Making light of metals

Bikes are changing

Look at the frame of your bicycle. It will be made of metal tubing, strong enough to carry your weight but light enough so you can carry its weight if you have to. It also needs to resist rusting and be affordable. In most bikes that metal is steel, which meets all these requirements, other than the rusting one (and painting and proper care can deal with that). But increasingly, ‘high-end’ bikes are being made of other, more exotic metals, alloys containing aluminium or magnesium, or even titanium. As time goes by, more ‘everyday’ bikes will be made the same way, and other goods as well. Why? Read on.

Increasingly bikes are being made of alloys containing light metals like magnesium. (Image: Paketa Bikes)

The metal miracle

Modern life would be impossible without metals. We can date the start of civilisation pretty much from the time our ancestors found out how to extract first copper and then iron and fashion those metals into tools, weapons and everyday items. The Industrial Revolution of the last 200 years, which created today’s world, was founded on iron and its alloy steel, letting us manufacture powerful, durable machines and build skyscrapers, bridges and railway tracks.

But these traditional metals have their drawbacks. Iron and steel are strong and cheap but they can rust and weigh a lot. Copper resists corrosion but is expensive and relatively soft. Now research is finding replacements, particularly for steel. We have known about aluminium, magnesium and titanium for a hundred years or more. We recognised that they are strong but relatively light and that they don’t rust. But for a long time they were too expensive for everyday use (much as steel was until the late 19th century).

That is now changing, and changing fast. Through advances in production and processing, these ‘light metals’ are becoming cheaper and more versatile. We are starting to see them in everyday items: in cars, aircraft, bicycles, the cases for laptops, mobile phones and even iPods. And Australian scientists are helping in this transformation of light metals.

Why light metals?

Industry is excited by light metals because they combine many of the traditional advantages of metals (Box 1: The magic of metals) with the virtue of being much lighter than the iron and steel we have used for so long. By replacing steel in things like cars and aircraft with lighter metals, they become more efficient, consuming less fuel and producing fewer greenhouse gases – big pluses in today’s world.

Everyday objects like mobile phones, cookware and soft drink cans, or exotic objects like medical implants, can be made both more convenient and more durable by using light metals. They are also readily recyclable. In fact, extracting metals from their ores usually takes so much energy it can be much cheaper to recycle them than to use a newly produced metal.
 
Light metal researchers have two big challenges if they want to make their metals more competitive and more widely used. They have to cut the cost of extracting the metals from their ores and they need to find new ways to process the metals once extracted, so they can be produced in more useful and versatile forms. Some of these forms will be alloys, where other metals or non-metals are added to improve the properties of the metal. (Box 2: Changing metals: Alloys).

And there is more. Rather than exporting mostly unprocessed ore, by developing and applying the new technology here at home we could add huge value to our resources. Exporting magnesium alloys could earn us up to 100 times more per tonne than the unprocessed magnesium ore. Components made from that alloy would be worth three times more again. At the moment, we make and export a lot of aluminium metal, but no pure magnesium or titanium. We have a long way to go.

Aluminium

Aluminium was the first of the light metals to hit the big time. We have been using it for decades in niche products; drink cans, cooking foil, planes and building materials. We are using it increasingly in other applications like iPod cases and cars, where its lightness and resistance to rusting is appealing. But it still can’t compete everywhere with the alternatives, largely because it costs too much.

There is no shortage of aluminium. It is the most plentiful metal in the crust of the Earth. We extract it by electrolysis from alumina, which in turn is produced by removing the impurities from bauxite.

Australia has immense deposits of bauxite, in northern Queensland, the Northern Territory and Western Australia. We are the world’s largest producer of the stuff. We also produce 30 per cent of the planet’s alumina, and a lot of that alumina is turned into aluminium metal in big smelters, at the cost of a lot of electricity. Australian scientists are working on reducing the cost of producing aluminium, to make it more competitive with other materials like steel. And the costs aren’t just financial. Like other metals, mining and processing aluminium has an environmental cost. But operators of today’s mines are careful not to repeat the environmental blunders of the past. Mining sites these days are carefully managed to minimise their impact, and after the ore has been extracted, the sites are remediated to return them as close as possible to their natural state.

Life cycle assessment shows that aluminium production produces more greenhouse gases than steel production. But when used in vehicles, aluminium’s lighter weight leads to better fuel efficiency, and so reduces greenhouse gas emissions from transport.

Better aluminium alloys are being developed and tested for a host of applications. We may be flying in some of them before long. Through an agreement with the second largest producer of alumina in the world, Monash University scientists are working on lighter, stronger aluminium alloys for Chinese-built commercial aircraft.

Magnesium

Everyone recalls magnesium from school as the metal that burnt with a blinding white light. However, in larger and thicker shapes magnesium is much less susceptible to burning. And when alloyed with aluminium, magnesium can be made even safer and more useful. Magnesium alloys are being used in motor vehicles, planes, and casings for mobile phones and laptops. It is an impressive metal, one third lighter than aluminium, stronger (kilo for kilo) than steel and resistant to shock and impact.

Unlike aluminium, magnesium can be extracted from several different minerals, as well as from sea water and even fly ash. Australia’s magnesium resources are in the form of magnesite. Extracting the metal requires conversion to magnesium chloride, then electrolysis to separate the magnesium and chlorine. This process takes a lot of energy, pushing up the price. As with aluminium, Australian scientists are looking into ways of cutting the energy needs and costs involved in producing magnesium.

Meanwhile, work will go on to develop even more sophisticated magnesium alloys, alloys that are stronger, cheaper, more ductile, able to stand higher temperatures, more resistant to corrosion, as well as better techniques to cast and shape those alloys into useful products. And magnesium is proving its worth, with alloys developed in Australia being tested in car engines in Europe and the US in the hope that they will produce lighter, more fuel-efficient cars.

Titanium

The major source of titanium is black ’beach sand’ which we often see on today’s beaches, but which can also be found in vast deposits from ancient beaches often many kilometres from the sea. Australia has the world’s biggest deposits of these titanium ores.

Titanium’s light weight makes it useful in planes, spacecraft, medical implants and sporting goods. (Image: Stockxpert)

Beach sands are a mixture of compounds, one of which can be rutile. When separated and purified, it becomes the brilliant white pigment titanium dioxide, much used in paint, paper making and plastics. Extraction using chlorine (the Kroll Process) yields titanium metal, but consumes magnesium along the way. Titanium costs a lot, mostly because it can be made only in batches rather than continuously. Australia mines more than half of the world’s rutile supply, but produces only 4 per cent of the pigment and no titanium metal at all.

The appeal of titanium is its high strength-to-weight ratio. Popular uses are in the aerospace industry (the Boeing 787 Dreamlineris 15 per cent titanium), medical implants or sporting goods where the need to reduce weight overcomes the problem of price. With its high melting point, titanium is particularly well suited to high temperature components in jet engines and spacecraft. Its resistance to corrosion makes it good for use in desalination plants and in the human body. Titanium is often alloyed with other metals to change its properties.

Although titanium is light, strong, and doesn’t corrode, its use is limited by its relatively high cost. Australian researchers are on a mission to lower the production and processing costs of titanium to allow it to be more widely used. The use of titanium powder technology is just one way they hope to achieve their goal, with scientists at CSIRO suggesting they could halve production costs.

Australian research reducing the load

Through advances in production and processing, light metals are becoming cheaper, more versatile and able to compete with traditional metals like iron and steel in a wide range of applications. That trend is sure to continue. You will see more titanium, aluminium and magnesium in your lives in future years, and enjoy the benefits. And much of the research into these metals, which is changing our future, is Australian.

Related Nova topics:

It's an advanced material world

Box 1 | The magic of metals

Many of the 90 or so naturally-occurring chemical elements are metals; indeed the remaining couple of dozen are known collectively as non-metals. We can pick a metal by looking at its properties. Metals have a lot of properties in common:

• All but one of the common metals (the exception being mercury) are solids at room temperature. Non-metals can be solids, liquids or gases.
• Metals are either naturally shiny or can be made to show a lustre.
• Metals are ductile; that is, they can be drawn out to form wires.
• Metals are malleable; that is, they can be hammered or rolled into thin sheets (think of gold leaf or aluminium foil). They can also be bent without breaking.
• Metals are generally good conductors of both heat and electricity. Of the non-metal elements, only carbon conducts electricity well.

Many of these properties come from the distinctive way that the atoms of metals are held together (or bonded). Metallic bonding is the result of the outer electrons of the metal atoms escaping and drifting freely in a sort of ’electron sea’ in which the atoms are embedded. The sharing of free electrons forms the bond (or force of attraction) of the atoms, holding them in a crystal lattice. The more electrons there are on the loose, the stronger the metal and, in general, the higher its melting point.

It is these wandering electrons that conduct electricity and heat so well.

Metals can be hammered and drawn out into wires because the metal atoms can slide past one another without breaking the bonds. The free electrons can rearrange with the new shape, holding the atoms together.

Metal atoms can also be packed tightly together, which is why metals are in general heavier (more dense) than non-metals.

Related sites

• Metallic bonding (The University of Melbourne, Australia)

• Chemguide (United Kingdom)
• Metallic bond

• Metallic structures

Box 2 | Changing metals: Alloys

Many of the metals we use in everyday life are not pure. They are a solid mixture of two or more metals (or metals with non-metals), what is generally termed an alloy. Steel is an alloy of iron, carbon and often various other elements. The brass used in trumpets and other wind instruments is an alloy that combines copper with zinc. Anything less than 24 carat gold has copper or silver mixed with the gold.

We make alloys because the pure metal does not have the properties we need; the mixture does. Iron is too brittle for many purposes; adding precisely controlled amounts of carbon turns it into steel which has a better combination of strength and toughness. Alloying copper with zinc (to make brass) makes it harder and more durable. Pure gold is very soft; adding silver or copper to gold changes its colour and hardness.

Alloys are a major line of research in the light metals business. Knowing which metals to mix and in what proportions is a complex process. For highly demanding situations, such as the blades of a jet engine, the mix might contain ten different elements, carefully chosen to deal with challenges like heat, wear and mechanical stress. Alloying can change the properties of a metal in more than one way. Mixing together atoms of different sizes can make it harder for the metal atoms to slide past each other, so the metal becomes stronger and less malleable. Also by adding atoms with extra valence electrons, the bonds between the metal atoms and electrons become stronger (Box 1: The magic of metals).

There are other ways to change the properties of metals. Heat treatments like quenching and tempering have been used by blacksmiths for millennia to harden and toughen iron to make swords or ploughs. Nowadays we have other ways to alter surface properties, say by adding very thin layers, implanting ions or heating with lasers. Getting the technology right depends very much on understanding the science.

Related sites

• Metals and alloys (British Broadcasting Corporation)
• Metals and alloys (San Jose State University, USA)
• Heat treatment (eFunda, USA)
• Heat treating: Process description (Metals Advisor, USA)

Activities

• Inspirational chemistry (Royal Society of Chemistry)

• Titanium – provides a printable worksheet covering the properties and cost (in pound sterling) of titanium relative to other metals.
• 21st century titanium – a printable worksheet covering the FFC Cambridge process of producing titanium by electrolysis.
• Titanium – from discovery to Mars – a worksheet (following on from the above activity) that examines the applications of titanium and challenges in commercialising the FFC Cambridge process.
• Alloys: Modelling an alloy – students model an alloy using plasticine and sand, then investigate its ductility.

• Discovery education (USA)
• Other metals – students review the types of metals and their properties. They then discuss the properties and uses of aluminium and create an advertisement for an aluminium product. (Note: the first part of this activity refers to the video Other metals but may be adapted for classroom use without the video).

• Australian Mines Atlas (Geoscience Australia)
• Mineral sands downunder – students learn about mineral sands then complete an online quiz.

• Scootle (The Learning Federation, Australia) (Note: these resources require registration to Scootle which is available to all Australian and New Zealand schools).
• Electrolytic cells: Aluminium extraction – an interactive resource in which students learn about production of aluminium through electrolysis.
• L7557 Metal munchers: Assessment – an interactive resource in which students protect a spaceship from metal-munching aliens by identifying metals to feed to them.

Activity 1. Race against the clock to sort the everyday objects by their metal.

Can you sort the everyday objects by the metal they are often made from before your time runs out? To beat the clock, compare aluminium, titanium and steel alloys before you start.

How do aluminium, steel and titanium alloys compare?1

Property	Aluminium	Steel	Titanium
Cost2 (relative to steel)	3	1	33
Hardness (Vickers)	126	268	368
Tensile strength (MPa)	298	785	1070
Melting point (°C)	660	1530	1600
Density (gcm-3)	2.7	7.9	4.5
Thermal conductivity (Wm-1K-1)	151	46	7

1. Approximate only, properties vary widely between alloys (www.matweb.com) 2. July 2009 data (www.metalprices.com, www.meps.co.uk/world-price.htm)

Activity 2. Properties of metals and the metallic model

1. Use the information and links in Box 1: The magic of metals to find out about the metallic model.

2. Draw diagrams in the table below to help explain the properties of metals using the metallic model. Two rows have been completed for you.

3. Find out the number of valence electrons for aluminium and sodium.

4. Using the metallic model, give a reason for aluminium being a stronger metal than sodium.

5. Why do you think aluminium is a much better conductor of electricity than sodium?

6. Considering that aluminium is softer than titanium, why do you think it is used more for building aircraft?

7. Find out and list below, the metals that the Australian twenty cent and two dollar coins are made from (include percentages of each).

8. Research the properties of each of the metals in the coins and suggest:

1. reasons why these particular metals were chosen; and






2. why the metals common to both coins are present in different proportions.

   
   
   
   

Further reading

Australasian Science
April 2006, page 11
Superbike on sale
Highlights the Australian development of a light weight, high performance aluminium bicycle.

Useful sites

National Research Flagships: Light Metals (CSIRO, Australia)

• Titanium production: Cutting the costs in half
  Highlights research aimed at creating a world-scale titanium industry in Australia.
  http://www.csiro.au/science/ps19i.html

• Magnesium production: New technologies to grow Australia's capability
  Covers research into magnesium production that has low cost and a reduced environmental impact.
  http://www.csiro.au/science/ps18r.html

• Aluminium production: Reducing costs and energy consumption
  Outlines research to reduce the energy costs and greenhouse effect of aluminium production.
  http://www.csiro.au/science/ps18o.html

Australian Research Council Centre of Excellence for Design in Light Metals

Provides information on the Centre’s activities, members and research programs in light metals.
http://www.arclightmetals.org.au

Australian atlas of minerals resources, mines and processing centres (Geoscience Australia)

Provides a range of clear information on metal resources in Australia. Specific information on titanium, magnesium and aluminium can be found in the Rock files, Fact sheets and Mineral sands downunder pages.
http://www.australianminesatlas.gov.au/index.jsp

The Australian industry (Australian Aluminium Council)

A good summary of the extent and significance of the Australian aluminium industry, with information on individual mines, refineries and smelters.
http://www.aluminium.org.au/Page.php?s=1005

Industry overview (The Aluminium Association, USA)

Covers the uses and production of aluminium. The transportation pages include information on aluminium use in cars, trucks, trains, boats, spacecraft and aircraft.
http://www.aluminum.org/Content/NavigationMenu/TheIndustry/Overview/default.htm

aluMATTER (University of Liverpool, UK)

An award-winning, interactive website with information aimed at university students on aluminium applications, materials science and processing. Some more technical information is presented; however, the products/applications section is suited to school students.
http://aluminium.matter.org.uk/content/html/eng/default.asp?CATID=&PAGEID=1

Information downloads area (Aluminium Federation, UK)

Presents a series of clearly written fact sheets on aluminium uses, properties and processing.
http://www.alfed.org.uk/page.asp?node=54&sec=Information_downloads_area

Mg Showcase (International Magnesium Association)

A clearly written, online newsletter that provides examples of the advantages and use of magnesium in a range of applications.
http://www.intlmag.org/mgshow.html

CAST Cooperative Research Centre (Australia)

Provides an overview of the Centre’s research in metals technology, including light metals and ferrous metals.
https://www.cast.org.au

Frames and materials (The Exploratorium’s science of cycling, USA)

Covers the materials used for bicycle frames and their properties.
http://www.exploratorium.edu/cycling/frames1.html

Glossary

alloy. A substance made of two or more metals, or a metal and one or more non-metals, that has mostly metallic properties. Alloys are often created to improve the properties of metals such as strength, resistance to corrosion and hardness. For example, steel is an alloy of iron with up to two per cent carbon and often small amounts of other elements. The properties of steel such as strength, malleability and machinability can be changed by adjusting the amounts of its component elements.

alumina. (aluminium oxide). A compound that occurs naturally and can also be produced from the mineral ore bauxite. Aluminium is produced from alumina via electrolysis.

bauxite. A naturally occurring rock with one or more minerals containing aluminium, oxygen and hydrogen. Globally, most bauxite is used to produce alumina, which is used to produce aluminium. For more information see Aluminium (Geoscience Australia).

cast. To pour a metal (or other material) as a liquid into a mould and allow it to solidify into the shape of the mould. For example, cast iron is often used to make manhole covers.

desalination. The removal of salts from water or soil. Desalination can be used to produce fresh water from sea water. For more information see Desalination (Ask an Expert, Australian Broadcasting Corporation).

ductile. Describes the ability of a material (mostly metals) to be drawn out into a wire without cracking or breaking.

electron. A negatively charged particle that is a constituent of an atom.

electrolysis. Chemical reaction brought about by passing electricity through a solution. Electrolysis is often used in industry to separate out the elements in a substance. The following equation illustrates the process of the electrolysis of water (H₂O).

fly ash. Fine particles of ash produced from the burning of fuels, particularly from power stations.

ion. A positively or negatively charged atom or group of atoms.

Kroll process. The process used to produce titanium (or zirconium) from its ore. The ore is converted first to titanium tetrachloride, and then reduced to produce titanium, usually by reacting it with magnesium. For more information see Titanium (Chemguide, UK)

lustre. The way that light interacts with the surface of a material, its sheen or gloss. Lustre is used to describe minerals eg, metallic lustre, greasy lustre.

magnesite. A mineral ore containing magnesium carbonate and used as a source of magnesium. For more information see Magnesium (Geoscience Australia).

malleable. The ability of metals to be shaped or hammered when cold without breaking. For example, aluminium can be hammered or rolled into sheets (aluminium foil).

ore. A naturally occurring mineral, or group of minerals, that is mined to extract materials such as metals. For example, bauxite ore is mined for the production of aluminium.

powder technology. (powder metallurgy). The production of metal as a powder which is then used to form shaped products. The powdered metal is usually placed in a mould, compacted and heated (sintered) to make the powder particles bond together.

quenching. A technique used by blacksmiths and in the metal processing industry which involves rapid cooling of a metal by immersing it into water or oil to achieve certain hardness or mechanical properties.

rutile. A mineral from which titanium is extracted. Rutile is black, yellow or brownish-red in colour and contains titanium dioxide.

smelter. An industrial plant that uses a high-temperature process to separate a pure metal, usually in a molten form, from an ore.

tempering. A heat treatment used by blacksmiths and in the metal processing industry that is applied to iron and steel. Tempering increases the toughness of the metal to improve performance in applications such as horseshoes and impact tools, for example, hammers.

valence (valency). The number of electrons in the outermost electron shell of an atom. These are the electrons involved in chemical reactions.