Processing of Metal Matrix Composites

Question 1

What are the main feature of processing metals matrix?

Answer 1

• Metals have higher melting points.
• When molten, quite low viscosity.
• Difficult to get wetting of ceramic fibres.
• Chemical reactions between fibres and matrix common.
• Secondary processing (machining, forging, press-forming) harder due to increase strength and stiffness.

Question 2

What is solid state processing?

Answer 2

• Relies on “diffusion bonding”.
• At high temperatures, but lower than melting point, diffusion rate increases.
• When surfaces brought together, diffusion across boundary joins the materials.

Question 3

How does diffusion bonding with foil work?

Answer 3

• Unidirectional fibres, or fibre mat.
• Sandwiched between metal foils.
• Plies are stacked up to desired thickness.
• Heated and pressed to from consolidated sheet.

Question 4

What are the advantages of diffusion bonding with foils?

Answer 4

• Lower temperature needed.
• Less chance of reactions.
• Especially with Ti alloys.

Question 5

What are the disadvantages of diffusion bonding with foils?

Answer 5

• Expensive
• Limited to fairly simple shapes
• Sheets
• Tubes

Question 6

Example of diffusion bonding with foils?

Answer 6

• Boron/Aluminium struts on space shuttle formed from single layer foils, wrapped on mandrel and HIP’d.
• Ti end fittings diffusion bonded on.

Question 7

What is powder metallurgy?

Answer 7

• Relies on diffusion bonding also.
• In this case from particle to particle.
• Short fibres or whiskers and metal powder mixed.
• Pressed into shape at elevated temperature.

Question 8

How can powder metallurgy be maintained?

Answer 8

• Inert atmosphere needed to prevent oxidation and explosion.
• Pressing can be done as part of rolling or forging, or simple pressing followed by secondary shaping.
• Limited to short fibres and low volume fraction (typically 30 % max).

Question 9

How does Liquid state processing work?

Answer 9

• Requires higher temperature.
• Fibres may need coating for protection and wetting.
  - C reacts with Al to from AL(4)C(3) (brittle and soluble). Ni coating applied by CVD.
• Possible to use continuous fibres and higher fibre volume fraction.

Question 10

What is conventional casting?

Answer 10

• Fibres (short) stirred into molten metal and cast.
• Problem of different densities can lead to setting or floating.
• Rheocasting can be used where the metal is partly solid, partly liquid.
• More fibre breakage.

Question 11

How does squeeze casting work?

Answer 11

• Preform used
• Liquid metal pushed through preform with a ram.
• Can now use continuous fibres.

has 3 component: RAM, preform, dye

add molten metal -> excess machined away

Question 12

What is squeeze casting limited to?

Answer 12

• Size of component limited by press size.
• Reinforcement generally limited to 30% (vol)
  With more compact preforms, it is not possible to infiltrate with molten metal.

Question 13

How does gas pressure infiltration works?

Answer 13

• Preform placed in mould.
• Molten metal pushed in by gas pressure,

Question 14

What are the advantages of gas pressure infiltration?

Answer 14

• Size not limited.
• Infiltration times are shorter
  Reaction between fibre and matrix
  less significant
• Fibre breakage is less of a problem.
• Reasonable properties, but some voids remains.

Question 15

What is spray deposition and characteristics?

Answer 15

• Molten metal atomised.
• Sprayed with short fibres onto substrate.
• Reduces time for chemical reactions.
• Quite slow and quite expensive.