Processing for Control of Material Properties Flashcards
What are the broad classifications of processing for the course?
Casting + Moulding Deformation processing Heat treatment and joining Powder processing Processing of hybrid materials
State the four parts of cost modelling.
Material
Tooling
Capital
Overhead
What is the capital cost relating to the cost of a component?
The maintenance of equipment cost.
What is the overhead cost relating to the cost of a component?
The cost of labor, admin, rent.
What can process modelling be used to optimise?
Furnace design
Baffle arrangement
Geometries that can be produced
Minimise scrap
What is processing?
The series of steps used in the manufacture of raw materials into finished, high value products.
What is the importance of process innovation?
It drives technological change and economic growth.
Defects determine what?
Control of behaviour and properties.
What die material is needed for metal die casting?
Ceramics such as alumina, silica or mullite.
What die material is needed for injection moulding of polymers?
Steel
What is casting for metals?
Where molten metal is poured into/onto a mould to allow for solidification and then cooling.
What us moulding of polymers?
Polymer heated until viscous (or melted for low molecular mass thermoplastics) then forced into a mould under pressure.
List the possible likely defects arising from casting or moulding due to heat flow.
Shrinkage
Cracking, tearing
Segregation
In polymers mix of viscoelastic and elastic behaviour
What possible resistances to heat flow are there in casting and moulding?
The liquid (often convects quickly so slow barrier)
The solid
The solid-mould interface
The mould
The interface between mould and surroundings (rarely important)
In investment casting and sand casting, what is the biggest barrier to heat flow?
The mould
State Chvorinv’s rule for solidification time.
Check L2 S7
What are the underlying assumptions in castings with insulating moulds?
Unidirectional solidification
No superheat in the liquid (it is at the solidification temperature)
Considered to have a single, unique melting point
The temperature of the solid is constant and equal to the melting temperature
The mould has infinite thickness.
Derive an expression for heat transfer in castings with insulating moulds (differential eqn).
Check L2 S9
Derive an expression for the penetration depth of solidification in a casting using the linear integral profile method.
Check L2 S10
Derive an expression for the penetration depth of solidification in a casting using the parabolic integral profile method.
Check L2 S11
Derive an expression for the penetration depth of solidification in a casting using the exact solution.
Check L2 S12
Set up an expression for the balance of latent heat evolved at the solid liquid interface in casting to the heat crossing the mould solid interface in casting limited by interface resistance.
Check L2 S115
State the key result of castings in conducting moulds.
Check L2 S16
When is solidification rate controlled by the heat transfer through the cast solid?
In castings with conducting moulds.
State an equation for fluidity of a material.
Check L2 S17
What is fluidity?
An empirical measure which depends upon viscosity and solidification rate. A spiral fluidity rate is often used.
State an expression for the shear thinning exponent of polymers at high strain rate.
Check L2 S20
State an expression for flow rate of a polymer.
Check L2 S21
By what devices can viscosity of polymers be measure?
Ostwald viscometer (most common)
Ubbelohde
Externally pressured
Ram extrusion
Derive an expression for traction of a material under steady flow in a capillary tube in both Newtonian and non-Newtonian systems?
Check L2 S23
Derive an expression for the flow rate under steady flow through a capillary with a Newtonian fluid.
Check L2 S24
Derive an expressionfor the flow rate under steady flow through a capillary with a non-Newtonian fluid.
Check L2 S27
Using elongation flow and conservation of volume show stat dε(t)/dt = dv/dz.
Check L2 S29
Derive an expression for elongation velocity
Check L2 S29
Why is branched PE said to tension-stiffen, whereas linear Pe is said to tension-thin?
The branches provide ‘hooks’ thus increasing the resistance to flow.
What is the advantage of tension-stiffening in branched PE?
The stress increases at the neck to counteract the increased velocity so drawing is facilitated (used in the polymer film market).
What is the effect of elastic response in polymers in injection moulding?
The stored energy is released upon exiting the mould resulting in some expansion “die swell”. Fringe patterns are observed under cross polarised light. Higher flow rates lead to higher strain rate and thus higher birefringence.
When do defects such as sharkskin become an issue with polymer moulding?
At very high shear strain rates the limit of molecular alignment is reached and the phenomena of melt fracture occurs (stick/slip at the die due to elastic turbulence). The change in internal strain due to stick/slip at wall leads to internal fracture and re-welding.
Most silica glasses also contain what?
Network modifiers. (Eg Na2O and CaO)
What do network modifiers do?
Disrupt the network by disconnecting some O atoms which are single bonded to Si. In turn the high T viscosity is reduced making the glass more workable. The trade off is that the glass is more prone to crystallise.
Describe the Pilkington process.
A method for producing glass sheets.
Molten glass is poured onto liquid which acts as a very flat mould.
Sheet moves at velocity.
Sn bath used as liquid due to higher density but lower melting point than the T at which glass becomes rigid.
Derive an expression for the time needed for steady state float glass processing.
Check L3 S10
What is Biot number?
The ratio of convention to thermal conduction in a material.
How can glasses be chemically toughened?
Ion stiffening: dip in molten potassium sulphate K+ ions replace Na+ placing the surfaces in compression.
How can glasses be thermally toughened?
Rapid cooling places the surface in compression due to contraction and interior in tension.
This causes greater resilience to surface flaws.
Describe the process of continually casting steel.
Molten steel taped from bottom of a ladle into a tundish.
Open mould consists of four water cooled plates waging hot steel slides.
Cooling by continuous quenching with water along the whole strand.
Sheet is glowing hot but solidified all the way through when cut into billets by oxygen lances.
Defects from continuous casting of steels
Level fluctuations Meniscus freezing non uniformly Poor flux infiltration Inclusions Flux entrainment Cracks from thermal stress, building, metallurgical embrittlement.
Derive an expression fir age solidification time in continuous casting with no air gap.
Check L3 S24
Benefits of continuous casting of steel,
Energy saving. Less scrap Improved labour productivity Improved steel quality Reduced pollution Can be automated
Challenges of continuous casting of steel
Contamination from various sources
Formation defects
Where does most energy to melt polymers in thermoplastic extrusion moulding come from?
Mechanical work
Describe the process of blow-film extrusion of polymers.
Molten polymer is extruded through a ring shaped die around a mandrel
Tube or sleeve is formed and expanded around an air bubble,
It is cooled below Tm and rolled up into a flattened tube and wound up.
Typical pressure range for injection moulding.
55-275 MPa
In injection moulding, what happens to pressure and flow rate over time?
Initially, the flow rate is high then pressure needs to increase over the cycle to avoid short shot and maintain constant flow rate.
Describe blow moulding of polymers.
Tube of molten polymer placed in hollow mould.
Air injected to inflate.
Once dimensionally stable, mould opened and bottle ejected.
Why isn’t Al cast often?
Shrinkage problems
What is added to Al to combat shrinkage in casting alloys?
Si
What is added to Al-Si alloys to refine the structure?
Al-Ti-B added to Al rich alloys
P is added to Si rich alloys
What modifier is used in Al-Si alloys?
Na (small additions of 0.1%)
Sr (0.02%)
How can light alloy materials such as Al or Mg be cast?
With pressure die casting.
What are general defects in Al castings?
Stable oxide still forms reducing toughness.
H is easily picked up and dissolved by molten Al alloys.
Turbulent flow can cause porosity on cooling
What is the Reynolds number?
The ratio of kinetic energy to degree of viscous damping of a fluid.
Why was the Cosworth process developed?
Due to problems in cast engines such as porosity, dimensional errors and fatigue failures.
Describe the Cosworth process.
Metals prepared by melting then degassing with argon.
Melt held for minimum of 10 hours then pumped through ceramic filter.
All prep done holding continuous metal level casting minimum oxide generation.
Metal then fills mould that contains metal positioning sensors in a closed loop.
Information is recorded about each casting and the metal cleanliness is the same as that of a furnace.
What are the desirable characteristics of Ni superalloys?
Ability to withstand loading close to melting T (T(opp)/T(m)>0.6)
Substantial resistance to mechanical degradation over time (ie creep)
Tolerance to severe operating environments (hot corrosion/oxidation resistance)
How can Ni turbine blades operate above their T(m).
Serpentine inner core structure in blade made vey Silica cores in casting that are removed by leaching with acid solvents.
Holes in the surface of the blade allow cool air from serpentine structure to form boundary layer over blade surface providing effective insulation.
Derive an expression for the heart transfer during directional solidification.
See L3 S81-83 and tube sheet 1
Why must heat management ve accounted for when casting components?
The component does not usually cool down uniformly which can lead to a lack of uniformity, thermal stresses, distortion defect formation etc.
Define deformation
The change of the shape/size of a continuous solid body over time resulting from the application of a force.
What is the quantitive measure of deformation of a body?
Strain
What is the quantitive measure of the deforming force?
Stress
Define what is meant by yield point.
Transition from elastic to plastic behaviour
Define engineering strain.
e = (l1-l0)/l0
Define true strain.
dε = dl/l
ε = ln(l1/l0)
What must be true about the stress tensor (Cauchy)?
It must be symmetrical
Which are the hydrostatic (dilation) components of the stress tensor?
The diagonals, they act only to change the volume of a material.
Which are the deviator components of the stress tensor?
The non-diagonals that act to change the shape of the material only.
Does the mean stress (hydrostatic stress) strongly affect yield?
No
What is Tresca yield criterion based on?
The assumption that yield occurs when the maximum shear stress reaches a critical value (based on dislocations).
What shape is the surface that defines the yield criterion around the hydrostatic line for Tresca yield?
Hexagonal
What is VM yield criterion based on?
The assumption that yield occurs when distortional energy exceeds a critical value.
What shape is the surface that defines the yield criterion around the hydrostatic line for VM yield?
Circular
State equations for Tresca and VM yield criterions.
Check
What happens to Mohr’s circle for transition from pure torsion to uniaxial tension under Tresca’s yield criterion?
Translates to right
What happens to Mohr’s circle for transition from pure torsion to uniaxial tension under VM’s yield criterion?
Translates to right and reduces in radius
What are the major assumptions in Tresca/VM?
Yield strengths are the same in compression and tension.
Volume is conserved by plastic deformation (“ν” = 0.5)
The mean stress (hydrostatic stress) does not strongly affect yield.
Which yield criterion is used for polymers and other non metal materials?
Mohr-Coulomb
State the Mohr-Coulomb criterion equation
Check
What is the Mohr-Coulomb criterion based on?
Friction
What material property controls the slip plane in M-C criterion?
Internal coefficient of friction.
tan2θ = -1/μ
What happens at plastic instability?
Necking
What happens to a material during necking?
The supportable load begins to decrease and localised necking becomes unstable.
How can stable plastic deformation be predicted if before peak in a σe vs e curve?
Strain hardening can be predicted by assuming uniform plasticity and empirically with power law equations (Hollomon equation and Ludwick equation)
State the Hollomon equation, Ludwick equation and Ramberg-Osgood equation,.
Check L4 S26
Derive the Considere criterion for plastic instability.
Check L4 S29 and tube sheet
What us the Considere criterion?
The increase in load in any given element of material is less than or equal to zero fir instability. The load increase is positive from work hardening (and dominates at first) but negative from the change in area.
What is plotted on a Considere construction?
The true stress is plotted against engineering strain. When the line between the stress and the point (-1,0) is a tangent to the curve, then this is the limit of plastic instability.
According to the Hollomon equation, derive a condition for when necking starts.
Check L4 S34
What is forging?
Forging is a manufacturing process involving the shaping of metal using localised compressive force.
Advantages of forging
Eliminates hidden defects (cracks and voids)
Refines the microstructure
Rearranges microstructure to conform with the metal flow
Considerably faster than cutting and machining.
Increase in directional and structural strength
Increase in homogeneity
Increase in density
Increase in ductility
Parts can easily be welded
Disadvantages of forging
Rapid oxidation at high T leads to scaling (leads to fast wearing of the dies)
Limited accuracy requires machining to high precision dimensions.
Not suitable for some materials
High initial cost of forging dies and their maintenance
Define cold forging
At or near RT
Strain hardening
Produce greater surface finish and dimensional accuracy
Define hot forging
Above recrystallisation T
When large amounts of plastic deformation needed to form the part.
Eliminates strain hardening
Provides lower dimensional accuracy
Define warm forging
At above 0.3T(recrystallisation)
Forging can also be defined by deformation rate, what types are there?
Hammer (drop) forging
Press (die) forging
Forging can also be defined by material flow and constraint, what types are there?
Open and closed die
In what type of forging is flash experienced?
Closed die
Can closed die forging be done so that there is no flash (flashless)?
Yes, ie if a ram drives the material into the die.
Why does barrelling occur in open die forging?
Friction between the die and material causes inhomogeneous deformation
Under what forging conditions is barrelling most prevalent?
Open die hot forging due to heat transfer between the material and mould.
Derive an expression for the total force and average pressure in open die forging due to slipping friction.
Check L5 S15-19
Derive an expression fro pressure and average load in open die forging for sticking friction.
Check L5 S20-22
Derive an expression for the stick-slip transition in open die forging.
Check L5 S23/24
What is fullering?
Open die forging with convex dies to reduce the CSA causing metal flow away from the worked area
What is edging?
Reduction in CSA in open die forging using concave dies causing metal flow in to the worked area
What is cogging (drawing out)?
Reduction in CSA using flat/contoured dies in a multi step process.
Why do interior and exterior cracks from in forging?
Excessive stress and imporoper stress distribution.
Why can laps form in forging?
caused by a buckling of the part and can result from insufficient material.
Why can warping result from forging?
Caused by temperature non-uniformities.
What tests of frog ability exist?
Upsetting test (appearance of cracks is measurement of failure) Hot twist test (turns to failure is measure of failure)
Considerations for forging manuafacturing process.
Forging die material
Forging die design
Lubrication
Describe the process of rolling.
Rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and make the thickness uniform.
It is measured by draft - the reduction of thickness after rolling.
Advantages of hot rolling.
REduces yield stress No strain hardening Lower loads required allows for higher speed Ductility increases Isotopic microstructure
Disadvantages of hot rolling
Poor surface finish
Possible surface oxidation/decarburisation for steel
REduced dimensional accuracy
Temperature variation may result in non-uniformity of microstructure
A lot of power needed for heating
Fast wear of rolling equipment.
Advantages of cold rolling
Anisotropic microstructure Better surface finish Work hardening Better dimensional accuracy Cheaper as less energy required
Disadvantages of cold rolling
High loads needed
Work hardening if undesirable
Increased residual stress (possible failure)
Typical materials for rolls
Cast irons and steels Forged steels Nickel steels Molybdenum steel alloys Tungsten carbide
Two types of rolling mills
Non-reversing
REversing
How are rolls usually arranged?
In clusters to prevent distortion of rolls during use
During rolling, what is the no slip point?
The point before which the rolls move faster than the material and after which the material moves faster than the rolls
How can rolling cause texture?
Hot rolling can cause recrystallisation along preferential orientations.
Derive a set of conditions for which the workpiece is rejected in rolling.
Check L6 S16-19
Derive approximate expressions for load in rolling.
Check L6 S20-22
Compare how load varies in rolling with front and back tension graphically.
Check L6 S23
List the general defects of rolling.
Internal porosity not closed Grain structure might not refine Can form rolled in scale Difference in thickness In plane stresses lead to deformation of the rolled sheet (centreline wrinkling, centre splitting, edge cracking).
During rolling, the rolls deflect in the middle, what is the solution to this?
Make non-uniform rolls, thicker in the middle to compensate for deflection (add camber).
Define extrusion.
The process used to create objects of a fixed cross-sectional profile.
A workpiece of certain length and cross section, is forced through a die of a smaller cross sectional area, thus forming the work to the new cross section.
What is a butt end?
The small portion of workpiece in extrusion that cannot be pushed through the die opening
What are typical extrusion ratios?
4-100
What metals and shapes is extrusion best suited to?
Stainless steel and Ni alloys, refractory metals (anything that is not easily formed through other routes).
Applicable to shapes that cannot be rolled such as re-entrant angles
What stresses are experienced in extrusion?
Only compressive and shear stresses.
When is hot extrusion used?
For large parts
For significant changes in shape
For less formable metals (eg steels)
When is cold extrusion used?
For easily extruded metals For small parts For less complex parts For discrete extrusions (a complete piece per extrusion) For impact extrusion
Advantages and disadvantages of direct extrusion.
Ads:
Simple in practice
Disads:
Strong friction between work piece and chamber walls
Oxide scale buildup on the surface (especially in hot working).
Advantages and disadvantages of indirect extrusion.
Ads: Not friction between the workpiece and chamber walls. Disads: Complicated setup Hollow rams are less strong and rigid.
Advantages and disadvantages of horizontal hydraulic press for extrusion.
Ads:
High capacity, can apply constant force over long stroke
Most extrusions of bars and shapes
Disads:
Large - a lot of room required
Non-uniform deformation
Alignment of long pieces may be difficult
Advantages of vertical hydraulic press in extrusion.
Cold extrusion Discrete pieces Provides uniform deformation Easier alignment Higher production rate Smaller footprint
Describe how polymers are extruded.
Continuous process
Feed powder/pellets via hopper & screw
Heat is applied in barrel to melt feedstock which is pushed along by screw. Temperature profile control is important
Filter mesh backed by the breaker plate removes solid contaminants.
Cooling and shaping occurs in nozzle and ie
Rapid cooling after leaving die
For hollow extrudes (ie tubes) internal pressure may be neede to prevent collapse until set cold.
Describe the process of co-extrusion of polymers.
Multiple polymer feedstock kept separate and combined at die to form distinct layers.
Polymers could be mixed to form a feedstock of multiple polymers or additives (eg pigment). Twin screw feeds in either co- or counter roasting arrangements can be used to improve mixing.
Sketch and label a plot of extrusion pressure against piston displacement for direct extrusion.
Check L7 S14-19
Sketch and label a plot of extrusion pressure against piston displacement for indirect extrusion.
Check L7 S20
Why do the plots of extrusion pressure against piston displacement differ for direct and indirect extrusion?
There is container friction when doing direct extrusion meaning the load must be increased to overcome this.
Describe the die material selection for extrusion dies.
High strength and hardness to be able to sustain dimensional accuracy.
Wear resistance
Thermal resistant and high temperature strength and hardness for hot extrusion
How is turbulent flow and friction affected by die angle in extrusion?
Higher angle more turbulent flow and higher force necessary.
Lower angle the higher the friction and higher force necessary.
Common lubricants for friction.
Needs low shear strength and stability at the extrusion temperature.
Tallow and graphite are used for non-ferrous alloys
Glass is used for ferrous and nickel based alloys (billet is coated in powdered glass before going into the container acting as a lubricant and a thermal insulator - glass plate is placed at die face which softens acting as a lubricant ≈25µm thick)
Sketch cross sections of fluid flow over time in extrusion.
Check L7 S24
How much for the extruded billet can be affected by skin of the billet being entrained in the centre of extrusion?
Up to 30% must be cropped.
Define wire drawing.
It is the metalwork process used to reduce the cross section of a wire by pulling the wire through a die (or series of dies).
What is the difference between wire drawing and bar drawing?
Bar drawing - for large diameter stock
Wire drawing - for small diameter stock down to tens of microns.
How is the wire prepared fro drawing?
Surface scale is removed
Wire is sharpened at the beginning by hammering, filing, rolling or swaying to fit through die
Prior annealing can be performed to increase ductility
What is typical reduction in area for wire drawing?
5-45% in smaller wires and 20-45% in larger wires (this is a lot smaller than extrusion)
How can very fine wires be produced?
Drawn in bundles and the wires are separated by a different material (sacrificial metal) which can be chemically removed.
Typical wire drawing die materials
Cemented carbides
Tool steels
Diamond for very thing wires
Sketch a die for wire drawing labelling the zones.
Check L7 S29
Components to the total work in wire drawing.
Useful, homogeneous, work to reduce the cross section
Work required to overcome frictional resistance
Redundant, inhomogeneous, work required to change the flow direction.
Derive an expression for the minimum tensile stress and maximum reduction in area in wire drawing.
Check L7 S 31-32
Derive the affects of friction in wire drawing.
Check L7 S 33-34
What lubricants are used in wire drawing?
Soaps and minerals or vegetable oils/animal fats
Polymers or soft metals
Wet drawing is where the dies and work are completely submerged in lubricant usually oil
Dry drawing applies lubrication to the material by use of a stuffing box containing lubricant located in front of the mould (works passes through the box and picks up lubricant before entering the mould).
When is sheet forming deep drawing or shallow drawing?
Deep drawing if h/D > 0.5
Shallow drawing if h/D < 0.5
Describe the process of deep drawing of a sheet,
Deep drawing is used extensively in sheet forming and is the process in which a sheet metal piece (a blank) is radially drawn into a forming die by the mechanical action of a punch.
Define the 3 regions of a drawn thin sheet.
Punch bottom region
Wall region
Flange region
Derive an expression for the stress in drawing a thin sheet when there is no hold down force.
Check L8 S7-8
Derive an expression for the limiting draw ratio in drawing of a thin sheet.
Check L8 S9
Derive an expression for the applied force in drawing a thin sheet with an approximate hold down force on the flange.
Check L8 S10
Derive an expression for the stress in the material of a drawn thin sheet with hold down force on the flange during processing.
Check L8 S11
Derive an expression for the limiting drawing ratio when there is hold down on the rim of a thin sheet being drawn.
Check L8 S12
What is formability?
The ability of a material to undergo plastic deformation without fracture.
Generally increases with Increased temperature and increases with decreased strain rate.
Describe the Erichsen test.
Test for formability
Sheet metal specimen is clamped with a circular die at constant load.
Tool 20mm diameter steel ball hydraulically pushed into the sheet until crack appears
Displacement of a tool on my where this occur is Erichsen number (measure of formability).
Assume equal biaxial stretching (rare in practice).
What is a forming limit diagram?
Sheet is marked with a circle.
Sheet is stretched over a punch
Tearing of the sheet occurs - strain state is measured via dimensions of circle.
Define heat treatment
The careful impositions of temperature/time cycles to improve material properties.
Why is heat treatment important in nickel based super alloys?
Careful heat treatment can improve high temperature performance by homogenising the residual micro segregation Fromm casting.
How can heat treatments be used in glass ceramics?
The grain size and distribution are controlled by the annealing programme and depend upon chemical composition.
How are heat treatments used in PMCs?
Autoclaves used to optimise cure and degree of cure of polymers.
List the aluminium series alloys.
1xxx 99%+ Al 2xxx Cu 3xxx Mn 4xxx Si 5xxx Mg 6xxx Mg+Si 7xxx Zn 8xxx Li (etc)
What do H, O, F and T mean in Al alloy naming?
H cold worked
O Annealed (wrought only)
F as fabricated
T heat-treatment.
What numbers can be used after H in the Al naming system.
1 Cold worked only
2 Cold worked and partially annealed
3 cold worked and fully annealed
After these
2 1/4 hard 4 1/2 hard 6 3/4 hard 8 hard 9 extra hard
What numbers can be used after T in Al naming system?
1 Partial solution and natural aging 2 Annealing cast products 3 solution and cold work 4 solution and natural aging 5 artificial aging only 6 solution and artificial aging 7 solution and stabilising 8 solution and cold work and artificial aging 9 solution and artificial aging and cold work
5 basic mechanisms for impeding dislocation motion,
Solid solution strengthening Precipitation strengthening Dispersion strengthening Work hardening Grain boundary or sub grain boundary strengthening
Describe the heat treatment of a Al-Cu 2000 series alloy.
Hold at 550C until all Cu is in solution in α phase.
Quench to RT causing Cu to super saturate
Temper at 200C to cause θ precipitates to form.
What happens when precipitates form at grain boundaries and how can it be avoided?
Precipitates at gbs reduce the nearby conc of solute atoms
This results in weak gbs and chemically different gbs
Quench and age is designed to avoid this as slow cooling would give nearly 100% gb precipitates
Uses of 2000 series alloys
Duralumin (3.5 Cu 0.5Mg 0.5Mn) is the original age hardened ally now used in rivets.
2014 (4.4Cu 0.5Mg 0.5Mn 0.9Si) standard Al-Cu alloy used in aircraft structures, automotive etc.
2219 (6.3Cu 0.3Mn 0.2Si 0.1Ti,Zn,V,Zr) Good creep strength for high T applications. Good cryogenic strength.
Typical properties of 7000 series alloys.
Very string age hardening response.
Highest strength Al alloys (add Cu to improve SCC resistance)
What is 7075?
5.6Zn 2.5Mg 1.6Cu 0.2Cr
High strength alloy for aero structural parts
7475 is more pure with reduced Fe content for greater fracture toughness.
Why do 8000 series alloys exist?
Li has low density
Li dissolves up to 14% in Al at high T
Large size difference in atoms
Li raises E by 6% for every 1% added.
Problems with 8000 series alloys
Li oxidises rapidly Severe gravity segregation in casting If Na or K is present as impurity form low mp phases Simple Al-Li alloys have poor properties High cost
What do additives do to Al-Li properties.
Al-Li-Cu good strength and creep properties, low toughness
Al-Li-Mg Al2LiMg plates out at gbs low toughness.
Al-Li-Mg-Cu Mg favours formation of S’ phase which is not easily sheared homogenising slip and improving toughness.
Al-Li-Mg-Cu-Zr ZrAl3 stabilises te bave structure further strength and toughness improvement
Sketch the strength vs toughness master curve for steels.
Check L9 S62
What is harden ability?
The capacity of a given steel to through harden to martensite.
What diagram is used for assessing harden ability and how do we arrive at it?
CCT diagram (continuous cooling diagram)
use Scheil for estimating CCt from TTT.
Describe what happens to quenching media during quenching (heat transfer).
Initially the fluid vaporises forming a vapour shield around the component.
Once the vapour envelope collapses heat transfer rate rapidly increases as the liquid touching the component boils (Leidenfrost effect)
At lower T the heat transfer slows again as it occurs by conduction and convection only.
Sketch reconstructive and displacive transformations in steels and give examples.
Check L9 S70
Why is martensite tempered?
It will undergo static fracture if untempered.
Excess carbon leads to high volume fraction of Fe3C which exacerbates coarsening and large carbides crack
Why is welding with ultra high strength steels difficult>
Risk of cold cracking in the HAZ means pesetas must be employed.
Decarburisation can also become a problem.
What is the tradeoff in tempering martensite?
Strength for toughness
What effect can alloy elements such as Cr, Mn and Si have on tempering martensite?
Cr and Mn inhibit coarsening of Fe3C
Si is relatively insoluble in Fe3C and rate controlling process becomes diffusion away from growing Fe3C.
What contributions to yield stress are there in tempered martensite?
Structural (lath boundaries) Dynamic (carbon jumping between sites) Work hardening Solid solution Carbon (pinning of dislocations)
3 groups of surface treatments
- Surface modification with no change of composition
- Surface modification by changing composition
- Deposited coating
Give two examples of surface modification with no change of composition.
Transformation hardening
Induction heating.
How can surfaces be heated for transformation hardening?
Oxy-acetylene flame with water quench spray
RF induction coil with water quench spray
LAser surface heating substrate quench
Electron beam heating with substrate quench.
High power density and short interaction time needed to get thin hardened layer.
Describe the process of induction hardening.
Used so that only regions which require heat treatment are heated (eg gear teeth).
Rapid cooling by condition to the bulk ensures martensitic transformation locally on the surface where it is needed.
Give two examples of surface modification with compositional change.
Carburisation
Nitriding
List the types of carburisation.
Gas carburisation - 1000°C in CO/H2/N2 or CH3OH/N2 gas mixtures to provide a carbon source with high carbon activity.
Pack carburisation - 900°C pack in box with charcoal and energiser (BaCO3). Carbon reacts with residual oxygen to form CO which provides a carbon source.
Vacuum carburisation - 1050°C low pressure CH3 or C5H12. Carburise and then diffuse into part.
Plasma carburisation - 1050°C components heated in low pressure CH3. Glow discharge deposits C on negatively charged surface. Carburise and then diffuse into the part.
Use stop-off paints or copper to prevent C diffusion in specific regions.
What elements must be present fir nitiriding in an alloy?
AL, Ti, V, Mo or Cr.
Why nitride a steel alloy?
Surface hardness is retained to 500°C (compared to 200°C for carburised steel)
Describe gas nitriding and plasma nitriding.
Gas nitriding - heat part in NH3 which decomposed on surface and N diffuses into the part. 3-4 days for 500µm thick layer.
Plasma nitriding - voltage applied between part and furnace wall in low pressure H2/N2 atmosphere. Glow discharge with high ionisation level (plasma) generated around part and N diffuses into surface layer.
Main characteristics of thermally sprayed coatings.
Metals, ceramics, polymers and cements can be sprayed
Normally feedstock powder but sometimes wire.
Vary thickness from 20µm to 500µm
Minimal chemical interaction with subsrate.
Minimal heat input to substrate as not to alter microstructure and properties.
Good adhesion from roughened substrate.
Wear, corrosion and oxidation resistance at both high and low T,
Describe plasma spraying
Powder fed into ionised Ar/H2 mix where it is heated and accelerated up to 300m/s melted and projected onto substrate.
Any material can be plasma sprayed due to v high T of plasma flame.
Used to coat with ceramics due to high T
Describe high velocity oxygen-fuel spraying (HVOF).
Poweder fed into chamber where chemical combustion generates heat and gas expansion.
Typical particle size 15-60µm.
Higher powder velocity than plasma spraying (300-700m/s) and temperatures generally lower.
Widely used for cements for wear resistance.
Lower porosity and less chemical degradation than plasma sprays.
What is the typical microstructure in thermal spraying and why does it form?
Often splat morphology with each solidified splat having a non-equilibrium microstructure.
Due to large differences in thermal mass of particles and substrate.
Rapid cooling causes non-equilibrium microstructures and super saturates.
List types of diffusion coatings.
Pack cermentation
CVD
Thermal barrier coating
Electron beam PVD
What is pack cermentation?
Components are immersed in a powder mixture within a sealed or semi-sealed retort.
Due to differences in activities in source and substrate, a layer can form between the two.
A interdiffusion zone can form (IDZ) with slow moving elements.
Describe how CVD can be used to form a coating.
Reactor filled with argon and heated. (1100°C).
Feedstock is introduced and fills chamber being deposited on surfaces.
Transport in gas phase is by gas flow and eddy currents that are set up in the reactor.
What is thermal barrier coating?
A coating system comprising ceramic top coat and a bond coat.
A low thermal conductivity ceramic layer is deposited and provides thermal insulation and lowers the temperature of the metallic substrate.
Modern ceramic top coats are based on zirconia and contain about 7wt% yttria (YSZ)
Describe EB-PVD
An electron beam is used to vaporise an ingot of the coating material (typically held in water cooled copper crucible). A vapour cloud forms above the ingot in which the coating is manipulated.
Columnar colonies grow perpendicular to the surface of the substrate competitively.
Describe the process of fusion welding.
Usually an intense moving heat source.
Heat can melt region of workpiece (fusion zone) and cause the adjoin gin regions to undergo a thermal cycle (heat affect zone).
Generally for fusion welding, as the heat source us highly localised, heating rate can be rapid (>500°C/s)
Workpiece usually large in comparison to joint so acts as heat sink.
Cooling rates rapid.
Describe the process of inertia welding.
One workpiece attached to the coating chuck along with a flywheel of a given weight,
Piece is spun to high rotation rate to store energy in the flywheel.
Once spinning at proper speed, motor is removed and pieces forced together under pressure,
Force is kept on pieces after spinning to allow weld to set.
Factors that affect the microstructure of fusion welding.
Heating up to fusion temperature.
Cooling down from fusion temperature
Holding at temperature during welding (interpose temperature)
Behaviour of molten metal, gas absorption, outgassing.
Solidification of molten weld metal.
How does weld metal differential from bulk?
It is often inferior to same alloy in wrought condition.
May use filler metal/electrode of slightly different composition.
Com position can change due to evaporation, dilution, filler metal.
How do welds cool?
Under highly non-equilibrium conditions.
Cooling rates between 10-10^3 °C/s
for EBW and LBW 10^3-10^6 °C/s
What are common defects in welds?
Solidification cracks
Porosity
Entrained ceramic inclusions
HAZ cracking
Factors that affect defects in welds
Introduction of impurities BAse metal dilution of filler Turbulence and mixing Large T gradients Large thermal stresses developed
Give an expression for surface melting time for a planar heat source on metallic surface for welding.
Check L10 S12
Give typical power densities for each type of fusion welding.
Air/fuel gas flame 10^2 W/cm Electroslag Oxyacetylene Thermal 10^3 Friction 10^3-10^4 Arce welding 10^3-10^4 Resistance welding (oxygen cutting) 10^5 Electron beam/laser beam 10^6
Describe arc welding.
Use of an electric arc as source of heat to melt and join metals
Joining is due to extreme heat of an electric arc drawn between electron and workpiece of two electrodes.
May or may not require use of pressure of filler metal
Electrode is either consumable wire rod or non-consumable carbon or tungsten rod.
Gas used to reduced amount of oxidation.
Describe LBW.
Thin vaporised column of metal used in keyhole mode
Narrow pool, thin HAZ
Usually inert shielding gas or vacuum.
If reflective material, special coatings used to increase efficiency.
Describe EBW
Heating caused by electron beam from tungsten filament
Requires vacuum.
Narrow HAZ and deep penetration.
Good fro difficult to weld materials such as Zr, Be and W.
LBW vs EBW
EB is high energy electrons striking metal
LB is heat generated from an extremely large no. photons
EB workpiece must be an electrical conductor
LB heat insulators with equal or even greater effectiveness compared with metals
EB generally requires a vacuum.
LB can be operated in ambient atmosphere.
EB distorted by magnetic fields.
Eb has limited max current of 0.5A due to repulsion of electrons causes defocusing at higher current
EB produces X-rays which require shielding (varies depending on energy)
Describe the process of friction stir welding.
Welding tool moves along area to be joined at high speed.
Works for soft alloys
Coating tool creates friction along with downward force to maintain pressure.
No melting of material
(similar to pearlage(note to self))
Weld is fine-grained hot worked condition
No entrapped oxide or gas porosity
Van have some defects if conditions are not perfect.
Describe the process of linear friction welding.
Solid state joint process where components are rubbed under axial pressure to create friction.
One piece is rigidly clamped, other moves in reciprocating manner.
More efficient material usage than machining.
No melting and fewer oxides compared to fusion welding.
Reduces weight compared to conventional joining methods.
Popular in aero for eliminating mass
Sketch a plot of temperature against time for a point on the fusion boundary during a welding pass.
Check L10 S36
Sketch a plot of how temperature varies with distance from the fusion boundary in the HAZ.
Check L10 S36
What are the main assumptions in mathematical modelling of fusion welding.
Heat source moves with uniform speed
Heat is delivered from a point source
Latent heat of fusion of weld metal and any phase trans are ignored
No heat loses from surface of plate
Thermal properties are independent of T
Convection effects in weld pool and other electromagnetic forces neglected.
Speed of heat source and rate of heat input are constant.
State an expression for how T changes with time at location r along a weld for a thick plate.
Check L10 S45
Derive an expression for cooling rate at rear of weld pool for thick plate.
Check L10 S47
State an approximate expression for peak temperature outside of the fusion zone in welding.
Check L10 S48
How can distortion during welding be controlled?
Pre-positioning of workpieces prior to welding leaves them in right location after welding.
Restrain workpiece so that it cannot move during welding (this generally increases the magnitude of residual stress field).
Design joint so weld deposited are balanced on each side
Select suitable welding process and weld sequence to minimise the amount of welding.
Preheat to reduce cooling rate
Correct it by mechanical means or thermal methods after welding.
What is powder processing?
A process whereby a metal, alloy or ceramic in the form of a mass of dry particles is converted into an engineering component of pre-determined shape.
Powder production -> compaction -> sintering
density usually increases from sintering.
What does the rate of any diffusional transformation depend upon?
Diffusivity
Thermodynamic driving force associated with the change of state compared to the thermal energy.
A characteristic length scale for diffusion.
Derive an expression for the sintering potential thermodynamically.
Check L11 S7
State an expression for grain boundary mobility during sintering
Check L11 S8
State an expression for grain growth rate during sintering.
Check L11 S8
What will the ideal sintering temperature do?
Maximise densification rate
Minimise grain growth rate.
The application of pressure accelerates the rate of densification without influencing grain growth rate
Why use powders to make parts?
Can be energy and labour efficient.
Conserves material and environmentally clean with minimal waste
Suitable for production of small complex parts for low and intermediate mechanical loadings (eg gears)
Forming without machining can yield high adde value and reduces no. of plant tools
Refractory metals (W, Mo, Ta, Nb) are brittle when cast
List methods for manufacturing powders.
Mechanical Comminution Chemical processes Carbonyl process Electrochemical processes Atomisation
Describe how mechanical comminution can be used to form a powder.
Dry ball milling process that produces homogenous powder
Repeat cold welding and fracture of particles to produce mixing of constituents
Creation of high density of lattice defects because of severe plastic deformation.
Material transfer by diffusion accelerated by lattice defects and instantaneous high T during impacts
Tumbler mill
Attritor mill
Describe two chemical process for producing Fe powders.
Direct reduction:
-Fe powder from reduction of Fe ores
-Grind ore floatation then concentrate, dry, remove impurities, reduction and left with sponge like powder
Reduction of mill scale:
-Mill scale from rolling mills
Oxidise Fe3O4 to Fe2O3 then reduce with H2 at 980°C on continuous belt furnace.
Describe the carbonyl process for producing powders
Fe and Ni ores react with CO at high T and P to form Fe(CO)5 and Ni(CO)4 that are liquids at RT.
Increase T and reduce P to decompose to pure spherical 1-5µm Fe and Ni particles
Pt produced from PtNH3Cl
Powder is expensive and has poor flowability because of fine size.
How can electrochemical processes be used to form powders
Mainly used for Cu from aqueous CuSO4 soon at 60°C with cast Cu anodes.
Cu deposits directly on tank bottom then wash dry anneal and crush.
Describe centrifugal atomisation.
Molten metal is ejected tangentially from a rapidly spinning cup, plat or disc.
Rapid solidification rate forms 25-80µm droplets
Rotation perforated cup (RPC) up to 5mm droplets exit from perforations at 10K/s
Rotating electrode process (REP) arc between rotating feedstock and tungsten electrode, molten droplets at 103K/s
Plasma arc (PREP), laser beam (LREP) also melting options
Overall typical particle sizes ≈150µm
Describe twin fluid atomisation.
Melt is held in crucible with nozzle and atomiser at base.
High pressure gas or water streams incident on stream from bottom of crucible casting a droplet spray.
State an expression fro the mass mean powder size in twin fluid atomisation.
Check L11 S23
State an expression for the droplet mass probability in gas atomisation.
Check L11 S24
Find an expression for cooling rate of a particle in atomisation assuming particles are isothermal. (Newtonian)
Check L11 S25
Derive an expression for solidification time for small undercooling of an atomised particle under non-newtonian conditions
Check L11 S27/28
Derive an expression for DAS time for large undercooling of an atomised particle under non-newtonian conditions
Check L11 S28/29
Describe how and why powders are mixed before processing.
Aim to get a homogenous mix of powders and lubricant if present.
Lubricant is typically an organic compound or zinc stearate.
Reduces friction to try and ensure uniform compaction.
Overmixing impedes the compaction process and reduces the green strength; impedes flowability.
Describe how and why powders are pressed before processing
Powders are pressed to [art shape at 150-1000MPa.
Controls the final part shape and properties, porosity and uniformity.
Powders cold weld together by local heating during plastic deformation.
Little lateral flow of powder and density gradients result from friction between powder and die walls.
Single action pressing for components up to 3mm high.
Double action pressing a neutral zone of low density often result and is use for heights up to 80mm
Why is hot pressing done in powder processing?
Hot pressing is sometimes used to press to higher densities and in some cases to avoid sintering.
This requires the use of heat resistant dies, increased cycle times and controlled atmospheres.
Describe cold isostatic pressing.
Flexible, polyurethane mould immersed in water and pumped to high pressure = uniform compaction.
High densities with no lubricant geometrical constraints removed and can be used on metals and ceramics.
Describe hot isostatic pressing.
Metal mould sealed by electron beam welding.
Ar used as pressurising medium in a furnace.
HIP used for Ni superalloy turbine components and high speed steels.
Describe pressure assisted sintering (Sinter HIP)
Same as HIP but sinter to 92% dense when there are likely to be no interconnecting pores.
If vacuum sinter, pores are gas free so then HIP to 100% density without cannon in the same vessel.
List the steps of closed die compaction.
Filling Transfer Compaction Ejection Sinter&sizing
3 methods for filling a die when powder processing
Gravity filling
Fluidisation methods (gas blown into shoe)
Suction filling
Factors that affect final packing density in powder processing.
Kinematics of shoe and punch motion
Die and shoe geometries
Environment
Powder characteristics
List the basic powder measures
Relative density (powder density over solid phase density) Porosity (volume fraction occupied by pores) Apparent density (as poured density) Tap density (density after vibrated/tapped)
What us Hausner Ratio
A measure of flowability of powders
Given by tap density over apparent density
Measures of flowability of powders
Hausner ratio
Angle of repose (angle on side of pile)
Hall flow meter (time for 50g of powder to flow through finnel with 2.5mm orifice)
Freeman rheometer (torque requires to rate a blade that is gradually lowered into powder bed)
Why are we concerned with variations in powder packs in a die?
It can influence what happens during compaction.
Variation in density leads to variation in properties.
What are the basic mechanisms for sintering?
Yield
Diffusion
Power-law creep
Herring-Nabarro-Coble creep
What is the driving force for consolidation in sintering?
Reduction of specific free surface area and associated energy (primary)
Reduction of specific interfacial area and associated energy (secondary)
Elimination of non-equilibrium lattice defects arising from powder processing (minor)
Elimination of non-equilibrium phases/states and homogenisation of chemical/compositional gradients (minor)
List all the processes for movement of material into the sinter neck region.
Surface diffusion from the surface Evaporation/condensation in void Volume diffusion from surface (through bulk) Grain boundary diffusion from gb. Volume diffusion from gb Volume diffusion from bulk Plastic flow (early stages)
Derive an expression for the driving force for densification in sintering.
Check L11 S76
Sketch a sintering map of log(x/r) against T/Tm.
Check L11 S78
Sketch a sintering map of pressure assisted sintering with relative density against Log(P/σy)
Check L11 S79/80
Fastest mechanism usually dominates
Derive an expression for the rate of sintering at a given P, T and ρ
Check L11 S82-86
Derive an expression for rate of densification during sintering.
Check L11 S87/88
Describe how metal injection moulding works (MIM)
Binder (usually polymer or other organic) and metal powder mixed
As granulas injected into mould at 130-190°C
Debinding (burn off binder)≈600°C over many hours
Sinter
When is MIM most useful?
Small highly complex shapes in high quantities that cannot be made with conventional powder processing
What processes can happen post sinter in MIM
Resizing to improve tolerance
Infiltration to remove porosity if wetting is good can fill with liquid metal
Impregnation (pores filled with organic material)
What is the production process for WC-Co (a cermet) and why is it the ideal system for liquid phase sintering?
Mixing
Pressing
Liquid phase sintering
Ideal system as perfect wetting, no solubility of liquid Co in solid WC phase and fairly large solubility of components of solid phase W,C in the liquid phase.
What is the drive to create composites?
Monolithic materials can be improved
Metals strong tough but heavy
Ceramics are srtrong in compression but weaker and brittle in tension
Polymers are lightweight and cheap but strength and toughness are poor
Glasses are transparent but brittle
Give an expression showing how stiffness scales with relative density in foams (theoretical and experimental)
Check L13 S15
List the routes for processing metal foams.
Bubbling gas through molten alloy.
Stiring a foaming agent into molten alloy.
Consolidation (powder pressing) metal powder with particulate foaming agent that followed by heating into a mushy state allows foaming agent to release H2 allowing the material to expand.
Vapour phase deposition or electrodeposition of metal onto polymer foam precussor which is burned out.
Additive manufacturing.
Describe how bubbling of gas can foam Al.
Add insoluble particles (10-30%) to increase viscosity making foaming easier (Al2O3, ZrO2, SiC or TiB2)
Variety of gases can be used (CO2, O2, inert gases or water)
Bubbles float to surface, drain and begin to solidify; thermal gradient determines how long foam remains liquid or semi-solid
or
TiH2 powder in molten Al.
Begins to decompose at 465°C releasing H2.
Melt Al between 670 and 690°C add 1-2%Ca to form CaO/CaAl2O4 to increase viscosity. Stir vigorously and add 1-2% TiH2.
Control by adjusting overpressure, time and temperature.
What is the basic concept of a foam sandwich panel?
Move mass away from the neutral axis to increase flexural rigidity.
Methods for producing honeycombs
Expansion process Corrugation process Extrusion through die AM Casting Bicarbon templating
Describe the method of joining and expanding to make a honeycomb sheet
Blocks of sheets are laser welded together or adhesive bond striped
Sliced
Pulled apart to creat expanded panel
Describe how honeycomb are made using corrugated sheets
Sheets are rolled with corrugated rolls
Sliced to length
Bonded together to make a block
Sliced into sheets
How can honeycombs deform under loading?
Plastic instability (hinge effect)
Bending due to in-plane loading (x1 or x2)
Beading shear
Buckling
What is the response of a hexagonal honeycomb sheet when forcibly curved around one axis out of the plane?
The core reacts by bending in a reversed curvature along the axis at 90°
This is called anti clastic curvature.
Benefits of superplastic forming of Ti-alloys.
Can form large pieces in one operation.
Finished product has excellent precision and surface finish.
Does not suffer from spring back or residual stress
List the steps involved in manufacturing a Ti fan blade.
Three Ti layers diffusion bonded together with stopoff layers to prevent diffusion in certain areas.
Structure inflated with Ar into a mould forming an internal corrugated structure.
Twist and camber positioning Twists and camber process Hot creep forming positioning Hot creep forming process] Super plastic inflation.
What condition are most α-β Ti alloys in?
In the annealed condition. Lower strength but more stable at high T and better fatigue resistance.
What is the method of pre-preg for PMCs?
A tape or sheet of fibres impregnated with resin
Partial curing produces a flexible aggregate allowing for excellent alignment in unidirectional layers
Component built by stacking pre-pregs in predetermined patterns then cured by heating under pressure.
How are most Al MMCs produced?
By molten metal infiltration
Describe the methods for producing carbon/carbon composites.q
Infiltration of carbon bearing fluid into interstices between preforms of carbon fibres laid into the correct shape.
During liquid impregnating a pitch or resin is injected then heated to leave carbon deposit. (1-5 times repeated)
Chemical vapour infiltration (CVI) involves injection of hydrocarbon gas (eg CH4) together with H2 and N2. (1-3 times repeated)
List the reactions in the kiln for making Portland cement.
Check L15
List the shorthand nomenclature of concrete additives and check their oxide formulas
C3S C2S C3A C4AF CSH2
Check L15
List the stages of cement hydration reactions
Stage 0: Rapid hydration (initial dissolution) high heat releasing rate CaO + H2O -> Ca2+ +2OH-
Stage 1: Surface of cement covered buy silicate and aluminate gels (early stage C-S-H) water access becomes difficult
Stage 2: Induction period, Ca2+ concentration increases and needle like ettringite forms
Stage 3: Initials setting starts, cement paste becomes cohesive due to gel-to-gel contact if adjoining grains by interlocking ettringite crystals. Also plate-like portlandite and C-S-H formed.
Stage 4: C-S-H form on cement surface so hydration slows and reaction becomes diffusion controlled.
Stage 5: hydration process proceeds at berry slow rate.
Sketch a plot of stress against strain for concrete showing its assymetrical response in tension and compression.
Check L15
Sketch a plot of strength against water/cement ration for concrete.
Check L15
Sketch a stress-strain curve for rebar.
Check L15
Why is steel appropriate for rebar?
Good tensile strength
Thermal expansion coefficient matches concrete
pH environment favourable (corrodes in acidic environments but not alkaline)
Interfacial strength can be developed (chemical bonding and physical bonding)
What are the effects of over and under reinforcing concrete?
Over: explosive failure by shearing
Under: gradual failure by crack extension
Describe the process of pre-tensioning rebar
Pre-stress applied to steel by anchoring and drawing.
Apply concrete cast
Release steel when concrete set and compressive load is transferred to concrete
Pros and cons of pre-tensioned rebar
Pros:
Less cracking
Improved durability
Cons:
Need large equipment
Only pre-stressed once
Cracking at point of release is a risk
Describe the process of post tensioning concrete.
Cast concrete with a duct.
Stretch steel through duct, anchoring to concrete at one end.
Anchor steel tendon at the other end while in tension.
What are the pros and cons of post-tensioned rebar?
Pros: Reduced cracking Almost any shape Only way to build certain concrete Structures Steels can be adjusted in Service Duct can be curved
Cons: Stressing in the field Special equipment required Complicated to design] Cracking at anchors is a risk
Derive an amount of steel that can be used to reinforce concrete.
Check L15
Describe the phenomena of setting and hardening in cement and account for their origins.
Solution on L15 QQ1
How does the alkalinity in cement arise?
Solution L15 QQ2
Identify the major constituent minerals in cement and describe their roles.
Solution L15 QQ3
Discuss the function of water in cement and the effect it has on mechanical properties.
Solution L15 QQ4
Explain the principles of the design of reinforced-concrete beams.
Solution L15 QQ5
Describe and distinguish between the two techniques for prestressing concrete.
Solution L15 QQ6
Differentiate between additive and subtractive processes in terms of bottom up etc.
Additive: bottom up
Subtractive: top down
What was the main pre-cursor to AM?
Early stereolithography with photosensitive polymers
In the car industry, what was the main benefit of the invention of AM?
Rapid prototyping to promote shorter design cycles
Pros and cons of powder based metal deposition (AM)
Pros:
Near net shape of complex geometry
Rapid prototyping
Less scrap/waste
Cons:
Feedstock material is expensive
Defect rate high
Dynamic loading and residual stresses
Sketch a plot of cost vs quantity for AM and traditional manufacturing methods.
Check L16 S10
Sketch a plot of cost per unit against design complexity for AM and traditional manufacturing.
Check L16 S10
List the methods of metal AM.
Direct AM:
Powder bed fusion (PBF)
- Laser (PBF-L)
- Electron beam (PBF-EB)
Direst energy deposition (DED)
- Laser (DED-L)
- Electron beam (DED-EB)
- Plasma arc (DED-PA)
Indirect AM:
Binder jetting
-Metal injection moulding (MIM)
Sheet lamination
-Ultrasonic additive manufacturing
Material extrusion
-Fused filament fabrication (FFF)
What is the general side effect of switching from powder bed fusion to direct energy deposition?
Processes get more powerful, faster, coarser, larger and less accurate.
Where are filling wires for AM processes usually produced?
Produced for the welding industry in spooled form so are readily available with a rich history of alloy development and production.
Low cost of production
Compare and contrast electron beam source vs laser beam in AM.
Electron beam:
- High intensity of focussed electrons
- Always in vacuum to avoid scattering with air molecules.
- Materials to be fused must be electrically conductive. No it sensitive to surface optical properties.
- Power efficiency 80-90%
- X-ray can be generated during EBM processing, shielding required.
- Usually expensive.
- In PBF, substrate usually preheated. T typically 700-1000°C.
Laser beam:
- High intensity of focussed photons.
- Open atmosphere or shielding gases such as Ar.
- Material does not have to be conductive but relies on optical properties of workpiece surfaces.
- Power efficiency 7-10%
- No X-rays generated
- Low cost
- Highly reflective materials such as copper, silver have low efficiency (<5%)
- In PBF substrate not usually preheated. Max T preheat is <200°C
List processing defects from PBF.
Vapourisation (loss of elements)
Gas related porosities (keyhole/initial gas entrapment)
Lack of fusion defect (insufficient penetration)
Cracking (solidification/phase transformation)
Surface roughness
Microstructure of metal AM
Small melt pools built up layer by layer.
Optimal angle between passes is 67°
Geometrical features of the melt pool is significant in determining the shape of grains and solidification rate.
Factors that affect melt pool geometry and solidification in AM.
Thermal diffusivity of metal, heat source speed and power
Grain orientation is a function of heat source speed as it affect the thermal gradient and also the under cooling.
Undertake a simplified analysis of the origin of residual stress in AM parts.
Check L16 S36
What is the main factor that affects yield stress in AM metal parts.
Orientation (it is not affected by size of part).
What is the response of AM parts under dynamic loading?
A significant reduction of fatigue and creep life is observed with AM materials.
Machining and HIP could help improve the situation.
