MYP Design Metals and Alloys Notes

Ferrous or Non-Ferrous: Does It Contain Iron?

1. A ferrous metal contains iron, and a non-ferrous metal does not.
2. That single fact predicts two things: ferrous metals rust in damp air, and most of them stick to a magnet.
3. A magnet on the end of a string is the fastest sorting test in the workshop, and scrapyards use the same trick at industrial scale.
4. Ferrous metals are generally stronger and cheaper per kilogram, which is why building frames are steel rather than aluminium.
5. Non-ferrous metals are usually lighter or better at conducting, and they do not rust, so they take the jobs steel would lose.
   1. Stainless steel is the awkward case: it is ferrous, contains chromium, and mostly ignores both rust and cheap magnets.

Six Metals That Cover Most School Projects

1. Mild steel is iron with about 0.2 per cent carbon: cheap, strong, easy to weld and drill, and it rusts within weeks if left bare.
2. Cast iron carries around 3 per cent carbon, which makes it hard and good at soaking up vibration, but brittle enough to crack if you drop it.
   1. That is why lathe and pillar drill bases are cast iron: heavy, stiff and never expected to take a hammer blow.
3. Aluminium weighs roughly a third of steel for the same volume, does not rust, and is soft enough to cut with a hacksaw.
4. Copper is the best affordable conductor of heat and electricity, bends easily by hand, and turns green outdoors as it corrodes.
5. Brass is copper mixed with zinc: gold coloured, easy to machine to a crisp edge, and the standard choice for taps, hinges and instrument parts.
6. Stainless steel adds chromium to steel and resists rust completely, which buys it cutlery, sinks and surgical tools at several times the price of mild steel.

An Alloy Is a Recipe, Not a Mixture You Can Separate

1. Pure metals are often disappointing: pure iron is soft, pure copper stretches under load, pure aluminium bends too easily for a frame.
2. Melting a second element into a metal makes an alloy, and the result behaves differently from either ingredient.
3. Atoms in a pure metal sit in neat rows that can slide over each other, which is what makes the metal soft.
4. Adding atoms of a different size jams those rows, so they slide less and the metal becomes harder and stronger.
5. Steel is iron plus carbon, brass is copper plus zinc, bronze is copper plus tin, and solder is tin plus a little silver or copper.
6. More carbon makes steel harder but more brittle, which is why a chisel at 1 per cent carbon holds an edge and a car body panel at 0.1 per cent bends instead of shattering.
   1. Alloying can also drop the melting point, which is exactly what makes solder useful at 200 degrees Celsius when copper melts near 1085.

Heat Treatment Changes a Metal Without Changing Its Recipe

1. Heating a metal and controlling how fast it cools rearranges its internal grains, which changes hardness without adding anything.
2. Annealing means heating the metal and letting it cool slowly, which makes it soft and easy to bend again.
   1. Copper that has gone stiff from repeated bending, called work hardening, can be annealed back to soft and reused.
3. Hardening means heating high-carbon steel to bright red and quenching it in oil or water, which freezes a hard, brittle structure in place.
4. Tempering reheats that hardened steel gently, to somewhere between 200 and 300 degrees Celsius, trading a little hardness back for toughness so the tool does not snap.
5. A screwdriver tip that is hardened but never tempered will shatter the first time you lean on it.
6. Only steels with enough carbon respond to hardening, so mild steel cannot be hardened this way and neither can aluminium.

Corrosion Is a Slow Fire, and Coatings Starve It

1. Rust forms when iron meets both oxygen and water, so a steel part stays clean in dry air and fails fast outdoors.
2. Rust flakes off and exposes fresh metal underneath, so the damage keeps eating inwards until the part is gone.
3. Aluminium corrodes too, but its oxide forms a thin tight skin that seals the surface and stops the attack there.
4. Painting and powder coating block oxygen and water with a physical layer, and powder coating survives knocks far better than brushed paint.
5. Galvanising dips steel in molten zinc, and the zinc corrodes in place of the steel even where the coating gets scratched.
6. Oiling or waxing suits workshop tools, while anodising thickens the natural oxide on aluminium and lets it hold colour.

Picking a Metal Means Ranking Three Things at Once

1. Weigh strength, weight and corrosion against cost, because no metal wins on all four.
2. Metals score well on sustainability because they can be melted and reformed endlessly without losing quality, unlike most polymers.
3. Recycling aluminium uses roughly 5 per cent of the energy needed to extract it from bauxite ore, which is a strong point for Evaluating.

Matching Metal to Joining Method

1. Mild steel takes almost any join: welding, brazing, riveting or bolting.
2. Aluminium is difficult to weld in a school workshop, so plan on rivets, bolts or a bonded joint instead.
3. Copper and brass solder cleanly at low temperature, which is why plumbing and jewellery both rely on them.