How do ferrous metals, non-ferrous metals and alloys differ, and how does a designer choose the right metal for a product?
Metals used in product design: ferrous and non-ferrous metals and alloys, their key properties (strength, hardness, ductility, malleability, conductivity, corrosion resistance, cost) and how those properties guide material choice.
An SQA Higher Design and Manufacture answer on metals, covering ferrous and non-ferrous metals and alloys, their key properties such as strength, ductility, conductivity and corrosion resistance, and how a designer matches a metal to the demands of a product.
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What this key area is asking
The SQA wants you to know the three families of metal - ferrous, non-ferrous and alloys - their key properties, and how a designer matches a metal to a product. The question paper asks you to justify a metal choice or to compare two metals for a use, usually for 3 to 5 marks. As with timbers, marks come from reasoning from properties, not from naming a metal.
The three families of metal
Ferrous metals. Contain iron, are usually strong, hard and cheap, but most rust (corrode) unless protected by a finish.
- Mild steel - an alloy of iron with a little carbon; strong, tough, cheap and easy to weld, used for structures, car bodies and general fabrication, but rusts unless painted, galvanised or coated.
- Cast iron - high-carbon iron; very hard and good in compression but brittle, used for machine bases, manhole covers and engine blocks.
Non-ferrous metals. Contain no iron, so they resist rusting and are valued for lightness or conductivity.
- Aluminium - light (low density), corrosion-resistant and a good conductor; used for aircraft, bike frames, cans and cookware, though more costly and harder to weld than steel.
- Copper - an excellent conductor of heat and electricity, ductile and corrosion-resistant; used for wiring, pipes and electronics.
Alloys. Made to improve on the base metals.
- Stainless steel - steel plus chromium; highly corrosion-resistant, hard and hygienic, used for sinks, cutlery and medical tools.
- Brass - copper plus zinc; corrosion-resistant, easy to machine and attractive, used for fittings, valves and decorative work.
Key properties to reason from
These properties decide the choice. A drinks can needs lightness and corrosion resistance (aluminium); electrical wire needs conductivity and ductility (copper); a hard-wearing kitchen sink needs corrosion resistance and hygiene (stainless steel); a cheap, strong, weldable structure needs mild steel, accepting that it must be protected from rust.
Where this fits in the course
Metals are one of the three material groups in the Materials and Manufacture area, alongside timbers and polymers. The question paper asks you to justify and compare metal choices, and your design assignment must choose materials with reasons. Reasoning from properties such as corrosion resistance, weight and conductivity earns marks in both components.
Try this
Q1. Explain why copper is chosen for electrical wiring. [3 marks]
- Cue. It is an excellent conductor of electricity and is ductile, so it carries current efficiently and can be drawn into thin wire.
Q2. Explain why mild steel car body panels must be given a finish, and name a suitable one. [3 marks]
- Cue. Mild steel is ferrous and rusts, so a paint coating (or galvanising) is needed to protect it from corrosion.
Q3. Explain why cast iron is suited to a machine base but not to a part that may be dropped. [4 marks]
- Cue. Cast iron is hard and strong in compression, good for a heavy stable base, but it is brittle, so it can crack or shatter under a sharp impact.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher4 marksExplain why aluminium is often chosen instead of steel for a product such as a bicycle frame or a drinks can.Show worked answer →
Worth about 4 marks, so the marker wants two developed reasons, each a
property linked to the product. The mark scheme rewards properties of
aluminium applied to the named product.
Low density (light). Aluminium has about a third of the density of steel,
so a bicycle frame is much lighter for the rider to handle and a drinks
can uses less material by weight, cutting transport cost.
Corrosion resistance. Aluminium forms a tough oxide layer that resists
corrosion without painting, so the frame or can does not rust and stays
clean in contact with drink or weather.
A strong answer adds the trade-off: aluminium is weaker and more expensive
per kilogram than mild steel and is harder to weld, so it is chosen when
lightness and corrosion resistance matter more than lowest cost or maximum
strength.
SQA Higher3 marksExplain why stainless steel is chosen for kitchen sinks and cutlery.Show worked answer →
Worth about 3 marks. The markers want properties of the alloy linked to
the use.
Corrosion resistance. Stainless steel is an alloy of steel with chromium,
which makes it highly resistant to rust and staining, so it survives
constant contact with water and food without corroding.
Hardness and hygiene. It is hard and tough, takes a smooth polished finish
that is easy to clean and resists scratching, which suits hygienic kitchen
use.
A top answer notes that these properties justify its higher cost over mild
steel, because a sink or cutlery must not rust or harbour dirt.
Related dot points
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