2024-T42 and Natural AgeMatt Kerster
2024-T42 and Natural Age
2024-T42 is a specific temper of 2024 aluminum alloy that is solution treated and naturally aged. As part of the family of 2024 aluminum alloy variants, it is a commonly used and sought after material.
2024 is one of the best known of the high strength aluminum alloys. Because of its high strength and excellent fatigue resistance, these qualities are leveraged for use on structures and parts where good strength-to-weight ratio is needed. 2024 is readily machined to a high finish and can be easily formed in the annealed condition. Additionally, it can be subsequently heat treated, if desired.
While 2024 can be spot, seam or flash welded, it is not recommended to subject it to arc or gas welding. Because of its relatively low resistance to corrosion, 2024 is commonly available in clad form, or Alclad, which means it is given an anodized finish with a thin surface layer of high purity aluminum. However, this may reduce the fatigue strength.
2024 aluminum alloy uses copper as its primary alloying element. In older systems of metallurgy terminology, 2XXX series alloys were known as duralumin, and this alloy was named 24ST.Al.
Duralumin was developed in 1909 by a German metallurgist named Alfred Wilm at Dürener Metallwerke AG. In 1903, Wilm discovered that after quenching an aluminum alloy containing 4 percent copper, the metal would slowly harden when left at room temperature for several days. Further improvements led to the introduction of what he called duralumin. That name is obsolete today.
In certain aviation and aerospace applications, 2024-T3 is sometimes used as an alternative to 2024-T42 as both 2024-T3 aluminum and 2024-T42 aluminum are variants of the same material. While they share the same alloy composition and many physical properties, they develop different mechanical properties as a result of different processing.
2024-T42 and Natural Aging
2024-T42 aluminum is 2024 aluminum in the T42 temper. To achieve this temper, the metal is solution heat-treated and naturally aged. Unlike T4 temper, this is done by the receiver rather than the supplier.
What is “natural aging”?
According to an entry at Corrosionpedia,
“Natural aging is the spontaneous aging of a supersaturated solid solution at room temperature. This process is important for strengthening heat treatment of alloys containing aluminum, copper, magnesium and nickel.
Natural aging is a step in the heat treatment of aluminum alloys in which the metal is removed from the quench bath and allowed to gain its full strength at room temperature.”
The primary distinguishing feature of 2024-T42 is that it is naturally aged. And this is in contrast to tempers such as 2024-T5. Natural aging is contrasted with artificial aging, which is done at elevated temperatures. In artificial aging, the metal is held at an elevated temperature which allows it to gain its full strength in a shorter period of time.
Effects of Natural Aging on 2024-T42
With the heating and then natural aging process, the strength of the 2024 alloy increases as the metal transforms from a supersaturated solid solution to GP (Guinier–Preston) zones to intermediate, or coherent, precipitates.
It is the extra strength provided by these precipitates that give many alloys such as 2024-T42 the sufficient strength and toughness needed for use in lightweight aircraft structures. However, while aging improves many mechanical properties of the alloy such as strength and fatigue resistance, the aging process may also degrade some other properties.
For example, aging lowers the ductility of aluminum. This can be a concern although the elongation-to-failure of many fully-aged alloys is still above 5 to 10 percent. Also, stress corrosion cracking, SCC, resistance of aluminum alloys may also be affected by age-hardening. SCC involves the growth of cracks created from a combination of tension loads and corrosive fluids. The result is a lowered level of fracture stress of the material.
Aging time gradually decreases SCC resistance of an aluminum alloy until it reaches a minimum level when the alloy is fully hardened. One method used to protect age-hardened alloys against SCC for materials used in aircraft is to add corrosion-resistant protective coatings. Several types of coatings are used for aircraft, including cladding and anodized films.
Applications for 2024-T42
There are a number of applications with aluminum 2024-T42 including the manufacturing of truck wheels, aircraft wheels, screw machine products, crew machine products and rivets. Its high strength and fatigue resistance also make it a good choice for orthopedic braces, veterinary equipment and scientific instruments, as well as aircraft structures.
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