Technical Information - 04

ZINC BASE ALLOYS

There are two alloys which are mainly used Zamak-3 (ASTM AG40A, SAE 903, ZL3) and Zamak-5 ( ASTM AC41A, SAE 925, ZL5). Both containing about 4% aluminium and 0.04% magnesium, but one contains copper whilst the other is copper-free.

Structure

The major use of zinc as a structural material is in the form of alloys for die castings. The zinc die-castingalloys are low in cost and easy to cast and have greater strength than all die castingmetals except the copper alloys. They can be cast to close dimensional limits and are machined at minimum cost.

The Al-Zn phase diagram shows that a lamellar eutectic forms at 382 C and 5% aluminium, containing alpha and Beta solid solution. The alpha constituent of the eutectic is stable only at temperatures above 275 C. At that temperature it transforms by a eutectoid reaction into alpha and beta phases and both of the alloys are structurally unstable at room temperature. However, all commercial diecasting alloys are cooled fast enough to prevent the eutectoid transformation and retain the eutectic mixture of alpha and Beta.

The primary zinc solid solution phase continues to reject aluminium from solid solution (a precipitation or aging process). In a chill-cast copper-free alloy, the structure consists of primary dendrites of alphaZn/Beta-Al eutectic (Fig. 1). At a higher magnification (Fig. 2), the precipitate in alpha-Zn can be seen, but the precipitate in the Beta-Al of the eutectic is not resolved. The same alloy, when sand cast (Fig. 3), shows a coarser eutectic than when it is chill cast. In the sand cast alloy, at higher magnification (Fig. 4), the precipitate as well as being resolved in the Alpha-Zn phase is also resolved in the Beta-Al of the eutectic phase.

Typical Chemical Composition (%)

Impurities in both alloys are restricted to Fe <01, Ni <0.06, Pb <0.005, Cd <0.005, Sn 0.002, Th <0.001, In <0.0005, and for alloy A, Cu <0.1.

Typical Mechanical Properties

The aging process causes a continuous change in properties, as typified by the following data:

The aging process is also accompanied by dimensional contraction and castings may be given a stabilising beat treatment (6h at 1000 C followed by air cooling) which reduces the extent of mechanical and dimensional changes due to aging.

Specification

Zinc alloys are made to BS 1004 : 1972 and the copper-free alloy illustrated in Figs. 1, 2, 3 and 4 is likely to meet the following specifications.

Typical Applications

The 4%Al alloy is more widely used in the UK, but the 4%Al-1%Cu alloy is equally widely used in other countries. The main applications of both alloys are in the motor trade, followed by toys, hardware and hand tools industries and domestic applications. Both alloys are essentially pressure die cast but, on a smaller scale, they are also used for gravity die casting, as well as an alloy containing 11 - 13%Al.

ZINC BASE ALLOYS

Fig 1 - Zinc alloy, A, chill cast............x200.............Fig 2 - Zinc alloy, A, chill cast (same as Fig. 1) ..x1000

Etchant : 1% nital....................................................... Etchant : 1% nital

Fig 3- Zinc alloy, A sand cast ...........x200....Fig 4-Zinc alloy, A, sand cast (same as Fig.3)..x1000

Etchant : 1% nital .......... ....................................Etchant : 1% nital

ALUMINIUM BASE ALLOYS - AlSi

Structure

Al-Si alloys form the largest family of the aluminium base casting alloys, ranging from simple binary to more complex alloy systems. Structurally, however, all of the alloys belong to three distinct groups, hypoeutectic, eutectic, and hypereutectic, although a few alloys are borderline between the hypo- and eutectic groups. The constituents in all alloys are the same; however, only their relative volumes, size and distribution differ; these are alpha-Al, which is present in all alloys, and can occur even in hypereutectic alloys under strong nonequilibrium freezing and silicon, as divorced in hypoeutectic alloys and equally divorced in eutectic or hypereutectic alloys. Primary silicon occurs in hypereutectic alloys but can also be found occasionally in eutectic alloys. Other intermetallic constituents which can occur in some alloys are CuAl₂ or alpha-Al/CuAl₂, eutectic, Mg₂Si or a-AI/Mg₂ Si eutectic, (AlFeNi), and various alpha-(FeMnSiAl) compounds.

The matrix of hypoeutectic alloys, alpha-Al, is usually grain refined as in the case of the Al-Cu alloys. The eutectic and hypereutectic alloys are, however, seldom grain refined, but the structural condition of the silicon phase in the microstructure is controlled by melt treatments as well as by the cooling rate. The macrostructure of a chill-east untreated eutectic Al-Si alloy, consisting of eutectic grains or cells, is shown in Fig. 5, and that of the sand cast-alloy is shown in Fig. 6

Dendrite arm spacing in hypoeutectic alloys is controlled by both grain refinement and the cooling rate during freezing. This in turn will affect the size and the distribution of all dispersed constituents including silicon. The case of silicon is different, however, and requires special considerations. In the plane of the micro-section, silicon particles appear as elongated plates of lamellae apparently disconnected from one another. In fact, deep etching and scanning photomicrography reveals that silicon particles are interconnected into a single, coral-like silicon mesh, the sections of which appear in the plane of the photomicrograph. Clearly, therefore, the state of silicon in the microstructure is dependent upon the number of silicon "corals" and the degree of branching of such corals. Fine branching is achieved either by fast cooling during freezing or by melt treatments with small amounts of sodium (0.01%) or strontium (0.05%). This is often described as a "modification" treatment. Some hypoeutectic alloys may be modified, but eutectic and hypereutectic alloys are generally modified, particularly for sand castings. In hypereutectic alloys, primary silicon is grain refined by small additions of phosporus (0.01%) to form AIP compounds which nucleate a large number of cuboidal silicon crystals. The eutectic silicon can be also "modified" in these alloys as well as primary silicon refined, by introducing together with phosphorus small concentrations of strontium and sodium into the alloy (double refinement). Impurity intermetallics can occur as needles but with correct proportions of iron and manganese the morphology of alpha-(FeMnSiAl) may be rendered more equiaxial.

The microstructure of a chill cast AI-Si alloy in Fig. 7 shows primary alpha-Al, alpha-Al/Si eutectic (dark) and intermetallic a-(FeSiMn) (light). When sand cast, the microconstituents are the same but coarser (Fig. 8). When modified with 0.05%Sr, the structure (Fig. 9) consists of primary alpha-Al and a finer eutectic of a-Al and Si which is more eutectic-like than in Fig. 7. The sand cast alloy, similarly modified (Fig. 10) is similar to the chill cast alloy, but exhibits a coarser eutectic and acicular alpha-(FeSiMn). Scanning electron micrographs of the structures shown in Figs. 9 and 10, taken after deep etching, reveal the branching of silicon crystals (Figs. 11 and 12).

The microstructure of a chill cast AI-Si-Mg alloy shows silicon (dark) and script a-(FeSiMnAl) in an alpha-Al matrix. The sand cast alloy reveals dark grey silicon, light grey script and dark Mg, Si in an alpha-Al matrix.

The chill-east Al-Si-Cu alloy has dark grey silicon, light angular or globular alpha-Al/CuAl₂ in an a-AI matrix. When sand cast, the microstructure is similar but coarser showing dark grey silicon, globular a-AI/CuAI₂ eutectic and needles of alpha-(FeSiAl) in an alpha-Al matrix.

The chill-cast Al-Si-Cu-Mg-Ni alloy has dark grey alpha-Al/Si eutectic, light grey CuAl, and needles of alpha-(FeSiAl) in an alpha-Al matrix. A sand cast alloy shows dark grey silicon; light faceted or globular CuAl, needles of alpha-(FeSiAl) and black Mg₂Si in an alpha-Al matrix. A similar alloy but with approximately twice the silicon content when chill-cast, the structure consists of dark grey cuboids or primary silicon, dark grey eutectic silicon, black Mg₂Si and light NiAl₃ in an alpha-Al matrix. When sand cast, the structure is coarser but has similar microconstituents, namely, dark grey cuboids of primary silicon, dark grey eutectic silicon, black Mg₂ Si and light NiAl₃ in an alpha-Al matrix.

AI-Si alloys are amenable to hardening heat treatment by utilising either CuAl₂ or Mg₂ Si precipitation mechanisms.

Alloys LM4, LM6 and LM25 are widely used for gravity die and sand castings.

Typical Chemical Composition

Typical Mechanical Properties

(M = as-east; TB = solution treated only; TF = solution treated and precipitation treated).

Fig. 5 - Alloy LM6 macrostructure, chill cast untreated..x 5 ..Fig. 6 - Alloy LM6 macrostructure, sand cast untreated...x 5

Etchant : CuCI.2 soln. ......................................................Etchant : CUC12 soln.

Fig. 7 - Alloy LM6, chill cast...................x 400 ..Fig. 8- Alloy LM6, sand cast ................x 400
Etchant : 0.5%HF .....................................Etchant : 0. 5% HF

Fig. 9 - Alloy LM6, chill cast, modified with 0.05%Sr Alloy..x 400...Fig.10 - LM6, sand cast, modified with 0. 0 5%Sr...x 400

Etchant : 0.5% HF ............................................Etchant : 0.5% HF

Fig. 11- Alloy LM6, chill cast, modified with 0.05%Sr .x 2000 ....Fig. 12 - Alloy LM6, sand cast, un- modified with 0.05%Sr..x 2000
Scanning electron micrograph. Deep etched............. Scanning elec.tron micrograph. Deep etched

25% HCl, Aq. 15 mins .........................................25% HCl, Aq. 15 mins