Process routes Flashcards
P_____ –> S______ –> P_______
Process –> Structure –> Properties
List problems caused by rolling
- flatness poor (solution is to pull it)
- cavity
- edge defects
- crocodile defect (material explodes)
Why would crocodile defect (material explodes) occur during rolling
- incorrect temp
- too ‘greedy’ - doing too much too soon
- incorrect strain rate
Name a Ti-Al based alloy that is used in engine blades
Ti - 6Al - 4V
Why is Titanium so expensive
Due to the process it needs to undergo to become usable.
This process is called: The Kroll process
Describe the Kroll process
- Energy needed to separate is 125MWhr/Tonne
- It is a batch process over 5 days
- Produce TiCl4 from TiO2 and Cl2
- React TiCl4 with magnesium to produce Ti
- Chip out Ti sponge
- Cost £5/kg
Name three processes used to procure Titanium
- Kroll process
- Hunter process
- FFC process (electrolytic process)
What are the limitations of using FFC process (Fray, Farthing, Chen)
- produces powder
- therefore further processing is required (additive layer manufacturing)
What conditions should Titanium be melted in
Should be melted in a vacuum to avoid recombination with N
Why are higher grade alloys remelted once or twice
To ensure homogeneity and freedom of regions that may have picked up N, O or C
These are then forged more easily into ingots
What ways can Titanium be remelted and which is best
- Use a consumable arc (VAR method)
- Use a plasma beam with a water cooled copper crucible (skull melting
‘2’ has an advantage in that more time is available for inclusions to be dissolved or removed
Difference between slabs and billets
Slabs - further processed and rolled into sheet/plates
Billets - processed into bar, rod, wire or tubes
Definition of solidification
how metal solidifies from molten state and resultant structure
Definition of fluid flow
How easily the metal flows
Definition of heat transfer
Shrinkage, causing defects due to how slow/fast cooling takes
Name three types of casting
- Sand casting
- Investment casting
- Pressure/gravity casting (permanent mould)
List the advantages of casting in general
- Extremely versatile
- Porosity is kept to a minimum
- Can produce a final product
Describe sand casting
- A pattern is made in sand
- Molten metal is poured in
- It is left to cool
- The sand is broken away to procure product
List advantages and disadvantages of sand casting
A:
- can produce very large products
D:
- poor surface finish
- poor tolerances
Describe investment casting (lost-wax process)
- A pattern is made out of wax
- The wax mould is coated and toughened
- After heat treatment a ceramic mould is formed and the wax is melted out
- Molten metal is poured into ceramic mould and left to cool
- Ceramic mould is broken off using vibrations
List advantages and disadvantages of investment casting
A:
- a number of moulds can be combined into one larger mould for pouring (TREE)
- good surface finish
- good tolerances can be achieved
D:
- limited in size
- relatively expensive
Describe pressure/gravity casting
- Suck molten metal upwards into cast
- Solidifies under pressure
- This can the be repeated
List advantages and disadvantages of pressure/gravity casting
A:
- can create lots of complex components
- Best tolerances
- Best finish
D:
- limited size
Describe centrifugal casting
- Spin molten metal whilst solidifying
can be used for composites
List three solid state processes
- Forging
- Rolling
- Extrusion
Describe the extrusion process
- A round billet is placed into a chamber and forced through a die opening by a ram
- The die dictates the final shape
- This is carried out in elevated conditions depending on the material
What temperature should Mg, Pb, Steel and Ti be at during extrusion
Mg - 450
Pb - 22
Steel/Ti - 800/1100
What two types of extrusion are there
- Direct extrusion
2. Indirect extrusion
What is the difference between direct and indirect extrusion
Direct - the billet slides relative to the container walls
Indirect - the die moves towards the billet
What can be needed during extrusion to help the process
lubricant
What parameters effect extrusion
- temperature
- speed
- friction factor (effects finish)
Equation for extrusion ratio
R = Ao / Ar
Ao = xarea of billet Ar = xarea of die
Equation for extrusion pressure
P/σ = a + bln(R)
R = extrusion ratio
Equation for extrusion strain rate
Extrusion strain rate
= 6vln(R) / D
D = diameter of billet v = speed
What are the defects that can be caused due to extrusion
Surface cracks - sometimes not visible to naked eye
Piping - occurs when there are impurities in the billet
Describe the rolling process
The process of reducing the thickness (gauge) of a workpiece by compressive forces applied through a set of rolls
What type of rolling equipment is used for thin sheet of high strength materials
cluster (Sendzimir or Z) mill
List defects caused by rolling and how can they be reduced
Surface defects - due to inclusions in the material, scale, rust or roll markings
reduced by: prior treatment of the ingot
Structural defects - wavy edges, cracking, alligatoring
reduced by: better control of homogenisation prior to rolling
Anisotropy
Describe the forging process
The permanent deformation of a material by heating and using force to change its shape
What is forgeability
Capability of a metal to undergo deformation without severe surface cracking
Equation for the upsetting force
P = σ( (1 + 2μr) / 3h )
Why does barrelling take place during forging
Due to tools being a different temperature will cause non-homogenous cooling
Difference between open and impression die forging
open:
deformation of specimen between two platens
impression:
take 2 dies and squeeze material down into desirable shape
What is closed die forging
similar to impression forging but with no excess material
What is near net shape forging
3D printing basically, and creates a shape close to final product
What is isothermal forging
- Dies are heated to the same temperature as workpiece
- this means more control over homogenisation
List the defects caused by forging
- Surface cracking
- Buckling or lapping (due to excess material)
- Anisotropy
