Aerospace heat treatment

The purpose of heat treatment is to change the metallurgical structure and by doing this, the properties of metal parts. Heat treatment can affect the properties of most metals and alloys, but ferrous alloys, principally steels, undergo the most dramatic increases in properties. Annealing is when a steel is heated and then slowly cooled, producing a structure that has a low level of residual stresses locked within the part. However, the properties that interest heat treaters the most are those exhibiting high strength and hardness, usually accompanied by high levels of residual stresses. These are metastable structures produced by nonequilibrium cooling or quenching from the austenitic state.

Quenching refers to cooling at such a rate that transformations in the primary and bainite ranges are suppressed and martensite is formed. In this state, steels are truly hard.

Heat-treatment auditing is a growth industry because heat-treated aluminium alloys are extensively used in aircraft structures. Commonly used heat-treatable alloys naturally age hardened are 2117, 2017 and 2024. There are three stages in the heat treatment process: Heating, quenching, and aging. In its "as fabricated" condition, the aluminium is placed in a heat-treating oven and heated for a period of time determined by its alloying elements and by its size.

For example, the aluminium alloy 2024F is heated to a temperature of 920°F and held at that temperature until all parts are equally heated. After being heated, the metal is quickly transferred to a quenching tank. The quenching operation must be performed as quickly as possible in order to reduce the possibility of intergranular corrosion. After quenching, the aluminium alloy is kept at room temperature for 16 to 24 hours to ensure that the metal has age hardened. Age hardening, the final step in the heat treatment process, makes the metal naturally hard.

The designation of metal treated with this process changes from F, as fabricated, to T4, heat treated and age hardened. If the metal is further hardened by coldworking (that is, mechanically) its designation is T3.

Heat-treatable aluminium alloys commonly used in the construction of modern aircraft are 2117T4, 2017T4 and 2024T4 or T3. The alloy 2117T4 is manufactured only as rivet stock. It is the most widely used rivet alloy in the aviation industry. The 2117T4 rivet is driven in the condition received from the manufacturer without any further treatment.

The 2017T4 alloy is manufactured in two similar forms. The first form dates back to 1925, when it was called 17ST and was used for rivets and structural skins. When being driven in larger sizes, the rivet stock frequently cracked. Also, it had to be re-heat treated and driven in a softened condition.

Recently, Alcoa reworked 2017T4 by slightly reducing the amount of magnesium and adding small amounts of iron and silicon. The new crackfree 2017T4 meets the original 2017 standards and can be driven in the condition received. It is used in the construction of the new Boeing 757 and 737-200 series aircraft.

The alloy 2024T3 is widely used for skin covering and internal parts of all types of aircraft. It has exceptional resistance to fatigue loads, it is highly resistant to cracks, it can withstand heavy load limits, and it retains high strength after damage. As rivet stock, however, the 2024T4 rivet is being challenged by a new rivet, the (Boeing rivet) 7050T73.

Precipitation heat treatment 

Precipitation Heat Treatment: Artificial Aging

Aluminium alloys containing zinc, magnesium, silicon, or copper are given a precipitation heat treatment after natural heat treatment is completed. For example, the alloy 7075 is given a normal heat treatment at 870°F and quenched in cold water. After it is precipitation heat treated at 250°F for 24 hours, it becomes 7075T6. Alloys are precipitation heat treated by heating them in an oven; time and temperatures vary.

This treatment has the effect of locking together particles in the grain of the metal, thus increasing strength, stability, and resistance to corrosion. Natural heat treatment begins the grain-binding process; precipitation heat treatment completes it. In addition, artificially aged alloys are generally over-aged to increase their resistance to corrosion, especially if, like 2024, they are subject to intergranular corrosion.

Metals which are given precipitation heat treatment usually lose some malleability and ductility, and their mechanical properties are so changed as to reduce their ability to be reshaped cold without cracking.

The most commonly used precipitation heat treated alloys are those containing zinc. The alloy 7075T6 has high impact resistance and therefore is used where great strength is required. The 7079T6 aluminium alloys are excellent for making forgings for heavy channels that carry landing gears or flaps of large aircraft. The alloy 7178 is used where compression loads are the greatest, for example in the superstructure of wide-body jets.

The alloy 7050T73 is the newest aluminium alloy. It was developed in 1979 by Alcoa and the US Air Force. A combination of aluminum, zinc, and magnesium, it is primarily used as a solid-shank rivet. The 7050T73 alloy is the strongest of any rivet alloy in use today. It has a high resistance to stress corrosion and is much stronger than the alloy 2024T31, which it has replaced on some modern jet aircraft.