Precipitation Hardening – How It Works, Benefits & More

Precipitation hardening, also known as particle hardening or age hardening, is a heat treatment method used to increase the yield strength of many different metals, including most structural alloys.

The process was originally discovered by Alfred Wilm, who aimed to strengthen aluminium alloys. He decided to try to apply the quenching method used for carbon steels to aluminium alloys. The process of age hardening aluminium was patented by him in 1906. His research also led to the discovery of the earliest types of age-hardenable aluminium–copper alloy, Duraluminium.

Since then, age hardening has also been adopted for carbon steels and other metal alloys to enhance their strength, hardness, corrosion resistance and other mechanical properties.

What Is Precipitation Hardening?

Precipitation hardening is a type of heat treatment process used to increase the yield and tensile strength of metal alloys. in which material hardness improves as a function of time. 

The process uses dissolved impurities to improve the mechanical properties of a material. It works by separating specific constituents that bind the mixture together and inhibit relative motion.

The mechanism is somewhat similar to steel-reinforced concrete. Adding steel bars to a concrete mixture enhances the tensile strength of the concrete. When the concrete column encounters a tensile load, the steel beams restrict the expansion of the concrete by absorbing the tensile stress. The impurities in the precipitation hardening process improve the overall strength of the alloy similarly.

Age hardening can lead to a significant improvement in the yield strength and hardness of alloys. For instance, the yield strength of 0.3% carbon steel when annealed is around 300 MPa. The same steel, when work-hardened, has a yield strength of around 600 MPa. When it is precipitation hardened, the yield strength increases to 1500 MPa. Thus, precipitation hardening provides a fivefold increase in the yield strength in this case. 

What Is the Difference Between Tempering and Precipitation Hardening?

Both processes sound really similar in terms of heating the alloy to a certain range, holding it there, followed by rapid cooling, and then heating again below the critical temperature. However, tempering and ageing are thermodynamically really different, performed on different types of metals and result in different properties depending on the time and temperature they’re performed at.

In terms of the effects on the mechanical properties, tempering increases toughness and ductility and slightly decreases hardness of the material. On the other hand, ageing increases the hardness and strength, making the metal somewhat brittle.

Precipitation hardening hardens by forming a fine precipitate phase in the matrix of an alloy. The precipitates increase hardness by blocking dislocations in the crystal lattice. Tempering is actually stretching out the lattice because you have a Body Centered Tetragonal (BCT) lattice that is diffusing out carbon to reduce the internal stresses.

Ageing is generally performed at lower temperatures than tempering and takes longer to achieve the desired effects. Typically, precipitation hardening is a process performed mainly on aluminium, nickel alloys and stainless steels, while tempering is specific to alloy steels because it is dependent on the formation of the martensite phase.

Applications for Age Hardening

Age-hardened alloys are used in a wide range of applications. They are ideal for applications that require rigid materials that do not flex under stress.

Thus, you’ll find that components, such as valves, gears, shafts, engine parts, turbine blades, ball bearings, bushes, dies and fasteners are precipitation-hardened. The age-hardening process improves most structural aluminium alloys (2xxx, 6xxx and 7xxx series), magnesium, nickel and titanium. The process also finds use with mild steel, stainless steel and duplex stainless steel.

Here are a few examples from various industries that rely on precipitation-hardened alloys:

Aerospace and automotive

Both the aerospace and automotive industries use age-hardened aluminium alloys as the parts need to be light and strong. Age-hardening further improves the excellent strength-to-weight ratio of aluminium. Aircraft parts, such as wings and fuselages, as well as automobile parts like engine blocks, cam covers and other critical engine components, utilise precipitation-hardened aluminium alloys.

Telecommunications

Copper beryllium hardens copper in the compound. The copper starts behaving like a spring within the material. Age-hardened copper-based compounds are used in electrical contacts and switchgear, mainly because of their high conductivity.

Mining & power generation

Age-hardened materials with copper-based compounds are used in mining tools since they have very high hardness and do not spark. When underground, preventing sparks is critical for safety.

Apart from the aforementioned sectors, precipitation-hardening alloys are used in many more types of products, such as bicycle frames, rifles, stamping tools and pressure vessels. 

How Does the Precipitation Hardening Process Work?

A typical precipitation hardening process consists of three stages: solution treatment, quenching, and ageing.

1. Solution Treatment

The solution heat treatment phase, or solutionising phase, is where the homogeneity of the solid mixture is improved with the constituent metal. This could be beryllium in copper or copper in aluminium.

The solution is heated above the solvus temperature to ensure maximum dissolution of the precipitate in the alloy. The mixture sits at this temperature for a specific duration. This stage is therefore also known as soaking.

Let’s try to understand what exactly happens in this stage: Imagine you want to dissolve sugar in water. Increasing the temperature of the water enables to dissolve more sugar. On the other hand, if you decrease the water temperature, some of that sugar which was previously dissolved might come out of the solution.

The solvus temperature is the maximum temperature at which you can dissolve the most amount of a solid in a solution. Thus, by holding the alloy at a temperature above the solvus temperature, we can ensure that the maximum amount of solute is uniformly dissolved into the solid solution. This solution is also known as a supersaturated solid solution.

2. Quenching

Once maximum dissolution of the impurity occurs, the alloy is rapidly cooled to room temperature. This step is known as quenching. Quenching enables to trap the supersaturated solution as a metastable phase.

A metastable phase is an intermediate energy state that should not exist at lower temperatures. However, by rapidly cooling the material, we can freeze the phase at lower temperatures.

Quenching achieves this by preventing the diffusion of nucleation sites. Rapid cooling prevents the solute from precipitating out of the solution, which would typically occur during slow cooling processes. Materials that are just quenched, are soft solid solutions with low strength.

3. Ageing

In the ageing stage, the solution is heated again. However, the temperature is lower than in the solution treatment phase and does not reach the solvus temperature. The elevated temperature initiates precipitate formation but within very short ranges. Fine precipitates develop in the solution. These precipitates form massive strain fields and enhance the strength and hardness of the final product.

Ageing can also occur at room temperature for some alloys. This is referred to as natural ageing while ageing above room temperature is referred to as artificial ageing.

We must remember that ageing is a function of time. As time passes, the hardness of the material increases and reaches a peak value. At that stage, the material is optimally aged and exhibits the highest strength. This is because an optimally aged material interferes the most with atomic motion.

If you heat the material further, it passes the peak strength point and the strength starts to decrease again. This is known as overageing. Overaged materials are not as hard as optimally aged materials.

Age Hardening Benefits

Age hardening provides the following benefits:

• Improved strength – For many materials, the strength can be increased up to four-five times. This provides many benefits, such as smaller parts, lower costs, reduced weight and safer operation.

• Improved hardness – Particle hardening makes metals more durable as they won’t wear out as easily.

• Improved corrosion resistance – Depending on the alloy, age hardening can enhance the corrosion resistance. This is particularly desirable in components that are frequently exposed to corrosive compounds, such as chemicals and seawater.

• Improved ductility – Precipitation hardening can also improve a product’s ductility. This enhances resilience and prevents cracking and breakage.

Conclusion

Precipitation hardening is a widely used process to improve the strength rating of many types of metals and alloys. However, there are two main limitations: cost and the potential for distortion. The process is costlier than many other heat treatment methods as it requires additional heating after quenching. This may take a few hours up to several days in some cases. Material distortion is a risk with certain alloys in the quenching phase.

However, the unique benefits precipitation hardening offers make it a compelling choice. The process guarantees a harder and stronger metal when carried out properly which is especially useful for soft metals like aluminium.