Materials Engineering (Lectures 5-8)

Question 1

Name four important factors for safety performance in a structure

Answer 1

• Shape of components
• Thickness of material
• Properties of material
• Spotwelds and connections

Question 2

Steel production process

Answer 2

Iron ore along with additives like limestone, and oven by products like coke, are placed into a blast furnace which produces molten iron. The molten iron can then be transferred into a basic oxygen furnance (BOF) which produces the molten steel.

The molten steel then goes through a slab caster, producing solid steel slabs. These slabs are then hot rolled; going through edging and roughing mills before a finishing mill. The steel can then be cold rolled as well. The rolling process reduces the thickness of the steel by 60-80%.

The steel might then be coated. 95% of automotive steel is coated, mostly using hot dip galvanising with zinc to protect the body from corrosion.

Question 3

The mechanical properties of steel are majorly determined by:

Answer 3

• Steel chemistry

- Processing route

Question 4

When a steel is cooled very quickly, what structure is formed?

Answer 4

Martensite.

Martensite has a very hard micro structure. It is therefore strong, but not ductile, making it difficult to shape which is necessary for automotive manufacturing.

Question 5

List three metal alternatives to steel for automotive body material

Answer 5

• Titanium
• Magnesium
• Aluminium

Question 6

Why might it be desirable to lightweight a vehicle?

Answer 6

• Improved performance: Better handling, braking, faster acceleration
• Improved fuel consumption
• Lower CO2 emissions

Question 7

Pros and cons of aluminium vs steel.

Answer 7

Pros: High strength to weight ratio, good formability, high corrosion resistance, easy to work with, easy to repair.

Cons: Difficult to weld, cost, thermal expansion coefficient to steel.

Question 8

Pros and cons of FRPs

Answer 8

Pros: Complex shapes can be readily achieved
High specific stiffness, high specific strength, good fatigue strength, high impact resistance.

Cons: VERY high cost (up to x40 steel per kg), specialist design and manufacturing skills, limited temperature resistance.

Question 9

State a few thermosetting processes

Answer 9

• Wet (hand) lay up
• Vac-bagging
• Autoclave
• Pre-preg
• Resin transfer moulding

Question 10

How are thermoplastics commonly produced?

Answer 10

Injection moulding

Question 11

Describe wet (hand) lay up and its pros and cons.

Answer 11

Woven fabrics are laid over a mould and impregnated with resin by hand. A release agent is applied to mould before the process begins. The fibres are impregnated using a roller.

Pros: Lowe investment, complex shapes can be achieved, cure occurs at room temperature.

Cons: Labour intensive, potential safety issues, violate organic compounds (resin).

Question 12

What is vac-bagging?

Answer 12

Vac bagging is typically used after hand lay up to provide better consolidation and lower porosity in the polymer. The pressure for vac bagging is limited to 1 bar.

Question 13

Describe the autoclave process and its pros and cons.

Answer 13

The autoclave process involves placing a vac-bag into a pressure chamber. The chamber goes through heat and pressure cycles ranging from 5 to 10 bar. This allows the resin to flow and cure. The temperature and pressures are controlled to achieve the desired mechanical properties.

Pros: Excellent mechanical properties and surface finish, high fibre factions.

Cons: High investment and cycle times, component size is limited.

Question 14

Describe the pre-preg process and its pros and cons.

Answer 14

Release agent is applied to a mould, along with an automatic or manual layout of pre-pregs. Any entrapped air is removed using rollers. On top of the layer a bleeder is used to remove excess moisture and resin. Vaccum plastic bag and rubber sealant is added (vac-bagging). The polymer might then be cured using the autoclave process.

Pros: Complex shapes achievable, high fibre factions, low tooling cost but high investment cost.

Cons: Expensive production process, labour intensive, expensive raw materials (pre-preg sheets), long cycle times, requires a clean room.

Question 15

What is resin transfer moulding? What are its advantages and disadvantages?

Answer 15

Resin transfer moulding is the transfer of liquid resin into a closed mould containing the fibres. A release agent is applied to the mould and fibres are placed in. The mould is heated and the resin injecting in. The heating allows the resin to cure. Water or air is used to cool the part. Curing takes 6-30 mins. 2-10 bar.

Pros: Low/medium mould pressure: medium tooling cost, complex geometries, medium-high volume and automated process, accurate fibre placcement with preforms, no harmful VOCs due to closed moulds, good surface quality.

Cons: Tool design is complex and requires experience - resin gating etc. Long cycle times.

Question 16

What is injection moulding? What are its advantages and disadvantages?

Answer 16

TP pellets or powder is injected into a mould. The mould cools the part. The process is high speed and highly automated.

Disadvantage: No control of fibre direction, short fibres.

Question 17

What are the 5 steps of the bayer process in aluminium production (step I)?

Answer 17

1) Mixing - Bauxite is crushed and pumped into huge digesters, alongside caustic soda.
2) Digestion - The caustic soda dissolves the alumina due to the high pressure and heat. Sodium Aluminate is formed.
3) Clarification - Solid impurities such as iron oxide drop to the bottom of the settling tank, where they are pumped into a disposal pond.
4) Precipitation - The sodium aluminate is further cooled, then agitated and seeded with aluminium hydroxide crystals. These form larger crystals, which gradually settle out of the solution.
5) Calcination - The aluminium hydroxide crystals are roasted at over 1000 degrees celsius to remove the water and form pure alumina.

Question 18

What is extrusion?

Answer 18

A process by which a block of metal is reduced in cross section by forcing it to flow through a die orifice under high pressure.

Question 19

Pros and cons of magnesium as an alternative metal

Answer 19

Pros: Even lighter than aluminium (30%), 60% lighter than steel, better recyclability than aluminium, abundant, NVH reduction, good combination of strength, stiffness, dent resistance, thermal conductivity etc.

Cons: Primary production localisation in China using highly polluting pidgeon process, limited formability, too expensive sheet material and semi finished products for extrusion. Mg is enigmatic, lack of understanding.

Question 20

Pros and cons of titanium as an alternative metal

Answer 20

would be an ideal material for many automotive applications, however has exceptionally high production and processing costs.

Question 21

What is a composite?

Answer 21

A material made of of at least two constituent materials, which are bonded together
along the interface in the composite, each of which originates from a separate ingredient
material which pre-exists the composite.