Metals Flashcards
Describe Sand Casting
- Mould material (sand/clay) is pressed into pattern and removed
- Metal poured into mould and solidifies
- Mould is broken away to leave metal part
Advantages of Casting
- Can produce intricate shapes, esp. internal cavities
- Can achieve uniform properties with good control
- Most metals can be cast
Sand Casting Advantages and Disadvantages
Adv : Very cheap
Disadv: low dimensional accuracy and surface finish, low production rate
Describe Investment Casting
- Wax or polymer pressed into pattern and removed
- Metal poured into mould and solidifies
- Mould is melted away to leave metal part
Investment Casting Advantages and Disadvantages
Adv: Better surface finish than sand casting, higher production rate
Disadv: Can only produce small parts, relatively expensive
Describe Permanent (Die) Casting
- Liquid metal pushed into permanent mould with external pressure
- Metal solidified and removed from mould
Die Casting Advantages and Disadvantages
Adv: Best finish and tolerances, high production rate
Disadv: Very expensive
Describe Rolling Process
- Ingot formed by casting
- Ingot is heated and pressed between rollers
- Forms plate or sheet metal
Defects caused by Rolling
Surface - inclusions, rust, roll markings
Structural - bumps, cracks, holes, splitting
Describe Extrusion Process
- Billet is forced through a die with external pressure
- Produces part with uniform cross-section
Defects caused by extrusion
- Piping: impurities diffuse to centre of cross-section
- discard: not all of the billet is extruded, leaving excess
Extrusion force equation
F = a + b * ln(Ao/Af)
Where a and b are material constants
Ao = Original area
Af = Area after extruding
Upsetting force equation (in forging process)
F= sigma0 + (2*mu*r)/(3*h) Where sigma0 = material constant mu = frictional force r = initial radius of workpiece h = final height of workpiece
Describe Open Die Forging
- Ingot deformed between 2 platens (usually hydraulic press)
- Often an initial step for other forging operations
Describe Impression Die Forging
- Material placed between 2 dies
- Dies pressed together to form shape
- Important to use excess material so that cavity is filled
Describe Closed Die Forging
- Material placed between 2 dies
- Dies sealed together
- Volume of material must be exactly volume of part as there is no space for excess material to go
Describe Isothermal Forging
- Heated workpiece pressed between heated dies
- Gives much better properties and tolerances, but is very expensive
Describe Additive Manufacturing
-Powder laid layer-upon-layer and heated to consolidate
Advantages of Additive Manufacturing
- Produces near-net shape
- Good buy to fly ratio
- Easy to iterate designs
- Low lead times
Give a disadvantage of using a Laser system in additive manufacturing and suggest an alternative method
Laser system causes residual stresses due to the difference in temperature between the deposited material and the surrounding powder
Electron beam preheats the layer before laying the powder
Describe Metal Injection Moulding
- Metal and binder are mixed and ground up
- Powder is injected into the mould (NOT MELTED)
- Binder is removed by heating
- Compacted part is sintered to densify
Advantages and Disadvantages of Metal Injection Moulding
Adv: Low energy footprint
Disadv :Part will reduce in size during sintering which can lead to dimensional errors
Advantages and Disadvantages of Powder Metallurgy
Adv: Produces less waste which is more efficient for expensive alloys, can produce complex geometries
Disadv: Producing metal powder is expensive and time consuming, and not all metals can be powdered, powder must be fine and spherical
Describe Hot Isostatic Pressing
- Capsule is filled with powder and gas is removed (to prevent oxidation)
- Soaked in high pressure and temperature for several hours
- Capsule dropped in acid to remove mould, leaving metal part
- Can only produce small parts, very expensive
What are the light metals?
Beryllium, Magnesium, Aluminium, Titanium
Advantages of Aluminium
- Good corrosion resistance
- Low density
- Easy to process
- Recyclable
Disadvantages of Aluminium
- Low modulus
- Low melting point
Advantages of Titanium
- High corrosion resistance
- Lighter that steel
- Higher modulus that aluminium
- Compatible with carbon fibre composites
Disadvantages of Titanium
- Very expensive due to melting and extraction processes
- Hard to machine
What are the Non-Heat Treatable Aluminium Alloys
1xxx - 99% Al
3xxx - 1% Mn
5xxx - 6% Mg
What are the Heat Treatable Aluminium Alloys
2xxx - Al-Cu
6xxx - Al-Mg-S
7xxx - Al-Zi
What series of Aluminium Alloy would you use for the upper and lower skin of a wing and why?
Upper - 7xxx - very strong in compression
Lower - 2xxx - damage resistant, not damaged by spray from runway
Describe Homogenization Treatment
- Cast ingots heated for varying lengths of time
- Reduces segregation in part (composition differences due to diffusion in the solidification process)
- Vital pre-hot working step
Describe the Ageing process
- Alloy is first heated to redissolve alloying elements at high temperature for a short time to avoid melting or contamination
- Alloy is quenched to form a supersaturated solution (must be controlled)
- Alloy is soaked at 150C for several hours so alloying elements precipitate within grains, increasing strength
How do Heat Treatable alloys derive their strength?
Precipitation Hardening
How do non-Heat Treatable alloys derive their strength?
Grain size reduction, solid solution hardening, cold working
What defects are caused by quenching too fast or too slow?
Too fast: leads to cracking or residual stresses
Too slow: Solution is not saturated, precipitate forms at grain boundaries which embrittles material
How does cold working strengthen a metal?
Dislocations are formed as the material is deformed, which entangle with other dislocations and reduce dislocation movement
How does grain refinement strengthen a metal?
Dislocation movement is obstructed grain boundaries, so increasing the number of grain boundaries decreases dislocation movement
Smaller grains also improve ductility and toughness
How does precipitation strengthening strengthen a metal?
Dislocations are obstructed by precipitates in the metal, and can only move past them by shearing the precipitate or by the bow mechanism
How does precipitation size and the distance between particles affect strength for the shearing and bow mechanism?
Shear mechanism: str = K * (precip size)^0.5
Bow mechanism: str = K/(dist between particles
What is the Hall-Petch equation?
Yeild str = sigma0 + K*d^-0.5
where sigma0 and K are constants
d = average grain diameter
How does solid solution strengthening strengthen a metal?
Alloying elements diffuse towards dislocations and ease strains around them due to the atomic size difference between the solute and solvent atoms
What are the advantages of alpha titanium alloys?
- Lower density
- Higher creep strength
- Improved weldability
What are the advantages of beta titanium alloys?
- Better heat treatment response
- Higher short-time strength
- Improved fabricability
Why is titanium welding performed in an argon environment?
Oxygen can react with titanium to produce a very brittle alpha phase, so argon is used as it is non-reactive
Which property of titanium is improved by adding chromium?
Burn resistance
