Lesson 7 - Aluminium

Question 1

Is aluminiums melting point low or high?

Answer 1

Low, at about 660 C which limits its applications at high temperatures

Question 2

What are the two main types of alloy strengthening/hardening?

Answer 2

Solution hardening and Age hardening

Question 3

What is solution hardening?

Answer 3

• Pure metals can be hardened when alloying element is added. A single phase solid solution (alloy) is formed by:
  
  • heating the metal to a temperature that can
    maximally take the alloying elements
  • holding it sufficiently long to let one or more
    alloying elements enter into the solid solution
  • then cooling it rapidly to hold these elements
    in the solid solution

Question 4

How does solution hardening affect aluminium alloys?

Answer 4

• It adds certain alloying elements to aluminium crystal in a way that results in increased strength of the Al-alloy
• These added elements take the form of substitutional solid solutions
• As a result, deformation processes are made more difficult which is known as solid solution hardening
• In practice the quantities of alloying elements in Al crystal is not high, depending on both temperature and alloy element

Question 5

What are some examples of Al-alloys?

Answer 5

Al-copper
Al-manganese
Al-silicon
Al-magnesium

Question 6

What is Age hardening of alloys?

Answer 6

• A heat treatment process used to increase strength by producing precipitates of the alloying material within the metal structure.
• In the age hardening process, soluble constituents are released in a controlled way either naturally (room temp) or artificially (higher temps) and not all of them can join the crystal structure to form solid solution.
• Therefore come extra constituents will reside between the grains, to provide more strength and hardness

Question 7

How does Age Hardening work on Al-alloys?

Answer 7

• It is a relatively low temp heat treatment technique that causes precipitation of constituents or phases of the alloying material within from the supersaturated solid solution
• General sequence of the process:
  1. Dissolve more alloy element by increasing temps
  2. Rapid-cooling to produce supersaturated solid solution
  3. Hold it at selected temp for a designed time
• It is used to increase strength and mechanical properties of some Al-alloys (e.g. Al-Cu alloys)

Question 8

What are some Age hardened Al-alloys commonly used in construction? And what are they used for?

Answer 8

Al-Mg-Si : used for low loading elements such as glazing bars and curtain walling
Al-Mg-Si-Mn : can be used in structures and also for scaffolding tubes.

Question 9

Grain refinement can also increase the strength of aluminium and its alloys. How does it work?

Answer 9

• Innoculation is a process to use secondary particulate to refine and control the grain structure of solidified metals in many Liquid Metal processing operations
• Very small particles of nucleating agents are added to the molten aluminium to promote the formation of solid aluminium grains (reduce grain size).
• Therefore (as-cast) grain size in some alloy scan be reduced from 1mm to <100 micro metres.

Question 10

downsides to Al-alloys?

Answer 10

• Under extreme conditions, they cannot be used
• Both hardening methods can be applied to other metal alloys to make them more applicable to extreme conditions
• Fore example, for really high temperature applications, titanium and nickel may be used as the main component of the system
• These are of use in aerospace and turbine applications