Light Alloys

Question 1

How are the properties of alloys edited when casting?

Answer 1

Control of the cooling rate and use of grain refiners

Question 2

How are the properties of wrought alloys edited?

Answer 2

Heat treatment and Age(precipitate hardening)

Question 3

What properties of the precipitate affects the properties of the alloy?

Answer 3

The precipitate’s distribution, sizing and coherency

Question 4

Main requirements for materials of a petrol piston?

Answer 4

1. Low thermal expansion - to prevent collision with casing
2. Good Wear resistance - High wear at ring grooves and piston pin hole
3. Good creep and fatigue strength - piston working at high T and high loading environment
4. High specific strength - minimize weight to maximize fuel efficiency

Question 5

Function of a piston?

Answer 5

Forms a combustion chamber by sealing hot gases

Question 6

Piston Constraints?

Answer 6

High operating temperature, Low thermal expansion, no seizure, high fatigue at high temperature, low cost

Question 7

Piston Objectives?

Answer 7

Transmit combustion power
gas exchange
transmit heat from piston head to wall
low cost
light and efficient

Question 8

What do these alloy suffixes represent?

• Tx
• F
• O
• Hx

Answer 8

Tx - precipitation hardened where x is specific treatment
F - fabricated
O - annealed
Hx - strain hardened, x denotes extent of hardening

Question 9

Why use Al-Si alloys?

Answer 9

• Si has low density -> improves str to weight ration
• Large solubility range -> potential for solid solution or precipitation hardening
• SI has low thermal expansion
• Improves alloy hardness -> reduces piston seizure

Question 10

How is Aluminium alloys generally processed?

Answer 10

Typically by casting

Question 11

What element is added to Al alloy and what does it do to the microstructure?

Answer 11

Phosphorous is added and it decreases the primary Si grain size. This improves the fracture toughness.

Question 12

How does the spacing of precipitates affect strength

Answer 12

Inversely proportion str = 1/s

Question 13

What happens when the Al alloy is overaged?

Answer 13

Al alloy will have coarsened precipitates causing a reduction of strength.

Question 14

What intermetallic elements are added to form intermetallic compounds with Al, how does it affect the mechanical properties?

Answer 14

Fe, Cr, or Ni. It strengthens the alloy at higher temperatures. However too much intermetallic compounds causes a coarse, brittle structure. Heaver elements adds weight.

Question 15

What is one disadvantage of casting Al alloys that lead us to use alternative methods of processing?

Answer 15

Thick sections of casting cool slower leading the coarser and weaker microstructure

Question 16

What are the alternate processing methods of Al alloy? Briefly explain each method

Answer 16

Forging and powder metallurgy

1. Forging is limited by the ductility of the alloy. Forged components rarely near net shape hence requires finishing step
2. Molten alloy sprayed into powders with high cooling rate. Rapidly cooled alloy contains finely dispersed intermetallics. Powder is then cold compacted in a die to produce green cylinder compacts. Then sintered, then hot forged.

Question 17

What are some further improvements that could be made to alloys?

Answer 17

Anodization - produces a hard and wear resistant oxide layer

Insert wear resistant material - cast iron with high hardness and thermal expansion coefficient or fibres of alumina.

Question 18

Why are magnesium alloys not used?

Answer 18

1. Poor workability at room temperature
2. Poor corrosion resistance
3. High chemical reactivity
4. High process cost
5. AT low temperature, drop in mechanical and creep properties.