Introductory Chapter: Introduction to Aluminum Alloys

1. Introduction

With the advancement of technology and scientific studies, new areas of use have been created for existing materials. In this way, the interest in new materials with high quality that can meet current needs has increased [1]. The developing technology and scientific studies to date are in search of improving the desired properties of existing materials and the emergence of new materials that can be alternatives. This search continues with the emergence of materials with new qualities [2]. Compatibility of physical and characteristic properties is very important in material selection. For this reason, many methods have been developed and continue to be developed today. Particularly in the aviation, space, gas turbines, automotive, and maritime industries, materials that are high-performance, light, and durable, and can combine features such as stability at high temperatures are needed [3, 4, 5].

Aluminum and its alloys, which are widely used especially in the aviation industry, can be preferred and used in long-lasting applications due to their features such as low density, high specific strength, and resistance to oxidation [6, 7]. Among the reasons why aluminum metal and its alloys have different usage areas in different sectors are that they can be easily produced, shaped, and processed, have high corrosion resistance, are lightweight, and have good strength properties. Additionally, aluminum is extremely suitable for recycling [8, 9]. Aluminum alloy is almost infinitely recyclable, and the recycling process requires only 5% of the energy of primary aluminum production. High-purity aluminum is a soft material with an ultimate strength of approximately 10 MPa, which limits its usability in industrial applications. To compete with other building materials, the strength of Al-based materials needs to be significantly increased. There are several ways to increase the strength of metallic materials: alloying with sufficient elements, adding appropriate strengthening particles, plastic deformation, or grain size reduction [10].

Aluminum alloys are the most used metallic engineering materials after steel today. Aluminum and its alloys, which are widely used especially in the aviation industry, can be preferred and used in long-lasting applications due to their features such as low density, high specific strength, and resistance to oxidation. Among the reasons why aluminum metal and its alloys have different usage areas in different sectors are that they can be easily produced, shaped, and processed, have high corrosion resistance, are lightweight, and have good strength properties. In addition, aluminum alloys provide significant advantages over other engineering materials due to their features such as high thermal conductivity, nonflammability, and being completely recyclable and weldable [11, 12].

During the production phase, material scientists constantly improve the properties of the materials they have obtained by moving them to macro dimensions under more minimized conditions. The purpose of this is that today’s technology is competitive in the market and the features of the materials needed are no longer affordable. Above all else, one of the most distinctive features required from materials is to ensure continuity [13, 14, 15]. In other words, the material produced gives the same physical results and reactions when used at different times but under the same conditions. Therefore, it is important that the material has a homogeneous microstructure and an equal stress distribution on the material [16, 17, 18]. In recent years, different production methods have been developed in addition to traditional powder metallurgy methods in the production of some critical parts. Mechanical alloying is one of these methods. This method is used to alloy metals without exposure to any chemical or heat treatment [19, 20, 21]. With mechanical alloying, metals can be alloyed without the need for melting or heat treatment. The mechanical alloying process involves cold welding of solid powder particles to each other and breaking them after deformation hardening. At the same time, particle-reinforced composites are produced using the mechanical alloying method, in which the best microscopic or macroscopic combinations of different materials are provided in order to obtain fine microstructures in powders [22, 23, 24, 25].

2. Aluminum and aluminum alloys

Aluminum is found in the form of aluminum silicate in rocks, feldspars, feldspathotites, and micas in clay soils formed by the disintegration of bronze, bauxite, and iron-rich laterite. Bauxite, the most important aluminum ore, contains 52% aluminum oxide [26, 27].

Aluminum is the most abundant metallic element in the earth’s crust after iron, and it became economically produced in engineering applications toward the end of the nineteenth century. With the emergence of the first internal combustion engine vehicles, the engineering value of aluminum as an automotive material began to increase. Aluminum, which meets the need for a light and conductive material for the transmission of electricity over long distances, has also taken its place in the new industry that was born on strong, light, and break-resistant parts, along with the work of the Wright brothers on aviation. Nowadays, aluminum is used to obtain value-added products in many transportation sectors such as automotive, defense industry, aviation, rail systems, and maritime. Aluminum production is generally made from ore and recycled scrap. Today, aluminum production from ore is approximately twice that of aluminum production from scrap [28, 29]. Aluminum is the third most abundant element after oxygen and silicon, with a content of 8% in the earth’s crust. Even though there are so many, it was discovered after minerals such as iron, copper, tin, lead, gold, and silver. Aluminum is found in nature as a compound. The existence of this abundant element was detected only in 1808 by the British Sir Humphry Davy [30].

The most distinctive feature of aluminum, which is used in all areas of human life and especially in engineering applications, is its lightness. It is the lightest metal after magnesium and beryllium. Due to the superior properties provided by aluminum and its alloys, their consumption is increasing rapidly and new areas of use are opening up every day. Although pure aluminum is a very active metal in the galvanic series, the protective oxide layer that easily forms on its surface makes it widely used. This impermeable, hard, and protective oxide layer consisting of aluminum oxide (Al₂O₃) significantly increases the corrosion resistance of aluminum. Accordingly, as aluminum is purified, its corrosion resistance and conductivity increase. For this reason, aluminum alloys, which are very sensitive to corrosion, are now protected from corrosion by cladding in pure aluminum [31]. On the other hand, the very low strength of pure aluminum can be increased by cold working. Today, aluminum and its alloys have, due to its properties, become one of the most important construction and engineering materials used in the industry. While it has properties, such as high thermal and electrical conductivity and corrosion resistance in its pure form, these properties have spread to a much wider spectrum with alloying and have become widely used. Today, more than 100 aluminum alloys are widely used in industry. The most important features are summarized below [26]:

• Aluminum is lightweight. It weighs only one-third of the weight of a steel material of the same volume.

• Aluminum is resistant to weather conditions, foodstuffs, and many liquids and gases used in daily life.

• Aluminum has high reflecting ability. With its silvery white color contributing to this feature, it has an attractive appearance for both interior and exterior architecture. This beautiful appearance of aluminum is achieved by anodic oxidation (anodizing), lacquer materials, etc. with applications, such as period can be preserved. In fact, in many applications even a natural oxide layer is sufficient.

• The strength of various aluminum alloys is equal to or higher than that of ordinary structural steel.

• Aluminum is an elastic material, especially for chipless manufacturing processes. Therefore, it is resistant to sudden impacts. Additionally, its durability does not decrease at low temperatures. (The resistance of steels against sudden impacts decreases at low temperatures.)

• Aluminum is a metal that is easy to machine. So much so that it can be turned into foil or wire with a thickness of less than 1/100 mm.

• Aluminum conducts heat and electricity, as well as copper.

• All methods, such as casting, forging, rolling, pressing, extrusion, and drawing, can be applied to shape aluminum.

2.1 Mechanical properties of aluminum

The modulus of elasticity of aluminum is approximately one-third of that of steel. In other words, the amount of elastic deformation of aluminum under the same load is three times that of steel. This feature is of great importance in design calculations. The low modulus of elasticity is considered an advantage when exposed to impact loads since the resistance of aluminum is higher than that of steel. It allows the absorption of large amounts of energy [32]. One of the main features of aluminum is its ease of shaping and processing according to conventional manufacturing methods. Since pure metal is soft and has the ability to become wire, it can be rolled, drawn, and shaped by applying various cold processes such as drawing, bending, pressing, and molding [33].

2.2 Chemical properties of aluminum

Aluminum is not available in pure form due to its high chemical activity. Therefore, its production is made from bauxite ore consisting of iron oxide and aluminum silicate. Due to a fixed oxide layer formed on the aluminum surface in contact with air, the metal and its alloys generally show great resistance to the corrosive effects of the atmosphere. Aluminum reduces the oxides of other metals due to its affinity for oxygen. Due to this feature, powdered aluminum is used in the production of metal oxides such as chromium, vanadium, barium, and lithium by reducing them [10]. Since aluminum is nontoxic, it is widely used in many areas, especially in the production of equipment in the food industry. Again, due to this feature, it is widely used in the packaging of food and medicines, cigarettes, and tea [34].

2.3 Physical properties of aluminum

Low density, one of the physical properties of aluminum, comes to the fore in many applications. The density of aluminum in the commercial group is approximately 2.70 g/cm³. When equal volumes are compared, aluminum has approximately one-third the weight of iron, copper, and zinc. In some applications, it is not enough to focus on the lightness advantage of metal alone. For example, a material that does not have sufficient strength but has low density is not very useful for the elements that make up the structure of an aircraft. In this case, while pure aluminum is not suitable for use, aluminum alloys are used, where strength and lightness are desired together [5].

Aluminum and its alloys also need different criteria in comparison with the traditional materials and manufacturing methods with which they have to compete. When expressed in terms of concepts such as specific strength, specific stiffness, and discontinuous yielding during forming, aluminum alloys exhibit equivalent and sometimes superior performance compared to traditional materials [2].

These properties of aluminum make it preferred for the automotive and manufacturing sectors. Saving fuel and reducing costs due to its lightness in the transportation sector and regulations on the emission number of vehicles on national and international platforms have made aluminum the best alternative material for the transportation sector [35].

2.4 Usage areas of aluminum alloys

Increasingly complex production methods, differentiating and diversifying consumer demands, increasing population, and the awareness that the limits of natural energy resources are rapidly approaching over time with production have necessitated radical changes in the production methods and raw materials that have been customary so far in many sectors. Although their industrial usage date is recent, aluminum alloys have rapidly taken part in production [28].

Aluminum is used extensively in the automotive industry because it is a light metal, and its use is constantly increasing. Aluminum is used in the production of radiators, engine parts, body sheets, and structural parts in the automobile industry. Aluminum is used in the construction of cargo transportation and passenger compartments in airplanes train transportation systems and in the production of ship hulls and propellers in the ship industry. Considering that energy will become more valuable in the future as a new area of use, aluminum batteries will find a wide range of applications (Figure 1). Aluminum-sulfur batteries are the first examples of these applications. With these batteries, it is possible to reach an efficiency of 250 Wh/kg. Another example is aluminum air-fuel cells [37].

Figure 1.

Diagram of applications of aluminum batteries along with other different types of batteries [36].

Esthetic applications of aluminum alloys in the construction industry have a longer history than manufacturing and other strategic applications. In the construction industry, the needs could be met without the need for high technology, but according to the strength and corrosion properties of aluminum and in most applications, where both are required together, the aluminum industry had to carry out basic studies in technology and production methods, which resulted in the development of alloys and different production methods [38].