METALS

DT

Categories of Metals​

• Ferrous – contain iron, are magnetic and can rust​

• Non-ferrous – do not contain iron, non magnetic, do not rust​

• Alloys – combination of two or more ​
  metals, or two or more elements, one ​
  of which must be a metal​

How is metal found

• Metal is mined from the ground in the form of ore:​

• Bauxite is an aluminium ore and is extracted by electrolysis ​

• Iron ore is the second most common ​
  ore in the earth’s crust and ​
  uses heat and carbon ​
  for extraction

Ferrous Metals

• Carbon (C) is a common additive used to increase the hardness of iron​

• Low carbon steel​

• Medium carbon steel​

• Cast iron​

Low Carbon steel

• malleable

• ductile

• relatively inexpensive compared to other metals

• good tensile strength but lower than ​
  higher carbon steels​

• It is easy to weld

• used in nuts, ​bolts and automotive parts​

Medium carbon steel

• less ductile than ​low carbon steel​

• hard

• malleable

• tough​

• good tensile strength​

• used for springs, gears,​ rail tracks and machine parts

Grey cast Iron

• distributes heat evenly

• withstands compression

• resists deformation​

• very hard

• good resistance to oxidation​

• brittle

• low impact resistance​

• used for radiators, cookware and stoves

White cast Iron

• excellent resistance to abrasion​

• hard ​

• brittle

• greater ​tensile strength​

• cast with little shrinkage​

• used for extrusion nozzles, dies, stone crushers​ and industrial rollers

Non-Ferrous metals

• Aluminium​

• Copper​

• Silver​

• Gold​

• Titanium​

• Tin

Aluminium

• excellent strength to weight ratio​

• soft

• malleable

• prevent corrosion​

• used for window frames, phone casings, ​aeroplane parts ​and bicycles

Copper

• malleable

• extremely ductile ​

• excellent conductor of electricity and heat ​

• used for electrical wire, ​motors, hot water pipes and pans

Zinc

• versatile

• often alloyed​

• ductile and ​easily worked ​

• excellent ​corrosion resistance​

• used for galvanising, ​roofing material and batteries ​

• Zinc is often used as a sacrificial anode on boats and ships to reduce corrosion

Silver

• best electric and thermal conductor​

• extremely ductile ​

• used for electronics, mirrors ​and jewellery

Gold

• ductile

• malleable​

• does not tarnish

• resistant to​ air, water, alkalis, and most acids​

• used for jewellery, electronics, ​ornamentation and dentistry

Titanium

• low density

• durable ​with a high melting point​

• highly resistant to corrosion ​

• excellent strength ​to weight ratio​

• used for plants, ​golf clubs, pacemakers, jewellery​ and aviation components

Tin

• soft and malleable​

• good corrosion resistance​

• used for ‘tin plate’​ coating and solder for electronics

Ferrous Alloys

• Ferrous alloys contain iron, carbon and ​
  one or more other element​

• Stainless steel​

• Die steel​

• High speed steel

Stainless steel

• very hard

• good impact ​resistance​

• good corrosion resistance ​

• used for medical and ​catering equipment and construction

Die Steel

• good dimensional stability​

• hard

• prevent corrosion​

• used for dies for ​die casting, wire drawing and tooling

High speed steel

• extremely hard and tough​

• withstands higher temperatures

• used for drill bits, ​tool blades, milling cutters ​and kitchen knives

Non-ferrous alloys

• Bronze​

• Brass​

• Duralumin​

• Pewter

Brass

• ow melting point

• low friction

• self lubricating​

• corrosion resistant​

• good conductor of ​heat and electricity ​

• used for valves, taps, ornaments and musical instruments ​

Bronze

• resistant to corrosion​

• brittle ​

• used for statues, medals and coins

Duralium

• ductile

• malleable

• excellent strength-to-weight ratio​

• It oxidises to create a protective layer ​

• used for aircraft ​parts, surgical instruments, ​telescopic tent and walking ​poles, automotive and ​
  military engineering

Pewter

• low melting point

• soft

• malleable​

• used for flasks and tankards, decorative​ items and trophies

plate

• ideal for plasma cutting, flame cutting and laser cutting​

• used for construction and fabrication, architectural features and military machines

Bar

• Flat and square bar is easily milled, drilled or welded​ or turned using a four jaw chuck​

• Round and hexagonal sections can be ​
  more easily turned on a lathe​

• used for tool making, gym equipment and construction

Tube

• used for bike frames, ​heating and plumbing, musical ​instruments and some medical applications

Mechanical properties

• Hardness​

• Toughness​

• Malleability​

• Elasticity​

• Tensile strength

Hardness

The hardness of metals determines how well they withstand wear and abrasion

Toughness

The toughness of metals determines their ​
ability to absorb energy without cracking

Malleability and Ductility

• Malleable metals can be bent, pressed ​
  and manipulated without fracturing​

• Ductile metals can be drawn or stretched without breaking

Elasticity

Elasticity is a material’s resistance to distortion and ability to return to its original shape

Material strength

• Compressive strength​

• Tensile strength​

• Bending strength​

• Torsional strength​

• Shear strength

Physical properties

• Density​

• Melting points​

• Thermal conductivity​

• Electrical conductivity​

• Resistance to corrosion​

• Ability to take applied coatings and finishes​

• Ability to be alloyed and joined with heat processes

Sand Casting method

1. The mould, including the flows to pour metal, must be made from MDF, so there is no grain. cut mould in half

2. pour green sand in both boulds with a ram 

3. join the cope and drag and pour the molten metal waiting for it to cool

4. remove cope and drag

5. Cut off pouring feet and put product into machine for a smoother finish 

Sand casting material, application, scale

• aluminium 

• Vices, Park Benches, Post boxes, Rail carriage wheels, Drain covers

• one off or batch 

Sand casting pros and cons 

pros

• bespoke parts

cons 

• slow, labour intensive 

• mould is single use only

• not a high quality finish

Gravity die casting

• Heat the metal until it is molton, then pouri it into the mould.

• This process purley relies on gravity to allow the metal to flow through.

• Once the mould has been casted it is cooled and the runner and riser removed. ​

​

Used for heavier components, with finer detail as it is much more accurate than sand casting.

gravity die casting material, application, scale

• aluminium, mild steel, copper

• wheels, engines, doorknobs, handles 

• mass production

gravity die casting pros and cons

pros 

• reusable 

• high finish

• more accurate than sand casting

cons

• expensive setup costs

Die casting (hot pressure, cold chamber)

Hot pressure 

1. Lower melt point metals such as aluminum and aluminum alloys. ​

2. Molten metal stored in chamber then forced into the die​

3. Ejector pins ensure the casting releases from the die

Cold chamber

1. A hydraulic ram pushes the molten metal into the cavity.

2. When the metal as solidified the pins are removed and the ejector pins push the die casting out.

Die casting material, application, scale

• aluminium, copper

• wheels, engines, doorknobs, handles 

• mass production

Die casting pros and cons

pros 

• high quality finish

• molds are reusable 

• low setup costs

cons

• cant do complex shapes 

Investment casting method

1. use of wax moulds to create intricate and accurate finishes to a metal cast. ​

2. wax mould is coated with a ceramic outer layer, dried and the wax is then heated to melt and removed from the ceramic out shell.

3. Then the metal is poured and the ceramic is broken away to leave the metal cast.

Investment casting material, application, scale

• stainless steel, brass, carbon steel

• pipe connector, valves, engine components 

• mass production

Investment casting pros and cons

Pros

• wide range of metals used 

• high finish

• wax is reusable 

Cons

• high setup costs

Plaster of paris method

Plaster of Paris material, application, scale

• Ornaments, valves​, Gears​, Lock components​, Decorative door knockers

Plaster of Paris pros and cons 

Pros:​

• Good for capturing fine detail.​

• cheap and accessible.​

• moulded relatively easily.​

• Non-toxic,

Cons:​

• Once set, it's brittle, not strong under tension.​

• Can be heavy.​

• Sensitive to moisture

• handled carefully during setting

• Limited reuse of moulds or limited lifespan.

Press forming method

1. Sheet metal is clamped over the die, this will determine the final shape ​

2. A hydraulic press pushes the die into the sheet metal. Cutting blades may be included to punch holes into the sheet and trim excess from edges​

3. The hydraulic die is lowered and pressed sheet component is removed​

4. The sheet may be placed into further process forming machines for additional pressing if a more complex shape is required

Press Forming material, application, scale

• low carbon steel, almunimium 

• car doors

• mass production 

Press forming pros and cons

Pros

• managed by automation ​

• Excellent for mass production ​

• Simple process

• High quality finish​

• No weakened parts of the material when formed ​

​Cons

• High set up costs​

• Specialists are required to make the moulds ​

• Can only be used on medium carbon steel and aluminium due to them being malleable and ductile materials

MIG method

1. Heat generated by an arc in a localised area 

2. workpiece clamped to the table with a negative collection 

3. co2 and argon is used as a flux

4. gas Shield acts as protextion to prevent oxidisation 

MIG materials, applications

• aluminium, copper, stainless steel, nickle, iron

• bike frames, climb frames, car exhausts

MIG pros and cons

Pros

• simple to learn

• high productivity 

Cons

• one off/batch

• components can be expensice

• can see the join 

• cant be used outdoors 

Oxy-acetylene welding method

1. mix of high pressure gasses (oxygen and acetylene) 

2. flame intesnity adjusted when changing mixture in gas

3. v shapoes are created to ensure the filler weld runs through the thickness

Oxy-acetylene materials

• low carbon steel

• cast iron

• stainless steel

• copper

• brass

• aluminium

• bronze

Oxy-acetylene pros and cons

Pros 

• use in romote areas

• does not need electricity 

• used for quick repair 

cons

• slower

• used for thinner metals

• requires high skill

Soldering method

1. parent metal should have no haps between joining area

2. flux to help clean and flow

3. solder tradilionalty led/tin alloy 320 degrees low melt point 

Soldering application, materials

• hard - gold, silver

• soft - copper

• jewellery, plumbing, electronics 

Soldering pros and cons

pros

• used when high heat will distrort parent metal

• can be set up portable 

cons 

• high level of skill

• time consuming 

Brazing method

1. material that is joined needs to be degreased 

2. clamped together 

3. flux is applied so material doesnt oxidise 

4. join is heated, brazing rod is applied to the join and will run flowing along the join 

Brazing mateirals, applications 

• used to join dissimplar metals 

• low carbon steel bar/tube 

• aluminium 

• copper

• nickel

Brazing pros and cons

Pros

• used in school

• low skill

cons 

• brazing is more sustainable than welding 

• high setup cost 

• none permanent 

Screws method 

1. clearence hole is drilled 

2. followed by a pilot hole

3. then a counterisnk screwdriver or drill driver is used to secure the screw

Screw pros and cons

Pros

• low skill

• can be assembled anywhere 

• can be disassembled after sue 

cons

• semi permanent

Nuts, bolts and washers method

1. clearence hole drilled

2. bolt slits in the clearence hole

3. washer applied for a tighter join and the load is spreaded creating less damage

4. marks on the joining material

Nuts, bolts pros and cons

• used for permanent joining

pros

• great access for repair

cons

• not as secure or as effective

Rivets method

1. clearance holes are drilled, rivets is inserted and hammerd to secure and tighten

2. pop rivets can be used from a gun

• thin material that overlap, stainless steel, duralium, aluminium

• can join material that cannot be accessed from the other side

Press joining method

1. a punch and a die are used for this process

2. compression is used to press one sheet material into the die with the other sheet material 

• materials that are ductile 

• can be high setup for the punc and die

• little skill

• weak join

Annealing

Annealing works on a metal when it has been ‘work hardened’ which makes the materials easier to work with and more ductile

Annealing method

1. Heat until just below upper critical point

2. Soak at that temperature, duration depends on size of part

3. Allow it to cool SLOWLY, usually in furnace. The more slowly the metal is cooled, the more malleable it becomes

• Effect on Metal Properties: ​

  • Reduces hardness​

  • Increases Malleability​

  • Increases Ductility​

  • Less Brittle

• Effect on Grain Structure: Relieves internal stresses and makes it easier to work

Normalising 

Normalising is carried out on steel that has been worked hardened to obtain a uniform structure throughout the metal.

normalising method

1. Worked hardened component.

2. Heat until just above upper critical point

3. Soak to attain a uniform temperature throughout the part

4. Allow it to cool NATURALLY IN AIR

• Effect on Metal Properties: ​

  • Tough​

  • Some Ductility​

  • Less Brittle

• Effect on Grain Structure:Relieves internal stresses but doesn’t allow grains to grow

Hardening

with medium high carbon steel, they are heated and then held at a certain temperature for a given time, then quenched in water, oil or salt water baths. ​

​

The process increases the hardness but also increases the brittleness ​

Tempering Process

tempering the metal, the internal stresses can be relieved and the part becomes tough and is less likely to shatter. ​

1. Clean the metal

2. Heat between 230-300 degrees depending on the property required

3. QUENCH (cool rapidly) ​in water.

Case Hardening

produces a outer surface which has improved hardness and resistance to surface indentation, while the inner core of the metal retains the original properties. ​

Carbursing

this changes the chemical composition of the surface of low carbon steel so it can absorb more carbon to increase the surface hardness. ​

Paints Method

1. Spray or brush application on lower cost materials that provides a barrier against corrosion or decay.

2. A undercoat or primer should be used for a high-quality finish​

3. Each layer can take up to 24 hours to cure ​

4. VOC is harmful when applying to safety considerations must be considered. ​

5. Well-ventilated areas and mask is required for application

Paints features and materials.

Any metals but mostly low carbon steel and low cost products.​

Product examples:​

• Window frames​

• Bike frames​

• DIY projects​

• Roofing sheets

Paint pros and cons

Pro: ​

• Primarily used on low-cost materials such as low carbon steel. ​

Cons:​

• Metals must be degreased before paint is applied

Sealants

Sealant is a polymer-based and protects polished surfaces from corrosion ​

Sealants features

used in the automotive industry as an additional bodywork barrier against the external elements.

Sealants pros and cons

Pros:​

• Provides a smooth finish​

• Increases toughness to the material ​

Cons:​

• Mostly used as an additional barrier on top of paint and rarely used alone.

Preservatives

For medium to long term protection for metal surfaces. ​

Apply with a cloth by wiping, spraying or immersion of the product.

Preservatives features

Often used for molds or dies for preventing fingerprints showing on surfaces ​

Minor atmospheric corrosion or condensation builds up.

Preservatives pros and cons

Pros:​

• Easy to apply and no specific specialist tooling is required

Anodising

Similar to electroplating.

Electric current passes through a sulphuric acid electrolyte solution from the part to be treated to a negative cathode. ​

Current flows from the part to be treated (positive anode) to the negative cathode.

Current flows from the part to be treated (positive anode) to the aluminium oxide layer builds up on the treated part, producing the anodised finish. ​

5 steps ​

1. Clean the metal ​

2. Etch the surface off the metal ​

3. Anodise ​

4. Colouring​

5. Sealing

Anodising features

Improves hardness to the outer casing of;​

• Torches ​

• External hard drive covers​

• Casing for mobile phones ​

• Casing for digital music players​

​

Magnesium and titanium can be anodised but most commonly used for aluminium only.

Anodising pros and cons

Pros: ​

• Enhances the natural oxide layer, giving increased hardness and toughness​

• Products are finished with a clear lacquer to seal the finish and provide further protection against surface scratches.​

• Increases electrical insulation ​

• Improve lubrication​

Cons: ​

• Requires sulphuric acid electrolyte solution bath for the process and a electric current​

• Specific process for aluminium

Electroplating method

1. Product and donor material placed in a container with electrolyte solution. ​

2. The donor material is usually more expensive or with enhanced properties than the material to be electroplated. ​

3. The negatively charged material is the one to be plated meaning the positively charged in the donor material. ​

4. When the current is turned on the positive electrode is attracted to the negative material and therefore coats with the new improved properties and appearance ​

5. Direct current is applied to the product attracts the donor metal and the product is electroplated. ​

6. Like anodising but for other lower cost metals

Electroplating features

A protective layer that enhances the aesthetic properties ​

Materials include: ​

• Gold​

• Silver​

• Copper​

• Zinc​

• tin​

Electroplating pros and cons

Pros:​

• Used on low cost materials​

• Makes the surface more hard wearing ​

• Provides colour ​

Cons:​

• Needs container with electrolyte solution and electric current​

• Cost​

• Environmental impact​

• Limited thickness ​

• Complexity of process

Powder Coating method

1. The product to be coated is negatively charged. ​

2. Polymer powder (positively charged) is then sprayed through an airgun and it is attracted onto the product. ​

3. Product is then baked in a oven; the heats melts the powder over the product to give an even coating.

Powder coating features

Protection from corrosion and improves aesthetics; ​

• Washing machines​

• Fridges​

• Dishwasher​

• Gates​

• Fencing​

• Railings​

Powder coating pros and cons

Pros:​

• More hard wearing than paint​

• Provides colour​

• Good for low cost metal such as mild steel​

Cons:​

• Relatively simple process but needs electrical charge and oven of appropriate size for the product

galvanisation method

1. Metal is coated by dipping the product into a tank of molten zinc at approximately 460 degrees C​

2. Zinc carbonate forms when zinc is exposed​ to air and water

3. this layer is extremely ​resistant to corrosion and very tough

4. There are two methods to galvanise: ​
   electroplating and hot dipping​

Galvanising dipping method is better for larger pieces that are thicker materials than electroplating would be

galvanisation features

Used for mostly mild steel;​

• Beams ​

• Gates​

• Animal pens ​

• First protective layer of mild steel for car bodies ​

​Uses zinc to protect the surface

galvanisation pros and cons

Pros:​

• Provides a barrier against corrosion ​

Cons:​

• Prior to the dipping process the material must be cleaned and degreased​

• Needs a dipping bath, rather large setup

Cathodic protection

1. Impressed current: the part to be protected in connected to an electrical power supply. The current flows from the inert electrode through any liquid to the component to be protected. ​

2. Sacrificial anodes; this is more basic method because it uses a sacrificial metal to protect the metal product of value. ​

3. An electrochemically active mental is joined to the less active metal to provide resistance to corrosion. ​

4. On large structures, the sacrificial metal is monitored for signs of corrosion and replaced when the base metal is no longer protected.