In today’s world, lightweight technology has become the key to the development of many industries as environmental awareness increases and energy efficiency is pursued. Aluminium, as a lightweight metal, is widely used in industries such as automotive, aerospace, and packaging due to its unique physical and chemical properties, such as low density, high strength, corrosion resistance, and recyclability. Research on lightweight aluminium materials not only helps improve energy efficiency and reduce fuel consumption, but also significantly reduces carbon dioxide emissions, which plays a positive role in environmental protection. In this article, we will delve into applications of aluminium and its alloys in lightweight technology, and the ways to push the frontier development of aluminium science and technology through innovative technology such as integrated die casting and the development of aluminium foam, to make the future greener and more sustainable.

1. Basic properties of aluminium

In the 130 years since its discovery, aluminium has gone through a transition from being a “precious metal” to overcapacity, and to now being in a great demand in the lightweight materials market (Table 1). Aluminium is the second most widely used metal, following iron (ferrous metal) because of its properties of being lightweight, corrosion resistant, ductile, rollable, and recyclable.

Table 1. Consumption structure of aluminium in downstream industries in China in 2022 (Source: USGS Data)

1.1 Light weight

The relative atomic mass of aluminium is quite small among metals. Compared to that of iron (55.85 g/mol) and copper (63.55 g/mol), aluminium is only 26.98 g/mol, and its density is merely 2.70 g/cm³, much lower than that of iron (7.87 g/cm³) and copper (8.92 g/cm³). These properties make aluminium an ideal lightweight material. In the industries such as aerospace, automotive, transportation and machinery manufacturing, the use of aluminium can significantly reduce the weight of structures, thereby enhancing energy efficiency and environmental performance. Because of its light weight, aluminium is widely used in materials for aircraft, earning its reputation of a “metal with flying wings.”

1.2 Corrosion resistance

Although aluminium is chemically active, the oxide it forms in natural state is rather stable. As a result, elemental aluminium does not exist in nature. The fact that the surface of aluminium reacts with air to form an inert oxide film improves its corrosion resistance and promotes its application in industries and daily life. The corrosion resistance of aluminium depends on the quality and properties of the oxide film on its surface. Since the oxide film that naturally forms has limited protective capabilities, treatment of its surface is needed.

The thickness of the passive film that forms naturally on aluminium is only 5~20 nm, which is far from sufficient for the widely-used aluminium alloys. Therefore, it’s necessary to artificially create an ideal protective oxide layer. The thickness of chemical oxide film is generally 1~3 µm, while the thickness of common anodized film can reach 3~30 µm. As a result, surface treatment technologies such as anodic oxidation (Figure 1) and microarc oxidation for aluminium have been developed.

1.3 Plasticity

When a force is applied, the layers of atoms in the aluminium lattice can slide. Since the interaction between the aluminium ions and free electrons (i.e. the metallic bond) is non-directional and non-saturated, even after the move occurs, the interaction between the layers is still maintained and the metallic bond is not destroyed (i.e. the aluminium ions don’t need to absorb energy to overcome the metallic bond when they slide). Therefore, the slide is easy to occur. Additionally, the free electrons or electron cloud that move within the lattice act as a “lubricant”. The crystal structure of aluminium is face-centered cubic, and the distance between atoms is much closer than in the simple cubic structures of iron, which facilitates plastic deformation. Due to the good plasticity and ductility of aluminium alloys, deformation is very easy. The processing method to change the shape and structure of aluminium materials through plastic deformation is called plastic processing. This method can be achieved through extrusion, stretching, rolling, etc. Plastic processing of aluminium can be used to manufacture a variety of aluminium alloy products, such as aluminium alloy doors and windows, aluminium alloy pipes, aluminium alloy plates or even foils, etc. (Figure 2).

1.4 Recyclability

Being highly resistant to corrosion, aluminium is normally hardly corroded during its use, except for certain containers and devices made of it. With minimal loss, aluminium can be recycled many times [1]. Therefore, it is highly recyclable and the use of used aluminium to reproduce aluminium alloys has significant economic advantages over the use of electrolytic aluminium. It is known as a green metal with strategic importance due to its recyclability. Compared to the production of electrolytic aluminium, the production of recycled aluminium has advantages of lower energy consumption, lower costs, lower carbon emissions, and lower environmental impact (Figure 3). In the future, improvement of energy efficiency will continue to be a global focus. These technologies and other effective measures will be implemented throughout the entire aluminium industry, and in long-term production and operational practices.

2. Development of aluminium

2.1 Origin and history

Aluminium, is derived from the Latin word “alum”, originally meaning “potash alum”. Aluminium is known to be the most abundant metallic element in the Earth’s crust, but it exists in nature in the form of compounds. The reason is that the peripheral electronic configuration of the aluminium atom, 3s²3p¹, where there are three free electrons, makes it possible to lose electrons as well as to gain electrons. It thus manifests itself as an acidic and alkaline amphoteric substance. Aluminium doesn’t exist in the form of an elementary substance, which also leads to its discovery far later than iron, copper and other metals. In 1827, Friedrich Wohler produced aluminium through a displacement reaction with potassium, but aluminium production through this method was extremely low, making aluminium rare and precious. In 1884, construction of the Washington Monument was completed with the placing of an 8.9 inch tall, 100-ounce pyramid of solid aluminium atop the capstone to protect the monument against thunder strikes. This pyramid-shaped aluminium cap was the largest aluminium piece in the world at that time (Figure 4) [2]. It was later discovered that the top of the Washington Monument is not made entirely of pure aluminum. In an undated letter, an anonymous scientist pointed out that the composition of the top is 97.75% aluminum, 1.70% iron, and 0.55% silicon [2].

2.2 Electrolysis theory

To commemorate the alumina production method proposed by K. J. Bayer between 1889 and 1892, people named it the Bayer process. Over the years, this method has been upgraded and optimized but the basic chemical principle has remained unchanged and has been used until now (Figure 5). The essence of the Bayer process is to carry out the following chemical reactions under different process conditions:

The chemical equations of the process are very complicated, but can be simplified as follows:

• Reaction at the cathode: Al ³⁺ + 3 e^– = Al
• Reaction at the anode: 2 O ^2- = O₂ + 4 e^–
• Overall reaction: 2 Al₂O₃ = 4 Al + 3 O₂

The appearance of the Hall-Héroult process in 1886 was a landmark in the development of aluminium (Figure 6) [3]. In 1888, Bayer process, a low-cost method of extracting high-purity alumina from bauxite, met the demand of low-cost and high-yield aluminium production, and aluminium achieved large-scale industrial production.

2.3 Evolution of electrolytic technology

The development of modern aluminium electrolytic technology has two main origins.

• Aluminium Company of America (Alcoa): A pioneer in the aluminium industry

Alcoa was founded in 1888 by Charles Hall. As one of the world’s largest aluminium producers, Alcoa has been at the forefront of the aluminium smelting technology and manufacturing sector. Since the 1940s, Alcoa has developed a variety of pre-baked aluminium electrolysis cell technologies, including P155 (170 kA), Alcoa-697 (230 kA), and Alcoa-817 (280 kA), each marked by higher current intensities and corresponding increases in production efficiency. By pursuing high current, Alcoa has been able to increase production, reduce the unit cost, and improve energy efficiency to enhance economic growth and meet the market demand. However, the increase of current will significantly enhance the electromagnetic force and thermal effect in the electrolyzer, leading to an increase in the amplitude and frequency of the fluctuation of the cryolite-aluminium interface, and complexity of the fluctuation pattern, thus affecting the stability and efficiency of the electrolysis process [7]. Alcoa is a typical representative both in the world’s modern aluminium electrolytic technology and in the development of modern industry and science and technology. Similarly, its technical advancements have been significant in driving industrial progress.

• Pechiney (AP): A leader of modern aluminium electrolysis technology

Pechiney is the world’s preeminent representative of the aluminium industry, with international leadership in aluminium electrolysis technology. Pechiney uses its abundant hydroelectric resources in the south of France to obtain low-cost electricity and maximize the efficiency of aluminium electrolysis production through technological optimization and production improvements. From 1940 to 1978, the Saint-Jean-de-Maurlenne aluminium electrolysis plant underwent several modernization processes, leading to a continuous increase in the capacity of its electrolytic cells and driving technological advancement. It was here that the prototype for the AP series of aluminium electrolysis technologies was developed, making the plant a birthplace of modern aluminium electrolysis innovations.

However, the aluminium electrolysis process generates exhaust gases including fluoride, sulfur dioxide, fluorocarbons and dust, and these pollutants have a serious impact on the environment. In the 1970s, as the production scale of Pechiney Aluminium Plant continued to expand, pollution that it caused also became increasingly serious, especially the fluoride emission. Fluoride is a toxic substance that poses a serious hazard to the environment and human health. Due to the inadequate exhaust gas treatment system, a large amount of fluoride is emitted directly into the air, polluting the surrounding environment. As pollution intensified, local residents and environmental organizations began to protest, demanding that the company take measures to reduce pollution. Extensive media coverage also drew national attention.

Under the joint efforts of the government and enterprises, the exhaust gas treatment technology of electrolytic aluminium has been gradually improved. Today, more enterprises use dry purification to treat exhaust gases. Dry purification can deal with almost all exhaust gases, absorbing 99% of hydrogen fluoride, insoluble fluoride and dust, preventing a large amount of waste gas entering the living environment. The adsorbent of dry purification uses alumina, the raw material of aluminium electrolysis. After purification, the fluorine-carrying alumina can enter the electrolysis tank as raw material, which not only makes up for the loss of fluoride salts but provides the raw material for electrolysis. Dry purification greatly improves the environmental protection, and sustainable production of the electrolytic aluminium industry. Over the past 50 years, the achievements of Pechiney in improving the efficiency of electrolytic cell have positioned it as a world leader in the development of aluminium smelting technology [4].

• Rapid Development of Electrolytic Aluminium in China

The development of electrolytic aluminium in China has a relatively late start. With the support of preferential national policies and rapid economic growth, in 1979, the 160 kA pre-baked cell with center loading (current efficiency 87.5%, production capacity 8 wt/a) was introduced to China from Japan. In the mid-1980s, the 180 kA aluminium electrolysis test cell (current efficiency 93.5%) was developed. Later, the “mathematical model and simulation software system of aluminium electrolysis cell” was designed on the basis of imported technologies, which provided the fundamental theories and design tools for large aluminium electrolysis cells. Subsequently, China independently developed successful technologies for super-large aluminium electrolysis cells of 165 kA, 186 kA, and above 280 kA. The success of the “280 kA test cell in the national large aluminium electrolysis test base” marks that China has entered the forefront of modern aluminium electrolysis technology, establishing its own modern aluminium electrolysis technology system. Since then, the 320 kA (current efficiency 94.43%), 400 kA (current efficiency over 93%, production capacity 34 wt/a), 500-600 kA extra-large pre-baked anode aluminium electrolysis cell (current efficiency 94.6%) technologies were successively developed (Figure 7), with various technical indicators having reached or exceeded international advanced levels [4]. It is precisely because of the efficient development of the electrolytic cell technology that China’s aluminium production soared. The fact that China’s aluminium production accounted for 50% of the world’s total was ranked 10^th among the top 50 events in the aluminium history, as was evaluated by the International Aluminium Association (according to votes). China’s primary aluminium production surged from about 16 million tons in 2010 to 26.5 million tons in 2013, accounting for 50% of the world’s total. Since then, China’s aluminium production has continued to grow, reaching 39 million tons in 2021, accounting for about 58% of the world’s total production.

To date, in the newly established production lines of aluminium reduction cell of Weiqiao Group in Shandong province China, the current intensity has reached 600 kA (with a capacity of 100 wt/a). The 600kA-order aluminium reduction cell energy-saving and carbon-reducing technology represents a groundbreaking innovation in the global aluminium electrolysis research [5]. It excels in energy efficiency, carbon reduction, longevity and environmental protection, propelling advancements in the aluminium industry and bolstering the international competitiveness of core aluminium electrolysis technologies.

2.4 Alloy technology and innovation

• Duralumin

Back in 1906, German chemist Alfred Wilm discovered that by adding a small amount of copper to aluminium, the hardness of aluminium could be significantly increased. This finding led to the birth of tough aluminium, later known as duralumin, as it was first introduced into industrial production by the German company Dürener Metallwerke. Duralumin typically has a tensile strength ranging from 300 to 500 MPa, offering high rigidity and hardness. These features have led to its widespread application in fields such as aerospace and aviation.

The promotion of Duralumin has ushered in a new era of aluminium alloying, where other metal elements are added to pure aluminium through specific processing techniques to achieve diversified performance. The tensile strength of aluminium alloys can reach 200-600 MPa, offering not only good plasticity and corrosion resistance but also suitability for various industrial applications, making aluminium alloys the second-largest industrial metal after steel.

The strength and hardness of aluminium alloys are improved mainly due to the presence of different alloying elements such as copper, manganese, and magnesium, which enhance the aluminium matrix through solid solution strengthening. Other elements like titanium, vanadium, and boron enhance performance by refining grain size and increasing nucleation, while cadmium and scandium mainly improve performance through second phase strengthening. The diverse combinations and arrangements of these alloying elements result in a wide variety of applications and properties for aluminium alloys.

• High entropy alloys (HEAs)

In 1995, Jein-Wei Yeh, a professor at National Tsing Hua University in Taiwan, proposed a novel idea to design alloys – High Entropy Alloys (HEAs). These alloys typically comprise five or more elements in nearly equal proportions, each element’s concentration ranging from 5% to 35%, maintaining a largely atomic ratio. The defining characteristic of HEAs is their mixing entropy, which exceeds the entropy of melting. They usually form simple solid solutions, including face-centered cubic (FCC), body-centered cubic (BCC) and hexagonal close-packed (HCP) structures.

HEAs are the focus of significant attention, as they contain multiple elements of different sizes and chemical properties, leading to lattice distortion effect. This distortion affects not only atomic spacing and the stability of lattice structures, but also the motion of dislocations, thus improving the mechanical properties and thermostability of the alloys. Besides, the combination of multiple elements slows down the diffusivity of atoms, resulting in sluggish diffusion effect. This helps the alloys to maintain better stability at high temperatures, hindering atom migration and grain growth, thus improving their resistance to deformation and creep. The combination also brings about the so-called “cocktail” effect, where the alloy’s properties are influenced not only by individual elements but also by the collective action of many. This effect enables HEAs to exhibit diverse properties, achieving a balance and optimization of various performance aspects, thereby enhancing their overall performance.

Due to their unique lattice structures and properties, HEAs have emerged as one of the most promising research subjects in materials science in recent years. They offer high strength, hardness, wear resistance, oxidation resistance, and corrosion resistance, demonstrating broad application potential in many areas, such as tools, cutters, molds, golf club heads, turbine blades, and high-temperature furnace resistant materials.

3. The application of aluminium in critical industries

3.1 A change-maker in the automobile industry

Cars are our common means of transport in daily life. The earliest cars were not powered by petrol but by steam, making them heavy and energy-intensive. According to relevant research, if the curb vehicle weight (CVE) of a car can be reduced by 10%, the fuel efficiency of it can increase by 6%~8%; furthermore, a decrease in gross vehicle weight by 100 kg can lead to a reduction in fuel consumption per 100 km by 0.3 L to 0.6 L. Nowadays, energy conservation and emission reduction is a global mission, but cars have also become an indispensable part of our lives. Therefore, automotive lightweighting has become a global trend in the automotive industry and is the priority for carmakers to develop and produce next-generation products. Modern aluminium alloys, with their superior weight-to-strength ratio, low cost, and high wear resistance, are widely used in many structural components, making them ideally suited for the trend toward automotive lightweighting in today’s market (Figure 8).

The lightweighting of automobile is an effective approach to enhance vehicle braking, safety and acceleration. Aluminium alloy materials are widely used in the automotive body structure, significantly impacting weight reduction. In China, emerging domestic electric vehicle producers usually adopt lightweight aluminium alloys as the primary structural material and components to meet longer range requirements. Achieving lightweighting without compromising strength poses higher demands on the structural properties of aluminium alloys. This is typically achieved through optimizing structures and increasing material thickness. Using lightweight aluminium alloys is the optimal solution to address automotive lightweighting challenges. In the future, aluminium alloys are expected to see broader development in the automotive manufacturing industry.

3.2 Preferred materials for aerospace

In addition to the lightweighting requirements in the automotive industry, when we travel comfortably in planes soaring through the sky, we may also wonder: Why can such a seemingly thin fuselage safely carry so many passengers? Due to their moderate cost and excellent workability (including extrusion, rolling, bending, and additive manufacturing), aluminium alloys have become an excellent choice for aerospace materials (Figure 9), which accounts for up to 80% of aircraft materials, and are mainly used in the production of lightweight structures, upper and lower wing skins, and wing stringers. Besides, the most widely used aluminium alloys are Al-Cu alloys, Al-Zn-Mg-Cu alloys, and Al-Li alloys.

3.3 Leader in Packaging Material

In sweltering summer, various drinks in the store are favoured by most people, whose cans are made of aluminium alloys, as shown in the picture (Figure 10). The can factories have stringent requirements for aluminium can materials, demanding not only good inherent quality, uniform chemical composition, and low gas and inclusion content but also sheet materials with uniform thickness, good shape, and excellent surface integrity. At present, the most used cans are drawn and ironed (DI) cans, i.e. double-sided cans produced by a thinning and deep drawing process. To obtain high quality aluminium, each process of can making is strictly controlled, from casting to hot rolling, cold rolling, finishing and others. Through these processes, 3104 H19 aluminium alloy plate is produced, featuring good mechanical properties, deep drawing properties and surface integrity. The current trend for DI cans is towards thin-walled and lightweight designs, with the thickness of the aluminium alloy for can bodies decreasing from 0.35 mm to 0.27 mm and 0.25 mm. In the future, it is expected to be further reduced to approximately 0.20 mm.

Due to the widespread popularity of canned beverages, in 1970 the can industry developed a closed-loop recycling system based on the recyclability of aluminium alloys. As technology has evolved, this recycling system has become more environmentally friendly and efficient, greatly facilitating the reuse of aluminium. The closed-loop recycling process consists of multiple steps: collection, sorting and cleaning, smelting, casting, manufacturing and reuse. Each step emphasizes the high recyclability of aluminium. During the melting and recasting process, aluminium is able to maintain its metal purity and properties, allowing for multiple recycling. In this way, not only can we save up to 95% of energy, reduce greenhouse gas emissions, and lower production costs, but we can also significantly reduce the landfill footprint of aluminium products, thus achieving efficient use of resources and environmental protection.

4. The innovative application of aluminium

4.1 Efficient integrated “die casting” technology

“Die casting” technology represents a special casting method. With this technique, liquid and semi-liquid aluminium is filled into the die casting mold cavity under high speed and pressure conditions and allowing the metal liquid to solidify rapidly into a casting under certain pressure. Integrated die casting technology involves the redesign of multiple independent components, that would typically require assembly in industrial design. Using ultra-large die casting machines, these components are formed in a single casting, resulting in complete parts that fulfill their original functions. It is apparent that, compared to conventional die casting, integrated die casting emphasizes large-area integration and is fundamentally different from casting individual parts. Compared with the traditional processes of stamping, welding, painting, and assembly, integrated die casting is characterized by one-piece molding  [6] (Figure 11).

Compared to split welded casting, this casting method reduces the number of parts that need to be assembled, thereby reducing production complexity and assembly costs, lowering the total weight of the vehicle, improving fuel efficiency and reducing emissions, and improving vehicle handling and stability for an enhanced driving experience.

4.2 The development and application of aluminium foam

The traditional aluminium materials we are familiar with are characterized by their dense structures, but aluminium foam has a completely different structure. Unlike traditional aluminium, it comprises pores and skeleton matrix (which physically support pores). The other parts are filled with pores, making the material porous like a sponge. We refer to this type of aluminium material with a large number of dense bubble cavities as aluminium foam. The most prominent feature of aluminium foam is its lightweight and low density, which can even float on water (as shown in Figure 12). The emergence of aluminium foam has broken the dense structure of traditional aluminium materials. It also has many advantages, such as high-temperature resistance, sound insulation and noise reduction, and good electromagnetic shielding performance. It has been applied in aerospace, construction, and automotive industries and is gradually becoming a focal point in the research of new materials for the modern era.

With the outstanding combination of mechanical, thermal, acoustic, energy absorption, damping and electromagnetic shielding properties, aluminium foam has become a remarkable example of integrated functional materials. It represents a significant breakthrough in traditional aluminium materials application and opens up new avenues for diverse engineering projects and scientific disciplines. Today, “aluminium foam” stands at the forefront of material science research and has become a popular topic in the field.

However, as both structural and functional material, aluminium foam still confronts several challenges in engineering technology applications. Firstly, it exhibits relatively lower specific strength and stiffness compared to solid metals, which has limited its widespread applications in engineering. Additionally, current research on aluminium foam mainly focuses on room temperature conditions, with further in-depth studies needed on its properties and performance under harsh environments such as high temperatures and high pressures. Moreover, the uniformity of pores in aluminium foam remains an issue to be addressed.

Aluminium foam, as a newly developed lightweight alloy, is not as widely used as traditional materials. However, it is foreseeable that in the 21st century, known as the “century of materials”, the mounting pressure on environmental protection and China’s shift towards high-quality development will vigorously promote the advancement of new materials including aluminium foam.

5. Messages to remember

• The Bayer method, the basic process of which has remained virtually unchanged since its invention in 1888, remains the primary method of producing aluminium oxide.
• The Hall-Héroult Process of producing aluminium by electrolysis, a principle first invented independently by Charles Martin Hall and Paul L.T. Héroult in 1886, remains the key method of global aluminium production today.
• Aluminium cans were initially produced by companies such as Coors Brewing and Royal Crown in 1960. The weight of two-piece aluminium cans have been reduced from the original 85 grams to approximately 15 grams today. They are lighter and easier to be recycled than three-piece steel cans. Through closed-loop recycling, a used aluminium can can enter the market in less than 60 days. The Mobius Loop, designed by Gary Anderson in 1970, reinforces the concept of waste reduction, reuse and recyclability.
• The “International Aluminium Institute: The Top 50 Moments in the History of the World Aluminium Industry” covers the key historical events in the development of the aluminium industry since the 19^th century. These milestones collectively illustrate the important role of aluminium in modern industry and everyday life.
• Aluminium recycling consumes only 5% of the energy which is used to produce virgin aluminium and can reduce carbon dioxide and sulfur oxide emissions by more than 90%. Aluminium recycling can reduce production cost, protect the environment, and bring in great economic and social benefits.

 

Acknowledgements. This paper has been completed with great support from all parties. We would like to express our sincere thanks to all the people and organizations who have made efforts and contributions. Special thanks to Weiqiao Pioneering Group, special thanks to GAO Peng, ZHAN Aishuang, JIA Luanluan, ZHOU Weidong and others. and the University of Chinese Academy of Sciences, who provided valuable high-quality pictures and text materials, which greatly enhanced the richness and professionalism of the content. Thanks to the Lab of Electromagnetic Process of Materials (EPM) for providing technical support. Express additional thanks to the Binzhou Institute of Technology and the Metal Materials Center. I am also grateful to my master student FENG Qi for her hard work in collecting and editing this paper, and grateful to the professional translation team (SUN Libing, YANG Lingnan) from the Department of Foreign Languages of the University of Chinese Academy of Sciences for their quality translation work which ensured the accuracy and readability of the paper. Lastly, express gratitude to Academician LI Jiachun and Professor WANG Yanfen.

 

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Notes and references

Cover image. The world’s largest passenger aircraft, the Airbus A380. [Source © Roger Green from BEDFORD, UK, CC BY 2.0, via Wikimedia Commons]

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[2] https://www.nps.gov/articles/000/wamocap.htm

[3] Qiu Zhuxian. Prebaked Cell Aluminum Smelting [M]. 3rd Edition. Beijing: Metallurgical Industry Press, 2005.

[4] Liang Xuemin. Modern Aluminium Electrolysis Design and Intelligence [M]. Metallurgical Industry Press, 2020.

[5] Liu Jingqing. Iterative New Technology for Aluminium Electrolysis Successfully Developed at Weiqiao Pioneering Group. China Nonferrous Metals News, 2023-08-29. (chinania.org.cn)

[6] Wu, Yixuan, Wu Jixia. Performance de l’année dernière des sociétés cotées spécialisées dans l’intégration