Magnesium Alloys

Definition of Magnesium Alloys

The lightest structural metal, magnesium, has an approximately a third of the density of aluminium. Magnesium alloys derive their enhanced properties by alloying magnesium with multiple other metals.

• Wrought alloys that are meant to sustain substantial plastic deformation during manufacturing.
• Cast alloys developed for applications requiring intricate and complex shapes.

In both kinds, the commonly used alloying elements include aluminium, zinc, silicon, manganese, and more.

History of Magnesium Alloys

Magnesium alloys have considerably evolved over the past century based on the demands of the various industries and domains they are used in.

Importance of Magnesium Alloys in Engineering

Magnesium alloys have gained tremendous popularity in engineering applications, primarily due to their remarkable weight to strength ratio, recyclability, and acceptable cost. Given this, they are being extensively utilised in diverse areas such as:

• Automotive: Used in car parts for reducing weight.
• Aerospace: Frameworks and parts of airplanes and spacecraft.
• Consumer Electronics: Cell phone and laptop casings.
• Medical: Orthopaedic implants and devices.

Their low density, high strength-to-weight ratio, excellent machining characteristics, and good damping capacity make these alloys well suited for these applications.

Magnesium Alloy Material Properties

Properties of magnesium alloys can be broadly categorised as physical and chemical characteristics. Both sets of properties can greatly influence their functionality and performance in a wide array of applications. It is this diverse portfolio of properties that makes the alloys increasingly attractive to engineers and designers.

Physical Properties of Magnesium Alloys

Magnesium alloys possess several physical properties that impact their usefulness in various applications. These physical properties include:

• Low density: Magnesium is the lightest structural metal available in the world. Its light weight makes it a good choice for applications where weight reduction is crucial such as automotive, aerospace, and portable electronics.
• High strength-to-weight ratio: Although magnesium is lightweight, it exhibits significant strength. When alloyed with other metals, its strength magnifies, making it a robust and reliable material.
• Good damping capacity: Magnesium alloys possess high damping capacity which is a measure of resistance to vibrational or oscillatory motion. This particular property finds useful applications in automotive and railway industry.
• Excellent machinability: Magnesium alloys are renowned for their machinability. They can be handled with less power, and thus, they consume less manufacturing energy.

Alloying plays a crucial role in enhancing the physical properties of magnesium. For instance, addition of aluminium to magnesium forms an alloy that possesses significantly improved stress-corrosion cracking tendency. This alloy, known as AZ91, is extremely popular in high-pressure die casting applications.

Chemical Properties of Magnesium Alloys

Magnesium alloys, owing to their chemical nature, exhibit a few characteristic properties:

1. Reactivity: Being an active metal, magnesium has a high affinity for oxygen, which makes it susceptible to oxidation. To mitigate this challenge, magnesium alloys often incorporate heavy metals like nickel or rare earth elements.
2. Affinity for silicon: Magnesium exhibits considerable affinity for silicon. This is beneficial as it leads to formation of magnesium silicide layers, which can prevent diffusion of corrosive species to the metal surface, thus enhancing corrosion resistance.

It should be noted that the chemical properties of magnesium alloys can be further tailored according to application requirements by means of alloying and heat-treatment techniques.

Corrosion Resistance of Magnesium Alloys

Corrosion resistance is an essential consideration when working with any metal, and magnesium alloys are no exception. Magnesium alloys are usually more susceptible to corrosion than most other metals, primarily due to their active electrochemical behaviour. However, this can be mitigated using various techniques.

One widely employed method is anodising. This technique utilises electrochemical processes to create a protective oxide layer over the surface of the magnesium alloys. Surface treatments such as chromating, phosphating, and plating are also effective in improving their corrosion resistance.

Strength and Durability of Magnesium Alloys

When talking about the strength and durability of magnesium alloys, it's important to realise that these terms refer to how well a material withstands applied forces without deforming or breaking.

The yield strength of magnesium alloys depends heavily on the alloying elements and their concentration. The tensile strength, which measures a metal's resistance to being pulled apart, for magnesium alloys usually ranges from 130 to 300 MPa. Its high strength-to-weight ratio makes it an excellent candidate for aerospace and automotive industries.

Additionally, the durability of magnesium alloys, referring to their lifespan and ability to endure harsh conditions over time, is enhanced by various treatments such as hot working, cold working, and heat treatments (like solution annealing and precipitation hardening).

Types of Magnesium Alloys

While magnesium alloys are versatile, they can be specifically categorised based on their enhanced properties. There are various types primarily grouped as High Strength Magnesium Alloys, Corrosion Resistant Magnesium Alloys, and Heat Resistant Magnesium Alloys.

High Strength Magnesium Alloys

Within the domain of magnesium alloys, certain variants were specifically designed to offer higher strength. These high strength magnesium alloys are typically reinforced with elements such as aluminium, zinc, or rare-earth elements. When compared to standard magnesium alloys, these high-strength variants exhibit greater durability under mechanical stress, making them ideal for structural applications where stability is crucial.

A widely utilised high strength magnesium alloy is AZ91. Constituting of 9% aluminium and 1% zinc with the balance being magnesium, AZ91 is renowned for exceptional toughness and resistance to impact. More importantly, aluminium enhances strength whereas zinc ensures good crevice corrosion resistance.

Certain high-strength magnesium alloys are fortified with rare-earth elements such as yttrium. One such alloy is WE43, which consists of yttrium, heavy rare earth elements, and zirconium. The presence of these elements substantially elevates the alloy's strength even at elevated temperatures. Similarly, ZK60 is another superior high strength magnesium alloy, it constitutes of zinc and zirconium and demonstrated excellent strength and ductility.

Corrosion Resistant Magnesium Alloys

Though magnesium alloys inherently tend to be more susceptible to corrosion, through careful alloy design and heat treatment processes, corrosion-resistant magnesium alloys have been achieved. These alloys successfully slow down or prevent corrosion even when exposed to saline, acidic or moist environments.

The most effective way of enhancing the corrosion resistance of magnesium alloys is through alloying with elements that have a better natural resistance to corrosion, especially aluminium, manganese, and rare-earth elements. For instance, AM60, comprising mostly of magnesium, with 6% aluminium and a trace of manganese, is known for good corrosion resistance, ductility and excellent castability.

Another notable high corrosion resistance alloy is AE44, which is constituted by 4% aluminium and rare-earth elements. This alloy is predominantly used in automotive applications because of its improved corrosion resistance and high performance at elevated temperatures.

Various surface treatments have also been widely used to improve the corrosion resistance of magnesium alloys. Sealing, anodizing, and chromating are some of them that form an impervious layer of magnesium oxide on the alloy surface impairing the corrosion process.

Heat Resistant Magnesium Alloys

Heat resistant magnesium alloys are a specific class of alloys that are able to maintain their properties even at high temperatures. These are increasingly significant in environments where thermal stress is a regular occurrence such as engines, power transmission lines, and high-speed aircraft.

The addition of specific alloying elements like aluminium, zinc, strontium, and rare-earth elements is the common approach to formulate heat resistant magnesium alloys. For instance, AS41, composed mostly of magnesium with 4% aluminium and 1% silicon, is an alloy that shows improved resistance to high temperatures.

A series of alloys with a high content of rare-earth elements, QE22, WE43, and EV31A are also known for their excellent high-temperature performance. QE22 is considered to have one of the highest creep resistances among magnesium alloys making it a perfect choice for powertrains and turbochargers in vehicles.

It should be noted that the heat resistance of these alloys also depends greatly on their heat treatment processes. Processes such as solution annealing, quenching, and age hardening can all play vital roles in increasing the heat resistance of magnesium alloys.

Advantages Of Magnesium Alloys

Magnesium alloys, having unique physical and chemical properties, offer a series of advantages in engineering applications. The very attributes that characterise these alloys, such as their lightweight nature, high strength-to-weight ratio and cost-effectiveness, translate into their significant potential in diverse industrial sectors especially in automotive, aerospace, and electronics applications.

Lightweight Nature of Magnesium Alloys

Magnesium is the lightest of all structural metals, making magnesium alloys exceptionally lightweight. This characteristic has a profound influence in numerous applications primarily in weight-sensitive industries such as aerospace and automotive.

The aerospace industry constantly strives to reduce the weight of components to improve flight efficiency. Lightweight nature of magnesium alloys enables a significant reduction in the weight of various aircraft parts which, in turn, leads to improved fuel efficiency and reduced emissions. Similarly, in the automotive industry, lightweight vehicles have better acceleration, improved braking, and enhanced fuel efficiency. As a result, by replacing heavy steel components with light magnesium alloys, manufacturers can significantly increase the efficiency and performance of vehicles.

Magnesium alloys also play an instrumental role in portable electronics where weight is a crucial concern. From the housing of smartphones to the body of laptops, the low density of magnesium alloys helps keep the weight of these devices to a minimum without compromising their strength and durability.

Magnesium Alloy's High Strength-to-Weight Ratio

Despite being lightweight, magnesium alloys boast a high strength-to-weight ratio. This property measures the material's strength - defined by stress limits before yield and fracturing - against its weight. Mathematically, the strength-to-weight ratio is generally calculated as the yield strength divided by its density:

\[\text{Strength-to-Weight Ratio} = \frac{\text{Yield Strength}}{\text{Density}}\]

This high strength-to-weight ratio makes magnesium alloys a preferred choice for applications needing robustness and compactness. Due to their ability to withstand great stress while maintaining lightness, these alloys are perfect for creating long-lasting, high-performance parts ranging from automotive engine blocks to bicycle frames and even bulletproof vests.

Magnesium alloys offer robustness that is comparative to that of steel and aluminium, while being substantially lighter. This lends them an advantage in industries where both weight reduction and durability are sought-after, such as automotive and aerospace. For instance, in automotive applications, using magnesium alloy parts like engine blocks or body panels can not only reduce the vehicle's weight but also improve its performance and increase its lifespan.

Cost-Effectiveness of Magnesium Alloys

The economical benefits of magnesium alloys are often as attractive as their physical and chemical properties. First and foremost, magnesium is the eighth most abundant element on Earth and is readily available, making it a more economical option compared to rarer metals.

Moreover, magnesium alloys offer excellent machinability, meaning they can be easily shaped and worked on. This reduces processing costs, such as those associated with cutting, drilling and moulding, thereby making the overall manufacturing process more cost-effective.

The durability and long lifespan of magnesium alloys also contribute to cost savings. Due to their superior resistance to wear and tear, parts manufactured from these alloys generally have longer operation lives, reducing the need for frequent replacements and related maintenance costs.

Lastly, in industries like automotive and aerospace, the fuel savings resulting from reduced weight make magnesium alloys a more cost-effective solution over the lifespan of the vehicle or aircraft. Therefore, while the upfront cost of magnesium alloys may be higher than some traditional materials, the overall cost-effectiveness is often superior when considering the total lifecycle costs.

Disadvantages of Magnesium Alloys

While magnesium alloys are celebrated for their numerous advantages, it's imperative to consider their drawbacks that might limit their application in certain fields. The prominent challenges faced with magnesium alloys revolve around their vulnerability to corrosion, high flammability, and potential limitations in high-temperature environments.

Vulnerability to Corrosion in Magnesium Alloys

One of the significant limitations of magnesium alloys lies in their predisposition towards corrosion. Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide or sulphide. Since magnesium is highly reactive, its alloys are prone to corrosion when exposed to harsh conditions, especially in marine and acidic environments.

This high reactivity of magnesium arises from its position in the galvanic series, where it's found at the extreme end as the most active metal. The galvanic series ranks metals based on their electrochemical reactivity and their tendency to corrode, with those at the top being least corrosive and those at the bottom most corrosive. Since magnesium alloys sit at the lower end of the series, they are more likely to lose electrons during exposure to corrosive environments, leading to corrosion.

Frequent forms of corrosion seen in magnesium alloys include galvanic corrosion, crevice corrosion, and pitting corrosion. Galvanic corrosion occurs when a magnesium alloy is in electrical contact with a higher noble metal, and both are immersed in a conductive fluid. Due to the higher reactivity of magnesium, it corrodes preferentially, while the noble metal is protected. Crevice corrosion is a localised form of corrosion typically occurring inside small cracks or gaps where stagnant solution can accumulate. Pitting corrosion manifests itself as small, pinhole-like pits on the surface of the magnesium alloy, often resulting in failure under load.

Although several potential solutions have been proposed to mitigate corrosion in magnesium alloys, each has its own set of challenges. On one hand, alloying with elements like rare-earth elements, aluminium, and zinc can improve corrosion resistance but also makes the alloy more expensive. On the other hand, protective coatings can considerably slow down corrosion but can also introduce other issues like potential contamination during coating application or the eventual wear of the protective layer.

High Flammability of Magnesium Alloys

Another notable drawback of magnesium alloys is that they are highly flammable. Magnesium, being a reactive element, is known to combust at relatively low temperatures. This means that in environments where the temperature may rise above the magnesium ignition temperature of \( 473^{\circ}\, \mathrm{C} \) , also known as flash point, there's a risk of fire.

Magnesium combustion results in intense, bright, white flames which are difficult to extinguish with water or carbon dioxide, making magnesium fires dangerously persistent. Hence, dealing with fire incidents involving magnesium alloys demands special precautions. Sand, dry chemical powder, or special Class D extinguisher are the preferred extinguishing agents for magnesium fires.

Despite the associated risks, magnesium alloys are still widely used in various applications due to their advantageous properties. However, their flammability necessitates careful handling and specific safety precautions, particularly during the manufacturing, storage, and disposal stages.

Limits of Magnesium Alloys in High Temperature Environments

At high temperatures, magnesium alloys exhibit reduced mechanical properties, which could limit their applications. The reduced performance includes lower tensile strength and altered physical properties, which may negatively affect their functionality in a high temperature environment.

Another challenge of employing magnesium alloys at elevated temperatures is their potential degradation due to the progressive oxidation of magnesium. At high temperatures, magnesium in the alloy reacts with oxygen in the environment to form magnesium oxide, a process called oxidation. This oxidation leads to the formation of a brittle and non-protective layer of magnesium oxide on the surface of the alloy, which can then lead to fracturing and a consequent reduction in structural integrity.

It's also worth mentioning that, because of the high thermal expansion coefficient of magnesium, parts made from its alloys could change dimensionally when subjected to significant temperature changes. This lack of dimensional stability can be a drawback in high-precision applications where dimensional tolerance is critical.

Those who seek to use magnesium alloys in high-temperature applications often resort to alloying with elements that enhance heat tolerance. For instance, alloying magnesium with elements like aluminium, zinc, and rare-earth elements can enhance its thermal stability and increase its creep resistance. However, doing so often results in a trade-off with other properties like ductility and fracture toughness. Therefore, finding the right balance between high temperature performance and other desired properties is crucial when designing magnesium alloys for high-temperature applications.

Magnesium Alloy Applications

Magnesium alloys stand their ground as an attractive choice for various applications across different industries. This appeal is attributed to their impressive properties such as low density, excellent strength–to–weight ratio, and good machining characteristics.

Magnesium Alloys in the Automotive Industry

Magnesium alloys have earned their place in the automotive industry due to their high strength-to-weight ratio and low density. The production of lighter vehicles is a goal for most automotive companies, as reducing the weight of a vehicle leads to greater fuel efficiency, decreased emissions, and improved performance.

Across the automotive industry, magnesium alloys are utilised in various applications such as:

• Car engines: They are used in making engine blocks due to their excellent heat dissipation properties. The low density of these alloys results in lighter engines, contributing to overall vehicle fuel efficiency.
• Steering wheels: The alloys are used to make steering wheels because of their high impact resistance.
• Transmission cases: The excellent castability and dimensional stability of these alloys make them ideal for transmission cases.

Despite their advantageous characteristics, the wide use of magnesium alloys in the automotive industry is somewhat hampered by issues such as corrosion and high costs of alloy production. However, the introduction of protective coatings and other innovative techniques are progressively overcoming these challenges.

Use of Magnesium Alloys in Aerospace Engineering

The aerospace industry presents one of the most demanding applications for magnesium alloys. With the increasing need for lightweight materials that can withstand the extreme conditions experienced in flight, magnesium alloys have proven to be a sound choice.

In aerospace, magnesium alloys are used in applications such as:

• Aircraft structures: They are used in the construction of frames and structural components of the aircraft due to their excellent strength-to-weight ratio and good shock absorption characteristics.
• Engine parts: Given their good heat dissipation properties, magnesium alloys are effective in engine parts.
• Interior components: Their high dimensional stability and ease of machining make them perfect for interior components like seating structures.

However, the flammability of magnesium alloys at high temperatures poses a significant challenge in aerospace applications. This calls for special alloying and coating techniques to improve their overall fire resistance.

Magnesium Alloys in Electronics and Mobile Devices

Magnesium alloys are an excellent choice in the electronics and mobile devices industry, where the demand for lightweight, compact, and high-performing materials is ever increasing. These alloys are especially appreciated for their low density, high strength, excellent electromagnetic shielding, and good machinability, making them perfect for applications like laptop frames, mobile phone cases, and camera bodies.

That said, potential issues like susceptibility to corrosion in humid conditions and higher production costs compared to plastic counterparts must be addressed for wider uptake of magnesium alloys in this sector. But with advances in alloying techniques and protective coatings, magnesium alloys continue to hold promising potential in electronics and mobile devices spheres.