Aluminum is an excellent material that brings a lot of valuable qualities to the table. Its unique combination of properties makes it the most popular non-ferrous metal in the world today. And it&#;s also often compared to the world&#;s most widely used group of metals, steels.
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There&#;s a lot to like about aluminum, but product designers should still evaluate their material options carefully. You may be wondering what the advantages and disadvantages of aluminum are. But it&#;s essential to keep the following in mind&#;
The pros and cons of aluminum will vary depending on the application you&#;re using it for.
In this article, we&#;re not going to define the characteristics of aluminum as strictly pros and cons. Instead, we&#;ll evaluate aluminum&#;s defining properties and highlight when they are advantageous. We&#;ll also explain how it stacks up against other materials.
#1 Strength-to-Weight Ratio
Elemental aluminum on its own isn&#;t that strong, with an ultimate tensile strength of about 90&#; MPa (13 ksi). But when alloying elements and tempers are added, the strength of aluminum is much higher. One of the strongest aluminum alloys is AA-T6, with an ultimate tensile strength of 710 MPa (103 ksi).
Steel alloys are still typically stronger than aluminum alloys. But since aluminum alloys have a density of 2.64 g/cm³ to 2.81 g/cm³, aluminum is roughly three times lighter than steel! This is why aluminum alloys are known for their impressive strength-to-weight ratio.
When minimizing weight is important, aluminum can offer a higher strength for the same mass as steel. It&#;s invaluable for many everyday items, including personal electronic devices and sports equipment.
However, sometimes saving weight isn&#;t as important as achieving high strength. In these cases, designers may need to consider stronger materials like steel or titanium alloys. (See: Steel vs. Aluminum)
#2 Corrosion Resistance
Aluminum forms a passivating layer on its surface that makes it naturally corrosion-resistant. As a result, aluminum alloys are very durable in various humid or wet environments.
Some aluminum alloys are more corrosion-resistant than others. However, they all perform better than plain carbon steels. In extreme environments, stainless steel and Inconel alloys can provide better corrosion resistance. But you need to justify their higher cost.
Another option is to choose one of the many available aluminum finishing options. They can help further protect your product&#;s surface from the environment.
#3 Ductility and Hardness
Ductility and hardness are often inversely related. And aluminum is a relatively soft, ductile metal compared to alternatives like hardened tool steel.
Its ductility gives aluminum great formability and bendability. So, it&#;s easy to work with when making extrusions or pressed products. The end result can be lower processing costs and greater part complexity without risking damaging the material.
While ductility is great, a ductile material also tends to break down faster when exposed to repeated wear stress. So, if the product could be exposed to high friction, hardened steel or even tungsten carbide might be preferred.
#4 Melting Point
Aluminum has a much lower melting point than steel and titanium. It also softens at a lower temperature than many metals. That&#;s why processes like aluminum extrusion require a relatively small amount of energy to shape it.
On the flip side, this also means aluminum is unsuitable for many high-temperature service applications. But if sufficient cooling is available, aluminum&#;s high thermal conductivity allows it to transport heat rapidly away from the heat source to maintain integrity.
#5 Thermal Conductivity
Aluminum&#;s high thermal conductivity makes it a staple of many thermal management and heat exchange systems. But its thermal conductivity is actually about 40% lower than copper&#;s. As such, copper is a more conductive heat sink.
That said, aluminum is much lighter and cheaper than copper. So in most cases, product designers opt for an aluminum heat sink or heat exchanger to save cost and weight.
#6 Electrical Conductivity
Engineers sometimes find themselves comparing the electrical conductivity of aluminum and copper. And while aluminum is only about 39% as conductive as copper, it is much lighter and cheaper.
It is a popular consideration for many electrical applications, including high-voltage transmission lines. Keep in mind, though, that some aluminum finishes can impact conductivity.
Other Considerations
In addition to the material properties mentioned above, here are some things you should consider when deciding if aluminum is suitable for you.
Aluminum Finishing Options
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We&#;ve discussed attributes like corrosion resistance, thermal conductivity, and electrical conductivity. But remember that those properties can change when you apply surface finishes such as anodizing or powder coating.
For example, anodizing a part makes it more wear-resistant. It also becomes more corrosion-resistant and thermally conductive &#; but less electrically conductive.
Surface finishing can enhance the aesthetics of a part too. Sublimation, for example, allows you to add many types of wood-look patterns. So, when comparing aluminum to other metals, consider the impact and aesthetic appeal of surface finishes.
Cost of the Metal
Aluminum is less expensive than copper, stainless steel, titanium, and many specialist metals. But raw aluminum also tends to cost more than the common grades of low carbon steel and low alloy steel.
Depending on the application, you may also want to consider non-metal alternatives like plastics. These could lead to significant cost savings as well.
The cost of material selection can be hard to pin down as it is constantly in flux with global markets. You must determine which material offers the required properties at the most reasonable cost.
Choosing Aluminum for Your Product
The key thing to understand is that aluminum&#;s advantages and disadvantages will vary depending on the situation. For example, a low melting point is great for reducing some processing costs. But it limits the ability of aluminum to accommodate high-temperature applications.
Aluminum is a very popular metal because it offers an exceptional combination of properties. It has found its way into many different industries and applications &#; everything from aerospace and chemical processing parts to furniture and electronics.
It is an incredibly versatile metal, and there&#;s a good chance it&#;s the right metal for your next project. After choosing to work with aluminum, designers will need to select an aluminum fabrication method that fits their needs.
If you want to learn more about aluminum fabrication, you can read our article about some of the key aluminum fabrication processes available today.
Aluminium has become a crucial part of the global economy. In the United States alone, aluminium (Al) and its alloys contribute more than $174 billion to the economy every year. This figure represents close to 1% of the country&#;s entire economic output.
Why are more engineers deploying aluminium alloys?
Aluminium brings a number of unique benefits, making it the most widely deployed light alloy in industry.
One of the major strings to aluminium&#;s bow are the social and environmental benefits. Aluminium is helping businesses around the world manufacture more sustainably. It&#;s widely recycled, much more than heavy metals and even other light alloys. For this reason, a significant proportion of aluminium that has been chemically extracted in human history is still in use today.
The advantages are not solely environmental, though. Mechanically, aluminium has a number of advantages over heavy metals and even other light alloys. It has an exceptional strength-to-weight ratio combined with good levels of formability, enabling its use in a wide variety of applications. Naturally, it forms an oxide layer which extends its life in non-acidic environments, too.
And now, with the advent of innovative surface treatments for aluminium, the material is being deployed in even more testing conditions. Al alloys are frequently replacing heavier metals like steel, which are often selected for their performance in challenging use cases.
Innovating aluminium for the future
Aluminium offers a range of beneficial properties while uncoated. However, there are several drawbacks to aluminium which have historically limited it to certain use cases.
Improving corrosion resistance
Naturally, aluminium is a fairly reactive metal. Due to the material&#;s chemical affinity for oxygen, aluminium alloys oxidise readily, forming a protective layer of aluminium oxide. This layer provides some corrosion resistance in pH neutral environments &#; however, in alkaline or, in particular, acidic environments, this layer does not provide sufficient protection.
Certain aluminium alloys are more susceptible to corrosion than others. The 2xxx and 7xxx series are both prone to galvanic corrosion, due to the containing high levels of copper and zinc contents respectively.
Wear resistance
It is not just corrosion resistance that has been highlighted as a potential weakness. Aluminium&#;s vulnerability to friction-initiated wear is an issue of concern, too. This makes advanced coatings, and surface engineering, compulsory in many applications where friction is produced from abrasive wear (contact between components). This is important where aluminium alloys are used against other metals.
Electro-plating aluminium alloys
Various electroplating techniques have been used over the years to enhance the properties of aluminium. Some of these have proven more effective at improving corrosion resistance than others. Other techniques have been outlawed, due to their use of toxic chemicals and the carcinogenic chromium.
Electroplating is a well known electrochemical conversion coating process. There are several forms, including electroless and electrolytic techniques. In short, metallic ions are incorporated on the surface of the aluminium substrate to provide it with greater wear resistance and tribological performance, among other characteristics.
When uncoated, aluminium coatings tend to offer poor adhesion to further coatings after forming an oxide layer. It&#;s the lack of adhesion between layers that can make the metal prone to galvanic corrosion. This causes electrical conduction to drop, and weaken bonds with other metals - particularly if they have a large differential in their anodic index. One method of preventing galvanic corrosion is plating with metals that are highly resistant to corrosion, like nickel. The effectiveness of nickel plating depends almost wholly on the application.
Electroless nickel (EN) plating is an alternative to conventional electrolytic electroplating, and currently one of the more popular types of autocatalytic, chemical plating for aluminium. Used widely from computer motherboards to submarine engineering, Electroless nickel coatings deliver high-hardness, ductility, and abrasion resistance &#; ideal for sliding wear applications.
However, one of the main drawbacks of the technique is that the cleanliness of the substrate is a factor in the success of the coating. Any bacteria or direct will impact the quality of the coating. For more complex and intricate parts, this tends to raise several issues, for example in cylinder liners, whereby the inner surface area is difficult to clean. Also, EN plated aluminium is difficult to recycle, as the plating needs to be removed with mechanical or chemical stripping.
Hard anodising and aluminium alloys
Anodizing is perhaps the most conventional method of improving the performance of aluminium alloys. In more intensive applications, a &#;harder&#; anodizing approach is used. Hard anodizing involves the use of sulphuric acids and other additives to produce thicker, harder, anodized layers. This helps improve hardness and wear resistance while moderately improving corrosion resistance.
Hard anodized coatings are characterised by their distinctive microstructure. During their formation, coatings grow vertically, producing a columnar microstructure. The greater the thickness (height of these micro-columns), the better the protection provided from corrosion. In applications that require a combination of corrosion and wear resistance, this micro columnar structure can cause issues &#; especially when wear has reduced the thickness of the coating and has left gaps to the substrate.
Figure 1. Anodized coating surface microstructure (right; Coating by plasma electrolytic oxidation, by comparison).
Plasma electrolytic oxidation (PEO) for aluminium alloys
Plasma electrolytic oxidation (PEO) is a technique that shares many of the same principles of hard anodizing, but the incorporation of plasma-chemical reactions creates coatings with a set of characteristics. These characteristics offer significant advantages over hard anodizing and plating techniques.
The coatings formed through PEO are incredibly hard, wear resistant, adhesive, and provide an excellent &#;key&#; to improve adhesion to subsequent coatings. It&#;s this key that allows PEO-induced ceramic layers to act as the perfect complement to subsequent layers, thus improving corrosion resistance.
Furthermore, the 3-stage coating process is far more flexible than other comparable techniques, allowing for its use in a wide variety of applications and to achieve bespoke-to-application characteristics. For example, the composition of the electrolyte bath can be modified to introduce new materials into the coating for specific applications. The dynamism of the technique has attracted leading companies from the automotive, aerospace, consumer electronics and defence industries to adopt use the technique.
The technique offers a range of environmental advantages. By nature, the PEO process involves no toxic chemicals, and electrolyte solutions are typically benign, allowing for easy disposal. For this reason, the coating received a surge in interest from leading aerospace manufacturers following the planned eradication of toxic hexavalent (VI) chrome from the industry. Companies seeking REACH and RoHS compliance may consider PEO an apt alternative to these prohibited coating techniques.
Conclusion
Aluminium has rightly earned its place at the top of the light alloy food chain. Its use looks set to continue and even expand, now it is even being incorporated within moving components (like engine blocks) to reduce weight. Materials science companies like Keronite are continually developing new solutions for light alloys like aluminium to enable their use and extend their life in challenging applications.
Businesses seeking to improve the performance of light alloys, or to reduce the mass of their products should always begin the project with an investigation into light alloys and their surface treatments. It could spring a few surprises to learn just how well light alloys can perform with innovative surface technologies, like plasma electrolytic oxidation.
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