Metal forming processes

Forging

  • Forging is where metal is deformed due to compressive and high impact forces.
  • Two main functions of forging can be described as:
    • Upsetting
      • Where the metal is flattened by the use of a die or a hammer
      • This increases its cross sectional area but reduces its length
    • Drawing
      • Where the metal is drawn out by the use of repetetive hammering along the sides
      • This increases its length but reduces its cross sectional area.
  • The main advantage of forged objects is that it is stronger than machined objects due to a grainflow that follows the shape of the object, unlike machining.
Hot forging involves:
  • Heating to above the recrystallisation temperature
  • Working it while it is at the temperature
  • Leaving it to cool
  • It requires a lot less energy to hot forge due to the metal's more malleable state
  • Yet its dimensional accuracy and surface finish are poor, so it needs machining
  • Click here for a demonstration (hopy shit that press is huuuge)
Cold forging involves:
  • Applying large amounts of force to an object at room temperature
  • Usually has a good surface finish and dimensional accuracy
  • Click here for a demonstration
Drop forging involves:
  • Heating metal to above the recrystallisation temperature
  • Placed in a series of dies
  • Struck at high speed and force, usually with aid of gravity
  • This is then repeated in different dies in the series to achieve the desired shape
  • Leaving it to cool
  • Usually produces an excess flash, which is trimmed or machined off.
  • Click here for a demonstration

Rolling

  • Rolling is when metal is shaped by being squeezed from some sides on a roller.
Hot rolling
  • Is done above the austenitic range
  • Produces fine but equiaxed and unstressed grains
  • Causes less stress on machinery
  • Final products not that dimensionally accurate
  • A black oxide layer (firescale) could form over the finished product
  • Click here for a demonstration
Cold Rolling
  • Done below the recrystallisation temperature
  • Produces elongated and stressed grains
  • Harder and stronger final product
  • Better surface finish due to lack of oxides
  • Less malleability/ductility
  • Requires heavier machinery
  • Has only two directional strength due to elongated grains
  • Click here to hear an old guy talk about it

Casting

  • Casting mainly involves pouring molten metal into a mould and taking it out after it's cooled
  • There are many types of casting, and the biggest difference is how they make the mould
Sand Casting
  • Procedure is as follows:
    1. Half of a model is placed within a drag (essentially a box)
    2. This is rammed with a sand-lubricant mixture until it is full
    3. This is repeated with a second drag containing the second half of the model
    4. The two are placed together to check whether they fit
    5. The drag on top (known as the Cope) has holes drilled into the bottom to allow pouring
    6. The two halves are taken apart and the models taken out
    7. These two halves are now placed together again, and molten metal is poured into it.
    8. It is kept like this until it solidifies
    9. The two halves are taken apart and the completed casting is taken out, usually destroying the mould in the process
  • This is cost effective and very simple to do
  • But it also has a poor surface finish and machining is often essential to remove excess parts created by the runner and riser holes.
  • Click here for a demonstration accompanied by 90's porn music
Shell Casting
  • Procedure as follows:
    1. A heated pattern plate, which usually is the invert of half a mould, is placed over a dump box with a mixture of sand and resin inside it
    2. The dump box is placed upside down, and the sand becomes stuck together in the shape of the pattern due to the heat in the pattern plate
    3. Additional heating of the "shell" and the plate may be needed to ensure that resin has fully cured
    4. The shell is then ejected by ejector pins
    5. The process is repeated to form the second half of the shell
    6. The two halves are then bolted or screwed together and placed in a box surrounded by metal shot
    7. Molten charge is then poured in
    8. Upon solidification, mould is separated and object is removed
  • This process offers higher dimensional accuracy and better grainflow
  • Yet, it is more expensive due to the manufacture of pattern plates
Die/Permanent Mould Casting
  • This method, unlike the previous ones, uses a pre-made mould or die.
  • It can be done in two ways:
    • Gravity die-casting:
      • Similar to sand casting and shell casting, this involves pouring hot metal into a mould.
      • The mould must be able to be taken apart as mould cannot be broken (unless you want to lose like $300 000)
      • Costs less to produce than sand casting and has a better surface finish
      • Yet extremely high initial costs as custom moulds are very expensive
      • Many automotive parts such as pistons and engine blocks are cast using this method
    • Pressure die-casting:
      • This is vaguely similar to extrusion, where pressure, usually from hydraulic/pneumatic rams, forces the molten metal into a die.
      • This casuses the casting to be denser with a smooth surface finish
      • Yet this process requires more energy and the dies need to be impact resistant
      • Gearboxes are usually made through this method
Investment/Lost wax casting
  • Method is as follows:
    1. An entire pattern of the item is made with wax
    2. A ceramic is then poured over the pattern and allowed to set.
    3. By heating and tipping the melted wax out, the wax is removed, creating a cavity
    4. Molten metal is then poured into this ceramic mould and solidified
    5. The casting is then taken out usually by destroying the mould
  • This process creates high quality castings with high dimensional accuracy and great surface finish
  • Yet, it may be expensive as the moulds are required to be re-made each time this is done
  • This is often used for turbine blades and other shapes which have complex shapes

Extrusion

27y1id3.gif
  • Extrusion is where metal is squeezed into shape through a nozzle or a punch
Direct Extrusion
  • This is a hot worked process
  • This is where the ram pushes the metal into the other side through a nozzle
  • This usually requires more force and is used with more ductile materials
Indirect Extrusion
  • This is a hot worked process
  • This is when the Ram pushes material upwards as it has nowhere to go
  • This usually requires more intricate ram design and more machinery
  • Click here for a demonstration
Impact Extrusion
  • This is a cold worked process
  • This is when a punch hits a material blank in a die
  • As a result, the material is forced around the punch
  • Cans and tubes are often made this way
  • Click here for a demonstration

Drawing

  • Similar to Extrusion, this is when metal is pulled through a die rather than be pushed out of.
  • An extra die known as a mandrel can be added to make a tube rather than a wire.
  • The process increases internal stresses and reduces the cross sectional area.
  • It is usually done cold, and only with ductile materials
    • As a result most materials are annealed before drawing and then re-annealed during drawing phases.
  • It is used to make wires, tubes and cables, which can then be used.
  • Click here for a demonstration.

Powder forming/Metallurgy

  • This is when a shape is formed from powdered metal
  • Method is as follows:
    1. Metals to be used are powdered by:
      • Mechanical disintegration
      • Atomised powers
      • Chemical methods
      • Electrolytic deposition
    2. Powders are blended carefully in the correct proportions and mixed with lubricants and other materials to achieve desired properties
    3. These powders are then hydraulically/mechanically pressed at room temperature into a die which cold welds the powders together, deforming the particles and reduces porosity
    4. At this point, the object is in its final shape but has very little strength
    5. The product is then sintered (heated to evaporate lubricants and increase strength of metal) at 1100 ゜C
    6. At this point, it has about 80% of the strength of solid metal and can be considered finished
  • Powdered forming is usually used for materials which are hard to machine or are too complex to form any other way
  • It also allows for composite materials, where if one metal doesn't react with another they could be mixed in the powder to utilise composite qualities
  • However, it is expensive to set up the facilities and the article is not as strong