Comparing Aluminium Alloys: Find the Right Material for Your Part

Aluminium is a common and popular metal that’s used in a stunningly wide range of parts and products: automotive parts, aerospace components, boat and marine gear, consumer electronics including smart phones, industrial machinery parts, HVAC systems, the list goes on. It’s also the material most commonly requested by our customers.
Engineers and product designers frequently turn to aluminium and many of its alloys to design both prototypes and end-use parts in a variety of industries. Choosing the right aluminium alloy when designing parts is critically important because different alloys possess significantly different properties, which directly impact the part's performance, manufacturability, durability, and cost. Alloys vary in mechanical and physical properties, manufacturing/fabrication characteristics, cost, and response to the application environment.

Aluminium Alloy Properties

Aluminium has a high strength-to-weight ratio, making it lightweight, yet strong and flexible. It’s also affordable, corrosion-resistant, and works well in a multitude of applications. Most commonly, aluminium is alloyed with other elements such as silicon, magnesium, chromium, copper, zinc, and manganese.

Aluminium alloys are often used in cars because of the material’s versatility. Aluminium’s formability and corrosion resistance make it easy to work with and shape, but its structural soundness addresses the most important requirement for car bodies. Strength is important, but car bodies must be lightweight, affordable to produce, resistant to rust, and have the attractive qualities consumers seek, such as exceptional surface finishing characteristics. Aluminium nicely fits the bill. 

Aluminium Component Machined for the Harley-Davidson Factory Racing Team

The same holds true with aerospace parts. Aluminium alloys are often a preferred material for aerospace designs, again because of aluminium’s corrosion-resistance properties and high strength capabilities. Compared to steel, it is a lightweight option and an ideal material for a wide range of aircraft components and aerospace applications. 

Aluminium Parts for the NASA Goddard Space Flight Center Machined and Delivered in 36 Hours

Lightweighting is a core need of both the automotive and aerospace industries.

With cars, light-weighting helps reduce emissions and meet improved fuel economy standards.

For aerospace, the use of aluminium alloys dramatically decreases the weight of an aircraft because it is significantly lighter than steel, allowing aircraft to either carry more weight or increase fuel efficiency. For instance, a 20% weight reduction in a Boeing 787 is expected to generate a 10–12% improvement in fuel efficiency. Along these lines, common applications in aerospace abound—many, but not all, of which, are metal: Fuel nozzles, heat exchangers, manifolds, turbo pumps, liquid and gas flow components, conformal cooling channels, fasteners, and more. 

Comparing Common Aluminium Alloys

Aluminium, as a pure element, presents many desirable properties. However, by itself, aluminium might not be strong enough for a high-durability use or purpose. Accordingly, aluminium can be combined with other elements to form alloys, which are more durable and suitable for industrial applications.

And what do all of those numbers in the alloy name signify? The aluminium series names elements with four numerical digits where the first digit represents the principal alloying element, the second digit indicates a modification of a specific alloy, and the third and fourth digits are arbitrary numbers assigned to specific alloys in the series. 

Aluminium Materials Available Through Protolabs Factories

Aluminium Materials Available Through Protolabs Network

Aluminium 6061 vs. Aluminium 7075

These two alloys are the most requested by our CNC machining customers so we receive a lot of questions comparing the two. Here’s how they score. The primary advantage of 7075 lies in its exceptionally high strength-to-weight ratio, significantly exceeding that of 6061, making it ideal for high-stress applications like aerospace structures and performance sporting equipment.

However, this superior strength comes with major trade-offs: 7075 exhibits poor weldability using common fusion techniques, has lower general corrosion resistance, is susceptible to stress corrosion cracking, and is typically more expensive and less ductile than 6061. In contrast, 6061 offers a more balanced profile with good strength, excellent weldability, superior corrosion resistance, better formability, and lower cost, making it a versatile workhorse alloy suitable for a vast range of general structural, marine, automotive, and consumer applications where the extreme strength of 7075 is not required.

Finishing Aluminium Parts

First, let’s look at finishing options for CNC-machined aluminium parts.

Anodising is one of the most common finishing options, offering strong corrosion protection and enhancing the overall appearance of metal parts. Three types of aluminium anodising are available from Protolabs:

• Chromic Acid (Type I): Provides a whisper-thin but still durable coating. Commonly used for welded parts and assemblies, and as a primer before painting. Like all anodised surfaces, it is non-conductive.
• Sulfuric Acid (Type II): Harder than Type I, it offers an exceedingly durable finish. Usage examples include carabiner hooks, flashlight handles, motorcycle parts, and hydraulic valve bodies. 
• Hard Anodise or Hardcoat (Type III): This is the thickest and hardest anodise available and it has a wide range of applications for parts and products in automotive, aerospace, heavy equipment, marine, general manufacturing, and defense industries. 

Another option is plating, which electrically bonds protective elements to the aluminium. Plating is available only through Protolabs Network or when you order complete prototype parts. Adding plating often means adding lead time, so keep that in mind. Also note that plating is one of the more expensive finishing options. Here’s what’s available:

For a tough, aesthetic finish, you can choose an application of powder coating on your parts. It leaves a somewhat protective layer of polymer-based coloured powder.

Interestingly, many aluminium alloys tend to be self-healing when exposed to air or oxygen. With this natural passivation process, any exposure causes the metal to form a layer of aluminium oxide on its surface, which acts as a barrier to avoid further material degradation. 

Main Takeaways

Some customers ask us what the best all-purpose aluminium is. It’s a tough choice because choice of alloy is entirely predicated on what your part will be exposed to in each product. That said, many people consider aluminium 6061 to be the most well-rounded aluminium, balancing strength, corrosion resistance, weldability, and to machine or fabricate. While it costs a bit more, it offers solid performance across a range of attributes.

If cost-efficiency is your biggest concern, note that less expensive alloys may not provide the lifespan your product requires, so choose carefully, grounded in that knowledge. In terms of strength, 7075 is the strongest alloy we offer, which is why it’s often used in aerospace applications.