Aluminum (Al) and magnesium (Mg) are two important lightweight structural metals used in modern engineering. They are utilized in applications where minimizing mass is paramount, such as in aerospace and high-performance vehicles. Aluminum is the most abundant metal in the Earth’s crust, while magnesium is the lightest structural metal. Determining which metal is “stronger” is complex, as the answer depends entirely on the specific engineering definition of strength being applied.
Defining Strength: The Engineering Metrics
Engineers define a material’s absolute strength using metrics that measure its resistance to deformation and fracture under load. Tensile Strength is the maximum stress a material can endure before it breaks. Yield Strength is the point at which the material begins to deform permanently.
When comparing alloys of similar quality based on these absolute metrics, aluminum typically outperforms magnesium. High-strength aluminum alloys can exhibit tensile strengths up to 700 megapascals (MPa), while magnesium alloys generally top out around 420 MPa.
A third measure is the Modulus of Elasticity, or stiffness, which quantifies a material’s resistance to flexing. Aluminum alloys have a modulus of elasticity around 68 to 70 Gigapascals (GPa), whereas magnesium alloys are notably lower, closer to 45 GPa. This difference means that aluminum is stiffer and more resistant to elastic deformation than magnesium for the same volume of material.
Specific Strength vs. Absolute Strength
The absolute strength advantage of aluminum shifts when considering the metal’s weight, which introduces the concept of Specific Strength. Specific strength is calculated by dividing a material’s absolute strength by its density, measuring strength-per-unit-mass. This metric is used in applications where weight reduction is a primary design goal.
Magnesium’s claim to being “stronger” is based on its significantly lower density. At approximately 1.74 grams per cubic centimeter (g/cm³), it is about 35% lighter than aluminum (2.70 g/cm³). Due to this weight difference, magnesium alloys often possess a superior specific strength compared to aluminum alloys. A magnesium alloy may offer a specific strength of around 158 kilonewton meters per kilogram (kN·m/kg), surpassing an aluminum alloy’s typical 130 kN·m/kg.
This superior specific strength allows engineers to design a thicker structural component made of magnesium that is lighter than its aluminum counterpart and equally strong. The increased thickness also contributes to greater rigidity without incurring the weight penalty of a similarly thickened aluminum part. In weight-sensitive designs, a magnesium component can be considered functionally superior.
Comparing Durability and Practical Limitations
Beyond strength and weight, real-world application is governed by durability and manufacturing considerations that often favor aluminum. Aluminum naturally forms a thin, dense, self-healing oxide layer on its surface, giving it excellent corrosion resistance in most environments. Magnesium is more chemically reactive and highly susceptible to corrosion, especially in saltwater or humid conditions, requiring extensive protective coatings.
Aluminum exhibits better fatigue life, meaning it can withstand a greater number of stress cycles before developing cracks. Magnesium’s lower fatigue life and susceptibility to permanent deformation at elevated temperatures—known as creep resistance—limit its use in high-stress, high-heat engine components.
Manufacturing processes present distinct trade-offs for the two metals. Aluminum is easier to cast and machine without complex safety measures. Magnesium, particularly in its fine powder or molten state, carries a risk of flammability, necessitating specialized equipment and handling protocols. While magnesium is known for its excellent machinability, the enhanced safety required for its processing can increase production complexity and cost.
Where Each Metal Excels in Industry
The distinct profile of each metal dictates its dominant industrial applications, illustrating a clear division based on design priorities. Aluminum’s superior corrosion resistance, better fatigue life, and lower cost make it the material of choice for large-scale, long-life structures. It is the dominant material for aircraft primary structures, automotive body panels, and beverage cans.
Magnesium, with its advantage in specific strength and exceptional vibration-damping characteristics, dominates applications where low mass is the priority. It is commonly used for portable electronic casings, such as laptops and cameras, where lightness and structural rigidity are desired. In the automotive industry, magnesium is found in interior components, steering wheel frames, and high-performance racing parts where every saved gram contributes to better vehicle dynamics.