How do we change the hardness of an aluminium alloy?

Is the aluminum you're considering too soft for the job, or maybe too hard to work with? This is a common puzzle. You need your material to have just the right hardness for optimal performance and processability.

The hardness of an aluminum alloy can be changed primarily through heat treatment processes like annealing (to soften) or precipitation hardening (to harden specific alloys), and through mechanical work hardening (cold working).

At SWA Forging, we've been helping clients get the precise aluminum properties they need since 2012. Our expertise in manufacturing large-diameter forged rings and discs means we deeply understand how to manipulate aluminum alloys. Whether it's for traders needing certified materials or machining companies requiring specific mechanical traits, controlling hardness is key. Let's look at how this is achieved.

How do you strengthen aluminium alloys?

Your project needs strong aluminum, but the standard stuff might not cut it. This can be worrying if structural integrity is on the line. The good news is, there are proven ways to boost aluminum's strength.

Aluminum alloys are primarily strengthened through heat treatment (precipitation hardening for specific series like 2xxx, 6xxx, 7xxx), work hardening (cold working for non-heat-treatable series like 1xxx, 3xxx, 5xxx), and by the initial choice of alloying elements.

There are several effective methods to make aluminum alloys stronger. The most suitable method depends on the specific alloy series.
One primary way is Heat Treatment, specifically Precipitation Hardening. This technique is used for alloys in the 2xxx (copper), 6xxx (magnesium and silicon), and 7xxx (zinc) series. The process generally involves three steps:

1. Solution Heat Treating: Heating the alloy to a high temperature to dissolve the alloying elements into a solid solution.
2. Quenching: Rapidly cooling the alloy (usually in water) to trap these elements in solution.
3. Aging: This can be natural (at room temperature over time) or artificial (reheating to a lower temperature for a specific period). During aging, fine particles of intermetallic compounds precipitate out within the aluminum's crystal structure. These tiny precipitates act like obstacles, hindering the movement of dislocations (defects in the crystal structure), which makes the material much stronger and harder. The "T" tempers, like T6 (e.g., 6061-T6), signify these heat treatments.
   Another method is Work Hardening, also known as Strain Hardening. This is typically used for non-heat-treatable alloys like the 1xxx, 3xxx, and 5xxx series. It involves deforming the aluminum at room temperature through processes like rolling, drawing, or forging. This deformation increases the number of dislocations, and they get tangled, making it harder for them to move, thus strengthening the material. The "H" tempers indicate work hardening.
   Finally, Alloying itself is a fundamental way to influence strength. Adding elements like copper, magnesium, silicon, or zinc changes the base properties of aluminum, creating alloys with inherently different strength potentials even before specific treatments.

How do you soften aluminum?

Sometimes, aluminum is too hard or brittle for your next manufacturing step, like forming or bending. You might worry about it cracking. So, how can you make it more pliable?

Aluminum is primarily softened through a heat treatment process called annealing. This process relieves internal stresses, increases ductility, and makes the material easier to form or machine by altering its microstructure.

When you need to make aluminum softer, more ductile, and easier to work with, the main technique is Annealing. This is essentially a heat treatment process designed to reverse the effects of hardening, whether that hardening came from previous work hardening or a heat treatment like T6.
The annealing process involves:

1. Heating: The aluminum alloy is heated to a specific temperature. This temperature varies depending on the alloy and its current condition, but it's generally high enough to allow the metal's internal structure to rearrange. For example, for 6061-T6, you might heat it to around 415°C (775°F).
2. Soaking: The material is held at this temperature for a certain period to ensure the entire piece reaches a uniform temperature and the desired microstructural changes can occur.
3. Cooling: The cooling rate is also important. Often, controlled slow cooling in the furnace or sometimes air cooling is used.
   What happens during annealing? The heat allows the metal's crystal structure to recover from stresses. For work-hardened materials, it causes recrystallization, where new, stress-free grains form and grow, reducing the density of dislocations. For heat-treated alloys, it can dissolve the strengthening precipitates or allow them to grow larger and less effective (over-aging if not controlled carefully for full annealing). The result is a softer, more ductile material, typically designated with an "O" temper (e.g., 6061-O). At SWA Forging, we might receive material in an annealed state for easier forging, or we might anneal parts between forging stages if complex shaping is needed.

What is the hardness of aluminum alloy?

You're looking at an aluminum alloy, but "how hard is it" seems like a simple question with a complex answer. You need a way to quantify this to select the right material for wear resistance or machinability.

The hardness of an aluminum alloy is not a single value; it varies greatly depending on the specific alloy composition and its temper (heat treatment or work hardening condition). It's typically measured using scales like Brinell (HB) or Rockwell (HRB, HRE).

The hardness of aluminum isn't fixed; it's a property that can span a wide range. It describes the material's resistance to localized plastic deformation, such as a scratch or indentation. Several scales are used to measure this:

• Brinell Hardness (HB): This test uses a hardened steel or carbide ball indenter. It's often used for softer metals and castings or forgings like the ones we produce at SWA Forging. The resulting indentation diameter is measured.
• Rockwell Hardness (HRB, HRE, etc.): This test measures the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different Rockwell scales (e.g., B, E, F) using different indenters and loads, suitable for various hardness ranges.
• Vickers Hardness (HV): This uses a diamond square-based pyramid indenter and is suitable for a wide range of materials, including very hard ones.
  Here are some typical Brinell hardness (HB) values to give you an idea of the range:
• 1100-O (Commercially Pure Aluminum, Annealed): Very soft, around 20-25 HB.
• 3003-H14 (Work-Hardened): Moderate, around 40 HB.
• 5052-H32 (Work-Hardened): Moderate, around 60 HB.
• 6063-T6 (Heat-Treated): Medium, around 75 HB.
• 6061-T6 (Heat-Treated): Medium-hard, around 95 HB.
• 2024-T3 (Heat-Treated): Hard, around 120 HB.
• 7075-T6 (Heat-Treated): Very hard, around 150 HB.
  When we provide product quality certificates for our forged rings and discs, the hardness value is often a key parameter reported, assuring our clients that the material meets their specifications.

How can you increase the hardness of materials?

You often need materials to be harder to resist wear, indentation, or deformation. This isn't just about aluminum. So, what are the general ways to make various materials harder?

The hardness of materials can be increased through several methods including alloying, heat treatments (like quenching and tempering for steels, or precipitation hardening for aluminum), work hardening (cold working), and various surface hardening treatments.

Increasing the hardness of materials is a common goal in engineering, and the methods can apply broadly, though specifics vary.
For many metals, including aluminum, Alloying¹ is fundamental. Adding other elements into the base metal can distort the crystal lattice (solid solution strengthening) or allow for the formation of hard precipitate phases, both of which restrict dislocation movement and increase hardness.
Heat Treatment is a powerful tool. For steels, this often involves quenching to form martensite (a very hard phase) followed by tempering to improve toughness. For aluminum alloys like the 6xxx or 7xxx series, as we've discussed, precipitation hardening is key. This involves solutionizing, quenching, and aging to grow fine, hard precipitates within the metal matrix.
Work Hardening (or Cold Working) is another widely used method. Deforming a metal at a temperature below its recrystallization point increases the density of dislocations. These dislocations interfere with each other, making further deformation (and thus indentation) more difficult. This is common for many metals like copper, brass, and non-heat-treatable aluminum alloys.
Surface Hardening Treatments are also important. These modify only the surface of the material to make it harder, while the core retains its original properties (often toughness). For steels, common methods include carburizing, nitriding, or induction hardening. For aluminum, Hard Anodizing (Type III) is a very effective surface treatment. This electrochemical process creates a thick, dense layer of aluminum oxide on the surface, which is extremely hard (often much harder than hardened tool steel) and provides excellent wear and corrosion resistance. This is different from changing the bulk hardness of the aluminum itself but is a critical way to achieve a hard, durable surface on an aluminum part.

Conclusion

We can precisely adjust aluminum alloy hardness through controlled methods like heat treatment and work hardening, tailoring material properties to meet specific application demands for strength and durability.