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
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)
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
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)
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)
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.
7
Q
How is a hollow object defined?
A
Has significant cavities (bowls, containers etc)
8
Q
How is a solid object defined?
A
No significant cavities
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
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
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
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)
13
Q
What material is used to cast a high melting point metal?
A
Ceramic moulds.
Tend to be expendable, remade for each cast
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
15
Q
What are the two basic classifications of casting processes?
A
Permanent Pattern
Permanent Mould
16
Q
What are the two main types of permanent pattern casting?
A
Sand casting
Shell moulding
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
18
Q
In sand casting, where is the sprue and riser system?
A
In the cope
19
Q
What is the sand mould made of?
A
Ceramic material with a binder
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)
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
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
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
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)
25
How does shell moulding differ from sand casting?
Mould is a thin walled shell, not a cavity in a block
Shells are permeable to air
26
Briefly describe the process to manufacture using shell moulding
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
27
What are the general advantages to shell casting?
Higher repeatability
Allows for intricate details
Less machining post casting
Uses less sand
28
What is ‘permanent pattern’ casting?
Pattern of final product used to make many expendable moulds
Mould destroyed after each casting
29
What is ‘permanent mould’ casting?
Same mould used to make large number of products
Mould opens to release component rather than by being destroyed
30
What is gravity die casting?
Gravity causes flow of molten liquid to enter cast iron or tool steel mould
31
What characteristics does a gravity die mould have?
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
32
Briefly describe the gravity die casting process
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
33
What are the general advantages of gravity die casting?
Close dimensional tolerances
Superior surface finish (compared to sand casting)
Faster cooling rates leads to finer microstructure and improved mechanical properties (highly conductive die)
34
What are the general disadvantages of gravity die casting?
Low melting point alloys only
High tooling costs
Limitation on shape
Coatings necessary
35
What generic components is gravity die casting best suited for?
High production volume
Uniform wall thickness
Limited undercuts/internal coring
36
Generally describe pressure die casting
Metal injected into die at high velocity
Solidifies under externally applied pressure
Short filling times
Complex, thin-walled castings can be solidified quickly
37
What types of pressure die casting are available?
Hot chamber
Cold chamber
38
Briefly describe hot chamber pressure die casting
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
39
What types of alloy are generally used for hot chamber pressure die casting?
Zinc
Magnesium
40
Briefly describe cold chamber pressure die casting
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
41
What are the general advantages of pressure die casting?
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
42
Describe the general disadvantages of pressure die casting
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
43
Briefly explain the lost wax pattern process
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
44
What are the general advantages of the lost wax process?
Allows great complexity
Can use any castable alloy
Close tolerances
Jointless mould (reduced machining cost)
Inexpensive mould
Can prototype
Reliable
45
What are the general disadvantages of the lost wax process?
Long production cycle
Single use mould
46
What sort of defects can exist in castings?
Porosity (bubbles)
Inclusions
47
What is homogenous nucleation?
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
48
What is undercooling and why is it necessary?
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
49
What is heterogenous nucleation?
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*
50
Why is heterogenous nucleation more likely to occur than homogenous nucleation?
Fewer atoms are required to form a cap and less undercooling is required (only a few degrees)
51
What makes a good nucleating agent?
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.
52
How is a low interfacial energy between catalyst and nucleating solid achieved?
The nucleating agent has at least one crystal dimension similar to that of the solid being nucleated. (eg TiB2 used to nucleate Al castings)
53
Why is the nature of nucleation important for casting?
Affects grain size/shape that influence the mechanical properties of the casting
54
How does porosity arise in castings?
Evolution of dissolved gases (microporosity)
Inadequate liquid supply to compensate for contraction (macroporosity)
55
In more detail, describe how microporosity arises in castings
From air trapped in the metal when poured
Chemical reactions
Thermal dissociation of water vapour (leads to hydrogen absorption and embrittlement)
56
Why is it important to prevent gases being in a melt?
When the metal solidifies, gas solubility drops significantly, promoting gas evolution
57
How can microporosity be prevented when casting?
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
58
When nucleating gas bubbles, which type of nucleation is more likely?
Heterogenous
59
What are good gas nucleating sites in a casting?
Poorly wetted inclusions
60
Is undercooling necessary for solidification?
Yes, the greater the undercooling, the faster the rate of solidification
61
How can undercooling be controlled?
By the rate at which latent heat of solidification is removed
62
Briefly describe the cooling process for a PURE metal
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
63
Why is a planar growth front stable with a positive temperature gradient?
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
64
Why is the growth front generally unstable in single phase alloys?
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
65
What type of interface is formed with a high, positive temperature gradient?
Stable, planar interface
66
What effect does decreasing the temperature gradient have on the planar stability of the growth front?
Decreasing Gl increases undercooling, decreasing stability
67
What sort of structure results from a small amount of undercooling?
Cellular; small instabilities may form primary stems in a parallel array
68
What type of structure is formed with a high level of undercooling?
Dendritic; high instability of the growth front allows any disturbance to rapidly grow
69
What conditions are likely to cause constitutional undercooling?
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
70
Explain the problems caused by dendritic growth
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)
71
What is the freezing range of alloys?
Range of temperature over which the metal fully solidifies (difference in temperature between solidus and liquidus line)
72
What problems are associated with a longer freezing range?
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
73
Give some examples of popular, short freezing range alloys
Zn-4%Al
Al-11%Si
Cast Irons
74
What is microsegregation in castings?
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
75
What controls the grain size in a casting?
Number of nucleation sites
76
What controls the scale of dendritic growth in castings?
Solidification rate
77
What two features of a dendritic structure form barriers to dislocation motion?
Secondary phases in the interdendritic region
Grain boundaries
78
What effect on the dendrite structure does increasing the solidification rate have?
Secondary dendrite arm spacing is reduced
79
What effect on mechanical properties does increasing the rate of solidification have?
Increases mechanical properties
May also increase porosity
80
How is the rate of solidification increased?
By removing excess heat from the melt and latent heat of solidification
81
What thermal resistances exist in a typical casting?
In liquid
In Solid
Solid/mould interface (air gap maybe)
In mould
At mould/environment interface
82
What problems arise from gravity pouring into a mould to cast?
Turbulent flow (air entrapment)
Impurities (oxides forming, erosion damage to mould)
83
How can turbulent flow be avoided in casting?
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
84
How can inclusions be avoided when casting?
Dross trap catches first metal entering mould (contains most oxides)
Avoid melt contact with air
85
Describe the Cosworth Process
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
86
What benefits does Zircon sand offer compared to Silica sand?
Better thermal stability, so better dimensional accuracy
Better conductivity, so faster solidification, finer microstructure
87
What is grey cast iron?
Iron with a graphitic microstructure
On cooling, graphite precipitates
88
What affects the porosity of grey cast iron?
Grey cast iron expands on solidification, reducing porosity
89
What effect on the mechanical properties of grey cast iron does the graphite have?
Graphite in flake form
Internal stress raisers make the structure brittle
90
How can grey cast iron be made tougher?
Add magnesium
Forms nodular graphite (or spheroidal graphite, SG cast iron)
91
What function do feeder heads perform in casting?
Provide reservoir of molten material to compensate for solidification shrinkage
92
What causes macroporosity?
Large cavities in the casting as a result of insufficient feeding
93
What is meant by directional solidification?
Getting the casting to solidify first at the furthest point from the feeder head and the progress closer to the head
94
What is Huevers construction?
Circles inscribed on casting section must increase in diameter in direction of the feeder head
95
Briefly describe forming?
Shaping materials in the solid state by plastic deformation
96
What are "bulk" workpieces?
Objects with small surface area to volume ratio
97
How are bulk workpieces usually deformed in forming processes?
Triaxial compressive loading
98
For cold temperature forming, describe the mechanism that causes an increase in yield stress with plastic strain
Work-hardening
Dislocation density increases with the plastic strain, thus increasing the yield stress
99
How can the effects of work-hardening be reversed?
Annealing
100
Describe the annealing process, including relevant stages and temperatures
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
101
What effect does increasing temperature have on the mechanical properties of most metals?
Increases ductility
Increases toughness
Lowers E
Lowers yield stress
Lowers tensile strength
Decreases strain hardening exponent
102
While increasing the temperature may mean lower forming forces are required, what is the main disadvantage?
Oxidation of the workpiece increases
103
Why are softening processes (recovery, recrystallisation and grain growth) time and temperature dependant?
They rely on diffusion
104
What effect on mechanical properties does a higher strain rate during forming have?
Increased yield stress, due to not enough time for diffusion to redistribute stresses
105
What is forging?
Workpiece deformed plastically using compressive forces
106
Describe open die forging
Solid, cylindrical workpiece is compressed between two platens (upsetting)
107
What is the "friction hill" in forging?
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
108
How can the upsetting force be calculated for forging?
Integrating the area under the tooling pressure curve (friction hill)
109
Why is forging pressure higher with friction compared to without friction?
Horizontal displacement means work is done to overcome the frictional resistances
110
What is the main consequence of sticking friction?
Barrelling
111
What is barrelling?
Non uniform horizontal displacement of the workpiece
112
What are the two main causes of barrelling?
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
How can tool deformation be reduced when forging?
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
What is impression die forging?
Billet is compressed between shaped dies
Material takes the shape of the dies
115
What is the "flash"?
Excess material that flows out from between the impression dies
116
Why is flash formation important for impression die forging (two reasons)?
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
How is an aligned fibre structure achieved in a forged component?
Inclusions in the material flow in the direction of plastic deformation, creating a fibre structure
118
Why is the direction of fibre alignment important in a forged component?
Encourages anisotropic properties
119
What function do draft angles perform in a forging?
Allow easy removal of a component
120
Why are small radii a problem in forging operations?
Metal in the solid state cannot flow easily into a small radii
121
What properties must a forging die have to be useful?
High temperature strength and toughness
Hardenability
Resistance to thermal and mechanical shock
High wear resistance
122
Name two examples of common forging die materials
Tool Steel
Die steel
123
What defects commonly occur when forging?
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
What benefits do using lubricants have with forging operations?
Reduce friction and wear
Provide a thermal barrier between forging and die to slow rate of cooling
Act as parting agents
125
What lubricants are used for hot forging?
Graphite
Molybdenum
Disulphide
Glass
126
What lubricants are used for cold forging?
Mineral oil
Soaps
127
What is Rolling (forging)?
Reducing thickness or changing cross section of long workpiece using rolls
128
Briefly describe the rolling process
Performed at temperature to reduce yield stress
Multiple passes (roll sets) to avoid excessive deformation
129
Briefly describe the change in microstructure that occurs during rolling
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
What is cold rolling?
Rolling, but carried out at ambient temperatures
131
What benefits does cold rolling offer?
Improved surface finish
Better dimensional tolerances
Better mechanical properties
132
Where are frictional forces set up in rolling processes?
On the rollers by the material speeding up as it reduces in thickness
133
Why are smaller diameter rollers commonly used?
Reduce r/h and change in height per pass ratios so lower rolling pressures are needed
134
Why might forward/backward tension be used for rolling processes?
Applied tension reduces the forming pressure
135
Why are rolled screw threads superior to cut threads?
Cold working increases strength
Good surface finish
Compressive residual stresses at thread root minimise effect of stress concentration
136
What is rotary tube piercing?
A hot working process that produces long, thick-walled seamless tubing
137
How does rotary tube piercing work?
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
What defects occur when rolling?
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
What is extrusion?
Metal is forced under pressure through a single or series of dies until the desired cross section in achieved
140
What are the main benefits to using extrusion?
Wide variety of shapes
High production rates
Improved microstructure and mechanical properties (mechanical work)
Close tolerances
Economical
141
What are the three types of extrusion?
Forward (billet pushed through die by ram)
Backward (die pushed onto billet)
Hydrostatic (fluid pressure exerted on billet that is extruded through die)
142
Describe the dead metal zone in the context of unlubricated extrusion
Dead metal zone at die entrance due to friction
Metal has to shear past this zone
Extruded product has fresh metal surface
143
What three components of energy make up the required energy for extrusion?
Energy required for ideal (frictionless) extrusion
Energy to overcome frictional forces
Redundant energy
144
What is redundant work?
Work that does not contribute to the shape change of the workpiece
145
What three characteristics are controlled in extrusion?
Temperature
Extrusion speed
Die design
146
What effect on the extruding material does increasing the ram speed (extrusion rate) have?
Strain rate and thus yield stress
147
How can complex (hollow) cross-sections be extruded?
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
Why can lubricants not be used to extrude hollow cross sections?
Lubrication prevents rewelding
149
What benefits does cold extrusion have?
Improved mechanical properties from strain hardening
Good dimensional tolerances
Good surface finish
No oxidation
High production rate
Relatively low cost
150
Briefly describe drawing
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
What is inhomogeous deformation?
Deformation where redundant work has to be taken into account
152
What are the general differences between hot and cold forging?
Cold forging characterised by work hardening, hot forging by dynamic recovery/recrystallisation
Cold forging improves tolerances and surface finish
153
Why aren't compressive forces wanted in sheet forming?
Cause buckling or wrinkling
154
What three mechanical deformations do sheet processes use?
Bending
Stretching
Shearing
155
What is yield point elongation?
An increase in strain without an increase in stress
156
What defect can be caused by yield point elongation?
Lüder's Bands (stretcher-strain marks)
157
How can stretcher-strain marks be avoided?
Temper/skin rolling
158
Why might there be a time limit on sheet forming?
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
How can strain ageing be avoided completely?
Using steels deoxidised with aluminium, nitrogen is present as a precipitated nitride (stabilised steel)
160
Excluding yield point elongation, what 5 factors affect the formability of sheet metals?
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
What is shearing (sheet metal operation)?
A combination of shaped die and punch shear a shape out of a sheet
162
Describe the fracture surface of a sheared sheet
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
What is fine blanking?
All sides of the sheet are supported at all times
Impingement ring holds sheet in place with compressive forces
Small clearances
164
What main benefit does fine blanking give compared to regular shearing?
Smooth, perpendicular fracture surface
165
What is the main limit on bend radius for bending a sheet?
Localised necking of the outer surface fibres
166
What is meant by complete bendability?
When the sheet can be completely folded over onto itself
167
At what point is the minimum bend radius reached?
When cracking occurs on the outer bend surface
168
What is the main factor affecting sheet bindability?
Outer surface fibre texture
If inclusions (as stringers) are aligned transverse to the bending stress, cracks will more easily occur
169
What is springback?
Plastic deformation followed by some elastic recovery
170
How can springback be compensated for?
Overbending part
Bottoming the bend (high compressive stresses at the bend radius)
Bend at high temperature
Thicker sheet
Stretch bending
171
What is stretch forming?
Sheet metal edges clamped
Sheet stretched over a male die into shape
172
Sheet forming is usually restricted to low volume production. How is it adapted for high volume?
Both male and female dies are used, so the shape is better defined
173
What two geometric features are not possible to manufacture using stretch bending?
Sharp contours
Re-entrant angles
174
What is deep drawing?
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
What stress state is the base of a deep drawn part in during the process?
Biaxial tension
176
What stress state is the side wall/s of a deep drawn part in during the process?
Longitudinal tensile stress
Tensile hoop stress
177
What happens to the material in transition from the flange to the side wall of a deep drawn part during the process?
Subject to bending and then rebending (straightening out)
178
What stress state is the flange of a deep drawn part in during the process?
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
What is the effect of the radial compressive stress on the flange of a deep drawn component during the process?
Blank thickens
Can lead to wrinkling
180
What is the limiting draw ratio (LDR)?
Maximum ratio of blank diameter to punch diameter that can be drawn without failure
181
What three components make up the max punch force in deep drawing?
Ideal work of deformation
Redundant work
Friction work
182
What happens if the punch force is too excessive (deep drawing?
Tearing occurs in the side walls
183
How can wrinkling be avoided (deep drawing)?
Shallow draws
Thicker blanks
184
What is the purpose of the blankholder (deep drawing)?
Exerts pressure to avoid wrinkling
185
If the blank holder exerts too much pressure, what may happen (deep drawing)?
Excessive thinning, possible failure
186
Where is the most likely location of failure in deep drawing?
Side walls, due to high longitudinal tensile stress
187
What is normal anisotropy and why is it important for deep drawability?
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
What is the parameter Rav
Average anisotropy, calculated across various orientations of R in the material
189
What is the parameter delta R?
Planar anisotropy
190
What is the relationship between Rav and LDR?
LDR increases with Rav
191
What is earing in deep drawing and what causes it? Why is it undesirable?
Wavy cup rim causes by anisotropy
Have to be trimmed away, means wastage
192
How to Rav and delta R influence deep drawability?
Deep Drawability enhanced with high Rav and low delta R
193
What common defects occur with deep drawing?
Flange wrinkling
Wall wrinkling
Tearing
Earing
Surface scratches
194
What is formability?
Ability of a sheet to undergo the desired shape change without failure
195
What extra tests are needed (besides tensile tests) to characterise a sheet metal behaviour?
Total elongation
Uniform strain
Strain hardening exponent
Planar anisotropy
Normal anisotropy
196
What is the Nakazima test?
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
What is a forming limit diagram?
Minor and major engineering strains from the Nakazima test plotted
Higher the curve, the better the formability
198
How can splitting be avoided in deep drawing?
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
What are the 5 basic steps of powder metallurgy?
Powder Production
Mixing & Blending
Powder Consolidation
Sintering
Finishing
200
In what situations would powder metallurgy be most appropriate for?
High melting point materials
Part is too hard to machine
Very large number of components (long production run)
201
What methods are used for powder production?
Atomisation
Reduction of oxide powders
Electrolytic deposition from an aqueous solution
Metal carbonyls
Communition (pulverisation)
202
What characterisations of powders are there?
Acicular
Irregular Rodlike
Flake
Dendritic
Spherical
Rounded
Irregular
Porous
Angular
203
How is powder size measured?
Screening, passing through various screens (meshes)
204
How do the size an shape of particles influence the powder metallurgy process?
Flow of powder
Packing density
Compressibility
Green density
Strength of final component
205
How can alloy powders be made?
Mixing elemental powders
Mechanical milling
Diffusion bonding
Atomised alloys
206
Describe in more detail mechanical milling to produce alloy powders
Powders mixed in a ball mill
Particles repeatedly fracture and weld together
207
Describe in more detail diffusion bonding for alloy powder production
Powder mixture heated to yield a sintered powder cake
Powder cake ground down into agglomerates of the components of the powder
208
How does the addition of lubricant affect the powder?
Improves flow characteristics
209
How do binders affect the powder?
Create a thin film to which additives can adhere to
Improves green strength and uniformity
210
What benefit would mixing powders of various sizes have?
Can customise final fill density
Finer particles occupy the interstices between the larger particles
211
How are powders compacted?
Cold compacted in a die
Single or multiple punches
Green density depends on compaction pressure
212
What are the limitations of single punch compaction?
Pressure rapidly tapers off due to wall friction and interparticle friction
213
What is isostatic pressing?
Powder places in a flexible, elastomeric mould and pressurised in a chamber with water, usually to 400MPa
214
Besides isostatic pressing, what powder compaction methods are available?
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
Describe powder sintering
Powder is heated to 0.7-0.9 Tm
Compacted mechanical bonds replaced by strong metallic ones
216
What is the driving force behind sintering?
Reduction in surface energy when particles join by solid state diffusion
217
What is liquid phase sintering?
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
What properties change during sintering?
Reduction in pore volume (shrinkage)
Density increases
Higher density means improved mechanical properties
219
Why should volume changes during sintering be avoided? How can this be done?
Achieve better dimensional accuracy
Use powder with better compressibility (high green density)
Sinter at moderate temperature
220
Why may finishing processes be needed for powder based components?
Sintered part has significant porosity
To improve mechanical properties, use finishing processes
221
What finishing processes are available for finishing powder based components?
Cold Restriking + Resintering
Heat Treatment
Impregnation of Heated Oil
Infiltration with Metal
Machining
222
What methods of improving powder products properties exist?
Hot pressing
Hot Rolling/Extrusion
Hot forging
Hot Isostatic pressing (HIP)
223
What design considerations are involved with powder processing?
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
What are the main advantages to powder processing?
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
What are the disadvantages to using powder metallurgy?
Limited part size
Limited geometric complexity
High powder cost
High tooling cost
Weaker than wrought parts
226
Why might a part be machined after manufacture?
Improve dimensional tolerances
Improve surface roughness
Create geometrical features not possible with other manufacturing techniques
227
What is orthogonal cutting?
Cutting edge is perpendicular to direction of cutting
228
What are the two main faces of a cutting tool? What function does each perform?
Rake face - front surface, chip travels along this face
Clearance face - prevents excess rubbing of tool on workpiece, at an angle above piece
229
What is the rake angle?
The angle between the rake face and the surface normal
230
What is the clearance/relief angle?
The angle between the clearance face and the workpiece
231
What is the cutting ratio? how is it defined?
Indication of the efficiency of the cutting operation
Feed thickness divided by chip thickness
232
How is a chip formed?
Feed material is sheared by the tool at the shear plane angle and travels up the rake face as a chip
233
How are the shear plane angle and the rake angle related to the shear strain on the chip?
The lower the angles, the greater the shear strain
234
What angle should be maximised for the most efficient cutting?
Shear plane angle
The larger the shear plane angle, the smaller the shear plane area, the smaller the shear force.
235
How can the shear plane angle be maximised?
Increase rake angle
Decrease friction angle (lubrication)
236
What is oblique cutting?
Cutting edge at angle to direction of cutting
237
Why is oblique cutting more widely used?
Has a larger effective rake angle, so the cutting force is lower compared to an orthogonal tool
238
What affects the scale of the shear zone in orthogonal cutting?
Materials strain hardening properties and strain rate sensitivity
239
How does sticking friction occur in orthogonal cutting?
If pressure on rake face is high and frictional stress exceeds shear yield stress of material, sticking friction occurs
240
What is the consequence of sticking friction in orthogonal cutting?
No flow at material-tool interface, only in material, developing a secondary shear zone
241
Assuming constant cutting speed and depth of cut, what is the relation between cutting speed and cutting force?
Higher the cutting speed, the lower the cutting force (for most materials)
242
Why does cutting slower increase the cutting force?
At low strain rates, temperature remains low so the material work hardens
243
Describe the chip formed when a ductile material is cut at a low speed
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
Describe the chip formation when a ductile material is cut at moderate/low speed with a cutting fluid
Chip is continuously formed
| New material surface is smooth
245
Describe the chip formation at higher speeds
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
Describe chip formation at high speeds
No work hardening occurs because friction generates significant heat
Secondary shear zone established
Good surface finish
247
What problems may a continuous chip cause?
May wrap around tool and block up work zone
248
What can be done to prevent a continuous chip from being formed?
Use a chip breaker (give rake face extra curvature to impart additional strain)
249
Where is heat mainly generated in the cutting operation?
60% in primary shear zone
30% in secondary shear zone
10% in tertiary shear zone
250
How is heat removed from the cutting operation?
80% removed via chip surface
Heat from secondary shear zone needs to be conducted by tool
251
What is the relationship between tool wear and tool temperature?
Higher the temperature, the higher the tool wear
252
How does a cutting fluid affect the tool temperature?
Doesn't not reduce peak temperature, but reduces the volume off the tool at high temperature
253
What is the main purpose of cutting fluid?
Prevents distortion of workpiece by minimising thermal strains
254
Where on the tool does wear occur?
Clearance face - flank wear
Rake face - crater wear
255
What are the consequences of flank wear?
Lose dimensional control
Surface finish deteriorates
More heat generated
256
What is crater wear? Where does it occur?
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
What does crater wear depend on?
Temperature and degree of chemical affinity between tool and workpiece
258
How is machinability of a workpiece defined?
Hardness
Surface texture
Max rate of material removal
Tool life
Chip formation
259
How can the microstructure of a workpiece affect its machinability?
Workpieces usually contain hard or soft particles which can be ductile or brittle
Hard particles cause abrasive wear
Soft particles improve machinability
260
What can be added to a workpiece to improve machinability?
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
What properties make a god tool?
Hardness at high temperature
Thermal shock resistance
Wear resistance
Chemical stability
262
What are the two types of high speed steels?
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
What effect does the addition of tungsten or molybdenum have on the high speed steels?
Hardened by the complex carbides that form
264
What are cemented carbide tools?
Carbides are bound to a metal using powder processing
265
What are the two types of cemented carbide tools?
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
What happens when a WC in Co tool is used to machine steel?
WC readily dissolves in steel at 1200K so tool eventually is dissolved into the chips
267
What tool would be used to machine steels?
Diffusion resistant grades, 10-40% TiC or TaC
268
How can tools be coated?
PVD or CVD
269
What types of coatings are available for use on metal tools?
Titanium nitride
Titanium carbide
Titanium carbonitride
Titanium Aluminium nitride
Aluminium titanium nitride
Ceramics
Multiphase
Diamond
270
What are the two main types of ceramic tool?
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
What is sialon?
Based on silica, but some Is replaced by Al, some of N replaced by O, add Yttria
272
What advantages does Sialon offer?
Higher resistance to thermal shock compared to silica, used to machine cast irons and nickel-based superalloys
273
Can silica based tools machine steel?
No, there is a high dissolution wear rate
274
What are cBN cutting tools?
Made by bonding layer of polycrystalline cubic boron nitride to a carbide substrate
275
What are cBN tools used to machine?
Hardened ferrous and high temperature alloys
276
Can diamond tools be used to machine steel? Why?
No, above 650 degrees, diamond reverts to graphite which is very soft
277
What costs are involved in cutting?
Fixed costs (material)
Machining costs (labour costs)
Cost of tool change (tool wear)
278
Does using the highest production rate always yield the minimum component costs for cutting?
No, depends on tool wear rates and hence tool costs
