Alloy Comparison: 7075-T6 Aluminum VS 7075-T62 AluminumMatt Kerster
Alloy Comparison: 7075-T6 Aluminum VS 7075-T62 Aluminum
Both 7075-T6 aluminum and 7075-T62 aluminum are variants of the same material. They share alloy composition and many physical properties but develop different mechanical properties as a result of different processing.
7075 is part of the 7000 Series of aluminum alloys, which is considered the highest strength series of aluminum alloys for aircraft applications. 7075 contains zinc as the primary alloy addition and possesses excellent fatigue properties. However, in the T6 temper its fracture toughness can be inferior to other alloy choices.
It has excellent mechanical properties, and exhibits good ductility, high strength, toughness and good fatigue resistance. While, more susceptible to embrittlement than many other aluminum alloys, it has far better corrosion resistance than the 2000 alloys. It is one of the most commonly used aluminum alloys for highly stressed structural applications such as with aircraft parts.
7075-T6 aluminum is 7075 aluminum in the T6 temper. To achieve this temper, the metal is solution heat-treated and artificially aged until it meets standard mechanical property requirements.
7075-T6 is heat-treated at the mill and shipped, while T62 aluminum is heat-treated after leaving the mill by third party, usually the fabricator or manufacturer purchasing the material.
Heat-Treating and Aging
Aluminum alloys do not undergo a phase or structure change like steels do when heated. However, if the right alloying elements are present, these alloys can be heat treated by solution heat treating and precipitation hardening.
Back in the 1930s, the process of solution heat treatment was known as “ST”, and precipitation hardening was often referred to as “aging.” A quick read through the metals industry literature will reveal that these terms are still used by many.
Solution heat treatment applies temperatures very close to the melting point of the aluminum alloy, usually just 200 to 300 degrees below the melting point. The purpose is to provide enough thermal energy to dissolve, in a solid solution, the alloy elements present in the aluminum. This is known as dissolution, which can only occur at this high temperature. However, if the part is allowed to cool naturally, the copper wants to come back out of solution, which is where the important step of quenching comes in.
Quenching is simply a very rapid cool down, using water. The result is that all the alloy elements that have been dissolved at the high temperature solution heat treatment are suddenly locked in. What results from this step is what is known as a “super saturated solid solution”, which is an unstable condition of the material.
Once the quenching step is completed, aging precipitation Hardening can now be accomplished. With 7075 aluminum, the dissolved zinc slowly comes back out of solution over an extended time and forms precipitants. The precipitants, or precipitated zinc particles, strengthen the alloy by pinning or locking up numerous microstructural features in the aluminum.
According to a paper from : Omega Research, Inc.,
“The way that metallurgists control the formation of these precipitants will determine the mechanical and corrosion properties later. In the case of artificial aging or precipitation hardening, the previously solution heat treated, and quenched parts are subjected to elevated temperatures (instead of room temperature) in the range of 225-375 deg. F over extended periods of time (4-24 hours). The precipitants formed and grown here are more controlled and substantial in nature, resulting in higher mechanical properties as compared to naturally aged conditions.”
How 7075-T6 and T62 is Used
Originally developed in secret by the Japanese during WWII, it became the world’s first mass production aluminum alloy when it was used to build the Mitsubishi A6M Zero fighter.
An entry in Wikipedia lists some of the more common applications of the 7000 series aluminum alloys including 7075,
“[7000 series alloys] are often used in transport applications due to their high Specific strength, including marine, automotive and aviation. These same properties lead to its use in rock climbing equipment, bicycle components, inline skating-frames and hang glider airframes are commonly made from 7075 aluminum alloy. Hobby grade RC models commonly use 7075 and 6061 for chassis plates. 7075 is used in the manufacturing of M16 rifles for the American military as well as AR-15 style rifles for the civilian market. In particular high quality M16 rifle lower and upper receivers as well as extension tubes are typically made from 7075-T6 alloy.”
In addition to these varied uses. 7075 is widely used in mold tool manufacturing because of its high strength, low density, thermal properties, and its ability to be highly polished.
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