Magnesium vs Aluminium: Which Light Metal Reigns Supreme for Your Application?

There is no overall winner of the game here; as each metal is best suited for certain applications... whilst equally unsuitable for others. Knowing that in advance saves a lot of money and effort down the road.

Density: Key Advantages of Magnesium

The AZ31B magnesium alloy has a density of 1.8 g/cm³ and hence is ⅓ lighter than aluminium alloys - that have a density of 2.7-2.8 g/cm³. The discrepancy in density becomes a critical determinant in industries that are weight sensitive such as aerospace, motorsport, and top-end automotive industry.

However, the density should not be the only consideration when making decisions on which metal to use. Magnesium has an elastic modulus of 45 GPa while aluminium has an elastic modulus of 70 GPa. This means that aluminium does not easily deform due to pressure. The parts of magnesium are always thick walled and have deep ribs; this makes magnesium less effective.

Aluminium has also started to catch up with the rest in a lot of ways. For example, 2099 aluminium-lithium alloys are a combination of aluminium and lithium that reduce density and increase stiffness. This is the opposite of what usually happens. If stiffness per unit of weight is critical, then aluminium may work well.

Strength and Heat Resistance

Aluminium offers the broader range of alloy systems and heat treatments. 2014 is a copper-based alloy and highly malleable besides having high strength. 7075 zinc-based alloys have high tensile and yield strengths, depending on the temper. When elevated temperatures are required, 2618 (with additions of Cu, Mg, Fe and Ni) can be used to retain high strength levels when other aluminium alloys already start losing theirs.

Magnesium is one of the narrower groups of metals in terms of overall range. AZ31B magnesium alloy is a widely used wrought magnesium alloy that has good formability properties and average strength. At temperatures ranging from 100°C to 150°C, AZ31B loses its strength; however, ZK60A (zinc zirconium alloy) is significantly stronger. ZK60A is selected when lightening up is essential and no exposure to heat is expected. As a rule of thumb, magnesium has less tolerance to elevated temperatures than aluminium.

Magnesium alloys have one advantage that often goes unnoticed – they provide good vibration damping qualities. Whenever housings, casings and other components with reduced vibration and fatigue loading are needed, magnesium can be better than aluminium.

Corrosion Resistance and Surface Finish

The alloys of 2000 and 7000 series and their tempers need some additional protection from corrosion in certain cases. Therefore, it cannot be said that all aluminium alloys have corrosion-resistant properties.

As magnesium is more active element than aluminium, it can form galvanic pairs with virtually all building materials. If the magnesium alloy is electrically connected with steel, copper, and aluminium in humid environment, it is highly probable that galvanic corrosion will take place. For this reason, the proper coating and insulation from other metals are needed to prevent magnesium corrosion.

Quite often, the structural elements, made of magnesium alloy, fail at the stage of operation due to the wrong mixture of magnesium and other metals.

Manufacturing and Processing

Aluminium is widely regarded as straightforward to machine, form, weld and finish, and the knowledge base around it is deep. The hexagonal crystal structure of magnesium makes it poorly formable at room temperature. That is why, forging of magnesium is carried out at elevated temperatures. AZ31B magnesium alloy is much more easily welded than other magnesium alloys, which contain higher amounts of zirconium.

Magnesium is the material that requires special approach during the process of machining. Although the solid magnesium will not catch fire, the magnesium chips are good burning materials. Therefore, one should use sharp tools and special handling of the magnesium chips, as well as fire safety measures. A skilled machinist can machine the magnesium alloy very efficiently.

How to Choose an Alloy

Aluminium suits those applications where temperature tolerance, corrosion resistance, elasticity and easy manufacturability are at least as important as the weight. Magnesium finds its place in cases when weight reduction is the main criterion, vibration damping is needed or the assembly allows for proper protection and manufacturing of this alloy.

Although the price of the raw material may not be all there is to it, the cost of the part may still be significantly increased or reduced by processing, tooling, and machining.

At Dynamic Metals, we provide in various finishes - both aluminium and magnesium alloys, documentation included. Feel free to contact us for further information regarding your specific requirements.