Chapter 3 · 2 hours
Interaction of Materials, Processing and Design
Practice questions
Practice questions and answers
2 exam-style questions on this chapter, written for this site from the official syllabus. We haven’t found past IOE papers for this subject yet; if you have some, share them in the community.
- Practice · 6 marks
Explain how the choice of manufacturing process affects design. Differentiate between casting and forging with respect to their influence on part shape, properties and design rules.
Answer
Role of processing in design: the process fixes the achievable shape, tolerance, surface finish, material properties and cost. A part should be designed for the process chosen, so that it is made economically and the properties assumed in the stress calculation are actually obtained. Changing the process after design often requires redesign.
Casting vs forging
| Point | Casting | Forging |
|---|---|---|
| Process | Molten metal poured into a mould and solidified | Hot or cold metal shaped by compressive force |
| Shape | Very complex shapes, internal cavities, large parts | Simpler shapes; limited undercuts and cavities |
| Grain structure | Random, coarse; possible porosity, shrinkage | Refined grain flowing along the part contour |
| Strength and toughness | Lower; lower fatigue strength | Higher; better impact and fatigue resistance |
| Tolerance and finish | Sand: poor; die/investment: good | Good, especially with closed-die/cold forging |
| Tooling cost | Low for sand casting | High die cost; economical for large quantity |
| Typical parts | Engine blocks, pump casings, machine beds | Crankshafts, connecting rods, gears, spanners |
Design rules
- Casting: uniform wall thickness, gradual section changes, generous fillets and radii, avoid sharp corners and hot spots, provide draft (1 to 3 degrees) and machining allowance, ribs for stiffness instead of heavy sections.
- Forging: place parting line on a plane of symmetry, give draft (3 to 7 degrees), fillets and corner radii for metal flow, avoid thin webs and deep pockets, align the grain flow with the main stress direction.
Example: a heavily loaded crankshaft is forged for fatigue strength; a gearbox casing is cast for its complex shape and damping.
- Practice · 3+3 marks
Write short notes on: (a) powder metallurgy and its design limitations; (b) heat treatment processes for steel (annealing, normalising, hardening and tempering) and their effect on design.
Answer
(a) Powder metallurgy (P/M)
Metal powder is blended, compacted in a die under high pressure (hundreds of MPa) and sintered below the melting point (about 70 to 90% of the melting temperature in a protective atmosphere). Optional sizing, impregnation with oil (self-lubricating bearings) or infiltration follows.
- Advantages: near-net shape, little scrap, good for gears, bushings, filters, cutting tool tips, and alloys that cannot be cast (tungsten carbide).
- Design limitations: part must be ejectable from the die (no undercuts or cross holes), uniform section, limited size and length-to-diameter ratio, porosity lowers strength and ductility, sharp corners and thin walls crack, die cost high so it suits large quantity.
(b) Heat treatment of steel
| Process | Operation | Result |
|---|---|---|
| Annealing | Heat above critical temperature, soak, cool slowly in furnace | Soft, ductile, stress-free; improves machinability |
| Normalising | Heat above upper critical, cool in still air | Fine uniform grain, moderate strength |
| Hardening | Heat to austenite, quench in water/oil | Hard, brittle martensite |
| Tempering | Reheat hardened steel to 150 to 650 °C | Reduces brittleness and internal stress, gives toughness |
Effect on design: quenching causes distortion and cracking, so avoid sharp corners, sudden thickness changes and blind holes; keep sections uniform; allow grinding allowance after hardening; choose a steel with enough hardenability for the section size; design stresses must use the strength of the heat-treated condition.
Written from the official syllabus. Questions and answers are written for this site; check them against your class notes.
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