Metals and alloys (AQA GCSE Design and Technology): Revision Notes
Metals and alloys
What are metals?
Metals are fascinating crystalline materials that form the backbone of modern engineering and manufacturing. These materials are created from various elements and compounds, and they share several important characteristics that make them incredibly useful. All metals are typically solid at room temperature, possess considerable hardness, appear opaque (you can't see through them), and excel at conducting both heat and electricity. Understanding these fundamental properties helps explain why metals are so widely used in everything from kitchen appliances to aircraft construction.
The ability of metals to conduct heat and electricity makes them essential for electrical wiring, cooking equipment, and electronic devices. Their crystalline structure gives them their characteristic strength and durability.
Ferrous metals
Ferrous metals represent a major category of metallic materials that all share one crucial characteristic - they contain iron, specifically in the form of ferrite. This iron content gives ferrous metals two distinctive properties: most are magnetic, meaning they're attracted to magnets, and they're susceptible to rusting when exposed to moisture and oxygen over time.
Key distinction: Ferrous metals contain iron (ferrite), which makes them magnetic and prone to rust. This is the fundamental difference that separates them from non-ferrous metals.
The world of ferrous metals includes three primary types, each with different carbon content that dramatically affects their properties and applications.
Mild Steel Contains 0.15% to 0.25% carbon. This relatively low carbon content makes mild steel tough and malleable, meaning it can be bent and shaped without breaking. It's also magnetic and easily joined through welding or other methods. However, mild steel has poor corrosion resistance, making it prone to rust. With a melting point of 1400°C, mild steel finds widespread use in everyday items like screws, nails, bolts, and girders, as well as in car body panels and fridge doors.
Cast Iron Takes a different approach, containing significantly more carbon - 2% to 6%. This higher carbon content creates a unique structure with a hard outer skin but a brittle nature overall. Interestingly, cast iron has a soft core and is self-lubricating, making it excellent for applications where parts move against each other. It melts at 1200°C and is commonly used for vices, brake discs, and manhole covers.
High Carbon Steel (Tool Steel) Contains 0.8% to 1.5% carbon, making it extremely hard but difficult to cut or machine. This type of steel can be hardened and tempered through heat treatment processes, allowing manufacturers to adjust its properties for specific applications. With a melting point of 1600°C, high carbon steel is perfect for making chisels, plane blades, saws, and drill bits - tools that need to maintain sharp edges under heavy use.
Non-ferrous metals
Non-ferrous metals offer a completely different set of characteristics because they contain no iron whatsoever. This absence of iron means they won't rust, aren't magnetic, and tend to be more malleable than their ferrous counterparts. These properties make non-ferrous metals ideal for applications where corrosion resistance and workability are more important than magnetic properties.
Critical advantage: Non-ferrous metals do not rust because they contain no iron. This makes them essential for applications where corrosion resistance is vital, such as plumbing, marine equipment, and outdoor installations.
Aluminum stands out as one of the most versatile non-ferrous metals. It's highly malleable and ductile, meaning it can be formed into complex shapes without breaking. Aluminium also boasts an excellent strength-to-weight ratio, making it strong while remaining lightweight. It polishes beautifully, giving it an attractive appearance. With a melting point of 660°C, aluminium is commonly used for foil, window frames, engine parts, and drink cans.
Copper has been used by humans for thousands of years and remains incredibly valuable today. It's malleable and tough, capable of working well whether hot or cold. Copper polishes to a beautiful shine and has exceptional electrical and thermal conductivity. Melting at 1084°C, copper is essential for electrical wire, gas and water pipes, printed circuits, and roofing materials.
Tin offers unique properties with its soft, malleable nature and high corrosion resistance. It has an unusually low melting point of just 232°C, making it easy to work with. However, tin is relatively weak compared to other metals. Its primary applications include food packaging and as a component in solder for electrical connections.
Zinc presents interesting characteristics - it's brittle at room temperature but becomes malleable when heated. With a melting point of 419.5°C, zinc's most important application is galvanising ferrous metals like steel to prevent corrosion, essentially providing a protective coating that prevents rust.
Alloys
Alloys represent one of humanity's greatest metallurgical achievements. These materials are created by mixing two or more metals or elements together, resulting in combinations that have improved properties and characteristics compared to the individual components alone. This mixing process allows engineers to create materials with exactly the properties needed for specific applications.
The concept of alloying is fundamental to modern metallurgy. By carefully controlling the composition of different metals and elements, engineers can create materials that are stronger, more corrosion-resistant, or have other enhanced properties that pure metals cannot achieve.
Worked Example: Brass Composition
Brass demonstrates the power of alloying by combining 65% copper with 35% zinc. This mixture creates a metal that's easily machined, casts well into complex shapes, and polishes to an attractive golden appearance. Brass melts between 900°C and 940°C and is widely used for plumbing fittings, door hardware, and musical instruments like trumpets and trombones.
Stainless steel revolutionised many industries by solving the rust problem of regular steel. This alloy combines carbon steel with 10.5% to 18% chromium, 8% nickel, and 8% manganese. The chromium forms an invisible protective layer that prevents rusting, while maintaining the strength and hardness of steel. It also exhibits magnetic properties. With a melting point of 1400°C, stainless steel is essential for kitchenware, sinks, cutlery, and medical equipment where hygiene and corrosion resistance are crucial.
High speed steel represents the pinnacle of tool-making alloys, designed specifically for cutting applications. This sophisticated alloy combines carbon steel with 18% tungsten, 4% chromium, and 1% vanadium. These additions give the steel exceptional hardness even at temperatures up to 600°C, along with excellent wear resistance and toughness. Also melting at 1400°C, high speed steel is used for drill bits, laith tools, and milling cutters that need to maintain their cutting edge even when generating significant heat through friction.
Key Points to Remember:
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Ferrous metals contain iron - this makes them magnetic and prone to rust, but they're often stronger and harder than non-ferrous alternatives
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Non-ferrous metals don't contain iron - they won't rust or attract magnets, and they're generally more malleable and easier to work with
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Alloys combine the best of both worlds - by mixing different metals and elements, engineers can create materials with exactly the properties needed for specific applications
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Carbon content matters in steel - more carbon generally means harder but more brittle steel, while less carbon creates tougher, more workable materials
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Material selection depends on the application - consider factors like strength, corrosion resistance, weight, magnetic properties, and cost when choosing materials for engineering projects