Magnesium, the lightest of all structural metals, presents a mechanical paradox to manufacturers and engineers. Although many metals are easily formed at ambient temperature, magnesium exhibits a unique reluctance to change shape when cold. Understanding this behavior requires examining the metal’s internal atomic structure and the conditions under which its properties change.
Defining Metallic Properties
Ductility and malleability are two distinct properties that describe a material’s capacity for permanent shape change, known as plastic deformation. Ductility refers to the material’s ability to undergo significant plastic deformation under tensile (stretching) stress without fracturing, allowing it to be drawn into a thin wire. Malleability is the capacity for plastic deformation under compressive (pressing or hammering) stress, allowing it to be rolled into thin sheets without cracking. While both properties are measures of a material’s plasticity, they depend on the type of stress applied.
Magnesium’s Workability at Room Temperature
Pure magnesium is characterized by a low degree of workability at room temperature. When subjected to common forming processes like rolling or bending at ambient temperatures, the metal is prone to cracking and fracture. This behavior classifies magnesium as relatively brittle under cold working conditions. However, the metal’s mechanical properties undergo a significant shift when heat is applied. When heated, magnesium becomes highly workable, exhibiting both ductility and malleability. This temperature dependence means that magnesium components are almost always shaped using elevated-temperature forming processes.
The Atomic Explanation for Resistance to Shaping
The underlying reason for magnesium’s resistance to cold shaping lies in its crystalline arrangement, known as the Hexagonal Close-Packed (HCP) structure. In metals, plastic deformation occurs when layers of atoms slide past each other along specific crystallographic planes, referred to as slip systems. For a material to be easily deformable, such as copper or aluminum, it must have a sufficient number of these independent slip systems.
Magnesium’s HCP structure possesses a limited number of active slip planes at room temperature, unlike the Face-Centered Cubic structure of highly ductile metals. Deformation is primarily confined to the basal plane, which is the flat, closely packed layer of atoms. According to the von-Mises yield criterion, a material needs at least five independent slip systems to accommodate general plastic deformation. Since only a small number of slip systems are easily activated, the metal cannot deform plastically to relieve pressure, leading to a sudden, brittle fracture.
Practical Consequences in Manufacturing
The poor room-temperature workability of magnesium necessitates specialized manufacturing techniques, primarily involving high heat. To overcome the structural limitations of the HCP lattice, engineers must perform hot working, which involves heating the metal to a temperature range typically between 250°C and 400°C. Applying heat activates additional, non-basal slip systems that are dormant at lower temperatures, and the activation of these pyramidal and prismatic slip systems increases the number of available deformation pathways. This allows the metal to be successfully rolled, forged, or extruded into complex shapes without fracturing. Despite the added manufacturing complexity, magnesium’s exceptionally low density ensures its continued use in weight-sensitive applications, particularly in the automotive and aerospace industries.