casting alloys would not be considered. Alloy 8
can be cast in the hot chamber process commonly used with conventional zinc
die casting alloys. Alloys 12 and 27 must be cast using the cold chamber
process. The life of iron components in the hot chamber process would be
unacceptably short at the required casting temperatures for alloys 12 and 27.
Corrosion resistance and machining
The excellent corrosion resistance of zinc in many environments has led to
its extensive use for corrosion protection. The Zinc-Aluminum Foundry
Alloys, like unalloyed zinc, also possess excellent resistance to corrosion
in a wide variety of environments.
Castings exposed outdoors normally develop a dark, gray patina which slows
further oxidation while leaving part performance unaffected. Corrosion data
developed for zinc and zinc die casting alloys are a useful guide for
estimating the corrosion performance of Zinc-Aluminum Foundry Alloys in
specific environments. When castings are to be subjected to environments
which are known to be aggressive to zinc, protective finishes should be
Finishing. Zinc alloy castings exhibit clean as-cast surfaces which
can be anodized, painted, chromated, polished, brushed or plated. The type
of finish selected will largely depend on service conditions, aesthetics
Anodizing. Zinc anodizing electrochemically produces a thin, abrasion
resistant, ceramic-like film. The film has a fritted structure and is a
complex mixture of chemical compounds - mainly zinc ammonium phosphate and
Anodized castings possess excellent resistance to corrosive attack from
most natural and industrial corrosive agents including detergents, road
salts, soft waters and most organic solvents.
Painting. The alloys lend themselves well to, pigmented organic
coatings, including those that require baking. Surface pretreatments,
such as chromating or phosphating, are necessary to ensure good adherence
of paint or lacquer finishes. Coatings can be applied by brushing, spraying
or dipping - the method used will depend largely on casting shape,
complexity and quantity.
Chromating is a low cost chemical conversion treatment used to
provide additional corrosion protection to metal products. It provides
corrosion protection of the order of 90-100 hours in a 5% neutral salt
spray exposure. To obtain the bright, iridescent type finish associated
with zinc die-castings or galvanized coatings, foundry alloy castings
must be given a cadmium or zinc flash prior to chromating. Without the
flash, chromated castings will exhibit varying brownish tones depending
on the alloy coated and process variables.
Machining. The Zinc-Aluminum Foundry Alloys have excellent
machinability and can tolerate wide variations in machining conditions.
Tool life compares favorably with that experienced with copper and
aluminum alloys and is significantly longer than with cast iron.
In general, high-speed steel tools perform well. Best results are obtained
with tools having large clearance angles and polished flutes and cutting
surfaces. The use of water-soluble coolants is strongly recommended to
prevent metal pickup on tools.
Joining. Adhesives, mechanical devices and certain solders are
suitable for joining the alloys. Each application must be considered
separately since the selection of the joining method is dependent on
service conditions and required joint strength.
Welding of the alloys can be done using inert gas welding techniques.
Zinc-aluminum wire and standard aluminum filler rods have been used in
TIG (tungsten inert gay) welding the alloys with good results.