What is the Hardest Aluminum Alloy, and What’s the Real Cost?

You search for the "hardest" aluminum alloy, assuming it is also the "best" for your job. This leads to choosing a material that is too expensive, difficult to machine, and prone to corrosion.

Hardness is bought, not found. The hardest alloys, primarily in the 7000 series, "pay" for their peak strength with reduced corrosion resistance¹ and difficult machinability. An expert doesn't ask "what is hardest?"; they ask "what is the acceptable cost for the required performance?"

I remember a project with a client who builds high-performance custom machinery. They came to us wanting a quote for several large forged discs made from 7075-T651. Their drawings specified it because it is one of the hardest and strongest aluminum alloys available. I asked what the parts were for. They were non-structural, cosmetic cover plates. They just wanted them to be "tough." I explained the "cost" of that hardness: 7075 is more expensive, much harder to machine, and needs better surface protection to prevent corrosion. For a cover plate, this was all cost with no benefit. I recommended they switch to 6061-T6. It's still very hard and durable, but it's more corrosion-resistant and significantly easier on their cutting tools and budget. They were thankful we had the conversation. It showed them we were partners invested in their success, not just an order-taker.

Does 7075 aluminum, the hardest alloy, rust?

You know 7075 is a top-tier alloy, but you see a slight haze or pitting on a part. You worry it's rusting like steel, signaling a failure in the material.

No, 7075 aluminum does not rust, as rust is iron oxide. However, due to its high copper content, it has lower corrosion resistance than other alloys like 6061. It can corrode if not properly protected, especially in harsh environments.

This is one of the "costs" of peak performance. The small amount of copper that gives 7075 its incredible strength also makes it more susceptible to galvanic corrosion, especially when in contact with moisture and other metals. Unlike the tough, self-protecting oxide layer that forms on 5xxx or 6xxx series alloys, the corrosion on 7075 can be more aggressive, often appearing as pitting or exfoliation where the material flakes in layers. This is why for many of our clients, especially those in the Middle East with humid or coastal climates, we emphasize proper protection. If an application requires the strength of 7075, the final part must be anodized, painted, or alodined to create a barrier against the environment. For less critical applications, choosing an alloy like 6082, which has better natural corrosion resistance, is often the smarter engineering and business decision.

Why can't you easily MIG weld high-strength aluminum?

You are a skilled welder, great with steel. You try to weld a high-strength aluminum alloy like 7075 or even 6061-T6 and get a cracked, weak, and ugly result.

The intense, localized heat from welding destroys the T6 heat treatment in high-strength alloys like 6061 and 7075. This turns the area around the weld back into a soft, weak state, completely compromising the part's structural integrity.

This is a critical concept for our machining clients to understand. The "T6" in 6061-T6² or 7075-T6 signifies a very specific heat treatment process that gives the alloy its hardness and strength. Welding essentially performs a reverse heat treatment on a small area. The metal melts and then cools uncontrollably, ruining the carefully engineered temper. In the case of 7075, the alloy is also "hot short," meaning it cracks when subjected to heat and stress during cooling. This is why we advise our clients: if your final assembly requires welding, you must design with that in mind from the start.

Choosing the Right Path

This is why we focus on forging. We create a single, solid part with uniform properties, eliminating the weak points that welds introduce.

Can 5052 aluminum be hardened?

You have a part made from 5052 aluminum³. It is tough and corrosion-resistant, but you need it to be harder to resist surface scratching and wear for a new application.

No, 5052 aluminum cannot be hardened by heat treatment like the 6xxx or 7xxx series alloys. It is a non-heat-treatable alloy that gets its strength and hardness from strain hardening, which is work done on the metal at the mill.

This is a fundamental difference in aluminum families. The strength of alloys like 6061 and 7075 comes from their response to a carefully controlled heating and cooling cycle (heat treatment). The strength of alloys like 5052 comes from "work hardening" or "strain hardening." Imagine kneading dough; the more you work it, the stiffer it gets. Similarly, when 5052 is rolled, bent, or drawn at the factory, its internal structure becomes stressed and tangled on a microscopic level, which makes it harder. This is designated by an 'H' temper, like H32 or H34. The 'H' number tells you how much it has been strain hardened. Because this hardness comes from a mechanical process, not a thermal one, you cannot simply heat a finished 5052 part to make it harder. In fact, heating it will do the opposite—it will anneal the metal, making it softer.

Is aluminum really stronger than steel?

You are designing a part and need maximum strength. You immediately think of steel, but then wonder if a high-tech aluminum alloy could be a better, lighter alternative.

On a pound-for-pound basis, yes, a high-strength aluminum alloy like 7075 is significantly stronger than most common steels. However, steel is denser and stiffer, so for a part of the exact same size, steel will be stronger.

This is the most important concept in modern material selection: strength-to-weight ratio⁴. Steel is about 2.5 times denser than aluminum. So while a one-inch steel bar is much stronger than a one-inch aluminum bar, an aluminum bar that weighs the same as the steel bar will be much larger and can be designed to be much stronger. This is why aerospace engineers use aluminum. They can design larger, thicker, and more robust structures that weigh far less than their steel equivalents, delivering superior performance. For our clients, the question is not "which is stronger?" but "what is the goal?" If you need maximum strength in a very limited space and weight is not a concern, steel is a great choice. But if you are trying to reduce weight, save energy, or improve dynamic performance (like in a moving part), a high-strength forged aluminum alloy is almost always the superior engineering solution.

Conclusion

The "hardest" alloy is often not the "best." True expertise lies in understanding the costs—in machinability, corrosion, and price—and choosing the alloy with the exact performance your mission requires.