Selecting the right lightweight metal

Many industries are looking for innovative ways to reduce their costs, lower the weight of their products, and reduce their overall energy consumption. As a result, lightweight metals, such as aluminium, magnesium, and titanium are considered increasingly as alternatives to steel. With new research into alloys and surface technologies like PEO, engineers are able to use these lightweight metals in ways that would previously have been considered inappropriate. To find the right material solutions, it is important to have a sense of the potential advantages and drawbacks of each metal, and how they might impact on the project at hand.

Aluminium

Aluminium has long been used as an alternative to stainless steel:

• It is cheaper than steel to cast and fabricate, and the cheapest of the metals we’re looking at pound for pound.
• Its passive oxide layer gives it high corrosion resistance, which can be further improved through anodising or PEO.
• It is about a third of the density of steel, giving it a useful strength-to-weight ratio. Its that is easily further improved through alloys and coating techniques.
• Aluminium has a high ductility and malleability. As a result it can be precision machined with ease. This saves time in the process of fabrication, making it a greener and more economic option.

Despite these advantages, it is worth keeping in mind:

• The low hardness of aluminium tends to give it poor abrasion and wear resistance. Hence, hard wearing coatings are required in many circumstances to enable its use where it otherwise provides suitable mechanical properties.
• While aluminium does have a fairly low tensile strength, there are alloys that can raise it from 70 MPa to around 700 MPa, providing a very high strength-to-weight ratio. It should be noted that the price for such high strength tends to be a significant loss of corrosion resistance. Coatings are normally essential to prevent corrosion where high-strength alloys such as 7xxx and 2xxx series are employed.
• Although it is widely used in food packaging and cooking utensils, there is some concern about aluminium’s biocompatibility and potential links to Alzheimer’s disease. Again, protective coatings can provide the answer in many cases, helping to ensure no reaction of the substrate occurs.

From aircraft fuselage to coke cans, aluminium, with its light weight, low cost, and ease of fabrication lends itself to a myriad of engineering applications:

• Apple have led the way in the widespread use of aluminium to make the distinctive bodies of their MacBooks, iPhones, and iPads. Steve Jobs’ enthusiasm for the metal even led him to order a custom aluminium yacht. Since Apple’s pioneering use of aluminium, it is now the choice material for laptops and phones.
• Many cars have a lightweight aluminium hood and other body panels. Typically, major engine components such as engine blocks and pistons are now almost exclusively made from cast aluminium alloys. Other lightweight aluminium components such as brake callipers, electrical housings, interior trim parts all help to reduce vehicle weight and increase fuel efficiency.

Magnesium

A surge in interest over the past decade has revealed how magnesium alloys and coating techniques can make the most of its attractive properties:

• Magnesium is extremely light: it is 75% lighter than steel, 50% lighter than titanium, and 33% lighter than aluminium.
• It has the highest known damping capacity of any structural metal, capable of withstanding 10x more than aluminium, titanium, or steel.
• It is very easy to machine, and can be injection moulded.
• Magnesium is entirely biocompatible, posing no toxicity hazards.

On the other hand, it has some well known shortcomings that limit its wider applicability.

• The metal is chemically highly active, so chemical and corrosion resistance tends to be low
• Low surface hardness, like aluminium, makes it difficult to use in tribological applications without a coating
• Perennial concerns about flammability sometimes rule out the use of magnesium, sometimes without justification. Nonetheless, this aspect should still be considered as part of a holistic material selection process.

Since the 1998 ACEA agreement, legislation limiting carbon emissions has led the automotive industry to investigate ways in which the extremely light weight of magnesium can be made fit for purpose. Prior to this surge in interest, magnesium had seemed unusable in many industrial contexts:

• Magnesium’s high reactivity had made it susceptible to corrosion. However, recently discovered alloys and higher-purity variants of traditional alloys have a much greater resistant to corrosion, and new coating techniques such as plasma electrolytic oxidation (PEO) make a thoroughly resistant neutral oxide from the metal’s substrate.
• Magnesium’s poor creep resistance had made it unsuitable for high temperatures, but recently discovered alloys such as ZE41 & ZWO8203 are heat resistant at extreme temperatures (c. 400 F). PEO coatings also make magnesium extremely heat resistant.
• Magnesium’s low tensile strength had made it unsuitable for structural uses, but new alloys and coatings mean this is no longer the case.

As a result of these developments, magnesium is increasingly being used in a range of settings:

• Car seats, power tools, luggage, and cameras have all been designed to make the most of lightweight, strong magnesium.
• Military engineers have begun using magnesium in helicopter gearboxes and generator housings, as a means of providing lightweight resistance to extreme temperatures.
• High-performance mountain bike frames and wheels are increasingly made of lightweight, corrosion-resistant magnesium.
• The aviation and automotive industries are increasingly looking at ways magnesium can increasing fuel efficiency and reducing greenhouse gases.
• Complex, light, and strong components such as those found in engines can easily be moulded out of magnesium.

Exciting developments in magnesium alloys, manufacturing methods and coating technologies are making magnesium an increasingly viable candidate for a strong, lightweight, and cost-effective solution.

Titanium

Titanium is significantly stronger than both aluminium and magnesium, although its higher density means that strength-to-weight ratios for the three metals tend to be similar. It is often the first port of call for engineers looking to replace steel in a lightweighting exercise for stressed components. It has the additional advantage of being highly corrosion-resistant and also has very high biocompatibility.

Unfortunately the high cost of extraction and fabrication may rule out its use for the general consumer market. 

In industry, titanium can be found:

• On ship hulls, submarines, and other structures exposed to seawater, due to its high corrosion-resistance
• In hip replacements and dental implants, due to its high biocompatibility and strength.
• In aircraft, spacecraft, and missiles.

If money is no issue, titanium is an excellent choice for a strong, lightweight material. Thanks to developments in coating technologies and newly researched alloys, cost-effective magnesium is increasingly emerging as the lightest solution. These three metals are often being considered concurrently in lightweighting exercises, along with composite materials and even high strength steels.

One other consideration that is often overlooked is the question of stiffness. Creating a steel or light alloy (e.g. aluminium) component of similar strength will, in many cases, require the use of higher wall thicknesses for the aluminium component compared to the steel component. One positive consequence of this at the aluminium component may actually be stiffer than its steel counterpart. This is noticeable in automotive body panels for example, where an aluminium monocoque body can be stiffer than its steel counterpart. In this case, there is a benefit in vehicle handling, for example, and also crash resistance.