Materials and Engineering Processes
Know the main engineering materials and their properties. Ferrous metals include mild steel (low carbon, weldable, ductile), medium and high carbon steel (harder, less weldable), cast iron (brittle, good compressive strength) and stainless steel (corrosion resistant due to chromium). Non-ferrous metals include aluminium (light, corrosion resistant, good conductor), copper (excellent conductor, ductile) and brass/bronze (alloys). Polymers (thermoplastics vs thermosetting plastics) and composites (fibre-reinforced) behave differently and must be selected for purpose.
Understand properties: strength, hardness, ductility, malleability, toughness, brittleness, conductivity, and corrosion resistance. Know basic processes: casting, forging, rolling, extrusion, welding, brazing and soldering, plus heat treatments such as annealing (soften, relieve stress), hardening (quench) and tempering (reduce brittleness after hardening).
Traps: assuming stainless steel cannot corrode (it can in chloride environments); confusing hardness with toughness; thinking thermosetting plastics can be remelted (they cannot); and forgetting that tempering follows hardening.
Rule of thumb: select material by matching required properties to service conditions, not by cost alone. Worked example: a bicycle frame needs high strength-to-weight ratio and fatigue resistance, so aluminium alloy or carbon-fibre composite is chosen over mild steel, even though mild steel is cheaper and easier to weld.