Magnesium alloy is a metallic mixture where magnesium serves as the primary base element. It is recognized as the lightest structural metal available to engineers, with a density of about 1.74 grams per cubic centimeter, significantly less than aluminum or steel. This low mass combined with desirable mechanical properties makes it a highly valued material in modern engineering design. Magnesium alloys address the growing demand for lightweight components across various industries, improving performance and efficiency.
Composition and Basic Metallurgy
An alloy is created by introducing other metallic elements to pure magnesium, dramatically improving its mechanical properties like strength and corrosion resistance. The most common alloying elements include Aluminum, Zinc, Manganese, and Zirconium. Aluminum is the most frequently added element, enhancing strength and hardness. Zinc addition often improves creep resistance, which is the ability to resist deformation over time at elevated temperatures.
Magnesium naturally possesses a hexagonal close-packed (HCP) crystalline structure, which is less symmetrical than the cubic structures found in steel and aluminum. This HCP structure limits the number of slip systems available for plastic deformation, leading to poor ductility and formability in pure magnesium. Alloying elements help modify this structure, and processing, like hot-working, is required to overcome these limitations and create complex shapes. Manganese improves resistance to saltwater corrosion, and Zirconium is used as a potent grain refiner, ensuring a finer, more uniform microstructure.
Key Material Characteristics
The primary characteristic driving the use of magnesium alloys is their extremely low density, approximately one-third lighter than aluminum alloys and nearly 75% lighter than steel. This low density leads to a high specific strength (the material’s strength-to-weight ratio), making it possible to design strong components with minimal mass.
Magnesium alloys also exhibit an exceptional damping capacity, meaning they effectively absorb mechanical vibrations and noise. This quality is valuable in components subjected to high frequency vibrations, such as engine parts, helping to reduce noise, vibration, and harshness. Magnesium is also known for its superior machinability, allowing for very high cutting speeds and low tool wear during manufacturing, which reduces production time and cost. While pure magnesium is susceptible to galvanic corrosion, modern high-purity alloys and protective coatings mitigate this challenge, making the alloys suitable for corrosive environments.
Major Categories and Designation Systems
Magnesium alloys are formally classified using a standardized system developed by the American Society for Testing and Materials (ASTM B275). This system uses a combination of letters and numbers to indicate the alloy’s composition. The designation begins with two letters representing the two main alloying elements by weight, listed in order of decreasing concentration (e.g., ‘A’ for Aluminum, ‘Z’ for Zinc).
Following the two letters are two numbers corresponding to the rounded-off nominal weight percentages of those two elements. For example, AZ91 contains approximately 9% Aluminum and 1% Zinc. A final letter, such as ‘A’ or ‘B’, distinguishes between alloys with the same nominal composition but slight differences in impurity levels or processing. Alloys are broadly categorized as casting alloys (shaped in a molten state) or wrought alloys (shaped through processes like extrusion or forging).
Real-World Applications
The distinctive properties of magnesium alloys have made them indispensable in industries focused on efficiency and portability. In the automotive sector, the pursuit of better fuel economy and reduced emissions has driven the use of magnesium alloys for components like engine blocks, transmission cases, and steering wheel armatures. Using alloys like AZ91 in die-cast parts achieves substantial weight savings.
The aerospace industry utilizes magnesium alloys, often strengthened with rare-earth elements, in secondary structures and non-structural parts where weight reduction is important. Examples include gearbox housings, brackets, and interior components, where low density translates into increased payload capacity and improved performance. Magnesium alloys are also widely adopted in consumer electronics for creating casings for laptops, cameras, and cell phones. Their low mass provides a lightweight and sturdy enclosure, and they offer the benefit of electromagnetic interference shielding.