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Materials Selection and Processing > Metals > Flashcards

Metals Flashcards

1
Q

Describe Sand Casting

A
  • Mould material (sand/clay) is pressed into pattern and removed
  • Metal poured into mould and solidifies
  • Mould is broken away to leave metal part
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2
Q

Advantages of Casting

A
  • Can produce intricate shapes, esp. internal cavities
  • Can achieve uniform properties with good control
  • Most metals can be cast
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3
Q

Sand Casting Advantages and Disadvantages

A

Adv : Very cheap

Disadv: low dimensional accuracy and surface finish, low production rate

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4
Q

Describe Investment Casting

A
  • Wax or polymer pressed into pattern and removed
  • Metal poured into mould and solidifies
  • Mould is melted away to leave metal part
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5
Q

Investment Casting Advantages and Disadvantages

A

Adv: Better surface finish than sand casting, higher production rate
Disadv: Can only produce small parts, relatively expensive

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6
Q

Describe Permanent (Die) Casting

A
  • Liquid metal pushed into permanent mould with external pressure
  • Metal solidified and removed from mould
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7
Q

Die Casting Advantages and Disadvantages

A

Adv: Best finish and tolerances, high production rate
Disadv: Very expensive

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8
Q

Describe Rolling Process

A
  • Ingot formed by casting
  • Ingot is heated and pressed between rollers
  • Forms plate or sheet metal
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9
Q

Defects caused by Rolling

A

Surface - inclusions, rust, roll markings

Structural - bumps, cracks, holes, splitting

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10
Q

Describe Extrusion Process

A
  • Billet is forced through a die with external pressure

- Produces part with uniform cross-section

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11
Q

Defects caused by extrusion

A
  • Piping: impurities diffuse to centre of cross-section

- discard: not all of the billet is extruded, leaving excess

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12
Q

Extrusion force equation

A

F = a + b * ln(Ao/Af)
Where a and b are material constants
Ao = Original area
Af = Area after extruding

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13
Q

Upsetting force equation (in forging process)

A 
F= sigma0 + (2*mu*r)/(3*h)
Where sigma0 = material constant
mu = frictional force
r = initial radius of workpiece
h = final height of workpiece
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14
Q

Describe Open Die Forging

A
  • Ingot deformed between 2 platens (usually hydraulic press)

- Often an initial step for other forging operations

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15
Q

Describe Impression Die Forging

A
  • Material placed between 2 dies
  • Dies pressed together to form shape
  • Important to use excess material so that cavity is filled
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16
Q

Describe Closed Die Forging

A
  • Material placed between 2 dies
  • Dies sealed together
  • Volume of material must be exactly volume of part as there is no space for excess material to go
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17
Q

Describe Isothermal Forging

A
  • Heated workpiece pressed between heated dies

- Gives much better properties and tolerances, but is very expensive

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18
Q

Describe Additive Manufacturing

A

-Powder laid layer-upon-layer and heated to consolidate

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19
Q

Advantages of Additive Manufacturing

A
  • Produces near-net shape
  • Good buy to fly ratio
  • Easy to iterate designs
  • Low lead times
20
Q

Give a disadvantage of using a Laser system in additive manufacturing and suggest an alternative method

A

Laser system causes residual stresses due to the difference in temperature between the deposited material and the surrounding powder
Electron beam preheats the layer before laying the powder

21
Q

Describe Metal Injection Moulding

A
  • Metal and binder are mixed and ground up
  • Powder is injected into the mould (NOT MELTED)
  • Binder is removed by heating
  • Compacted part is sintered to densify
22
Q

Advantages and Disadvantages of Metal Injection Moulding

A

Adv: Low energy footprint

Disadv :Part will reduce in size during sintering which can lead to dimensional errors

23
Q

Advantages and Disadvantages of Powder Metallurgy

A

Adv: Produces less waste which is more efficient for expensive alloys, can produce complex geometries
Disadv: Producing metal powder is expensive and time consuming, and not all metals can be powdered, powder must be fine and spherical

24
Q

Describe Hot Isostatic Pressing

A
  • Capsule is filled with powder and gas is removed (to prevent oxidation)
  • Soaked in high pressure and temperature for several hours
  • Capsule dropped in acid to remove mould, leaving metal part
  • Can only produce small parts, very expensive
25
Q

What are the light metals?

A

Beryllium, Magnesium, Aluminium, Titanium

26
Q

Advantages of Aluminium

A
  • Good corrosion resistance
  • Low density
  • Easy to process
  • Recyclable
27
Q

Disadvantages of Aluminium

A
  • Low modulus

- Low melting point

28
Q

Advantages of Titanium

A
  • High corrosion resistance
  • Lighter that steel
  • Higher modulus that aluminium
  • Compatible with carbon fibre composites
29
Q

Disadvantages of Titanium

A
  • Very expensive due to melting and extraction processes

- Hard to machine

30
Q

What are the Non-Heat Treatable Aluminium Alloys

A

1xxx - 99% Al
3xxx - 1% Mn
5xxx - 6% Mg

31
Q

What are the Heat Treatable Aluminium Alloys

A

2xxx - Al-Cu
6xxx - Al-Mg-S
7xxx - Al-Zi

32
Q

What series of Aluminium Alloy would you use for the upper and lower skin of a wing and why?

A

Upper - 7xxx - very strong in compression

Lower - 2xxx - damage resistant, not damaged by spray from runway

33
Q

Describe Homogenization Treatment

A
  • Cast ingots heated for varying lengths of time
  • Reduces segregation in part (composition differences due to diffusion in the solidification process)
  • Vital pre-hot working step
34
Q

Describe the Ageing process

A
  • Alloy is first heated to redissolve alloying elements at high temperature for a short time to avoid melting or contamination
  • Alloy is quenched to form a supersaturated solution (must be controlled)
  • Alloy is soaked at 150C for several hours so alloying elements precipitate within grains, increasing strength
35
Q

How do Heat Treatable alloys derive their strength?

A

Precipitation Hardening

36
Q

How do non-Heat Treatable alloys derive their strength?

A

Grain size reduction, solid solution hardening, cold working

37
Q

What defects are caused by quenching too fast or too slow?

A

Too fast: leads to cracking or residual stresses

Too slow: Solution is not saturated, precipitate forms at grain boundaries which embrittles material

38
Q

How does cold working strengthen a metal?

A

Dislocations are formed as the material is deformed, which entangle with other dislocations and reduce dislocation movement

39
Q

How does grain refinement strengthen a metal?

A

Dislocation movement is obstructed grain boundaries, so increasing the number of grain boundaries decreases dislocation movement
Smaller grains also improve ductility and toughness

40
Q

How does precipitation strengthening strengthen a metal?

A

Dislocations are obstructed by precipitates in the metal, and can only move past them by shearing the precipitate or by the bow mechanism

41
Q

How does precipitation size and the distance between particles affect strength for the shearing and bow mechanism?

A

Shear mechanism: str = K * (precip size)^0.5

Bow mechanism: str = K/(dist between particles

42
Q

What is the Hall-Petch equation?

A

Yeild str = sigma0 + K*d^-0.5
where sigma0 and K are constants
d = average grain diameter

43
Q

How does solid solution strengthening strengthen a metal?

A

Alloying elements diffuse towards dislocations and ease strains around them due to the atomic size difference between the solute and solvent atoms

44
Q

What are the advantages of alpha titanium alloys?

A
  • Lower density
  • Higher creep strength
  • Improved weldability
45
Q

What are the advantages of beta titanium alloys?

A
  • Better heat treatment response
  • Higher short-time strength
  • Improved fabricability
46
Q

Why is titanium welding performed in an argon environment?

A

Oxygen can react with titanium to produce a very brittle alpha phase, so argon is used as it is non-reactive

47
Q

Which property of titanium is improved by adding chromium?

A

Burn resistance

Materials Selection and Processing (4 decks)

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