Light alloys are used widely in aerospace, automotive, architectural, lithographic, packaging, electrical, and electronic applications. They have been the prime material of construction for the aircraft industry throughout its history. About 70% of commercial civil aircraft airframes are made from aluminium alloys, and civil aviation would not be economically viable without aluminium. One kilogram of metal saved in the design and construction of an aircraft can result in important weight savings in construction costs and total fuel requirements.
Other metals, such as steel and titanium, are sometimes used to build aircraft. Steel is heavy, though, so not too much is used. Titanium is almost as strong as steel, has a medium weight, is heat resistant, and is corrosion resistant. For example, the Lockheed SR-71 Blackbird, the world’s fastest jet-propelled aircraft, is made of titanium. In some cases, these light alloys may be replaced by composite materials, especially those made from glass, carbon, and Kevlar fibers. These composite materials are strong but can weigh half as much as aluminium. These lightweight, customizable materials are becoming more popular. More than half of the materials used to make the Boeing 787 Dreamliner are composites.
Metal-based light alloys may also be used for parts that operate at high speeds and thus must be lightweight to minimize inertial forces. Other commercial applications include hand-held tools, laptops, luggage, and ladders, automobiles (e.g., steering wheels and columns, seat frames, transmission cases).
Types of Light Alloys
As was written, magnesium, aluminium, and titanium are light metals of significant commercial importance. These three metals and their alloys comprise the bulk of the high strength-to-weight ratio metallic materials used in industrial systems. Aluminium is the most versatile of these materials, titanium is the most corrosion resistant with very high strength, and magnesium has the lowest density. In addition to these metals, beryllium is a high-strength, lightweight metal with a very high elastic modulus (303 GPa) that is increasingly used as a structural material in aerospace vehicles. The elastic modulus of beryllium is nearly three times that of titanium.
- Aluminium Alloys. The mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. Among others, high strength can be achieved by introducing a high volume fraction of fine, homogeneously distributed second phase particles and refining the grain size. In general, aluminum alloys are characterized by a relatively low density (2.7 g/cm3 as compared to 7.9 g/cm3 for steel), high electrical and thermal conductivities, and resistance to corrosion in some common environments, including the ambient atmosphere. The chief limitation of aluminum is its low melting temperature (660°C), which restricts the maximum temperature at which it can be used. For general production, the 5000 and 6000 series alloys provide adequate strength, good corrosion resistance, high toughness, and ease of welding. Aluminium and its alloys are used widely in aerospace, automotive, architectural, lithographic, packaging, electrical, and electronic applications.
- Magnesium Alloys. Magnesium alloys are mixtures of magnesium and other alloying metal, usually aluminium, zinc, silicon, manganese, copper, and zirconium. Since the most outstanding characteristic of magnesium is its density, 1.7 g/cm3, its alloys are used where light weight is an important consideration (e.g., in aircraft components). Magnesium has the lowest melting (923 K (1,202 °F)) of all the alkaline earth metals. Magnesium alloys are typically used as cast alloys. Despite the reactive nature of the pure magnesium powder, magnesium metal and its alloys have good resistance to corrosion. We must add that pure magnesium is highly flammable, especially when powdered or shaved into thin strips, though it is difficult to ignite in mass or bulk. It produces intense, bright, white light when it burns. Flame temperatures of magnesium and some magnesium alloys can reach 3,100°C.
- Titanium Alloys. Titanium alloys are metals that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures). They are light in weight, have extraordinary corrosion resistance, and can withstand extreme temperatures. Although “commercially pure” titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications, titanium is alloyed with small amounts of aluminum and vanadium, typically 6% and 4%, respectively, by weight. This mixture has a solid solubility that varies dramatically with temperature, allowing it to undergo precipitation strengthening.
The density of Light Metals and Alloys
The density of typical aluminium alloy is 2.7 g/cm3 (6061 alloys).
The density of typical magnesium alloy is 1.8 g/cm3 (Elektron 21).
The density of typical titanium alloy is 4.43 g/cm3 (Ti-6Al-4V).
Strength of Light Metals and Alloys
In the mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. The strength of materials considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. The strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
The ultimate tensile strength of 6061 aluminium alloy depends greatly on the temper of the material, but for T6 temper, it is about 290 MPa.
The ultimate tensile strength of Elektron 21 – UNS M12310 is about 280 MPa.
The ultimate tensile strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1170 MPa.
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress that a structure in tension can sustain. Ultimate tensile strength is often shortened to “tensile strength” or “the ultimate.” If this stress is applied and maintained, the fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after achieving the ultimate strength. It is an intensive property; therefore, its value does not depend on the size of the test specimen. However, it depends on other factors, such as the specimen preparation, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for aluminum to as high as 3000 MPa for very high-strength steels.
Yield Strength
The yield strength of 6061 aluminium alloy depends greatly on the temper of the material, but for T6 temper, it is about 240 MPa.
The yield strength of Elektron 21 – UNS M12310 is about 145 MPa.
The yield strength of Ti-6Al-4V – Grade 5 titanium alloy is about 1100 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically. In contrast, the yield point is where nonlinear (elastic + plastic) deformation begins. Before the yield point, the material will deform elastically and return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behavior termed a yield point phenomenon. Yield strengths vary from 35 MPa for low-strength aluminum to greater than 1400 MPa for very high-strength steel.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of 6061 aluminium alloy is about 69 GPa.
Young’s modulus of elasticity of Elektron 21 – UNS M12310 is about 45 GPa.
Young’s modulus of elasticity of Ti-6Al-4V – Grade 5 titanium alloy is about 114 GPa.
Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to limiting stress, a body will be able to recover its dimensions on the removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position, and all the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions, and no permanent deformation occurs. According to Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
