Notes · Design and TechnologyUK · A-Levels

Enhancement of materials

This chapter explains how the raw properties of metals, polymers and timbers are deliberately improved so that a material meets a demand it could not meet as supplied. It covers alloying and the heat treatment of metals, the additives that transform polymers, and the seasoning, lamination and preservation of timber, always linking the treatment to the change in properties and the reason a designer chooses it.

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

T·0333 / 18

Exam profile

AO4 · Explain how metals, polymers and timbers are treated and modified to enhance their propertiesAO3 · Analyse and evaluate the effect of an enhancement on a material's suitability for a productAO2 · Apply knowledge of enhancement when selecting and preparing materials for making

Operators:explaindescribecompareanalyseevaluatejustify

basic level

AS-Level expects recognition of the main enhancement methods and the property each improves - alloying, hardening and tempering, additives, and seasoning.

higher level

The full A-Level expects the mechanism and sequence of a treatment (for example harden-then-temper) and reasoned justification of an enhancement against a product's requirements.

Depth

Text

Contents · 4 sections

Enhancement of materials

Alloying and the enhancement of metals#

StandardAQA 7552 3.1.3DfE GCE D&T - enhancement of materials

Enhancement methods by material family

Fig. 1How each material family is enhanced: metals by alloying, heat treatment and work hardening; polymers by additives; timbers by seasoning, lamination and preservation.

Enhancement is the deliberate modification of a material to improve a property it lacks - strength, hardness, toughness, corrosion resistance or dimensional stability - so that it can do a job it could not do as supplied. Every material family has characteristic enhancement routes: metals are alloyed and heat-treated, polymers are modified with additives, and timber is seasoned and preserved. Recognising which route improves which property, and why, is the aim of this chapter.

Alloying is the most fundamental enhancement of metals: mixing a base metal with one or more other elements to engineer new properties. The added atoms disrupt the regular crystal lattice, which generally makes the alloy harder and stronger than the pure metal, and can transform its corrosion resistance or appearance. Steel is iron alloyed with carbon; adding chromium makes stainless steel corrosion-resistant; copper alloyed with zinc gives corrosion-resistant, machinable brass, and with tin gives hard bronze; aluminium alloyed with copper gives strong, light duralumin.

Work hardening is a further enhancement that happens during cold forming. Hammering, rolling, bending or drawing a metal cold deforms its grains and makes it progressively harder and stronger but less ductile and more brittle - which is why a repeatedly bent wire eventually snaps. Work hardening can be exploited to stiffen a formed part, but it also means a metal being heavily worked must sometimes be softened again (annealed) to continue forming it without cracking.

A designer chooses an enhancement by starting from the property the product demands and the material's shortfall. If mild steel is strong but rusts, the choice is between alloying to stainless steel, applying a finish, or accepting maintenance; if a copper part is too soft, work hardening or an alloy may serve. Enhancement is therefore not an end in itself but a tool for closing the gap between a material's natural properties and a product's requirements - and it usually carries a cost, so the benefit must justify it.

Worked example

Justifying an alloy over a pure metal

A manufacturer must choose between mild steel and stainless steel for kitchen sinks. Explain, in terms of alloying, why stainless steel is chosen and what property is gained and at what cost.

Identify the shortfall
Mild steel is strong, cheap and formable but rusts, which is unacceptable for a sink in constant contact with water and food.
Explain the alloying
Stainless steel alloys iron with chromium (and often nickel); the chromium forms a self-repairing passive oxide layer that resists corrosion, so the sink does not rust and stays hygienic.
Weigh the trade-off
The gain is corrosion resistance and hygiene; the cost is a higher material price and slightly harder forming, which is justified for a product that must last years in a wet environment.

Result: Stainless steel is chosen because alloying iron with chromium gives corrosion resistance and hygiene the mild steel lacks - a property gain that justifies the higher cost for a wet, food-contact product.

Typical mistakes

Describing an alloy as a compound - it is a mixture of a metal with other elements, engineered to change properties.
Forgetting that work hardening reduces ductility, so a heavily cold-worked part becomes brittle and may need annealing.

Active revision

Explain why cutlery is made from stainless steel rather than mild steel, referring to the specific enhancement and the property it provides.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Design and Technology: Product Design (7552) specification (AQA)

Heat treatment of metals#

AdvancedAQA 7552 3.1.3DfE GCE D&T - heat treatment

Hardening and tempering a high-carbon steel

Fig. 2The harden-then-temper sequence: heating and quenching make high-carbon steel hard but brittle; tempering then restores toughness at the cost of a little hardness - the order is essential.

Heat treatment enhances a metal by controlling how it is heated and cooled, changing its internal grain structure without changing its composition. The main treatments for steel are hardening, tempering, annealing, normalising and case-hardening, and they let a single steel be made hard for cutting, tough for impact, or soft for forming, according to the sequence used. Because the effect depends on the cooling rate, the same heating followed by different cooling gives very different results.

Hardening makes high-carbon steel hard by heating it above its critical temperature and then quenching (cooling it rapidly in water or oil). The rapid cooling traps a hard, brittle structure. The problem is that fully hardened steel is too brittle to use - it would shatter - so hardening is almost always followed by tempering. Tempering reheats the hardened steel to a lower temperature and cools it more slowly, sacrificing a little hardness to restore toughness; the tempering temperature (judged by the oxide colour on the surface) sets the balance, lower for a hard cutting edge, higher for a tougher spring.

Annealing and normalising do the opposite - they soften. Annealing heats the steel and cools it very slowly (often in the furnace), producing the softest, most ductile and stress-free structure, ideal before heavy cold forming or to remove work hardening. Normalising heats and then cools in still air, giving a slightly harder, tougher, more uniform grain than annealing, often used to refine the structure after forging. Case-hardening gives a low-carbon steel a hard, wear-resistant skin over a tough core by adding carbon to the surface (carburising) and then heat-treating - ideal for gears and shafts that need a wear-resistant surface but must not be brittle throughout.

The designer's skill is choosing the treatment - and the sequence - that gives the required combination of properties. A cold chisel needs a hard edge that will not shatter, so it is hardened and then tempered; a gear needs a wear-resistant surface on a shock-resistant core, so it is case-hardened; a sheet to be deep-drawn needs maximum ductility, so it is annealed first. Getting the sequence wrong - hardening without tempering, for example - produces a part that fails in service, so the order matters as much as the treatment.

Worked example

Choosing a heat-treatment sequence

A workshop makes a cold chisel from high-carbon steel. The cutting edge must be hard enough to cut mild steel but the tool must not shatter when struck. Recommend the heat treatment and justify the sequence.

Harden the edge
Heat the steel above its critical temperature and quench it in oil or water; the rapid cooling traps a hard structure so the edge resists wear and can cut metal.
Recognise the problem
Fully hardened, the chisel is dangerously brittle and would shatter under hammer blows, so hardness alone is unsafe.
Temper to restore toughness
Reheat the hardened chisel to a tempering temperature (judged by the oxide colour) and cool more slowly; this sacrifices a little hardness to regain the toughness needed to survive being struck.

Result: Harden by heating and quenching to give a hard cutting edge, then temper to restore toughness so the chisel resists wear yet does not shatter when hammered - the sequence, not just the treatment, delivers the required combination of properties.

Typical mistakes

Saying a part is 'hardened' and stopping there - fully hardened steel is brittle and must be tempered to be usable.
Confusing annealing (softest, slow furnace cooling) with normalising (air cooling, slightly harder and tougher).

Active revision

A screwdriver tip wears and rounds off quickly. Recommend a heat treatment (and any sequence) to give it a hard, durable tip that will not snap, and justify each step.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for design and technology (Department for Education) · AQA A-level Design and Technology: Product Design (7552) specification (AQA)

Enhancing polymers with additives#

StandardAQA 7552 3.1.3DfE GCE D&T - additives

Raw polymers are rarely used as they come; almost all are enhanced with additives - small quantities of other substances mixed into the polymer before moulding - to tailor their properties, appearance, processing and lifespan. Additives are what turn a single base polymer such as PVC into a whole family of materials ranging from rigid pipe to flexible cable insulation, which is why they are central to how designers exploit polymers.

The main additives each target a property. Plasticisers make a rigid polymer soft and flexible by letting the chains slide past one another - rigid uPVC pipe becomes flexible PVC for cable sheathing and clothing. Stabilisers, including ultraviolet stabilisers and antioxidants, slow the degradation that sunlight and heat would otherwise cause, extending outdoor life. Fillers bulk out the polymer to reduce cost and can add stiffness or heat resistance. Flame retardants reduce flammability for electrical casings and furnishings; antistatic agents dissipate static charge for electronic packaging; and pigments and dyes colour the material throughout so no painting is needed.

Additives are chosen to close the gap between the base polymer's properties and the product's requirements, just as heat treatment does for metals. A garden chair moulded from polypropylene needs ultraviolet stabilisers so it does not become brittle and fade in sunlight; a child's toy needs pigments for colour and may need flame retardants; a flexible phone case needs plasticisers. The designer specifies the base polymer and then the additive package that makes it fit the job and its environment.

Additives also carry trade-offs and consequences that must be weighed. Some plasticisers and flame retardants have raised health and environmental concerns and are regulated; fillers that cut cost can reduce strength; heavy pigment loading can affect processing. Additives can also complicate recycling, because a heavily-modified polymer is harder to reprocess cleanly. So while additives are a powerful enhancement, a responsible designer considers their safety, their effect on properties and their impact on end-of-life recycling.

Worked example

Specifying an additive package

A manufacturer moulds garden furniture from polypropylene that will stand outdoors year-round. Specify the additives needed and justify each against the product's environment.

Analyse the environment
The furniture is exposed to sunlight, weather and temperature swings, must look attractive without painting, and should be safe - so it needs protection from ultraviolet degradation, integral colour and durability.
Specify the additives
Ultraviolet stabilisers and antioxidants to stop the polymer becoming brittle and fading in sunlight; pigments to colour the material throughout so no finishing is needed; and fillers if cost must be reduced without losing too much stiffness.
Justify and weigh trade-offs
The UV stabilisers extend outdoor life, the pigments remove a painting stage, and fillers cut cost - but the designer checks that fillers do not weaken the chair too far and that the additive package does not undermine recyclability.

Result: UV stabilisers (weather resistance), pigments (integral colour) and possibly fillers (cost) are specified in the polypropylene, each matched to the product's outdoor environment while watching the effect on strength and recycling.

Typical mistakes

Confusing additives with separate materials - they are blended into the polymer to modify it, not bonded on afterwards like a finish.
Ignoring that additives can reduce recyclability or raise health and environmental concerns, so praising them uncritically.

Active revision

Outdoor cable insulation must be flexible, coloured and survive years of sunlight. Identify the additives a designer would specify in the PVC and explain the property each provides.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Design and Technology: Product Design (7552) specification (AQA)

Enhancing timber: seasoning, lamination and preservation#

StandardAQA 7552 3.1.3DfE GCE D&T - timber enhancement

Seasoning: moisture content falls over time

Fig. 3Seasoning reduces a timber's moisture content from green (very high) towards an equilibrium set by its environment. The curve falls quickly at first and then levels; kiln seasoning reaches lower values than air seasoning.

Freshly felled 'green' timber holds a great deal of water and is unusable for quality work: as it dries it shrinks, warps, twists and splits, and it is prone to decay. Seasoning is the controlled reduction of a timber's moisture content to a level in equilibrium with its intended environment, which stabilises it, increases its strength and stiffness, improves its resistance to decay and lets it be glued, finished and worked reliably. Seasoning is the essential first enhancement of natural timber.

There are two main methods. Air seasoning stacks sawn boards under cover with spacers ('stickers') so air circulates, slowly bringing the moisture content down over months to a year or more; it is cheap and low-energy but slow and limited to about 15-18% moisture. Kiln seasoning dries the timber in a heated, humidity-controlled chamber in days, reaching a lower moisture content (down to about 8-10% for indoor use) and killing insects and spores; it is faster and more controllable but uses energy and equipment. The right moisture content depends on where the timber will live - lower for centrally-heated interiors, higher for outdoor use.

Lamination enhances timber by bonding thin layers (laminae) together with adhesive, often over a former. Because the layers are thin they can be bent to curves impossible in solid timber, and gluing them in a stable arrangement resists the warping of solid wood - the principle behind curved laminated furniture, roof beams (glulam) and plywood. Lamination lets a designer create strong, stable, curved and large components from small, cheaper sections of timber.

Preservation protects timber from decay, insects and moisture where it will be exposed. Pressure treatment (tanalising) forces preservative deep into the timber for outdoor structures such as decking, fencing and cladding; surface treatments such as preservative stains, oils and paints protect and decorate. Choosing whether and how to season, laminate or preserve a timber is a design decision driven by the service environment: an indoor cabinet needs kiln-dried, stable timber; an outdoor bench needs pressure-treated or naturally durable timber and a maintainable finish.

Worked example

Applying seasoning and lamination

A designer produces a curved, laminated timber chair back for indoor use in centrally-heated homes. Explain how the timber would be enhanced and justify the choices.

Season to the right moisture content
Kiln-season the timber down to about 8-10% moisture content so it is in equilibrium with a centrally-heated interior; this stops the finished chair shrinking or splitting in service and makes it strong, stable and ready to glue.
Laminate to form the curve
Bond thin, seasoned laminae over a former with adhesive; the thin layers bend to the required curve that solid timber could not, and gluing them in a stable arrangement resists warping.
Justify the combination
Seasoning guarantees dimensional stability indoors; lamination delivers the curved, strong shape from small, cheaper sections - together they give a stable, elegant chair back that would be impossible in a single piece of green timber.

Result: Kiln-season the timber to about 8-10% for indoor stability, then laminate thin layers over a former to create the curved back - seasoning ensures it will not move in a heated home and lamination gives the curve and strength.

Typical mistakes

Thinking seasoning only removes water - it also stabilises the timber, increases strength and improves decay resistance and workability.
Using unseasoned or wrongly-seasoned timber, so a finished product later warps, splits or shrinks as it reaches equilibrium with its environment.

Active revision

A designer makes a curved-back dining chair for a centrally-heated home. Explain how seasoning and lamination would each be used and justify the moisture content chosen.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Design and Technology: Product Design (7552) specification (AQA)

References & sources

Sources

AQA

AQA A-level Design and Technology: Product Design (7552) specification

Department for Education

GCE AS and A level subject content for design and technology

Enhancement of materials

4 sections~15 min reading time3 competenciesLevel Standard 3 · Advanced 1

Enhancement of materials

Key points

Justifying an alloy over a pure metal

Key points

Choosing a heat-treatment sequence

Key points

Specifying an additive package

Key points

Applying seasoning and lamination

Sources

Enhancement of materials

Key points

Justifying an alloy over a pure metal

Key points

Choosing a heat-treatment sequence

Key points

Specifying an additive package

Key points

Applying seasoning and lamination

Sources