Forming Flashcards
What is metal forming?
Any process which utilises the ability of a metal
to deform plastically under applied forces, with
or without the use of external heat.
General Advantages of Forming?
Compared to cutting: low materials waste
Compared to casting: no solidification processes or handling of molten metals
Processes can be very fast
Processes can improve mechanical properties and integrity of components as well as creating their shape
General Disadvantages of Forming?
Very high forces required
Equipment generally very large and very expensive
Tooling needs replaced as it wears out
Only justified for large scale production
Temperature of hot working
0.4-0.6 x Melting temp (K)
Explain re-crystallisation
Dislocations form new grain boundaries
Hot working advantages
Lower flow stress
Improved ductility
Large amounts of deformation possible
Improved mechanical properties through refinement of
metallic crystal structure
Directional flow lines (‘fibre’) give improved strength
Part has better machining properties than cold working
Promotes internal diffusion which removes chemical
inconsistencies
Pores and vacancies removed
Hot working disadvantages
Heating costs
Poor surface condition (reaction of surface with environment - oxides)
Handling problems
Dimensional tolerances are poor
Possibility of distortion on cooling and residual stresses
Cold working temperature
Room temperature, may be slightly heated to improve ductility
Cold working advantages
No heating required, Good quality surface finish, Good dimensional accuracy, Strength, fatigue, wear properties improved, Contamination problems reduced, Products have good reproducibility.
Cold working disadvantages
Higher forces required to initiate and complete
deformation,
Heavier equipment and stronger tools required -
expensive,
Strain hardening occurs - annealing may be
required to relieve strain,
Residual stresses may be present,
Surface must be clean and scale free,
Elastic memory may present problems
Warm working temperature
0.3-0.6 x Melting temperature (K)
Applicability of warm working
Suited to aluminium,
Energy savings are attractive - carbon tax,
Not that well developed
What is rolling?
Rolling is the squeezing of metal between two rolls,
thereby reducing thickness and increasing length.
Products of rolling?
Plates - thickness greater than 6mm - ship hulls and bridges.
Sheets - thickness less than 6mm - car bodies, food containers.
Rails and pipes
Flat rolling process
Rolls pull material between them through a
net frictional force. The entry velocity of material V0 is lower than the exit velocity Vf. Therefore there is a no slip point at which the velocity of the strip is the same as the roll. The frictional force on the exit side of the
no slip point must be higher than on the entry side.
What is the maximum draft?
Difference between initial and final thickness. Dependent on friction and roll radius. Friction can be reduced with the use of lubricants
Defects in flat rolling
Wavy edges, zipper cracks, edge cracks and alligatoring
What is roll bending?
Bending of straight cylindrical rolls
Types of rolling
Two high, three high, four high, tandem rolling and a cluster (Sendzimir mill).
Rolling based processes
Shaped section rolling - H-section part. Gear/Thread rolling.
Outcome of hot rolling followed by cold rolling
Converts cast structures into wrought metal before subsequent cold rolling improves strength, hardness and finish
What is forming?
Forging is the forming of discrete metal
parts by the application of compressive
forces using tooling and dies.
What is this F=Yf πr2(1+ 2µr/3h)
Forging force for upset forging
Yf is the flow stress of the material at the given true
strain;
r is the radius of the part (assumed cylindrical);
h is the height of the part;
µ is the coefficient of friction.
Since Yf , r and h change during the forging stroke so
does F
Real forging forces are higher because of redundant
work
Outcome of forging
Forged parts have good strength and toughness and so are
often used in critical applications. For other applications it is only
viable for high volume manufacture.
Steps in forging
Descale the blank if necessary, Lubricate Forging, Preforming, Finishing, Trimming and other finishing operations.
