Image Credits: Dabarti CGI/shutterstock.com
When the topic of heat treatment comes up, aluminum is not the first metal that comes to mind. Aluminum is considered a soft metal, but there are alloys of aluminum that respond to heat treatment in a similar manner to steel and other iron based metals.
Pure aluminum and aluminum alloyed primarily with manganese or magnesium does not respond to heat treatment, so this article will focus on the aluminum alloys that contain copper, zinc, or a blend of magnesium and silicon, as these respond to heat treatment favorably.
When casting aluminum alloys, the outside edge in contact with the mold cools first, forming a skin of aluminum crystals, also known as grains. As the cooling process continues towards the center, the casting has regions of pure aluminum near the skin and other regions near the center, where the alloying element(s) precipitate out and lock crystal regions in place. This results in areas where the casting is soft and others that are strong.
Homogenizing is the heat treatment process that redistributes the precipitating element(s) evenly through the part. For many alloys the homogenizing temperature is 900°F to 1000°F, which is just under the melting point. Once the whole part reaches the homogenizing temperature it is allowed to cool slowly, resulting in a part that has a uniform internal structure ready to take advantage of other heat treatment processes or cold working.
The process of shaping aluminum alloys causes the grain structures to slide against one other, along areas known as slip planes. After a while there are less easy slip planes, and increased force is required to shape the part. This state is referred to as work hardened. The annealing process resets the crystalline structure and creates a new batch of unused slip planes, making it easy to work the part again.
Annealing requires heating the alloy between 570°F to 770°F for thirty minutes to three hours, depending on the composition of the alloy and the size of the part. This causes recrystallization, where the original crystalline structure reforms and easy slip planes are evident again. Unlike many heat treatment processes, the rate of cooling after annealing is not critical.
Image Credits: Yulia Grigoryeva/shutterstock.com
If the alloy is not kept at the proper temperature for the correct duration, recrystallization does not occur throughout. This partial annealing is referred to as recovery. Recovery reduces the internal stress and the material becomes easier to work, but it is not as easy to work as a fully annealed part. Recovery leaves stresses in the part that could interfere with future cold working applications, as well as create fracture points.
Solution Heat Treatment
Solution heat treatment is similar to annealing, but it involves quenching, which is the rapid cooling of the alloy to preserve the distribution of the elements. In solution heat treatment the elements that cause age hardening dissolve, undissolved elements become spheroids, and the whole structure becomes homogenized. The quench traps dissolve elements in the solution that will later precipitate out and create the age hardening effect. Right after the quench the alloy is usually easy to work with, but as time passes, it will harden and become difficult to work.
Solution heat treatment occurs at a range from 825°F to 980°F, with the specific temperature depending on the alloy. A big difference in this process from many other heat treatment methods is that there is a very narrow margin for error - around ±10°F of the alloy’s target temperature. If the alloy is below the narrow range, strength is lost. Above the target widow and there is a risk of discoloration, increased strain, or other elements melting, which requires a complete reprocessing of the alloy.
Soaking time is another key component of solution heat treatment, and it is a measure of time from when the coldest metal reaches the minimum limit of the desired range until quenching. For thin parts, the soaking time may be 10 minutes, while a heavy part may need 12 hours to soak. The general rule of thumb is one hour of soaking for each inch of cross-sectional thickness. Excessive soaking time can increase oxidation and negate the benefits of protectively cladded parts.
The elements that dissolve during the solution heat treatment process precipitate out over time after the quench and lock the grains into position, increasing the strength of the material. For naturally aged alloys, this happens at room temperatures and takes four or five days to reach full strength, but 90% of the hardening happens in the first 24 hours. The process starts immediately after the quench, but because it is a slow process there is a window to shape the alloy with relative ease immediately after quenching.
Image Credits: Therina Groenwald/shutterstock.com
Artificial Aging/Precipitation Hardening
Some alloys require heat to fully precipitate out the dissolved elements and reach their maximum hardness. This process is called precipitation hardening. These alloys will harden some at room temperature, with the amount depending on the specifics of the alloy. Precipitation hardening happens between 240°F and 460°F with each alloy having a specific temperature. To get the best results the temperature needs to be within ±5°F of the alloys specific temperature. This process takes from six to twenty-four hours, depending on the alloy. Once soaking is complete, the material is often air cooled to room temperature.
While not all aluminum alloys benefit from heat treatment, we can heat-treat several alloys to increase the ease of forming or the strength of the finished product. Unlike steel or iron, aluminum requires rigid heat control to achieve optimal results, so special equipment is often required. Attention to detail results in aluminum alloys that are easy to work during the forming process, followed by a hardened part that will resist wear and corrosion for years to come.
Sources and Further Reading