Zinc Alloys: Properties, Production, Processing and Applications

After iron, aluminium and copper, zinc is the fourth most widely used metal in the world [1]. About 13.2 million metric tons of refined zinc metal was produced globally in 2018, and this level of production has remained largely the same over the last decade. China is the world's largest producer of zinc, contributing almost 40% of the global output, while Australia has the largest zinc reserves in the world [2].

Zinc is the 16^th most abundant metal and 23^rd most abundant element in earth's crust, respectively. It is more abundant than copper, although global copper production is considerably more than that of zinc. Global zinc production is primarily from its ore, zinc sulphide, commonly known as sphalerite.

Almost half of all the zinc produced is used for galvanisation; the process of coating iron or steel with a thin layer of zinc to protect it from corrosion. This process can drastically increase the recyclability of steel and iron. Aside from being used for galvanisation, zinc is heavily used for making alloys, the most common being brass. Brass is an alloy of copper and zinc with smaller amounts of lead and tin.

In this article, you will learn about:

• The properties of zinc alloys
• The production and processing of zinc alloys
• Applications of zinc alloys
• Example grades and standards of zinc alloys

Properties of zinc alloys

Aside from zinc, zinc alloys usually contain aluminium, copper, magnesium and iron.

Aluminium has a considerable solubility in zinc, and it is often added in the foundry process to increase fluidity and reduce the melting temperature. Aluminium also improves some mechanical properties such as elongation.

Copper is another common constituent in zinc alloys. Copper-zinc alloys, also known as brass, have enhanced properties such as tensile strength, hardness, wear resistance, and creep.

Magnesium, in relatively small quantities, is an important zinc alloying metal as it improves the grain structure and prevents inter-granular corrosion typically caused by impurities.

Table 1. Properties of Zinc Alloys

	ZA-8 Die Cast	ZA-8 Perm. Mould	ZA-12 Die-Cast	ZA-12 Perm. Mould	ZA-12 Sand Cast	ZA-27 Die Cast	ZA-27 Perm. Mould	ZA-12 Sand Cast
Density (g/cm³)	6.3	6.3	6	6	6	5	5	6
Elastic modulus (GPa)	86	85	83	83	83	78	78	83
Tensile strength (MPa)	370	210	400	290	260	430	400	260
Elongation (%)	8	1.3	5.3	1.9	1.6	2	3.6	1.6
Thermal conductivity (W/(m·K))	120	120	120	120	120	130	130	120
Melting point  (°C)	380	380	380	380	380	380	380	380
Coefficient of thermal expansion (K-1)	2.3 ✕ 10-5	2.3 ✕ 10-5	2.4 ✕ 10-5	2.4 ✕ 10-5	2.4 ✕ 10-5	2.6 ✕ 10-5	2.6 ✕ 10-5	2.4 ✕ 10-5
Yield strength (MPa)	290	210	320	270	210	370	260	210
Electrical conductivity (S/m)	1.624 * 10-8	1.624 * 10-8	1.624 * 10-8	1.624 * 10-8	1.624 ✕ 10-8	1.74 ✕ 10-8	1.74 ✕ 10-8	1.624 ✕ 10-8

Production and processing of zinc alloys

Historically, zinc had been produced from its oxide ores before the discovery of its sulphide ores, which are far more abundant in nature. The most common ore of zinc is zinc sulphide, also called zinc blende.

There are two main methods of producing zinc: the pyro-metallurgical method and the hydrometallurgical method.

By 1916, the hydrometallurgical process had replaced the pyro-metallurgical process as the primary production process of zinc [1].

Pyro-metallurgical process

This process involves the ‘roasting’ of concentrated forms of zinc ore, the concentration of which is achieved via the froth floatation process. This concentrated ore is finely ground and suspended in a stream of air which serves the dual purposes of converting the ore into the highly reactive zinc oxide ZnO and also to reduce the sulphur content [3]. The reaction that takes place is as follows:

2ZnS + 3O₂ → 2ZnO + 2SO₂

The resultant zinc oxide is further ground and mixed with coke (a high-carbon content fuel) and then heated to 1400 ^oC. The coke serves as a reducing agent, and the zinc oxide is reduced to metallic zinc via the following reaction;

2ZnO + C → 2Zn + CO₂

The zinc obtained in this method is in a gaseous state, as its boiling point is below 1400 ^oC. Therefore, it must be condensed in order to yield solid zinc. The gaseous state of zinc has the advantage of being easily separated from impurities such as cadmium, lead and iron, which have higher boiling points and are left behind.

Hydrometallurgical process

Crude zinc oxide, which is obtained via floatation and subsequent oxidation, is dissolved in dilute sulphuric acid, H₂SO₄, to produce a solution of zinc sulphate, ZnSO₄.

ZnO+H₂SO₄ → ZnSO₄ +H₂O

ZnSO₄ is then electrolysed using a (Pb-1%Ag) anode and an aluminium cathode. Oxygen is released at the anode while zinc is deposited on the cathode. The zinc metal (which is 99.995% pure) can then be removed from the cathode and processed via methods such as zinc die casting. About 80% of global zinc production is via this hydrometallurgical process [4].

Zinc alloys are produced by mixing pure zinc with other metals in specific ratios. This provides metals with properties suitable for a range of applications, described below. Zinc alloy parts are produced by casting methods which, depending on the mixture of alloying elements, may be via hot chamber die-casting, cold chamber die-casting, gravity and sand casting. These processes typically involve the injection of the molten metal mix into a permanent mould at high pressures, after which the mould is cooled to produce the metal part directly or an ingot for secondary use. Post-processing of zinc alloys is usually limited to surface finishing as techniques such as cold-working and heat treatment are typically not carried out due to the possibility of crack formation [5].

Applications of zinc alloys

Zinc alloys (together with other metals such as copper and titanium) are used for building and architectural applications such as rainwater systems, claddings, fittings and roofing. Zinc alloys also find application as sacrificial components such as fuses, shear bolts/pins and sacrificial anodes in corrosion protection. Zinc alloys are also used in electromagnetic shielding to protect devices from electromagnetic fields.

Brass has an excellent combination of mechanical properties such as strength, ductility, wear and corrosion resistance, electrical and thermal conductivity, hardness and machinability, which makes it suitable for a wide range of applications. These can be broadly divided into two; decorative and mechanical. Such applications include household fittings, jewellery, engine parts, pumps, valves, fasteners and clock components.

Other zinc alloys are used in certain types of machine bearings, die-casting and stamping dies. Zinc is also used for medical equipment, rubber products, paint pigments and ceramics. One interesting fact about zinc itself is that it is the second most abundant trace metal in the human body, after iron. 

Some applications of commercially available zinc alloys are given below.

Table 2. Typical applications of zinc alloys.

Alloy designation	Alloy composition in %	Alloy applications
Alloy 3	Zn–4Al–0.04Mg	Suitable for the fabrication of complex shapes through hot-chamber pressure die-casting
Alloy 5	Zn–4Al–0.04Mg–1Cu
ZA 8	Zn–8Al–1Cu–0.02Mg	Most suitable for gravity casting and electroplating
ZA 12	Zn–11Al–1Cu–0.02Mg	Suitable for sand or gravity casting
ZA 27	Zn–27Al–2Cu–0.015Mg	Suitable for pressure die-casting and extrusion
Zn–Cu–Ti	Zn–1Cu–0.1Ti	High strength and creep resistance

Example grades and standards of zinc alloys 

There are many grades of zinc alloys, with more than 25 currently in use. Many of them have proprietary or commonly known names. 

The most well-known zinc alloy is brass, which is composed of copper and zinc. There currently exist over 600 EN standards of brass. The main types of brass are differentiated by their crystal structure, which depends on the ratio of copper to zinc, and are separated into alpha brass, beta brass, alpha-beta brass, gamma brass and white brass.

Zamak is a common family of zinc alloys which are composed of aluminium, magnesium and copper. They are distinct in that they contain a fixed amount of aluminium at 4%.

Tombak is a brass alloy with a low quantity of zinc (below 28%) and a high quantity of copper (above 78%).

Zinag alloys consist of zinc, aluminium and silver. They have low density, are resistant to corrosion, and have good mechanical properties. Silver imparts superplasticity on the alloy making it deformable without significant loss of its mechanical properties.

Other zinc alloys include nickel silver, lead-free solder and commercial bronze.

Some relevant zinc alloys are shown below, along with their composition. The measurements are as per the ASTM B86 standard. Trace amounts of other metals such as cadmium, tin, chromium, silicon, nickel and lead are not shown.

Table 3. Chemical composition of some zinc-based alloys [6].

Alloy	Aluminium (%)	Copper (% )	Magnesium (% )	Iron (%)	Zinc (%)
Zamak 2 (AC43A)	3.7 – 4.3	2.6 – 3.3	0.02 – 0.06	<0.05	Rest (approx)
Zamak 3 (AC40A)	3.7 – 4.3	<0.1	0.02 – 0.06	<0.05	Rest (approx)
Zamak 5 (AC41A)	3.7 – 4.3	0.7–1.2	0.02 – 0.06	<0.05	Rest (approx)
Zamak 7	3.7 – 4.3	<0.1	0.005 – 0.02	<0.05	Rest (approx)
ZA-8	8.0 – 8.8	0.8 – 1.3	0.01 – 0.03	<0.075	Rest (approx)
ZA-12	10.5 – 11.5	0.5 – 1.2	0.01 – 0.03	<0.075	Rest (approx)
ZA-27	25 – 28	2 – 2.5	0.01   –   0.02	<0.075	Rest (approx)
ACuZinc 5	2.5 – 3.3	5.0 – 6.0	0.02 – 0.05	<0.075	Rest (approx)
ACuZinc 10	2.5 – 3.3	10 – 11	0.02 – 0.05	<0.075	Rest (approx)
ALZEN 305	30	5	0.01 – 0.02	-	Rest (approx)
ALZEN 501	50	1	0.01 – 0.02	-	Rest (approx)
ZEP®	14 – 16	0.8 – 1.2	0.025 – 0.035	<0.03	Rest (approx)

 

[1] “Metals: Market & Opportunities,” KPMG & India Brand Equity Foundation, 2008. [Online]. Available: https://www.ibef.org/download/Metals_210708.pdf [Accessed May 1 2020]

[2] M. Garside, “Zinc - Statistics & Facts,” statista, Jan 17, 2020, [Online]. Available: https://www.statista.com/topics/2306/zinc/ [Accessed May 1 2020].

[3] “Extraction Of Zinc: Application Of Metallurgy,” BYJU’S, [Online]. Available: https://byjus.com/chemistry/zinc-extraction-metallurgy/ [Accessed May 1 2020].

[4] P.A.Tasker, P.G.Plieger and L.C.West, “Metal Complexes for Hydrometallurgy and Extraction,” in Comprehensive Coordination Chemistry II, J. A. McCleverty and T:J. Meyer, Ed. Elsevier, 2003, pp.759–808.

[5] Gross, Douglas K. “Zinc Alloys: Specifications and Processing.” SAE Transactions, vol. 96, 1987, pp. 1039–1046. JSTOR, www.jstor.org/stable/44472868. [Accessed 7 May 2020].

[6] A. Pola 1, M. Tocci and F. E. Goodwin, "Review of Microstructures and Properties of Zinc Alloys," Metals, vol. 10, 253, 2020.