Annealing is a heat treatment process that improves the machinability of metals by altering their internal properties.
But is annealing required for soft metals like aluminum?
Annealing aluminum is one of the most popular heat treatment processes adopted to further soften aluminum. Annealing makes aluminum amenable to various metal-working operations. The process involves heating followed by controlled cooling, which helps reduce internal stress and improve the material's endurance.
This article explains annealing aluminum in detail such as its process, its purpose, its applications, and the alloys of aluminum suitable for annealing.
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Annealing Aluminum - The Process
Annealing is a heat-treatment process that involves heating the workpiece to improve its properties, unlike anodizing, where the aluminum workpiece undergoes an electrochemical reaction, without heating.
The annealing process is performed periodically on a cold-worked aluminum to improve its workability (ability to bend and deform without crack formation).
Cold working is a metalworking process where the metal is deformed (extruded or rolled) at a temperature below recrystallization temperature to improve its hardness or strength.
It is also referred to as the strain hardening process, where the material is hardened by applying strain.
Thus, cold working results in dislocated or distorted grain structures, which makes aluminum fragile for further working. In such a state, the annealing process helps restore its original structure and properties.
However, before annealing the workpiece, it is advised to clean the workpiece to remove any unwanted impurities which can catch fire and stain the workpiece during annealing.
You can grind the aluminum workpiece to remove a fine layer from its surface, thereby eliminating surface impurities.
The process of annealing improves the properties like ductility, toughness, etc., by raising its temperature above the upper critical temperature, followed by controlled cooling or quenching it in water.
The critical temperature, Tc, is the temperature at which the metal undergoes a phase change.
Though the critical temperature of aluminum is not well established due to its higher value, it is approximated to be around 350-510oC.
This is so because of the observable change in the material microstructure in this temperature range.
The process of annealing occurs in three stages: Recovery, Recrystallization, and Grain Growth.
Stage 1: Recovery
Recovery is the initial stage of annealing, where aluminum is heated in a furnace to a temperature higher than or equal to 400OC.
An inert environment is maintained inside the furnace to avoid metal oxidation. This stage of annealing helps to restore its original structure and properties partially.
With the increase in temperature and time, the grain size of aluminum increases.
Stage 2: Recrystallization
In this stage, defects are completely recovered by heating the aluminum beyond its recrystallization temperature. Distortion-free grains are formed in the recovered metal.
The following table shows the time taken for the appearance of the first recrystallization nuclei for different annealing temperatures of the AA3003 series aluminum alloy.
|Annealing Temperature||Time taken to observe first recrystallization nuclei|
|450℃||Less than an hour|
Stage 3: Grain Growth
Grain growth refers to an increase in the grain size of the newly formed grains.
In this stage, the aluminum workpiece is allowed to cool inside the furnace (controlled cooling). This stage completes the phase of crystallization.
How to Anneal Aluminum for DIY Projects?
Can we anneal aluminum for DIY projects? The answer would be “Yes.”
To perform annealing on a smaller scale, you would need a blow torch and a permanent marker.
Steps to follow for the DIY annealing project
- Using a permanent marker, scribble a few lines or markings on the specimen.
- Choose a work area where the surrounding lighting is dull or dark.
- Use a blow torch (preferably an oxyacetylene torch) to supply heat to the specimen.
- Heat the workpiece throughout uniformly without concentrating the flame at any specific point. This is done to avoid any hotspot that might cause the workpiece to melt or develop non-uniform properties.
- After a few minutes, markings on the workpiece will disappear, followed by the appearance of a light-colored red spot at the interjection of the flame and workpiece.
- The red flame spot indicates that the material has reached its annealing temperature. In this instance, switch off the blow torch.
- To complete the annealing process, cool the workpiece in still air or dip it in a water tank.
- Once cooled, the workpiece is all ready to be reworked. All sorts of operations like bending, twisting, and hammering, can be done on the workpiece without causing any failure or formation of cracks.
The process mentioned above is the most commonly used one. However, apart from this method, an alternative approach of using bar soap is also followed.
Instead of permanent markings, bar soap is rubbed over the material.
On heating the workpiece, when soap markings turn into black soot, it indicates the specimen has reached its annealing temperature.
However, this method is not recommended as it might affect the appearance of the workpiece.
Annealing can also be performed using a wood-fired oven, which is not economical for small workpieces.
The size of the workpiece chosen to work upon also has a role in determining the process to be used. It is advisable to use thin sheets over thick ones to assure uniform heat distribution throughout the workpiece (distribution of heat from the outer surface to the inner core).
Purpose of Annealing Aluminum
Finished cold-worked aluminum parts provide desirable strength and hardness. But, if the part still requires to be worked upon, the annealing process comes in handy.
The primary purpose of annealing aluminum is to relieve internal stresses. Internal stresses lead to premature failure or brittleness (tendency to snap off) in the material.
Performing annealing on aluminum improves the material's ductility and refines the grain structure. Thus intermittent annealing helps deform the aluminum workpiece into complex shapes without triggering any cracks.
Annealing also helps in improving the corrosive resistance of the material. Thus, we can infer that annealing enhances the overall mechanical properties of the material.
Aluminum Alloys Suitable for Annealing
|Series||Alloying Elements||Annealing Temperature||Type of alloy|
|6xxx||Magnesium and silicon||413℃||Heat-treated|
Copper, Manganese, Silicon, Magnesium, Magnesium-silicon, and Zinc are the most commonly used alloying elements with aluminum.
Adding these elements to aluminum increases the strength, corrosive resistance, workability, and conductivity compared to pure aluminum metal.
An aluminum alloy is classified into two groups: Wrought alloys and Casting alloys.
These, in turn, are classified into heat-treated and Non-heat-treated alloys.
In heat-treated alloys, the alloys are heat treated at high temperatures, enhancing strength and hardness. Thus the process of annealing does not play an influential role.
A non-heat-treated alloy is hardened through strain hardening, and annealing helps soften the alloy to relieve internal stress and avoid cracking of the workpiece during the process.
Alloys with a lower percentage of aluminum undergo structural changes if annealed at higher temperatures.
Therefore, it is essential to know the annealing temperatures for different alloys.
Applications of Annealed Aluminum
Based on the versatile properties of aluminum alloys, each type has its own specialized applications.
|1xxx||Pure Aluminum||High thermal and electrical conductivity||Electrical and Chemical industry|
|2xxx||Copper||High strength-to-weight ratio||Aircraft fuselage, wing skins, truck suspensions|
|3xxx||Manganese||Good workability||Architectural industry|
|4xxx||Silicon||Low coefficient of thermal expansion and high wear resistance||Brazing sheets, welding rods, forged engine pistons|
|5xxx||Magnesium||Good resistance to corrosion||Marine equipment, boat hull|
|6xxx||Magnesium and silicon||Good formability and machinability||Automotive components, architectural extrusions|
|7xxx||Zinc||Reduced resistance to stress corrosion||Pressure vessels|
1100 alloy or commercially available pure aluminum is used in making chemical processing equipment, fan blades, nameplates, fin stock, and many more.
1350 series is commonly used in making electrical conductors and is often annealed to improve its ductility to be drawn into wires.
3003 and 3004 are the most commonly found general-purpose alloys, used in manufacturing kitchen utensils, agricultural appliances, fuel tanks, freezer liners, and furniture.
3105 alloy is usually used in making medical equipment, parts of appliances, and architectural products.
5005 alloy is used in areas where anodizing aluminum parts are needed—for example, automotive parts, marine applications, and containers.
5052 alloys find their applications in the manufacturing of pressure vessels, electronic panels, fasteners, and highway signs.
5056 is used in manufacturing screen wires, zippers, fence wires, etc.
5083 is used in cryogenic applications, bridges, drilling rigs, and missile components.
Limitations of Annealing Aluminum
The limitations of annealing are few when compared to its wide applications. The main restriction in performing the annealing process is increasing and maintaining the heat at elevated temperatures.
Similarly, a few alloys, like alloy 3003, usually require rapid heating to prevent grain growth.
Though annealing helps soften the material to rework, it should be adopted only if it is necessary, i.e., to shape the workpiece into the desired shape.
This is because repeated annealing would weaken the material as it reduces hardness. Also, the formation of stress-concentrated regions can trigger the formation of cracks.
Further, the energy cost of annealing is high when used with thicker sections.
The heat-treating method should be able to distribute heat uniformly over larger sections.
Though aluminum has better thermal conductivity than commonly machined materials like iron or steel, some of its alloys may have lower conductivity. This should be kept in mind when adopting heat treatment.
Frequently Asked Questions (FAQ)
What are the materials that can be annealed?
Can annealing be performed on a glass?
Yes, annealing can be performed on glass. It helps relieve its internal stresses, improving its stability and heat resistance.
What are the different methods of heat treating an aluminum alloy?
Age hardening, homogenizing, annealing, and precipitation hardening are commonly used heat treatment processes for an aluminum alloy.