Magnesium Alloys: Types, Properties and Applications

Magnesium alloys are well-known for being the lightest structural alloys [1]. They are made of magnesium, the lightest structural metal, mixed with other metal elements to improve the physical properties. These elements include manganese, aluminium, zinc, silicon, copper, zirconium, and rare-earth metals [2].

Some of magnesium’s favourable properties include low specific gravity and a high strength-to-weight ratio. As a result, the material lends itself to a range of automotive, aerospace, industrial, electronic, biomedical, and commercial applications.

Here, you can learn about the various types of magnesium alloys and their designations, the physical properties of magnesium alloys, and the applications in which magnesium alloys are used.

Types and designation

Magnesium alloys can be categorized into two groups: cast alloys and wrought alloys.

Cast alloys are basically made by pouring the molten liquid metal into a mould, within which it solidifies into the required shape. Common cast alloys of magnesium consist of different amounts – but not exceeding 10% – of aluminium, manganese and zinc as principal alloying elements. Other alloying elements have been recently used, as well, mostly to enhance creep resistance, such as zirconium and rare-earth metals. Besides, mechanical properties of cast alloys are augmented by heat treatments.

Wrought alloys, on the other hand, are alloys subjected to mechanical working, such as forging, extrusion, and rolling operations, to reach the desired shape. Aluminium, manganese and zinc are also the main alloying elements. Wrought alloys of magnesium are sorted into heat treatable and non-heat treatable alloys.

In order to understand the compositions of the alloys, designation systems have been created showing the alloying elements and their relative information. One of the most widely used designation systems is the ASTM Standard Alloy Designation System. It is made of four parts, described in the following example [3]:

Magnesium Alloy: AZ91E-T6

• First part (AZ): designates the two main alloying elements (aluminium, zinc)
• Second part (91): designates the percentage amount of the main alloying elements (9% and 1%, respectively)
• Third part (E): differentiates alloys having the same amounts of the main alloying elements (fifth standardised alloy with the above percentages)
• Fourth part (T6): designates the condition of the alloy (temper)

 

So, in the ASTM designation system, magnesium alloys are named and grouped by means of their main alloying elements. Table 1 shows the principal alloying elements and their relative designations.

Principal Alloying Element	ASTM Designation
Manganese	M
Aluminium-Manganese	AM
Aluminium-Zinc-Manganese	AZ
Zirconium	K
Zinc-Zirconium	ZK
Zinc-Zirconium-Rare Earth Metal	ZE
Rare Earth Metal-Zirconium	EZ
Zinc-Copper-Manganese	ZC
Aluminium-Silicon-Manganese	AS

 

Physical properties

Magnesium alloys are materials of interest mostly due to their high strength-to-weight ratios, exceptional machinability and low cost. They have a low specific gravity of 1.74 g/cm³ and a relatively low Young’s modulus (42 GPa) compared to other common alloys such as aluminium or steel alloys [4]. They suffer, however, from brittleness and poor formability at room temperature [4]. Their formability increases with increasing temperature, but that requires high energy. Furthermore, studies have shown that formability can be enhanced at the expense of strength, by weakening the basal texture of the Mg alloys [1].

Figure 1 illustrates the inverse relationship between the Index Erichsen (IE) – the measure of ductility in a sheet metal – and the yield strength of different Mg alloys at room temperature. This shows that as the yield strength increases, the IE value decreases, thus demonstrating the poor formability of Mg alloys at room temperature.

^{Figure 1 Yield strength and stretch formability represented by the Index Erichsen (IE) value at room temperature of various Mg alloys sheets. Higher IE values mean that the alloys exhibit better formability. Retrieved from Ref. [4]}

Magnesium alloys are the third most popular non-ferrous casting material. The physical properties of the alloys change based on their chemical compositions. Adding different alloying elements would result in different properties at different conditions.

• Aluminium improves strength, hardness and ductility, facilitating the alloy’s casting process.
• Zinc increases room-temperature strength, fluidity in casting, and corrosion resistance.
• Manganese increases the resistance of AM and AZ alloys to saltwater corrosion by forming intermetallic compounds with iron-like metals, to be removed during melting.
• Rare earth metals help increase strength and resistance to high-temperature creep and corrosion, and decrease porosity and weld cracking.
• Zirconium is a strong grain refiner when added to alloys containing zinc and rare earth metals.
• Beryllium helps decrease surface oxidation during casting and welding.
• Calcium increases grain refinement, which helps in controlling the metallurgy of the alloy [4].

Applications

Magnesium alloys cover a wide array of applications, from automotive and aerospace applications to electronic and biomedical uses.

Structural applications

Automotive, aerospace, industrial, and commercial applications are examples of structural applications. The advantage of magnesium alloys to be used in such applications is their light weight, high strength-to-weight ratio, high stiffness-to-weight ratio, castability, machinability, and great damping [4].

• Automotive: support brackets for brakes and clutch, housing for transmission
• Aerospace: landing wheels, helicopter rotor fittings, gearbox housings
• Industrial: high-speed operating machinery, such as textile machines
• Commercial: luggage, hand tools, computer housings, ladders

Electronic applications

Electronic applications include electronic packaging, hard drive arms, cell phone and portable media device housings. Magnesium alloys are being used instead of plastics due to their light weight, strength and durability. They also are relatively better for heat dissipation and protection against electromagnetic and radio frequency interference [5].

Medical applications

Portable medical equipment and wheelchairs that require light material make good use of magnesium alloys. Also, cardiovascular stents and orthopaedic devices are potential applications of some magnesium alloys due to magnesium’s biocompatibility and bioabsorbability [4].

[1] Trang, T. T. T. et al. (2018) Designing a magnesium alloy with high strength and high formability, Nature Communications 9, 2522

[2] National Research Council. (1975) Properties of Magnesium and Magnesium Alloys. In Trends in Usage of Magnesium. (pp. 37-42). Washington, DC: The National Academies Press

[3] ASM International. (2017) Introduction of Magnesium Alloys. In C. Moosbrugger (Ed.), Engineering Properties of Magnesium Alloys. (pp. 1-10). Novelty, OH: ASM International

[4] Woodhead Publishing. (2010) Overview. In P.K. Mallick (Ed.) Materials, Design and Manufacturing for Lightweight Vehicles. (pp. 1-32). Woodhead Publishing

[5] (n.d.) Magnesium Applications. International Magnesium Retrieved from: https://www.intlmag.org/page/mg_applications_ima