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Manufacturing And Materials > Manufacturing With Metals > Flashcards

Manufacturing With Metals Flashcards

1
Q

Why is the surface quality an important consideration for a manufacturing process?

A

Most failure begins at the surface of the component

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2
Q

List two ways a surface can be altered after manufacture.

A

Coating (corrosion/wear-resistant surfaces)

Altering surface structure (shot peening, carburising/nitriding etc)

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3
Q

What types of coatings can be applied to a surface after manufacture?

A

Corrosion resistant - paint, varnishes, metal (electrochemical - galvanising)

Wear resistant - CVD, PVD

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4
Q

How can the surface quality of a component be changed after manufacture?

A

Physically - shot peening

Chemically - carburising/nitriding (case hardening with residual compressive stress, improves fatigue/wear resistance)

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5
Q

How is surface roughness defined?

A

Ra, the sum of areas above and below the mean line divided by the length of the measured profile (L)

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6
Q

What are the two classifications of shape?

A

2D (continuous) - profile is constant across the length of the object (pipe, rod etc)

3D - everything else.

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7
Q

How is a hollow object defined?

A

Has significant cavities (bowls, containers etc)

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8
Q

How is a solid object defined?

A

No significant cavities

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9
Q

What are the six measures of process performance?

A

Cycle time - time to process one item

Quality - surface condition, dimensional accuracy, integrity (pores, voids), undesired anisotropy

Reliability - reproducibility or consistency

Flexibility - adaptable for more than one part

Material Utilisation - wastage

Operating costs - capital, tooling, labour, setup, running

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10
Q

Briefly summarise how a manufacturing process is selected.

A

1) Filter out process that cannot produce shape/process right materials
2) Find best using performance ratings
3) Subtract 3 from Process Performance Ratings, Combined Score = SUM(weightings) x (PPR-3)
4) Select process with highest CS

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11
Q

Briefly define casting

A

Material flows into a mould

Heat is transferred from the mould and the material solidifies in the shape of the mould

Solid product is removed from mould

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12
Q

What mould material is used to cast a metal with a low melting point?

A

A high melting point metal (called a die)

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13
Q

What material is used to cast a high melting point metal?

A

Ceramic moulds.

Tend to be expendable, remade for each cast

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14
Q

What are the key mould design considerations for casting?

A

Casting material

Component size

Component shape (complex shapes require mould made of several pieces, hollow components need core)

Quality

Quantity

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15
Q

What are the two basic classifications of casting processes?

A

Permanent Pattern

Permanent Mould

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16
Q

What are the two main types of permanent pattern casting?

A

Sand casting

Shell moulding

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17
Q

Briefly describe the sand casting process

A

1) Sand packed around moulding board (pattern)
2) Moulding board removes, leaving mould (drag)
3) If hollow, a core is inserted. Cope is fitted to the drag
4) Molten metal poured into sprue
5) When cooled, mould and core shaken off
6) Excess material (in sprue/risers) removed

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18
Q

In sand casting, where is the sprue and riser system?

A

In the cope

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19
Q

What is the sand mould made of?

A

Ceramic material with a binder

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20
Q

How can the sand mould be cured?

A

Thermosetting resin where reactants are combined and curing begins

Curing by heating (heat cured binder system)

Curing by passing a catalyst gas through the mixture (cold box)

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21
Q

Describe in more detail the heat-cured binder system

A

Liquid thermosetting binder and catalyst mixed with dry sand

When heated, catalyst releases acid that induced rapid cure

Pattern is removed

Mould post cured in an oven

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22
Q

What are the general advantages of sand casting?

A

Low cost mould material

Can cast high melting point alloys

Wide range of component sizes

Economical for low and large numbers of components

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23
Q

What are the general disadvantages of sand casting?

A

Poor dimensional accuracy/surface texture

Sand easily deforms

Final accuracy/surface finish achieved with machining after casting

Labour intensive

Slow

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24
Q

List the ways in which sand casting can be improved

A

Use a precision metallic pattern

Use fine sands or coatings to improve surface finish

Use thermally stable sand (Zircon, ZiSiO4)

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25
Q

How does shell moulding differ from sand casting?

A

Mould is a thin walled shell, not a cavity in a block

Shells are permeable to air

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26
Q

Briefly describe the process to manufacture using shell moulding

A

1) Pattern made
2) Pattern sprayed with parting agent, heated and placed into dump box
3) Sand/resin mixture dropped onto heated pattern. Resin next to mould cures, leaving a shell
4) Excess sand tipped away
5) Shell removed
6) Shell halves combined to make full mould
7) Mould placed in mould box packed with pellets to support it
8) Molten metal poured into mould and left to cool
9) Pellets removed and casting removed

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27
Q

What are the general advantages to shell casting?

A

Higher repeatability

Allows for intricate details

Less machining post casting

Uses less sand

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28
Q

What is ‘permanent pattern’ casting?

A

Pattern of final product used to make many expendable moulds

Mould destroyed after each casting

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29
Q

What is ‘permanent mould’ casting?

A

Same mould used to make large number of products

Mould opens to release component rather than by being destroyed

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30
Q

What is gravity die casting?

A

Gravity causes flow of molten liquid to enter cast iron or tool steel mould

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31
Q

What characteristics does a gravity die mould have?

A

Vertical split through die cavity

Running, feeding, venting systems in same plane

Die has locating pins, clamps and ejection systems built in

May need cores for hollow parts

May need to be made of several parts for more complex geometry

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32
Q

Briefly describe the gravity die casting process

A

1) Die preheated to 300-400 degrees (maintained)
2) Die coated with a dressing/lubricant
3) Molten metal slowly poured to prevent turbulence
4) Component must be ejected as soon as possible to allow cooling contraction

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33
Q

What are the general advantages of gravity die casting?

A

Close dimensional tolerances

Superior surface finish (compared to sand casting)

Faster cooling rates leads to finer microstructure and improved mechanical properties (highly conductive die)

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34
Q

What are the general disadvantages of gravity die casting?

A

Low melting point alloys only

High tooling costs

Limitation on shape

Coatings necessary

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35
Q

What generic components is gravity die casting best suited for?

A

High production volume

Uniform wall thickness

Limited undercuts/internal coring

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36
Q

Generally describe pressure die casting

A

Metal injected into die at high velocity

Solidifies under externally applied pressure

Short filling times

Complex, thin-walled castings can be solidified quickly

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37
Q

What types of pressure die casting are available?

A

Hot chamber

Cold chamber

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38
Q

Briefly describe hot chamber pressure die casting

A

Reservoir of molten metal held above melting point

Metal injected through gooseneck into die by piston

New shot of metal pulled into cylinder when piston withdraws

Minimal exposure to air, turbulence and heat losses

Piston does contact molten metal so could lead to contamination

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39
Q

What types of alloy are generally used for hot chamber pressure die casting?

A

Zinc

Magnesium

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40
Q

Briefly describe cold chamber pressure die casting

A

Molten metal held in a holding furnace

Metal loaded into chamber via an aperture

Piston forces metal into die at high pressure

Operation completed in a few seconds, minimising contamination problems

Lower metal temperatures allowed by higher pressures

Dies sprayed with lubricant

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41
Q

What are the general advantages of pressure die casting?

A

High precision from die rigidity

Smooth surface finish

Can cast thin and intricate features

Suitable for components with high surface area/volume ratio

High production rate with automation

Economical for large quantities

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42
Q

Describe the general disadvantages of pressure die casting

A

Size of casting limited by available pressure

Limited to low melting point metals

Very expensive tooling

Long lead times (need to make die)

Turbulent filling causes internal porosity

Castings cannot be further machined (would remove non-porous skin)

Subsequent heat treatment would cause distortion (expanding gas bubbles)

Lack of pressure tightness from porosity

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43
Q

Briefly explain the lost wax pattern process

A

1) Pattern made from low melting point material (any cores located before wax injection)
2) Separate patterns may be arranged into a cluster around a gating/feeding system, creating a wax tree
3) Mould is built around the pattern with slurries or liquid refractories. Multiple coats applied
4) Mould hardens
5) Pattern is melted and removed (may be some stressed from differential thermal expansion)
6) Mould fired and metal poured in
7) Mould destroyed to remove casting

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44
Q

What are the general advantages of the lost wax process?

A

Allows great complexity

Can use any castable alloy

Close tolerances

Jointless mould (reduced machining cost)

Inexpensive mould

Can prototype

Reliable

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45
Q

What are the general disadvantages of the lost wax process?

A

Long production cycle

Single use mould

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46
Q

What sort of defects can exist in castings?

A

Porosity (bubbles)

Inclusions

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47
Q

What is homogenous nucleation?

A

When the equilibrium melting/freezing point is reached, some atoms momentarily cluster into embryos

Embryo will grow if its critical radius is greater than r* (at this point it is energetically favourable to grow)

Need undercooling of 20-30% of Tm

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48
Q

What is undercooling and why is it necessary?

A

When the melt is cooled below the melting point to initialise solidification

When solidification occurs, latent heat is released which causes a temperature rise , thus reducing the likelihood of solidification

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49
Q

What is heterogenous nucleation?

A

Instead of a spherical embryo freely floating in the melt, solidification begins at a solid boundary (on a catalyst or other surface) and forms a cap with the same critical radius r*

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50
Q

Why is heterogenous nucleation more likely to occur than homogenous nucleation?

A

Fewer atoms are required to form a cap and less undercooling is required (only a few degrees)

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51
Q

What makes a good nucleating agent?

A

The smaller the contact angle of the nucleating cap, the better the nucleating agent (fewer atoms required)

A good nucleating agent has a small interfacial energy between catalyst and nucleating solid.

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52
Q

How is a low interfacial energy between catalyst and nucleating solid achieved?

A

The nucleating agent has at least one crystal dimension similar to that of the solid being nucleated. (eg TiB2 used to nucleate Al castings)

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53
Q

Why is the nature of nucleation important for casting?

A

Affects grain size/shape that influence the mechanical properties of the casting

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54
Q

How does porosity arise in castings?

A

Evolution of dissolved gases (microporosity)

Inadequate liquid supply to compensate for contraction (macroporosity)

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55
Q

In more detail, describe how microporosity arises in castings

A

From air trapped in the metal when poured

Chemical reactions

Thermal dissociation of water vapour (leads to hydrogen absorption and embrittlement)

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56
Q

Why is it important to prevent gases being in a melt?

A

When the metal solidifies, gas solubility drops significantly, promoting gas evolution

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57
Q

How can microporosity be prevented when casting?

A

Venting and minimising turbulence to prevent air being trapped

Minimise moisture levels

Degassing (flushing with insoluble gas or vacuum degassing)

Make pore nucleating more difficult

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58
Q

When nucleating gas bubbles, which type of nucleation is more likely?

A

Heterogenous

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59
Q

What are good gas nucleating sites in a casting?

A

Poorly wetted inclusions

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60
Q

Is undercooling necessary for solidification?

A

Yes, the greater the undercooling, the faster the rate of solidification

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61
Q

How can undercooling be controlled?

A

By the rate at which latent heat of solidification is removed

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62
Q

Briefly describe the cooling process for a PURE metal

A

Temperature at solid-liquid interface is lower than the bulk of the liquid (heat removed through the mould)

Nucleation begins at the mould wall

Positive temperature gradient develops into liquid

A planar solidification front is stable with a positive temperature gradient and growth progresses

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63
Q

Why is a planar growth front stable with a positive temperature gradient?

A

Any instabilities the protrude into the liquid are advancing into a higher temperature region, so the growth rate at that location slows, allowing the rest of the growth front to catch up

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64
Q

Why is the growth front generally unstable in single phase alloys?

A

As nucleation occurs, the lower melting point constituent of the alloy is rejected into the melt, thus concentration of the melt changes.

This affects the liquid freezing temperature, increasing it above the melt temperature.

As a consequence, constitutional undercooling occurs and there is a negative temperature gradient.

Any disturbances in the growth front will grow faster due to this negative gradient.

Dendritic structures grow favourably in these conditions

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65
Q

What type of interface is formed with a high, positive temperature gradient?

A

Stable, planar interface

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66
Q

What effect does decreasing the temperature gradient have on the planar stability of the growth front?

A

Decreasing Gl increases undercooling, decreasing stability

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67
Q

What sort of structure results from a small amount of undercooling?

A

Cellular; small instabilities may form primary stems in a parallel array

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68
Q

What type of structure is formed with a high level of undercooling?

A

Dendritic; high instability of the growth front allows any disturbance to rapidly grow

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69
Q

What conditions are likely to cause constitutional undercooling?

A

High solidification rate (less time for diffusion to reduce concentration profile)

Low temperature gradient

Steep liquidus line, m, and lo value of distribution coefficient

High constitution of secondary alloying element

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70
Q

Explain the problems caused by dendritic growth

A

Generally leads to porosity

Very small interdendritic channels are difficult to feed during solidification shrinkage, high stress concentrations

Cause solidification cracking (hot tears)

Dendrites begin to interfere with each other at vf = 50%, causes higher melt viscosity (harder to fill mould)

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71
Q

What is the freezing range of alloys?

A

Range of temperature over which the metal fully solidifies (difference in temperature between solidus and liquidus line)

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72
Q

What problems are associated with a longer freezing range?

A

Alloys with a longer freezing range are more susceptible to constitutional undercooling

Have a longer semi-solid region, more prone to dendritic growth

More likely to have shrinkage porosity/cracking

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73
Q

Give some examples of popular, short freezing range alloys

A

Zn-4%Al

Al-11%Si

Cast Irons

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74
Q

What is microsegregation in castings?

A

In alloy systems with constitutional undercooling and dendritic growth, last liquid to solidify is in the interdendritic region.

High conc of lower melting point alloy

Inclusions are swept into these regions by the solidification front

Microsegregation is this varying composition between dendrites

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75
Q

What controls the grain size in a casting?

A

Number of nucleation sites

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76
Q

What controls the scale of dendritic growth in castings?

A

Solidification rate

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77
Q

What two features of a dendritic structure form barriers to dislocation motion?

A

Secondary phases in the interdendritic region

Grain boundaries

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78
Q

What effect on the dendrite structure does increasing the solidification rate have?

A

Secondary dendrite arm spacing is reduced

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79
Q

What effect on mechanical properties does increasing the rate of solidification have?

A

Increases mechanical properties

May also increase porosity

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80
Q

How is the rate of solidification increased?

A

By removing excess heat from the melt and latent heat of solidification

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81
Q

What thermal resistances exist in a typical casting?

A

In liquid

In Solid

Solid/mould interface (air gap maybe)

In mould

At mould/environment interface

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82
Q

What problems arise from gravity pouring into a mould to cast?

A

Turbulent flow (air entrapment)

Impurities (oxides forming, erosion damage to mould)

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83
Q

How can turbulent flow be avoided in casting?

A

Well designed running/gating systems

Introduce liquid at lowest point in casting (liquid slowly rises through mould)

Tapered sprue prevents liquid pulling away from sides

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84
Q

How can inclusions be avoided when casting?

A

Dross trap catches first metal entering mould (contains most oxides)

Avoid melt contact with air

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85
Q

Describe the Cosworth Process

A

Bottom filled Zircon sand mould

Vertical fill tube in middle of holding furnace only feeds cleanest metal at a controlled rate

Reduced turbulence, minimal inclusions

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86
Q

What benefits does Zircon sand offer compared to Silica sand?

A

Better thermal stability, so better dimensional accuracy

Better conductivity, so faster solidification, finer microstructure

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87
Q

What is grey cast iron?

A

Iron with a graphitic microstructure

On cooling, graphite precipitates

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88
Q

What affects the porosity of grey cast iron?

A

Grey cast iron expands on solidification, reducing porosity

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89
Q

What effect on the mechanical properties of grey cast iron does the graphite have?

A

Graphite in flake form

Internal stress raisers make the structure brittle

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90
Q

How can grey cast iron be made tougher?

A

Add magnesium

Forms nodular graphite (or spheroidal graphite, SG cast iron)

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91
Q

What function do feeder heads perform in casting?

A

Provide reservoir of molten material to compensate for solidification shrinkage

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92
Q

What causes macroporosity?

A

Large cavities in the casting as a result of insufficient feeding

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93
Q

What is meant by directional solidification?

A

Getting the casting to solidify first at the furthest point from the feeder head and the progress closer to the head

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94
Q

What is Huevers construction?

A

Circles inscribed on casting section must increase in diameter in direction of the feeder head

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95
Q

Briefly describe forming?

A

Shaping materials in the solid state by plastic deformation

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96
Q

What are “bulk” workpieces?

A

Objects with small surface area to volume ratio

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97
Q

How are bulk workpieces usually deformed in forming processes?

A

Triaxial compressive loading

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98
Q

For cold temperature forming, describe the mechanism that causes an increase in yield stress with plastic strain

A

Work-hardening

Dislocation density increases with the plastic strain, thus increasing the yield stress

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99
Q

How can the effects of work-hardening be reversed?

A

Annealing

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100
Q

Describe the annealing process, including relevant stages and temperatures

A

At 0.3-0.5Tm, recovery occurs; some strain fields annihilate each other and some rearrange into low angle boundaries. Ductility increases and yield stress decreases

0.5Tm and above, recrystallisation occurs; new, relatively dislocation free grains nucleate and grow from the old grains. Mechanical properties return to pre work-hardened state

At even higher temperatures or longer times, grain growth occurs; increased strength and ductility of metal. Poor surface finish from orange peel effect

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101
Q

What effect does increasing temperature have on the mechanical properties of most metals?

A

Increases ductility

Increases toughness

Lowers E

Lowers yield stress

Lowers tensile strength

Decreases strain hardening exponent

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102
Q

While increasing the temperature may mean lower forming forces are required, what is the main disadvantage?

A

Oxidation of the workpiece increases

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103
Q

Why are softening processes (recovery, recrystallisation and grain growth) time and temperature dependant?

A

They rely on diffusion

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104
Q

What effect on mechanical properties does a higher strain rate during forming have?

A

Increased yield stress, due to not enough time for diffusion to redistribute stresses

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105
Q

What is forging?

A

Workpiece deformed plastically using compressive forces

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106
Q

Describe open die forging

A

Solid, cylindrical workpiece is compressed between two platens (upsetting)

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107
Q

What is the “friction hill” in forging?

A

The distribution of forging pressure across the width of the billet. Highest in the centre and decreases towards the edges. Due to there being less material to push outwards the further from the centre you go

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108
Q

How can the upsetting force be calculated for forging?

A

Integrating the area under the tooling pressure curve (friction hill)

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109
Q

Why is forging pressure higher with friction compared to without friction?

A

Horizontal displacement means work is done to overcome the frictional resistances

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110
Q

What is the main consequence of sticking friction?

A

Barrelling

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111
Q

What is barrelling?

A

Non uniform horizontal displacement of the workpiece

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112
Q

What are the two main causes of barrelling?

A

Sticking friction at the billet-platen interfaces

Hot billet between cool platens; material closest to platens cools fastest and has highest strength so centre portion deforms easiest

113
Q

How can tool deformation be reduced when forging?

A

Reduce the coefficient of friction between the material and dies

Reduce a/h ratio of workpiece (geometry change)

Reduce yield strength by increasing temperature

114
Q

What is impression die forging?

A

Billet is compressed between shaped dies

Material takes the shape of the dies

115
Q

What is the “flash”?

A

Excess material that flows out from between the impression dies

116
Q

Why is flash formation important for impression die forging (two reasons)?

A

Creates high resistance to material flow at the join of the two dies, encouraging material to fill the dies instead of flowing out

At temperature, the flash will also cool faster than the rest of the material, creating extra material resistance at the seam, further encouraging die filling

117
Q

How is an aligned fibre structure achieved in a forged component?

A

Inclusions in the material flow in the direction of plastic deformation, creating a fibre structure

118
Q

Why is the direction of fibre alignment important in a forged component?

A

Encourages anisotropic properties

119
Q

What function do draft angles perform in a forging?

A

Allow easy removal of a component

120
Q

Why are small radii a problem in forging operations?

A

Metal in the solid state cannot flow easily into a small radii

121
Q

What properties must a forging die have to be useful?

A

High temperature strength and toughness

Hardenability

Resistance to thermal and mechanical shock

High wear resistance

122
Q

Name two examples of common forging die materials

A

Tool Steel

Die steel

123
Q

What defects commonly occur when forging?

A

Anisotropic properties from flow

Surface cracking

Web buckling

Laps (material folds on itself)

Grain boundaries being exposed at the edge of a casting (poor corrosion resistance)

124
Q

What benefits do using lubricants have with forging operations?

A

Reduce friction and wear

Provide a thermal barrier between forging and die to slow rate of cooling

Act as parting agents

125
Q

What lubricants are used for hot forging?

A

Graphite

Molybdenum

Disulphide

Glass

126
Q

What lubricants are used for cold forging?

A

Mineral oil

Soaps

127
Q

What is Rolling (forging)?

A

Reducing thickness or changing cross section of long workpiece using rolls

128
Q

Briefly describe the rolling process

A

Performed at temperature to reduce yield stress

Multiple passes (roll sets) to avoid excessive deformation

129
Q

Briefly describe the change in microstructure that occurs during rolling

A

Coarse grained microstructure of cast metal is broken down into a wrought (mechanically worked) structure with finer grains

Inclusions are aligned with the working direction

130
Q

What is cold rolling?

A

Rolling, but carried out at ambient temperatures

131
Q

What benefits does cold rolling offer?

A

Improved surface finish

Better dimensional tolerances

Better mechanical properties

132
Q

Where are frictional forces set up in rolling processes?

A

On the rollers by the material speeding up as it reduces in thickness

133
Q

Why are smaller diameter rollers commonly used?

A

Reduce r/h and change in height per pass ratios so lower rolling pressures are needed

134
Q

Why might forward/backward tension be used for rolling processes?

A

Applied tension reduces the forming pressure

135
Q

Why are rolled screw threads superior to cut threads?

A

Cold working increases strength

Good surface finish

Compressive residual stresses at thread root minimise effect of stress concentration

136
Q

What is rotary tube piercing?

A

A hot working process that produces long, thick-walled seamless tubing

137
Q

How does rotary tube piercing work?

A

When a round bar is subjected to radial compression, tensile stresses develop in the centre of the bar

Continuous cycling loading creates a cavity at the centre

Skewed rotational axes pull the bar through

Internal mandrel sizes the hole

138
Q

What defects occur when rolling?

A

Waviness (when rolls bend and the outer edges are thinner than the centre)

Zipper cracks (too much central rolling)

Edge cracks (too much rolling on outside edge)

Alligatoring (too much induced tensile stress splits the part down the middle)

139
Q

What is extrusion?

A

Metal is forced under pressure through a single or series of dies until the desired cross section in achieved

140
Q

What are the main benefits to using extrusion?

A

Wide variety of shapes

High production rates

Improved microstructure and mechanical properties (mechanical work)

Close tolerances

Economical

141
Q

What are the three types of extrusion?

A

Forward (billet pushed through die by ram)

Backward (die pushed onto billet)

Hydrostatic (fluid pressure exerted on billet that is extruded through die)

142
Q

Describe the dead metal zone in the context of unlubricated extrusion

A

Dead metal zone at die entrance due to friction

Metal has to shear past this zone

Extruded product has fresh metal surface

143
Q

What three components of energy make up the required energy for extrusion?

A

Energy required for ideal (frictionless) extrusion

Energy to overcome frictional forces

Redundant energy

144
Q

What is redundant work?

A

Work that does not contribute to the shape change of the workpiece

145
Q

What three characteristics are controlled in extrusion?

A

Temperature

Extrusion speed

Die design

146
Q

What effect on the extruding material does increasing the ram speed (extrusion rate) have?

A

Strain rate and thus yield stress

147
Q

How can complex (hollow) cross-sections be extruded?

A

Die fitted with a singular or multiple mandrels held in place by webs and spiders

Metal has to flow around webs/spiders but then rewards in the welding chamber under high pressure

148
Q

Why can lubricants not be used to extrude hollow cross sections?

A

Lubrication prevents rewelding

149
Q

What benefits does cold extrusion have?

A

Improved mechanical properties from strain hardening

Good dimensional tolerances

Good surface finish

No oxidation

High production rate

Relatively low cost

150
Q

Briefly describe drawing

A

Cross sectional area of a bar is reduced by pulling it through a converging die

Mandrel or plug allows wall thickness to be modified

151
Q

What is inhomogeous deformation?

A

Deformation where redundant work has to be taken into account

152
Q

What are the general differences between hot and cold forging?

A

Cold forging characterised by work hardening, hot forging by dynamic recovery/recrystallisation

Cold forging improves tolerances and surface finish

153
Q

Why aren’t compressive forces wanted in sheet forming?

A

Cause buckling or wrinkling

154
Q

What three mechanical deformations do sheet processes use?

A

Bending

Stretching

Shearing

155
Q

What is yield point elongation?

A

An increase in strain without an increase in stress

156
Q

What defect can be caused by yield point elongation?

A

Lüder’s Bands (stretcher-strain marks)

157
Q

How can stretcher-strain marks be avoided?

A

Temper/skin rolling

158
Q

Why might there be a time limit on sheet forming?

A

After rolling to prevent Lüder’s bands, strain ageing at room temperature occurs due to the C and N in solid solution, meaning the yield point deformation returns

159
Q

How can strain ageing be avoided completely?

A

Using steels deoxidised with aluminium, nitrogen is present as a precipitated nitride (stabilised steel)

160
Q

Excluding yield point elongation, what 5 factors affect the formability of sheet metals?

A

Anisotropy (from preferred grain orientation and mechanical fibreing)

Grain size (small grains cause higher yield stress, coarse grains cause a poor surface finish)

Residual stresses (cause distortion if a portion is removed)

Springback

Wrinkling (some compressive stresses may arise)

161
Q

What is shearing (sheet metal operation)?

A

A combination of shaped die and punch shear a shape out of a sheet

162
Q

Describe the fracture surface of a sheared sheet

A

Rollover at the top from plastic deformation

Rough fractured zone

Burr of material below lower surface

Fracture surface is not smooth or perpendicular to sheet surface

163
Q

What is fine blanking?

A

All sides of the sheet are supported at all times

Impingement ring holds sheet in place with compressive forces

Small clearances

164
Q

What main benefit does fine blanking give compared to regular shearing?

A

Smooth, perpendicular fracture surface

165
Q

What is the main limit on bend radius for bending a sheet?

A

Localised necking of the outer surface fibres

166
Q

What is meant by complete bendability?

A

When the sheet can be completely folded over onto itself

167
Q

At what point is the minimum bend radius reached?

A

When cracking occurs on the outer bend surface

168
Q

What is the main factor affecting sheet bindability?

A

Outer surface fibre texture

If inclusions (as stringers) are aligned transverse to the bending stress, cracks will more easily occur

169
Q

What is springback?

A

Plastic deformation followed by some elastic recovery

170
Q

How can springback be compensated for?

A

Overbending part

Bottoming the bend (high compressive stresses at the bend radius)

Bend at high temperature

Thicker sheet

Stretch bending

171
Q

What is stretch forming?

A

Sheet metal edges clamped

Sheet stretched over a male die into shape

172
Q

Sheet forming is usually restricted to low volume production. How is it adapted for high volume?

A

Both male and female dies are used, so the shape is better defined

173
Q

What two geometric features are not possible to manufacture using stretch bending?

A

Sharp contours

Re-entrant angles

174
Q

What is deep drawing?

A

Sheet blank is drawn into a cylindrical or box shaped part with a punch that presses it into a cavity

Blank held in place with a blank holder

175
Q

What stress state is the base of a deep drawn part in during the process?

A

Biaxial tension

176
Q

What stress state is the side wall/s of a deep drawn part in during the process?

A

Longitudinal tensile stress

Tensile hoop stress

177
Q

What happens to the material in transition from the flange to the side wall of a deep drawn part during the process?

A

Subject to bending and then rebending (straightening out)

178
Q

What stress state is the flange of a deep drawn part in during the process?

A

Radial tensile stress (pulled into cavity)

Compressive stress from blank holder

Radial compressive stress as it reduces in diameter while pulled into the smaller radius die

179
Q

What is the effect of the radial compressive stress on the flange of a deep drawn component during the process?

A

Blank thickens

Can lead to wrinkling

180
Q

What is the limiting draw ratio (LDR)?

A

Maximum ratio of blank diameter to punch diameter that can be drawn without failure

181
Q

What three components make up the max punch force in deep drawing?

A

Ideal work of deformation

Redundant work

Friction work

182
Q

What happens if the punch force is too excessive (deep drawing?

A

Tearing occurs in the side walls

183
Q

How can wrinkling be avoided (deep drawing)?

A

Shallow draws

Thicker blanks

184
Q

What is the purpose of the blankholder (deep drawing)?

A

Exerts pressure to avoid wrinkling

185
Q

If the blank holder exerts too much pressure, what may happen (deep drawing)?

A

Excessive thinning, possible failure

186
Q

Where is the most likely location of failure in deep drawing?

A

Side walls, due to high longitudinal tensile stress

187
Q

What is normal anisotropy and why is it important for deep drawability?

A

Expresses the ratio between wall strain and thickness strain for a material.

Important as wall strain can be high but thickness strain needs to be low to prevent excess thinning

Hence normal anisotropy needs to be high

188
Q

What is the parameter Rav

A

Average anisotropy, calculated across various orientations of R in the material

189
Q

What is the parameter delta R?

A

Planar anisotropy

190
Q

What is the relationship between Rav and LDR?

A

LDR increases with Rav

191
Q

What is earing in deep drawing and what causes it? Why is it undesirable?

A

Wavy cup rim causes by anisotropy

Have to be trimmed away, means wastage

192
Q

How to Rav and delta R influence deep drawability?

A

Deep Drawability enhanced with high Rav and low delta R

193
Q

What common defects occur with deep drawing?

A

Flange wrinkling

Wall wrinkling

Tearing

Earing

Surface scratches

194
Q

What is formability?

A

Ability of a sheet to undergo the desired shape change without failure

195
Q

What extra tests are needed (besides tensile tests) to characterise a sheet metal behaviour?

A

Total elongation

Uniform strain

Strain hardening exponent

Planar anisotropy

Normal anisotropy

196
Q

What is the Nakazima test?

A

Sheet blank is marked with grid pattern of circles

Sheet is stretched over a punch and circle deformation is observed

Major and minor strain calculated at failure locations

197
Q

What is a forming limit diagram?

A

Minor and major engineering strains from the Nakazima test plotted

Higher the curve, the better the formability

198
Q

How can splitting be avoided in deep drawing?

A

Increase minor strain by clamping in certain area with draw beads

Improve lubrication

Reduce major strain by decreasing depth of stretch

Use thicker material

199
Q

What are the 5 basic steps of powder metallurgy?

A

Powder Production

Mixing & Blending

Powder Consolidation

Sintering

Finishing

200
Q

In what situations would powder metallurgy be most appropriate for?

A

High melting point materials

Part is too hard to machine

Very large number of components (long production run)

201
Q

What methods are used for powder production?

A

Atomisation

Reduction of oxide powders

Electrolytic deposition from an aqueous solution

Metal carbonyls

Communition (pulverisation)

202
Q

What characterisations of powders are there?

A

Acicular

Irregular Rodlike

Flake

Dendritic

Spherical

Rounded

Irregular

Porous

Angular

203
Q

How is powder size measured?

A

Screening, passing through various screens (meshes)

204
Q

How do the size an shape of particles influence the powder metallurgy process?

A

Flow of powder

Packing density

Compressibility

Green density

Strength of final component

205
Q

How can alloy powders be made?

A

Mixing elemental powders

Mechanical milling

Diffusion bonding

Atomised alloys

206
Q

Describe in more detail mechanical milling to produce alloy powders

A

Powders mixed in a ball mill

Particles repeatedly fracture and weld together

207
Q

Describe in more detail diffusion bonding for alloy powder production

A

Powder mixture heated to yield a sintered powder cake

Powder cake ground down into agglomerates of the components of the powder

208
Q

How does the addition of lubricant affect the powder?

A

Improves flow characteristics

209
Q

How do binders affect the powder?

A

Create a thin film to which additives can adhere to

Improves green strength and uniformity

210
Q

What benefit would mixing powders of various sizes have?

A

Can customise final fill density

Finer particles occupy the interstices between the larger particles

211
Q

How are powders compacted?

A

Cold compacted in a die

Single or multiple punches

Green density depends on compaction pressure

212
Q

What are the limitations of single punch compaction?

A

Pressure rapidly tapers off due to wall friction and interparticle friction

213
Q

What is isostatic pressing?

A

Powder places in a flexible, elastomeric mould and pressurised in a chamber with water, usually to 400MPa

214
Q

Besides isostatic pressing, what powder compaction methods are available?

A

Rolling - Powder fed between rollers

Pressureless Compaction - die filled with powder and sintered (porous parts)

Metal Injection Moulding - powder combined with binder, injected into mould and binder removed

Spray Deposition - metal is atomised and sprayed onto a cool, rotating mould where it solidifies

215
Q

Describe powder sintering

A

Powder is heated to 0.7-0.9 Tm

Compacted mechanical bonds replaced by strong metallic ones

216
Q

What is the driving force behind sintering?

A

Reduction in surface energy when particles join by solid state diffusion

217
Q

What is liquid phase sintering?

A

One of the constituents melts completely and envelopes the other

Liquid drawn between particles by capillary action

Pores migrate to surface, driven by buoyancy

218
Q

What properties change during sintering?

A

Reduction in pore volume (shrinkage)

Density increases

Higher density means improved mechanical properties

219
Q

Why should volume changes during sintering be avoided? How can this be done?

A

Achieve better dimensional accuracy

Use powder with better compressibility (high green density)

Sinter at moderate temperature

220
Q

Why may finishing processes be needed for powder based components?

A

Sintered part has significant porosity

To improve mechanical properties, use finishing processes

221
Q

What finishing processes are available for finishing powder based components?

A

Cold Restriking + Resintering

Heat Treatment

Impregnation of Heated Oil

Infiltration with Metal

Machining

222
Q

What methods of improving powder products properties exist?

A

Hot pressing

Hot Rolling/Extrusion

Hot forging

Hot Isostatic pressing (HIP)

223
Q

What design considerations are involved with powder processing?

A

Length to thickness ratio limited to 2-4

Internal cavities require a draft (to release from die)

Avoid sharp corners

Avoid large wall thickness differences

Wall thickness greater than 1mm

224
Q

What are the main advantages to powder processing?

A

Virtually unlimited alloy choice

Uniform, fine microstructure

Controlled porosity for self lubricating parts/filtration

Economical at large production runs

Excellent material utilisation

Long term reliability

225
Q

What are the disadvantages to using powder metallurgy?

A

Limited part size

Limited geometric complexity

High powder cost

High tooling cost

Weaker than wrought parts

226
Q

Why might a part be machined after manufacture?

A

Improve dimensional tolerances

Improve surface roughness

Create geometrical features not possible with other manufacturing techniques

227
Q

What is orthogonal cutting?

A

Cutting edge is perpendicular to direction of cutting

228
Q

What are the two main faces of a cutting tool? What function does each perform?

A

Rake face - front surface, chip travels along this face

Clearance face - prevents excess rubbing of tool on workpiece, at an angle above piece

229
Q

What is the rake angle?

A

The angle between the rake face and the surface normal

230
Q

What is the clearance/relief angle?

A

The angle between the clearance face and the workpiece

231
Q

What is the cutting ratio? how is it defined?

A

Indication of the efficiency of the cutting operation

Feed thickness divided by chip thickness

232
Q

How is a chip formed?

A

Feed material is sheared by the tool at the shear plane angle and travels up the rake face as a chip

233
Q

How are the shear plane angle and the rake angle related to the shear strain on the chip?

A

The lower the angles, the greater the shear strain

234
Q

What angle should be maximised for the most efficient cutting?

A

Shear plane angle

The larger the shear plane angle, the smaller the shear plane area, the smaller the shear force.

235
Q

How can the shear plane angle be maximised?

A

Increase rake angle

Decrease friction angle (lubrication)

236
Q

What is oblique cutting?

A

Cutting edge at angle to direction of cutting

237
Q

Why is oblique cutting more widely used?

A

Has a larger effective rake angle, so the cutting force is lower compared to an orthogonal tool

238
Q

What affects the scale of the shear zone in orthogonal cutting?

A

Materials strain hardening properties and strain rate sensitivity

239
Q

How does sticking friction occur in orthogonal cutting?

A

If pressure on rake face is high and frictional stress exceeds shear yield stress of material, sticking friction occurs

240
Q

What is the consequence of sticking friction in orthogonal cutting?

A

No flow at material-tool interface, only in material, developing a secondary shear zone

241
Q

Assuming constant cutting speed and depth of cut, what is the relation between cutting speed and cutting force?

A

Higher the cutting speed, the lower the cutting force (for most materials)

242
Q

Why does cutting slower increase the cutting force?

A

At low strain rates, temperature remains low so the material work hardens

243
Q

Describe the chip formed when a ductile material is cut at a low speed

A

Material severely strain hardens and upsets, until enough strain is accumulated to initiate shear

Elastic components of system create a sudden acceleration, making the chip completely separate

Newly formed surface is of high roughness

244
Q

Describe the chip formation when a ductile material is cut at moderate/low speed with a cutting fluid

A

Chip is continuously formed

New material surface is smooth

245
Q

Describe the chip formation at higher speeds

A

Heat generation rom friction causes a rise in temperature

Friction prevents sliding on rake face

Tool effectively conducts heat away from rake face so material there work hardens

Material slightly further from rake face doesn’t work harden and flows over work-hardened material, creating a dead metal zone or Built up Edge (BUE)

BUE blunts tool and increases rake angle

Tool wear decreases

BUE occasionally breaks off and sticks to new material, creating a rough surface texture

246
Q

Describe chip formation at high speeds

A

No work hardening occurs because friction generates significant heat

Secondary shear zone established

Good surface finish

247
Q

What problems may a continuous chip cause?

A

May wrap around tool and block up work zone

248
Q

What can be done to prevent a continuous chip from being formed?

A

Use a chip breaker (give rake face extra curvature to impart additional strain)

249
Q

Where is heat mainly generated in the cutting operation?

A

60% in primary shear zone

30% in secondary shear zone

10% in tertiary shear zone

250
Q

How is heat removed from the cutting operation?

A

80% removed via chip surface

Heat from secondary shear zone needs to be conducted by tool

251
Q

What is the relationship between tool wear and tool temperature?

A

Higher the temperature, the higher the tool wear

252
Q

How does a cutting fluid affect the tool temperature?

A

Doesn’t not reduce peak temperature, but reduces the volume off the tool at high temperature

253
Q

What is the main purpose of cutting fluid?

A

Prevents distortion of workpiece by minimising thermal strains

254
Q

Where on the tool does wear occur?

A

Clearance face - flank wear

Rake face - crater wear

255
Q

What are the consequences of flank wear?

A

Lose dimensional control

Surface finish deteriorates

More heat generated

256
Q

What is crater wear? Where does it occur?

A

Occurs at a distance along rake face where temperature is highest

Wear develops in form of a crater due to some diffusion of atoms across the tool-chip interface

Tool essentially dissolves into the chip

257
Q

What does crater wear depend on?

A

Temperature and degree of chemical affinity between tool and workpiece

258
Q

How is machinability of a workpiece defined?

A

Hardness

Surface texture

Max rate of material removal

Tool life

Chip formation

259
Q

How can the microstructure of a workpiece affect its machinability?

A

Workpieces usually contain hard or soft particles which can be ductile or brittle

Hard particles cause abrasive wear

Soft particles improve machinability

260
Q

What can be added to a workpiece to improve machinability?

A

Lead - soft, easily fractured and smears across tool-chip interface like a solid lubricant

MnS particles - resulpherised steels, MnS particles are stress raisers and cause a small chip to be formed

261
Q

What properties make a god tool?

A

Hardness at high temperature

Thermal shock resistance

Wear resistance

Chemical stability

262
Q

What are the two types of high speed steels?

A

M (molybdenum) series - 10% Mo with Cr, V, W and Co alloying elements

T (tungsten) series - 12-18% W, with Cr, V and Co and alloying elements

263
Q

What effect does the addition of tungsten or molybdenum have on the high speed steels?

A

Hardened by the complex carbides that form

264
Q

What are cemented carbide tools?

A

Carbides are bound to a metal using powder processing

265
Q

What are the two types of cemented carbide tools?

A

WC in Co - 3-6% Co for hardness, 6-15% Co for toughness

TiC in Ni-Mo matrix - Higher wear resistance than WC but not as tough

266
Q

What happens when a WC in Co tool is used to machine steel?

A

WC readily dissolves in steel at 1200K so tool eventually is dissolved into the chips

267
Q

What tool would be used to machine steels?

A

Diffusion resistant grades, 10-40% TiC or TaC

268
Q

How can tools be coated?

A

PVD or CVD

269
Q

What types of coatings are available for use on metal tools?

A

Titanium nitride

Titanium carbide

Titanium carbonitride

Titanium Aluminium nitride

Aluminium titanium nitride

Ceramics

Multiphase

Diamond

270
Q

What are the two main types of ceramic tool?

A

Based on aluminium oxide (alumina) - high abrasion resistance and hardness, poor thermal conductivity and toughness

Based on Silicone nitride (silica) - lower thermal expansion coefficient compared to alumina so less thermal stress build up

271
Q

What is sialon?

A

Based on silica, but some Is replaced by Al, some of N replaced by O, add Yttria

272
Q

What advantages does Sialon offer?

A

Higher resistance to thermal shock compared to silica, used to machine cast irons and nickel-based superalloys

273
Q

Can silica based tools machine steel?

A

No, there is a high dissolution wear rate

274
Q

What are cBN cutting tools?

A

Made by bonding layer of polycrystalline cubic boron nitride to a carbide substrate

275
Q

What are cBN tools used to machine?

A

Hardened ferrous and high temperature alloys

276
Q

Can diamond tools be used to machine steel? Why?

A

No, above 650 degrees, diamond reverts to graphite which is very soft

277
Q

What costs are involved in cutting?

A

Fixed costs (material)

Machining costs (labour costs)

Cost of tool change (tool wear)

278
Q

Does using the highest production rate always yield the minimum component costs for cutting?

A

No, depends on tool wear rates and hence tool costs

Manufacturing And Materials (3 decks)

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