1. Metal forming processes

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 Material ductility
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 Fullering Edging Drawing Gutter Forging Die Flash
Swaging (Shaft is rotated)
Closed Die

24. Forged products High quality irregular shapes Coins (cold forgings)
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