Metals Flashcards
(47 cards)
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
