The Science Behind the Heat Treating of Aluminum
Heat treatment processes are used to enhance the strength and durability of aluminum components found in everything from airplanes to everyday consumer goods. These processes include solution heat treating, quenching, and aging. In this first installment of our blog series on aluminum heat treating, we will examine the science behind these processes.
Understanding the Metallurgy of Aluminum Heat Treating
Heat-treatable aluminum contains alloying elements like copper, silicon, and magnesium. These elements are added in varying combinations to provide a range of properties, including greater strength, increased hardness, improved machinability, corrosion resistance, and more. To create these properties within the aluminum, a specific sequence of time and temperature changes are needed to dissolve the alloys, fix them in the aluminum matrix by quenching, and precipitate the elements in an orderly manner within the aluminum via the aging process. This is where solution heat treatment comes in.
The Solution Heat Treatment Process
The first step in the aluminum heat treating journey is solution heat treatment, which sets the foundation for the material’s final properties. The process involves heating and holding the aluminum alloy at a sufficiently high temperature for a long enough period to achieve a homogenous solid solution. The purpose of this stage of solution heat treatment is to dissolve the alloying elements throughout the aluminum structure.
The Heating and Soaking Phase
During solution heat treatment, the aluminum alloy is heated to a specific temperature range, typically between 850°F and 1,050°F (454°C to 566°C). This high temperature enables the alloying elements, such as copper, silicon, and magnesium, to dissolve into the aluminum matrix. The alloy is then held at this temperature for a designated soak time. This soaking period allows for the complete dissolution of the alloying elements, creating a solid solution.
Time and Temperature Precision
At this production stage, the aluminum parts have already undergone various manufacturing processes (forging, extruding, casting, and even machining), so scrap caused by improper heat treatment is an expensive mistake. Maintaining precise temperatures and soak times is crucial, since over or underheating will create problems.
- Overheating Effects: Exceeding the recommended temperature range can lead to eutectic melting, a phenomenon where a mixture of metals melts at a lower temperature than its individual components. Overheating can degrade the properties such as tensile strength, ductility, and fracture toughness.
- Underheating Effects: If the alloy is not heated to a sufficiently high temperature or held there long enough, the alloying elements will not fully dissolve. This results in an incomplete solution and lower-than-expected strength.
Temperature tolerances for solution heat treatment are generally allowed a variation of ±10°F (±5.5°C) from the control setpoint, but certain alloys require even tighter tolerances. The time the alloy is held at the specified temperature varies from several minutes to many hours depending on the thickness of the material and loading arrangement. Once the alloys are evenly dissolved, the aluminum moves onto the next step where it is rapidly cooled in a quench.
Rapid Quenching
Quenching, in many ways, is the most critical step in the sequence of heat treating aluminum. A water or polymer solution is used for rapid and uninterrupted quenching of the aluminum. The choice of quench medium also plays a significant role in the outcome. Different alloys, part shapes, and material thicknesses require different approaches:
- Water: This is the most common and cost-effective method with the fastest quench rate. The part cooling is very rapid due to the high specific heat of water, and this can cause warpage in some thinner parts. A water quench is the normal for thicker or cast aluminum parts, and is also very common for thinner aluminum parts where some warpage can be tolerated.
- Polymer: This provides a slower, more controlled quench and reduces the risk of distortion and cracking. A polymer quench is often preferred for thinner sections or intricate shapes prone to warping.
The goal of quenching is to rapidly cool the aluminum to prevent the alloying elements from naturally reforming into their previous state. The rapid quench essentially captures and fixes the high-temperature, evenly distributed solid solution. This provides the optimal condition for the subsequent age (precipitation) hardening process.
The Role of Aluminum Aging (Precipitation Hardening)
After quenching, the aluminum is still relatively soft. Age hardening, or precipitation hardening, involves reheating the quenched material to a lower temperature range (212°F to 424°F) and soaking it at temperature for a defined period of time, typically several hours. This improves both the tensile properties and the yield strength of the aluminum while improving the hardness. The ductility, as measured by percent elongation, also decreases during the aging process.
Up Next: Choosing Your Solution Treatment Equipment
In our next blog post, we will shift our focus from the process, to the equipment used for aluminum heat treatment. We will explore one of the most traditional options for solution heat treatment: the drop bottom furnace. Our blog Drop Bottom Furnaces: The Traditional Powerhouse of Aluminum Heat Treatment will cover the design, operation, and unique advantages a drop bottom furnace offers for your solution heat treatment process. Learn how this workhorse can elevate your aluminum processing capabilities: