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Metal forming processes

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Title: Metal forming processes


1
Metal forming processes
  • EF420 lecture 2
  • John Taylor

2
Metal forming
  • Using forces to shape solid metal plastically
  • Avoids problems with solidification which can
    occur during casting
  • Minimises the high scrap loss of machining
    processes
  • Removes segregation and defects present in cast
    ingots
  • Promotes a desirable fibre structure

Machined thread
Rolled thread
3
Classification of methods
  • Primary metal working - Processing
  • Forming ingots or other cast forms into simple
    shapes (plate, sheet, bar)
  • Often performed hot
  • Rolling, extrusion
  • Secondary metal working - Fabricating
  • Producing components from simple shapes.
  • Often performed cold
  • Deep drawing, bending, shearing, machining

4
Typical stress states
5
Process analysis
  • Used to determine required forces, for selecting
    equipment
  • May predict failure
  • Study of yielding behaviour (flow stress)
  • Total work put into working operation work
    involved in shape change work to overcome
    friction redundant work.
  • Mathematical computer modelling

6
Cold working (forming)
  • Strength is increased and ductility reduced
  • Strength improvements can be dramatic
  • Work hardening rate depends on material
  • Grain structure is distorted
  • Dislocation population is increased
  • From 104 lines/mm2 in fully annealed metal
  • To 1010 lines/mm2 in fully cold worked metal
  • Dislocation locking

7
Cold work advantages
  • Strength, fatigue wear properties improved by
    cold working
  • Good surface finish dimensional control
  • No oxidation
  • Finishing processes may not be needed

8
Cold work limitations
  • Higher forces and therefore more powerful
    equipment is required
  • Only a limited amount of cold work can be
    undertaken before the material fails
  • Brittle materials cannot be cold worked at all
    (tungsten, silicon carbide, glass)
  • Intermediate anneals may be required
  • Undesirable residual stress may be created

9
Cold worked material
  • Many materials are available in the cold worked
    condition
  • The temper designation is the amount of cold work
  • Annealed O no cold work
  • 1/4 hard 25 of the maximum cold work possible
  • 1/2 hard and 3/4 hard designations
  • Fully work hardened no ductility left

10
Annealing heat treatment
  • Removes the effect of cold work,
  • Increasing ductility
  • Reducing strength
  • Improves other physical properties
  • Eg conductivity of copper
  • At a temperature of about 0.5 of the melting
    temperature in degrees Kelvin
  • Applied ONLY to cold worked metals

11
Effect of annealing
  • New grains are nucleated where deformation is
    highest
  • The more heavy the cold work, the more grains are
    nucleated the finer the grain size
  • The new grains take over the cold worked metal by
    diffusion annihilating the distorted structure
    (recrystallisation)
  • Dislocation density is reduced

12
Grain growth
  • Reduction of grain boundary energy
  • Occurs at higher temperatures or insufficient
    amounts of deformation
  • Grain boundaries straighten
  • Large grains tend to consume small grains
  • Yield strength and ductility are both reduced
  • Can be inhibited when second phase particles pin
    grain boundaries

13
Recrystallisation temperature
  • Depends on amount of initial cold work
  • Less than critical strain, no recrystallisation
  • Critical strain for iron - 10, for aluminium -
    1
  • High amount of cold work, lower recrystallisation
    temperature
  • Also depends on the time at temperature.
  • Long times reduce the recrystallisation
    temperature.

14
Typical recrystallisation temperatures
15
Hot working
  • Carried out at a temperature and strain rate at
    which recrystallisation is simultaneous with
    deformation.
  • Above about 60 of the absolute melting point
  • New grains are continually formed
  • Material properties (yield strength, ductility)
    largely independent of the amount of hot work,
    and are the same as if the material was cold
    worked and annealed
  • The amount of deformation is limitless

16
Effects of hot working
  • Crystal structure is refined
  • Original cast structure eliminated
  • Facilitates homogenisation
  • Defects can be welded closed
  • Improves strength, ductility, toughness by
    refining grain size.

17
Temperature limits
  • The maximum temperature is determined by the
    point at which constituents in the material melt
  • Low melting point phases may be present
  • High strain rates cause adiabatic heating
  • Must be above the recrystallisation temperature
  • Hot work can be at room temperature (Sn, Pb)

18
Formability of a metal
  • Load required for yielding
  • Reduced by increasing temperature
  • Material ductility
  • Ability to stand tensile stress without cracking
  • Stress system imposed by forming
  • Some processes more suitable than others for less
    ductile materials

19
Pure metals
  • Flow stress decreases with melting point
  • Low formability - tungsten
  • Good formability - tin, lead, zinc
  • Ductility increases with number of slip planes
  • fcc - large number of slip planes, Al, Cu
  • bcc - fewer slip planes, Ferrite
  • cph - limited slip planes, Mg

20
Alloying effect
  • Increasing alloying level and complexity
  • Usually lowers melting point
  • Raises work hardening rate
  • Generally higher alloy levels and more complex
    alloys are more difficult to form

Melting temperature
Hot working range
Temperature
Alloy content
21
Forging
  • Localised compression is used to form complex
    shapes
  • Usually a hot work process, seldom cold
  • Open die forging
  • Simple shapes, larger forgings, slow process
  • Closed die forging (stamping)
  • Small items, large numbers, complex shape,
    expensive dies
  • Drop hammers (Hammer and anvil)
  • Press forming - more deeply penetrating

22
Features of forging
  • A batch process, limited productivity
  • Capable of producing a wide variety of shapes
  • Components made by forging have better properties
    than those made by casting or machining from
    stock
  • Useful in reducing machining costs
  • Less machining time
  • Less scrap

23
Forging types
Drawing
Gutter
Forging
Die
Flash
Swaging (Shaft is rotated)
Closed Die
24
Forged products
  • High quality irregular shapes
  • Gears, levers, crankshafts, pipe fittings, gas
    cylinders, rings
  • Fasteners (nuts, screws)
  • Coins (cold forgings)

25
Rolling
  • By compressing between rolls, a material is
    reduced in thickness and increased in surface
    area
  • Can be regarded as a form of continuous open die
    forging.
  • Cylindrical rolls produce flat products (plate
    sheet)
  • Thickness variations by controlling roll spacing
  • Grooved rolls used for long products
  • Angles, channels, tees, beams, columns

26
Types of roll stands
3-high
4-high
2-high (reversing or Non-reversing)
Cluster rolls (12-high)
Backing rolls
27
Hot rolling
  • Breakdown of as-cast shapes (ingots strands) to
    slabs, blooms or billets
  • Finished steel sections and plates
  • Simple two- or three-high rolling systems
  • Product has mill scale and has to be finished
  • May require pickling
  • Structural steel can be sand blasted and primed

28
Cold rolling
  • Improved finish
  • Higher forces needed
  • For better finish and dimensional control, more
    complex rolls are required

29
Extrusion
  • Material compressed through a hole in a die to
    make a product of uniform cross section
  • A mode of deformation that occurs in other
    working processes, particularly closed die
    forging
  • Almost always performed hot
  • Flow is complex, with a lot of redundant work
    (bending and unbending)
  • Friction plays an important part
  • Surface of billet tends to stick to container,
    extrusion surface is new

30
Extruded products
  • Long products, uniform cross section.
  • Can be complex sections, which cannot be rolled
  • Reentrant angles
  • Window frame sections
  • Small billets used to make containers
  • Beer cans
  • Ductile materials (aluminium)

31
Extrusion processes
Direct extrusion - solid
Indirect extrusion - tube
Impact extrusion
32
Deep drawing and pressing
  • The formation of shapes, such as cups and dishes
    from sheet material
  • Often undertaken cold to allow work hardening and
    maintain a high surface quality
  • Stress systems vary over the surface and include
    biaxial tension, bending and unbending,
    circumferential and ironing compression.
  • A high work hardening rate is desirable so that
    distortion is shared over the whole surface

33
Final forming
  • Deformation less than primary forming
  • Bending is the most common process
  • Includes roll bending of plate, tube and sections
  • Induction bending
  • Local induction heating of increment
  • Spinning of dished heads.

34
Machining
  • Passing a tool through the metal, which should
    break off in chips.
  • Cold working with fracture of the chip from the
    component
  • Ease of machining depends on
  • Design of tool
  • Lubrication
  • Material being cut (machinability)

35
Machinability
  • Measured by speed of cutting
  • Strength of material affects the force necessary
    for machining
  • Ductility affects the type of chip formed and the
    ease of its removal
  • Very ductile materials such as copper or
    aluminium spread under the cutting tool, and can
    pressure-weld to it.

36
Machinability improved by
  • Low number of slip planes for dislocation glide
  • Compare fcc aluminium to cph magnesium
  • Presence of brittle or weak second phase
    particles
  • Graphite in cast iron, sulphides in free cutting
    steel
  • Cold work hardening mechanisms
  • Solid solution, second phases, etc
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