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148 Cards in this Set
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What do we need to know when we try to select a material? What are the two main considerations as engineers?
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L1
1. What materials are available 2. What processes for shaping these materials are available 3. Costs of both the materials and the production line 2 key considerations 1. What material 2. Which production route |
3 things for material selection
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In which two categories do we split processes? List examples for both categories.
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L1
Primary Shaping processes 1. Casting 2. Pressure Moulding 3. Deformation Processing 4. Power methods 5. Special methods Secondary Shaping processes 1. Machining 2. Heat treatment 3. Joining 4. Finish |
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Compare Aluminium vs Steel for the auto-mobile industry.
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L1
1. Cost of steel 1/3 to 1/4 that of aluminium 2. Density of steel is 3 times that of aluminium 3. Steel has a greater specific stiffness 4. Energy absorption rate for steel is higher, useful for impacts when crashes happen 5. Steels have distinct fatigue limits unlike aluminium which just degrades. 6. Steel has higher formability than aluminium (formability of Al 2/3 that of steel) 7. Hardness of steel higher 8. Steels are magnetic while aluminium isn't which negatively affects its recyclability |
In terms of cost, mechanical properties (density, moduli, fatigue, stiffness, energy absorption, recyclability, hardness)
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What do efficient designs have as an attribute?
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L2
They incorporate materials properties into each stage of the design process. |
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What are the considerations for a process selection?
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L2
1. Size and scale 2. Cost 3. Environmental impact 4. Energy requirements 5. Materials |
5 considerations
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Explain briefly the general stages of a casting process.
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L2
Molten metal is placed in a mould cavity where it is left to solidify. At the end there is a finishing stage. |
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What types of casting exist? Explain each type briefly.
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L2 - 1. Sand Casting: using sand as mould material/bad finish/cheap
2. Permanent mould casting: using reusable moulds, usually made from metal, gravity/vacuum/gas are used to eliminate gaps 3. Investment Casting: using wax patterns to create accurate moulds/expensive 4. Die casting: molten metal forced into a mould cavity under high pressure/tool steel die moulds |
4 types
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Construct a table of comparison for sand, investment and permanent mould casting in terms of applicability, finish, tolerances.
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L2
Sandcasting: Applicability WIDE, Finish POOR, Tolerance POOR Permanent mould: Appl. LIMITED, Fin. BEST, Tol. BEST Investment mould: Appl. WIDE, Fin. GOOD, Tol. GOOD |
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Construct a table of comparison in terms of economics for the different casting processes.
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L2
Sandcasting: Mould L, Equipment L, Labour L/M, Production rate: <20 pieces/hour Permanent mould: M. M, Lab. L/M, Eq. M, Pr. R.: <60/h Investment: M. M/H, Eq. L/M, Lab. H, Pr. R: <1000/h Die casting: M. H, Eq. H, Lab. L/M, Pr. R.: <200/h |
In terms of labour/mould/equipment/productionrate
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Draw a mould with a cast metal placed inside and indicate the different mould areas formulated during cooling. Explain briefly each one of them.
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L2
Columnar zone: grains growing towards the centre, grow faster, parallel to the direction of max temp Chill zone: fine grains, first solid layer around mould Shrinkage (aka pipe): solid becomes denser so it occupies less volume than liquid material, i.e. there is a shrinkage Equiaxed zone: fine grains at centre, last so solidify |
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What is the definition of a solid state process? List all solid state processes and their categories.
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L2
Processes that cause plastic deformation on a solid specimen through the use of forces by dies and other tools. PRIMARY: Rolling, Forging, Extrusion SECONDARY: Wire Drawing, Machining |
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What happens during rolling? What is the equation of the rolling force? What is the effect of an increased temp on rolling?
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L2
Rolling: The process of reducing the thickness or cross sectional area of a workpiece by compressive forces applied through a set of rolls. F=xw((R(h0-hf))^1/2)*flowstress As temp increases, flow stress decreases hence F rolling decreases. |
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What is the most popular cold worked process in the world?
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L2
90% of the metal produced by metal working is rolled. |
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Draw the defects that can occur during rolling.
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L2
Anisotropy an extra problem |
4 defects
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What happens during the extrusion of a metal? What is the equation of the extrusion force?
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L2
A round billet is placed into a chamber and forced through a die by a ram. Die dictates final shape. (Can produce, round, I, T cross sections). Usually takes place at high temperatures. Extrusion ratio R=A0/Af Extrusion force F=kA0lnR OR P/s=a+blnR |
Squeezing toothpaste from a tube example.
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What defects can occur during extrusion?
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L2
If not controlled, defects can occur during extrusion. If extrusion temp too high or too low for material deformation, surface cracking can occur. Anisotropy. |
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What happens during the forging process? Give examples of products produced by forging. What is the upsetting force equation?
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L2
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Examples of components are crankshafts, gears, wheels, turbine blades and other structural components. Can happen in room or higher temp. F=flowstr*(1+((2*mu*r)/(3*h)) ) |
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Explain the concept of forgeability. What does good forgeability mean?
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L2
The capacity of a metal to undergo deformation without severe surface cracking. Good forgeability=low forces needed for shaping, no cracking |
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Explain isothermal forging. What are the pros and cons?
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L2
Dies heated at same temp as work piece, i.e. no cooling of specimen during process. Low flow of material maintained. Allows much greater control over micro-structural and mechanical properties. Very good dimensional accuracy. Relatively expensive. |
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What are the defects that can occur during forging?
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L2
Surface cracking cab occur which can affect the fatigue limit of the component. Anisotropy. |
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What is anisotropy? Why does it occur?
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L2
The material properties are not uniform but they are directional. Occurs due to grain orientation and dislocations. |
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What does the friction stir welding technique involve?
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L2
Involves joining metals without the need for filler materials. |
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Advantages and Disadvantages of Friction stir welding?
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L2
Pros: 1. Excellent mech properties (fatigue and tensile strength) 2. No porosity/distortion 3. Non consumable tool 4. Possible to perform on the spot repair work Cons Costly, inappropriate for some materials |
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What is porosity in metals?
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L2
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0 and 100%. It can occur due to cooling in casting. |
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What is additive manufacturing? What are the advantages of this technique? Mention an example of where this process can be used.
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L3 Layer upon layer addition of material. Achieves near net shape. Low amount of scrap. Design iteration friendly (Formula 1). Customisable. Low lead times. Capable of manufacturing of parts deemed impossible with other techniques. E.G. Machines for medical purposes.
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How does the ARCAM laser work?
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L3 Most promising laser technology, preheats at each layer. Excellent fatigue properties compared to casting. Similar to forging.
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Disadvantes of additive manufacturing?
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L3 Expensive. Powders expensive and require special treatment. Highly skilled staff required.
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What happens during the Metal Injection Moulding Process?
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L3 Finely powdered metal is mixed with binder material to create feedstock being able to be treated appropriately.
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Give an example of a component that can be manufactured by MIM.
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Stator vanes, swirlers, levers. High precision aerospace components.
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Advantages of Metal Injection Moulding?
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L3 Very high component complexity possible. Competitor against precision casting. Disruptive competitor for machining small parts. Large parts can also be made.
Much lower energy footprint than casting. Very competitive for steels, great for small parts. |
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How does Foxconn produce very small metal parts for Apple?
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L3 Using Metal Injection Moulding.
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What happens during the Hot Isostatic Pressing?
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L3 A component is subjected to isostatic gas pressure in a high pressure vessel using an inert gas. During this compression, porosity in the material is reduced.
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Advantages and Disadvantages of Hot Isostatic Pressing?
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L3 Pros: Trusted process, reduces porosity in metals, improves workability and other mechanical properties. Unparalleled shape complexity and surface quality.
Cons: expensive, too niche, complex, low volume |
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List all solid state, liquid state and power metallurgy processes.
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L3 Solid state: Primary; Rolling, Forging, Extrusion. Secondary; Machining, Wire Drawing
Liquid State: Sandcasting, Investment mould casting, Permanent mould casting, Die Casting Powder state: ALM, MIM, HIP |
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What properties does the bond energy curve in metals determine? What happens to the melting point as binding energy increases?
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L4 Melting temp, elastic modulus, thermal expansion coefficient determined by bond energy curve.
As binding energy increases, so does the melting point. |
3 properties determined by bond energy curve
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Why do materials expand when the temperature increases?
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L4 When temperature increases, thermal expansion occurs. This happens due to the molecules starting to vibreate around bigger amplitudes because of excess energy.
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What is the equation for the thermal expansion coefficient? What does this coefficient depend on?
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L4 a=1/l * dl/dT
Material dependent |
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What happens to the thermal coefficient as bonding energy increases?
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L4 It decreases.
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Draw the following curves; density vs temp, Youngs modulus vs temp, resistivity vs temp, refractive index vs temp.
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L4 They all decrease linearly with temp apart from resistivity which increases linearly.
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Draw the FCC, BCC and HCP structures.
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L4
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Which of the microscopic packing structures are considered close packed?
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L4 FCC, HCP
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How many atoms do FCC, BCC and HCP pack per unit cell?
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L4 FCC = 4 atoms/cell
BCC = 2 atoms/cell HCP = 6 atoms/cell |
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What is the lattice distance of BCC, FCC and HCP?
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L4 FCC=2*(2^1/2)*R
HCP c/a=1.633 BCC=4R/(3^1/2) |
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What properties does atomic arrangement control?
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L4 Tensile behaviour, fracture behaviour, density, crystal structure can cause anisotropy in properties
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Explain the phenomenon of polymorphism in metals.
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L4 Polymorphism is a physical phenomenon where a material may have more then one crystal structure. A material that exhibits polymorphism exists in more than one type of lattice space in solid state.
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Definition of polymorphism.
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What is allotropy? Give an example of a material that exhibits allotropic behaviour.
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L4 If the changes in structure found in a polymorphous material are reversible then the polymorphic change is known as allotropy. Iron is an allotropic material.
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Draw a dL vs Temp curve showing the different grain stages (allotropy) of Iron.
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L4
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What happens to iron at 912 degrees Celcius?
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L4 It becomes an FCC structure from a BCC one.
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Identify all the three stages of iron crystal structures and the temperatures that these occur.
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L4 Before the 912 Celsius point, iron is BCC with ferrite type (alpha Iron) grains. Past 912 Celsius, Iron becomes FCC with austenite type grains (gamma iron) and finally at 1394 Celsius, delta iron occurs with a BCC structure.
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Examples of metals exhibiting allotropy.
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L4
Ti and Fe |
2 metals
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Why does contraction occur when there is a transition between a BCC to FCC structure?
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L4 FCC packs more atoms in the unit cell meaning that the density of the material will increase leading to an overall contraction.
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What is the physical mechanism responsible for plastic deformation and ductility?
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L5
The physical mechanism responsible for plastic deformation and ductility is the movement of atomic defects such as dislocations. |
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What kinds of defects do materials have?
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L5
Defects in macro, micro and atomic level. |
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How do defects occur? Give examples.
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L5
Due to impurities, processes or vacancies during solidification. |
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Give two examples of an material property improvement due to substitutional atoms.
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L5
Corrosion resistance e.g. in steels. or strengthening of Cu-Ni alloys by increasing Ni content. |
In steels and Cu-Ni alloys.
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How does iron's crystalline structure change when interstitial carbon atoms are present?
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L5
Body centred cubic becomes Body centred tetragonal BCC to BCT (Carbon in Iron) |
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Why does hardness increase when interstitial atoms are present?
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L5
The hardness increases because interstitial atoms increase the distortion of the lattice. |
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What is creep?
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L5 - Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. A result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials subjected to heat for long and increases as they near their melting point. Dependent on temp, time, stresses.
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What happens in the microstructure of the material during creep?
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L5
Voids appear on grain boundaries |
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Find the equivalent number of vacancies in 1 m^3 of Cu at 1000 C.
densityCu = 8.4 g/cm^3 Atomic mass = 63.5 g/mol Activ. energy = 0.9 eV/atom Avgdro's no = 6.02*10^23atoms/mol |
L5 example
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How do dislocations allow for plastic deformation?
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L5
It is because they move on slip planes under the application of a shear stress that plasticity occurs. |
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Explain the mechanism of slip.
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L5
The mechanism of slip consists of the movement of two adjacent crystal regions relative to each other. Dislocations separate slipped regions for unslipped regions. |
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What is dislocation motion? Where does it occur?
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L5
Dislocation motion is the movement of dislocations. It usually happens in the most close packed planes. |
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What is a slip system? How many slip systems exist in FCC, BCC, HCP?
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L5
A slip system is a combination of a slip plane and a slip direction. FCC: 12 slip systems BCC: up to 48 HCP: 3 slip systems |
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Why are HCP structures usually brittle?
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L5
HCP structures have a very limited number of slip systems meaning that dislocations are not able to move easily. Hence, strong but not ductile (brittle). |
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Define the terms slip plane, slip direction and slip system.
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L5 - Slip system: a combination of slip plane and slip direction
Slip plane: plane of greatest packing density Slip direction: closest packed direction within a plane |
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What is a slip band?
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L5
Slip does not occur on just one plane but on regions of parallel planes called slip bands. |
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What happens when a lot of obstacles to dislocation are present?
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L6
Lots of obstacles---> Limited dislocation movement resulting in high strength |
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What happens when a few obstacles to dislocation are present?
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L6
Few obstacles --> easy dislocation movement resulting in toughness and ductility |
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What assists dislocation motion?
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L6
High temperatures (diffusion) and availability of slip systems. |
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Mention five types of obstacles to dislocation motion.
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L6
Other dislocations, Grain boundaries, precipitates, cracks, other atoms. |
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What is the critical resolved shear stress?
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L6
The critical value of shear stress on a slip system for plastic deformation to occur. |
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When does plastic deformation occur?
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L6
When the critical resolved shear stress value is exceeded in a favourably oriented slip system. |
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What is the equation for the critical resolved shear stress?
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L6
t=sigma*cosfi*coslamda |
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What are the types of dislocations. Draw them.
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L6
Edge and screw dislocations. |
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When do dislocations move on slip planes?
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L6
When the critical resolved shear stress value is exceeded. |
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What is the DBTT?
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L6
Ductile to brittle transition. The temperature at which a material loses its ductility and starts exhibiting brittle behaviour. |
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What happens in DBTT? Why does the material start behaving brittlely? :P
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L6
At the ductile to brittle temperature point, the slip planes of bcc and hcp start becoming inactive allowing little dislocation motion. Hence limited ductile and more brittle behaviour. |
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Define Toughness.
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L6 Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Or The amount of energy per volume that a material can absorb before rupturing. It is also defined as the resistance to fracture of a material when stressed.
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What does toughness depend on?
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L6 - Toughness depends on dislocation motion which happens to be temperature dependent. Toughness is the most important property regarding DBTT.
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Sketch the curve of absorbed energy vs temperature for steel and aluminium. Explain why they are different.
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L6 - Aluminium does not have a distinct DBTT that's because the dislocation motion in the FCC structure of Aluminium is independent of temperature while the BCC structure of steel is highly temp dependent.
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What is the charpy test?
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L6 - An impact test used to determine the absorbed energy in different conditions (e.g. temperature)
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Compare Toughness and Strength. What's the difference?
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L6 - Toughness is the ability of the material to absorb energy and plastically deform without fracturing. Strength is the measure of how much load a material can resist without rupture. In order to be tough, a material must be both strong and ductile.
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What is the von Mises criterion for plastic deformation?
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L6 At least 5 independent slip systems must be operational for plastic deformation to occur.
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How many slip systems must be operational in order for plastic deformation to occur?
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L6 - According to the von miss criterion, 5 independent slip systems must be active for plastic deformation to occur.
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How many slip systems does FCC, BCC and HCP have? How many of them are temp dependent in each?
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L6 - FCC has 12 slip systems ans 5 of them are temp independent and operate at all temps. BCC has 48 slip systems all of them temp dependent. HCP has 12 slip systems highly temp dependent.
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Explain how grain boundaries impede dislocation motion. Draw relevant sketch.
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L6 - Grains have different orientations in a polycrystal and dislocation motion is hindered by the these grain boundaries (planar defects). (Misoerientation between grains)
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What happens to the yield stress of a material as the grain size reduces? Why does this happen?
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L6- Yield stress stress decreases as grain size increases. This is due to the fact that larger and fewer grains have less complex and fewer boundaries between them hence dislocation motion is improved.
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What is the equation relating grain size and strength?
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L6 Hall Petch equation
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What happens to ductility as grain size decreases?
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L6 Ductility decreases.
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How do we solve the problem of wanting metals easy to form and shape but with the final components being very strong?
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L6 - We use temperature sensitivity of strength. Metals are softer and easier to shape at high temperatures. Then we use them in cool temperature.
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What happens if we choose wrong temperature in forging?
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L6 - We will either exceed the maximum capacity of the forging press or produce defective material.
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How do we classify processes in terms of temperature? How are these classifications defined?
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L6 - We classify processes as cold, warm and hot working. Hot working defined as T/Tmelting>0.6
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At what temperature does warm working of lead occur?
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L6 - At room temperature. It is consider warm working.
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What is cold working?
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L6 The strengthening of a metal by plastic deformation in relatively cold temperature (usually room temperature).
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What are the effects of cold working?
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L6 - When a crystal is deformed, dislocations interact and obstruct each other. They are generated in large numbers. And hence this leads to work hardening.
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How and why does work hardening occur?
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L6 - Dislocations interact and obstruct each other while the crystal is deformed. They are also generated in big numbers --> this increases the strength of the material. As strain increases dislocation density increases and flow stress too.
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What happens as strain increases?
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L6 - Plastic deformation primary on glide planes. Flow stress increases with strain but with strain rate. Strength increases.
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Difference between hardness and hardenability?
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L6 - Hardness is a material property. A measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied. Hardenability is a way to indicate a material’s potential to be hardened by thermal treatment.
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What are the effects of cold working on mechanical properties. Plot graphs of yield strength, tensile strength and ductility for steel vs % cold work.
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L6 - As cold work increases, yield strength increases, tensile strength increases and ductility decreases.
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Why do cold worked materials exhibit reduced ductility?
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L6 - A cold-worked material is, in effect, a normal material that has already been extended through part of its allowed plastic deformation. If dislocation motion and plastic deformation have been hindered enough by dislocation accumulation, and stretching of electronic bonds and elastic deformation have reached their limit, a third mode of deformation occurs: fracture.
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How can we overcome the problem of work hardening?
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L6 - By annealing (heat treatment). Annealing undoes work hardening.
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What is annealing?
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L6 - Annealing is a heat treatment that alters a material to increase its ductility and to make it more workable. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling.
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What happens to the stored strain energy when a cold worked material is annealed?
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L6 - The stored strain energy reverts back to the pre-cold working state after annealing.
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What happens when we anneal a material?
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L7 - Restoration of mechanical properties through 3 mechanisms. 1. Recovery, 2. Recrystallisation, 3. Grain growth.
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What are the restoration mechanisms that are active when the temperature is increased (e.g. in annealing.) Sketch what happens in each stage.
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L7 - The Recovery, Recrystallisation and Grain growth stages.
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What happens to ductility and to the tensile strength as the restoration process goes through all 3 stages? Plot relevant diagram.
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L7 - As the restoration process reaches its end, the grain size increases hence the strength decreases and the ductility increases.
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Describe in detail the three stages of restoration during annealing.
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L7
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What happens during the recovery stage of the annealing process?
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L7 - 1. Recovery: 1. Some internal strain energy is relieved due to dislocation motion as a result of enhanced diffusion at high temps. 2. Material softens, linear defects and their stresses are removed. 3. Grain size and shape do not change.
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3 things
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In which annealing stage do strain free grains appear?
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L7 - In the recrystallisation stage.
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Which stage of restoration takes place at the lowest annealing temperature?
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L7 - Recovery.
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What is the driving force behind the recovery taking place in the first stage of annealing?
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L7 - The movement and removal of dislocations due to the enhanced diffusion at high temps.
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What happens during the recrystallisation stage of annealing?
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L7 - 1. New strain free grains are formed and they nucleate replacing the previous deformed ones. 2. There is massive reduction in dislocation density. 3. Creation of dislocation and strain free grains.
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3 things
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In which stage of annealing do dislocation and strain free grains appear? What happens to the dislocation density and why?
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L7 - In the recrystallisation stage. The new grains nucleate and they replace the old deformed ones. This causes massive reduction to the dislocation density and strain free, dislocation free grains appear.
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What is the driving force of recrystallisation?
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L7 - The difference in internal energy between strained and unstrained material.
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What happens to the mechanical properties of the metal as the recrystallisation takes place?
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L7 - Material becomes less strong and more ductile as the recrystallisation reaches the end.
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What is the final stage of annealing?
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L7 - Grain growth
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What happens during the grain growth stage of annealing?
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L7 - The recrystallised strain free grains will continue to grow id a metal is held at an elevated temperature past the recrystallisation process.
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What happens if a metal os kept at an elevated temperature after the second stage of annealing?
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L7 - Strain free grains will continue growing. Microstructure starts to coarsen and there is a strength loss.
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What happens to the microstructure as the grain growth stage of annealing takes place. What happens to strength and ductility?
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L7 - It coarsens. Strength loss. Ductility increase.
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What is the process of hot working?
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L7 - It includes cold working with subsequent or simultaneous annealing.
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Why is hot working beneficial?
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L7 - It annihilates the dislocations caused by the cold working processing hence it restores the materials mechanical properties while keeping the shape as it is.
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When does dynamic recrystallisation occur?
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L7 - When annihilation of dislocations happens at the same time as deformation due to cold working. Simultaneous annealing and cold working.
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When does static recrystallisation occur?
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L7 - When the annihilation of dislocations caused by cold working are annihilated immediately after the cold working. Annealing after cold working.
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What is the difference between dynamic ans static recrystallisation?
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L7 - In dynamic recrystallisation there is simultaneous deformation and dislocation annihilation during forming the material (Annealing + Cold working at the same time). In static recrystallisation, annihilation happens after the cold working.
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What is the TMP process?
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L7 - Thermomechanical processing. A forming process that controls temperature and deformation conditions to achieve target microstructures for required mechanical properties.
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How do we achieve target microstructure using TMP? What is generally required to achieve this?
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L7 - By controlling deformation and temperature conditions throughout the whole process. Requirements are a good understanding of metallurgy and an advanced control of the industrial operation.
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What are the differences between cold, warm and hot working?
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L7 - Cold: no dynamic restoration, limited strain due to hardening, loss of ductility, requires annealing to restore formability, excellent shape dimensional tolerances and finish/ Warm: Between hot and cold, no dynamic recrystallisation but some recovery is present, cooler so tol. and finish are good. Popular in forging and automotive parts / Hot working: annealing + cold working - dynamic or recrystallisation
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Cold, warm or hot working achieves the best finish and dimensional tolerances?
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L7 - cold working
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What are the disadvantages of cold working?
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L7 - loss of formability due to work hardening, loss of ductility/limited strain, requires annealing to restore mechanical properties
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3 things
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What is the main advantage of cold working?
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L7 - excellent shape, finish and dimensional tolerances.
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What is the starting microstructure of TMP?
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L7 - As cast microstructure is the beginning microstructure.
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What is the Hall Petch Relationship?
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L7 - It relates strength and grain size. s0=si+kd^-1/2
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What does the hall petch relationship show for fine grains?
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L7 - Smaller grains, higher strength.
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Plot the hall petch relationship (yield stress vs grain size)
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L7
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What are the characteristics of the starting component for TMP in terms of microstructure?
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L7 As cast microstructure, large grain size and chemical segregation.
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What is the effect of chemical segregation on properties?
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L7 - Chemical segregation causes heterogeneity in properties
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Plot a traditional hot rolling process (Temp vs time) and a TMP. Compare.
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L7
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What are the disadvantages of traditional hot rolling?
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L7 - expensive and 2 stages instead of continuous.
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What are the advantages of TMP?
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L7 - Better result than hot rolling, cheaper
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What does accelerated and normal cooling produce in TMP?
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L7 - finer α grains and fine α grains respectively.
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What is the final microstructure of TMP based on?
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L7 - The starting microstructure hence it is important to trace the metal back throughout its whole processing to see full properties.
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What do we need for an accurate prediction of material behaviour after TMP?
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L7 - We need to track the metal throughout the whole processing since TMP produces initial microstructure.
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Draw a diagram showing how each process contributes to final process.
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L7
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Why do we study the TMP?
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L7 Greater understanding of TMP leads to better control hence reduced variability in properties and better tolerances. Less waste through improving properties of existing simple alloys. TMP reduces process steps.
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3 things
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What are the problems occurring during metal forming? How do we deal with them?
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L7 - Oxide scale formation: can decrease mech properties and cause bad finish (Use hot working to achieve large thickness reduction then cold working for correct dimensions)
Poor ductility even at elevated temps: use isothermal forming. Increased cost for materials that do not behave well. |
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How do we improve material properties?
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L8 - Through mechanical processes and alloying.
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What are the families of aluminium alloys?
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L8 - 2 families: 1. Derives strength from heat treatment (precipitation hardening) 2. Derives strength from grain size refinement, solid solution str., cold working
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