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Ever wished your aluminum parts were tougher or more resistant to wear? It's a common need, but how to actually make aluminum harder isn't always clear. You don't want to try something that ends up damaging the material.
Yes, aluminum can be hardened. The primary methods are work hardening (cold working) for non-heat-treatable alloys, and precipitation hardening (heat treatment involving solutionizing, quenching, and aging) for specific heat-treatable alloys.
At SWA Forging, we've been deep in the world of aluminum alloys since 2012. Our specialty is manufacturing large-diameter forged rings and discs, and a critical part of our process for many alloys involves heat treatment to achieve specific hardness and strength properties. Our clients, who are often traders or machining companies in regions like the Middle East, rely on us for aluminum that meets precise specifications, including hardness. So, let's explore how aluminum gets its strength.
How do you solidify aluminum?
Thinking about where aluminum comes from, you might picture it as a molten liquid. But how does it become the solid material we use every day? Getting this initial state right is fundamental to all subsequent processes.
Aluminum solidifies by cooling from its molten state (above its melting point, roughly 660°C or 1220°F for pure Al) to below this temperature, causing it to transition into a crystalline solid structure.
Aluminum starts its journey to becoming a usable solid material from a liquid, molten state. Pure aluminum melts at around 660°C (1220°F), though this temperature changes a bit depending on the specific alloying elements mixed in. To solidify it, you simply need to cool it down below this melting point. As it cools, the aluminum atoms begin to arrange themselves into an orderly, repeating pattern, forming crystals. This process is called crystallization or solidification.
The way it cools – quickly or slowly – can significantly affect the size of these crystals (also called grains) and, therefore, some of the material's properties. For instance, when large ingots or billets are cast, the cooling process is carefully controlled to achieve a desirable grain structure. These billets are the starting material for processes like extrusion, rolling, or, in our case at SWA Forging, forging. Even though we receive aluminum as solid billets, our forging process involves heating them until they are very soft and malleable (but still well below melting point for most forging operations), then shaping them. The final cooling after forging, and any subsequent heat treatments, are then critical for determining the final properties of our forged rings and discs. The initial solidification sets the stage for everything that follows.
How long does it take for aluminum to harden?
You need hardened aluminum for your application, but you're wondering about the timeline. Is it a quick process, or does it require a lot of patience before the material reaches its desired strength?
The time it takes for aluminum to harden varies greatly: work hardening is immediate during deformation, while heat treatment (precipitation hardening) can take from several hours to many days, depending on the alloy and specific aging process (natural vs. artificial).
The time frame for hardening aluminum really depends on which method you're using and the specific alloy.
If we're talking about work hardening (also called strain hardening), this happens virtually instantaneously. When you bend, roll, draw, or otherwise deform a non-heat-treatable aluminum alloy at room temperature (or below its recrystallization temperature), the internal structure of the metal changes, making it harder and stronger right away. The more you deform it, the harder it gets, up to a certain point.
For heat-treatable alloys (like the 6xxx or 7xxx series), the hardening process is called precipitation hardening, and it's a multi-step affair with distinct time components:
- Solution Heat Treatment: The aluminum is heated to a high temperature (e.g., around 530°C / 985°F for 6061 alloy) and held there for a period, often an hour or so per inch of thickness, to dissolve the alloying elements into a solid solution.
- Quenching: The aluminum is then rapidly cooled, usually in water. This step is very fast – seconds to minutes – to "trap" the dissolved elements.
- Aging (Precipitation): This is where the actual hardening occurs as fine particles (precipitates) form within the metal structure.
- Natural Aging (e.g., T4 temper): This happens at room temperature. For some alloys, like 2024, it can achieve significant hardness in a few days, continuing to harden slowly for weeks.
- Artificial Aging (e.g., T6 temper): This is done at an elevated temperature (e.g., around 160-175°C / 320-350°F for 6061-T6) for a specific number of hours. For 6061 to reach T6, this might take 8-18 hours, depending on the exact temperature and part thickness.
At SWA Forging, achieving the correct temper, like T6 for our 6061 forged rings, involves precise control over these heating and aging times to ensure our products meet the stringent mechanical property requirements our clients need.
Does aluminum get harder with age?
You've heard the term "aging" in relation to aluminum, and it sounds a bit like cheese or wine. Does aluminum actually change and get harder just by sitting around over time after it's made?
Yes, certain heat-treatable aluminum alloys (like 2xxx, 6xxx, 7xxx series) do get harder with age after solution heat treatment and quenching. This process, called age hardening or precipitation hardening, occurs as microscopic particles form within the alloy's structure, strengthening it.
It's a fascinating property of some aluminum alloys! This phenomenon is known as age hardening or precipitation hardening. It doesn't apply to all aluminum, only to the heat-treatable alloys which contain elements like copper, magnesium, silicon, or zinc in specific combinations.
Here's how it generally works:
- First, the alloy is solution heat-treated. This involves heating it to a high temperature to dissolve the alloying elements evenly into the aluminum, like sugar dissolving in hot water.
- Then, it's quenched – rapidly cooled, usually in water. This rapid cooling traps the alloying elements in a "supersaturated" state; they want to come out of solution but don't have time. The material is relatively soft at this point.
- Now, the aging part begins. Over time, these trapped alloying elements start to clump together to form extremely fine, dispersed particles called precipitates within the aluminum's crystal structure. These tiny particles act as obstacles to the movement of dislocations (defects in the crystal structure), which is how metals deform. By impeding dislocation movement, the precipitates make the aluminum harder and stronger.
This aging can happen in two ways:
- Natural Aging: Occurs at room temperature over days or weeks. This leads to tempers like T3 or T4. For example, alloy 2024 naturally ages to achieve good strength.
- Artificial Aging: The process is sped up by heating the quenched material to a moderately elevated temperature (e.g., 120-200°C or 250-400°F, depending on the alloy) for several hours. This creates tempers like T6 or T8. Many of our forged products at SWA Forging, such as those from 6061 or 7075 alloys, are artificially aged to achieve their final high-strength properties. Our clients, particularly machining companies, often require these specific tempers for their high-performance applications.
What happens when you heat up aluminum?
You're planning to heat aluminum, maybe for bending, welding, or it's part of its service environment. But you're unsure what effect this heat will have. Will it soften, get stronger, or just get hot?
Heating aluminum causes it to expand. Depending on the alloy and temperature, it can soften (anneal), lose work-hardened strength, prepare heat-treatable alloys for hardening (solution treatment), or if overheated/improperly treated, it can over-age and weaken.
Heating aluminum can lead to several different outcomes, and what happens depends a lot on the specific aluminum alloy, the temperature it reaches, how long it stays at that temperature, and its prior condition (e.g., if it was work-hardened or heat-treated).
First, like all materials, aluminum expands when heated. This is a purely physical change and is reversible upon cooling, assuming no other changes occur.
If you heat aluminum that has been work-hardened (made stronger by cold rolling, drawing, etc.), it will begin to soften if you exceed a certain temperature. This is called annealing or recrystallization. The heat allows the distorted internal crystal structure to rearrange into a less strained state, reducing its hardness and strength but increasing its ductility. This is often done intentionally to make the aluminum easier to form.
For heat-treatable alloys (like the 6000 or 7000 series that might be in a T6 temper), heating can have more complex effects:
- Solution Heat Treatment: If you heat it to a specific high temperature (e.g., ~530°C for 6061), you're performing the first step of the hardening process, dissolving the alloying elements. This prepares it for quenching and aging.
- Over-aging: If an already hardened alloy (like T6) is heated to moderately high temperatures (e.g., above 200°C / 400°F for extended periods), the fine precipitates that give it strength can grow too large or change form, causing the material to lose strength. This is generally undesirable unless carefully controlled for specific effects.
- Annealing: Heating to an even higher temperature will fully anneal a heat-treatable alloy, making it very soft (O temper).
In our forging processes at SWA Forging, we heat aluminum billets to make them malleable enough to shape into rings and discs. This is done very carefully. For heat-treatable alloys, this heating is often integrated with, or followed by, specific heat treatment cycles (solutionizing, quenching, aging) to achieve the precise mechanical properties, like hardness and tensile strength, that our customers require and that we certify with our product quality certificates.
Conclusion
Aluminum can indeed be hardened through work hardening or sophisticated heat treatments like aging. Understanding these processes is key to utilizing aluminum's versatility and strength in various demanding applications.
