As aerospace and aircraft technology develop to be more sophisticated, the need of more advance material improves as well. From the early beginning of wooden and aluminum aircraft, until the usage of titanium alloy and high-performance composite materials need intense development from a lot of scientific and engineering disciplines.
The use of light materials for aircraft are very important. For example, for transport sub-sonic aircraft, the payload only takes 20% of its total weight, however, its 80% consisted of its empty weight and fuel. It can be said that increasing its weight could improve the fuel usage, which is directly affects the operational cost.
These are the following considerations for aircraft materials selection:
- Static strength efficiency (strength to weight ratio)
- Fatigue characteristics
- Toughness and crack propagation characteristics
- Corrosion and embrittlement characteristics
- Environmental stability
Moreover, other important criteria which are important for production and manufacturing cost are:
- feasibility of the materials
- the materials cost
- Fabrication characteristics (easy to form, cut, mold, etc.)
Below are the following commonly used commercial aerospace materials:
- Aluminum alloy
For commercial flights, aluminum is used almost 80% of airplane’s structural materials. Aluminum in this context of course not the same as daily used aluminum for cooking devices or decorations. Aerospace grade aluminum alloyed (mixed) with some alloys such as copper, magnesium, zinc and manganese which improve its strength, stiffness as well as its toughness.
Bellow are commonly used commercial aluminum for aerospace application:
- Aluminum 2024-T3, T42, T351, T81: for high tensile applications, high toughness and good crack propagation characteristic. T42 has less strength than T3. While T81 commonly used for high temperature applications.
- Aluminum 2224-T3, 2324-T3: has 8% more strength than 2024, lower toughness, used for low tensile application. Has a good corrosion resistance
- Aluminum 7079: almost the same with 7075 but has better cross-sectional area properties (>3 inch)
- Aluminum 7150-T6: 11% stronger than 7075-T6, has better fatigue and toughness characteristics than 7075-T6.
- Aluminum 7178-T6, T651: used for compressive load. Stronger than 7075 but less toughness.
- Aluminum-lithium: 10% lighter and stiffer than conventional aluminum alloy.
- PM Aluminum: stronger, tougher, high temperature resistant, and more corrosion resistance than conventional aluminum.
Titanium material was not considered in the early of aerospace development because its chemical reactivity is very high and very hard to purify from its raw material as well as its fabrication characteristics are unfavourable. Despite its high machining cost, this material has strength per weight ratio higher than aluminium or steel. On the other hand, this material has superior corrosion resistance and high temperature resistance.
Its costly manufacturing cost restrict Titanium usage in the large production volume, hence just some few critical components are made of this material. Commonly used titanium in aerospace industry are Ti-6AI-4V and Ti-4AI-4Mo-2Sn-0.5Si
- Steel alloy
For some high tensile strength applications, steel alloy still a better choice compared with titanium or aluminium, especially for lower cost consideration. Below is the list of commonly used steel alloy in the aircraft industry:
- Martensitic stainless steel
Contains 12-18% of chromium and no nickel content then heat treated with quenching and tempering. Has a relatively low corrosion resistance.
Commonly used for kitchen utilities, turbine blade etc.
- Ferritic Stainless Steel
Contains 15-30% of chromium, without nickel and heat treatment hence has lower strength. Has good corrosion resistance at elevated temperature. Commonly used for piping, vessels and chemical industry.
- Austenitic stainless steel
Contains 18% or more chromium and 3,5 up to 22% of nickel. Stainless steel 321 and 347 contain titanium and columbion as stabilizer alloy for corrosion resistance. This material are very corrosion resistant even for salt water.
Commonly used for aerospace industry, chemical industry, piping and sea water applications.
- Precipitation Hardened Stainless Steel
Contains of very low carbon, 15-17% chromium, 4-7% nickel and a few amounts of other alloy elements. High corrosion resistance even for sea water applications. Commonly used for aircraft components which need high strength, corrosion and high temperature resistance.
- High strength low alloy steels
Iron-based material, which can be hardened to very hard condition. Commonly referred to 4130 and 4340 alloy material. These materials commonly used for frame and landing gear.
- Composite materials
Recently, composite materials are widely used in aerospace industry because of its superior strength and stiffness compared to its weight (which weight just like plastic). Moreover, the fiber direction could be engineered to optimize its strength and weight usage. From manufacturing point of view, this material could be shaped easily into aerodynamic shape more flexible compared to the other materials because the use of molding.
Commonly used aerospace composite materials are carbon fiber, boron, fiberglass and Kevlar. Not just those materials composition variables, aircraft structure sometimes combined the composite material with optimised sheet shape such as honey comb sheet to maximise stiffness to weight ratio. One of the restriction of composite material parts is the high initial cost for making the mold, but it will decrease significantly for high volume production.
By Caesar Wiratama
Reference: Chun-yung Niu, M. 1989. “Airframe Structural Design : Practical Design Information and Data on Aircraft Structures”. Comlit Press. California
Author: Caesar Wiratama