Material of the month: aluminium

Materials World magazine
,
1 Mar 2017

From aircrafts and skyscrapers to smartphones and packaging, aluminium is one of the most versatile materials used today. Natalie Daniels discovers why.

Aluminium is a young metal, with a relatively late discovery less than 200 years ago. Despite this, it has become the second most used metal in the world, after iron, and established a well-earned reputation for its versatility and diverse range of applications. Aluminium is the third most abundant element in the Earth’s crust. It is estimated that the solid portion of the crust, to a depth of ten miles, contains around 8% aluminium. However, aluminium hasn’t always been widely available. In the mid-1800s, it was considered more valuable than gold. Elite guests of the President of the French Republic, Napoléon III, were presented with aluminium cutlery, while fashionable and wealthy women wore jewellery crafted from the metal. 

Today, aluminium is a critical material because of its lightweight and high-strength properties, and has become popular in the aerospace, automotive and construction industries. It also transmits conducted heat and reflects radiant heat, making it a favourable material in cooking utensils, kitchen foil and building insulation.

The discovery

Scientists suspected that an unknown metal existed in the chemical compound alum as early as 1787, but they did not have a way to extract it until the 1800s. Aluminium was named after alum – which itself is derived from the latin Alumenen, meaning ‘a bitter salt’ – by Sir Humphry Davy, who in 1808 suggested that it could be produced by electrolytic reduction from alumina (aluminium oxide), but did not manage to prove the theory in practice. Although his efforts were unsuccessful, he made a significant step in separating the metal from its oxide. 

Hans Christian Oersted, a Danish chemist, was the first to produce tiny amounts of aluminium. On 8 April 1825, he isolated the metal for the first time, by a chemical process using an amalgam of potassium and chloride aluminium, obtaining a small lump of metal with the appearance of tin. However, it was unknown whether this was pure aluminium or an alloy with other elements that were used in the course of the experiments. Between 1827 and 1845, Friedrich Wöhler, a German chemist, had improved Oersted’s process, extracting aluminium in the form of small balls of solidified melted metal using mercury, potassium and platinum. Wöhler was the first to measure the density of aluminium and demonstrate its lightness. It has a density one-third that of steel at 2,700kg/m3. 

The first aluminium products are considered to be medals made during Napoléon III’s reign. Napoléon supported the development of aluminium production, and Friedrich Woehler designed a rattle for Crown Prince Louis Napoléon made of aluminium and gold. The new metal was introduced to the public in 1855 at the Paris Exposition, France, around the time that it became available in small amounts at great expense by the sodium reduction of molten aluminium chloride.

Patenting the process

Finally, in 1886, a French scientist named Paul Louis-Toussaint Héroult and Charles Hall, an American inventor and chemist, independently patented processes for melting aluminium oxide in cryolite and subjecting it to an electric current. Hall received US patent #400,666 in 1889. Hall opened the Pittsburgh Reduction Company, later known as the Aluminium Company of America (Alcoa) in Pennsylvania, USA, to commercialise the process worldwide, extracting aluminium using an electrolysis technique. During its first months of operation, the smelter produced around 22.5kg of metal a day, and by 1890, daily output reached 240kg. The Hall-Heroult process remains the most commonly used to produce aluminium today.

Around the same time, while working at the Tentelevsk factory in St Petersburg, Russia, Carl Josef Bayer, an Austrian chemist, introduced the Bayer process – extraction of alumina from an alkaline solution. He developed a method for supplying alumina to the textile industry, where it is used as a mordant in dyeing cotton. The extraction involved mixing ground bauxite into a solution of sodium hydroxide. By applying steam and pressure in tanks containing the mixture, the bauxite slowly dissolves. The alumina released reacts with the sodium hydroxide to form sodium aluminate. After the contents of the tank have passed through other vessels where the pressure and temperature are reduced and impurities are removed, the solution of sodium aluminate is placed in a tank where the alumina is precipitated out. The precipitate is removed, washed, and heated in a kiln to drive off any water present. The residue is a pure alumina.

Widespread production

The use of aluminium quickly spread as techniques to process the metal improved, making it suitable for aircraft and automotive applications. In 1903, the Wright brothers flew their Flyer-1, the first heavier-than-air steerable aircraft (see Materials World, December 2015, page 30). The engine built for the Flyer-1 aircraft incorporated parts cast from aluminium such as the cylinder block.

Later, aluminium gradually replaced wood, steel and other materials in the bodies of early aircraft and by 1917 German aircraft designer Hugo Junkers built the world’s first fully metallic aircraft with a fuselage made from duralumin, an aluminium alloy with copper, magnesium and manganese. This specific alloy was developed by Alfred Wilm in 1909 after discovering that these alloys could be made significantly stronger after being heated over time.

Aluminium has proved indispensable in the aerospace industry, given its combination of low weight and high strength. In the 1940s, the metal was used to produce aircraft frames, ship infrastructure, radar chaff and mess kits, with production reaching US$1.5bln worth of aluminium a year for military requirements alone. It has since become the primary aircraft material, comprising about 80% of an aircraft’s unladen weight. The body of the first human-built satellite launched in the Soviet Union in 1957 was made from an aluminium alloy, capable of withstanding high and low temperatures, vibration loads and radiation. In addition, aluminium alloys have cryogenic strengthening properties, meaning that as the temperature falls their strength and flexibility increases. 

Consumer boom

It wasn’t just in the air where the metal’s superior properties were recognised. The post-war years from 1945–1975 saw a consumer boom in the demand for aluminium as the developed world shifted gears towards consumer product manufacturing. In the world of food and drink, aluminium-manganese alloys were used to make cooking utensils such as frying pans, roasting pans and large cooking pots. It also found application in food containers, because it is highly resistant to corrosion. The first two-piece aluminium can was based on development work carried out in Alcan’s Banbury Laboratories by Alec Lovell and in 1958, the first beverage cans were manufactured in the USA by the Adolph Coors Company. The early generation of aluminium cans required what was known as a church key to open the can. In 1975, Daniel Cudzik of Reynolds Metals invented the stay-on tab, which is still used worldwide. 

Aluminium is 100% recyclable and can be reused. It is estimated that 75% of all aluminium ever produced – a stockpile of about 750 million tonnes – is still in use, and could all be recycled into new products. According to The Aluminium Packaging Recycling Association, UK, in 2015 around 110,000 tonnes of aluminium cans were sold in the UK and around 60% of the used cans were collected for recycling and went back into cans, mainly at the Novelis recycling plant near to Warrington, UK.

The automotive industry has also been adopting more and more aluminium, as they develop lighter, more fuel-efficient cars. The world-leading manufacturer of aluminium intensive vehicles is Jaguar Land Rover, using vehicle build technology originally developed as a joint venture between Alcan and British Leyland in the 1980’s. The same technology is used for the Ford F150, the most popular pickup car in America for the past 38 years. It has a full aluminium body, making it 315kg lighter than its preceding model, and more than a million of these vehicles are made annually. 

As more efficient processes for production are developed and prices remain stable, aluminium is becoming more widely used and demand is increasing. It has one of the world’s highest annual production levels and continues to make headway in a many industries. According to Geoff Scamans, Chief Scientific Officer at Innoval Technology, ‘The major challenges are to increase its mechanical properties to compete more and more strongly with advanced grades of steel and to maximise recycling to progressively decrease the use of primary metal in favour of aluminium returned from end-of-life scrap. The favoured use of aluminium must be in transportation applications, where it either reduces the use of fossil fuels or the size of the battery pack required for electric vehicle propulsion and acceptable range.’ As recycling performance challenges are progressively overcome, aluminium will continue to lead as a shining example of the circular economy.