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.
What is fullering?
In forging when the material is spread.
What is edging?
In forging when the material is being gathered
Why, in forging, is lubrication required?
Wear reduction,
Help metal flow,
Act as a thermal barrier,
Act as a parting agent.
Define forgeability?
Ability of a material to deform without cracking. Does not correlate closely with forging temperature
Forging design factors
Parting line should be a flat plane, Parting line should be through centre of forging, Adequate draft required, 3o Al, 5o-7o steel, Generous fillets and radii, Keep ribs wide and low, Keep tolerances as large as possible,
Forging defects
Web buckles, laps in finished forging
Extrusion forging versus drawing forging?
Extrusion produces parts of uniform cross section by pushing metal through a die.
Drawing produces small diameter parts, such as wire and rod, by pulling metal through a die.
Isothermal/hot die forging
The dies are heated to the same temperature
as the forging.
Swaging
Forming internal profiles of workpieces with or without a mandrel
Heading/Upset forging
Form heads on fasteners such as nails and rivets. Cold working operation.
Coining
Using dies to stamp an impression into the workpiece.
Extrusion
Extrusion is used to
force a metal billet
through a die. The part produced has
a uniform cross section
Drawing
Drawing is used to produce particularly small
diameter parts, e.g. wire and rod, where the
original billet would buckle if it were compressed
Ideal material properties for sheet metal forming
High elongation for formability, Large strain hardening component. Coarser the grain the rougher the surface.
What is shearing?
Shearing is the action of cutting the metal either between rotating blades or with a punch and die.
What is the relevance of clearance with regards to shearing?
Low clearance gives excessive tool wear but high clearance gives poor edge quality
When does failure occur during shearing?
Before the punch has moved through the whole thickness of the blank
Rake angle?
Rake angle reduces contact area so reduces hearing force.
Punch and die determine what…
Punch determines hole size whereas die determines blank size.
How much more times do dies last over punches
2 to 3 times
Why is slitting and nibbling used?
Slitting and nibbling used to make shapes without
production of special dies and can make both straight or curved cuts in the sheet
Fine (Fein) blanking produces…
Very smooth and square edges, but a fine grain material must be used
How is springback (elastic recovery) prevented during bending?
Springback (elastic recovery ) corrected by
overbending or by localised compressive stresses
When bending tubes what is required?
Tubes need to have internal support to prevent
buckling, this depends on the thickness of the tube
Forming and drawing processes…
Forming and drawing
processes are similar to
bending but include stretching
and shaping in two directions
Products of deep drawing
Used for production of cups, tubes, boxes.
Variables of deep drawing
Sheet properties, Clearance, Force of punch, Friction/lubrication, punch radius
Draw limit ratio =
Original blank diameter /
Cup diameter
Why might multistage drawing be used?
Often a part cannot be made in one drawing
operation because the material will fail
Stretch forming?
Sheet metal is clamped and stretched over a die or
forming block, which moves upward, downward or
sideways
Peen forming?
Curvatures on thin sheets, The surface of the sheet is subjected to compressive stresses, which tend to expand the surface layer.The material below the peened surface remains rigid,
the surface expansion therefore causes the sheet to
develop a curvature.
Spinning
Involves forming of axi-symmetric parts
over a mandrel with tools or rollers
What is tube spinning?
In this process the thickness of cylindrical parts is
reduced by spinning them on a cylindrical mandrel
using rollers
Bulging?
The basic process involves placing a tubular, conical
or curvilinear part in a split female die and expanding
it against the walls of the die
What is rubber forming?
When replacing a metal die with a rubber/flexible
one, you get a process where the die does not
damage or scratch the surface of the sheet
used for bending and embossing
surfaces
What does sheet metal forming rely on?
Highly uniform and isotropic elongation
Hydroforming/Fluid forming
in contrast to the ordinary deepdrawing process, the pressure in the dome forces the cup walls against the punch. The cup travels
with the punch; in this way, deep drawability is improved
Explosive forming
An explosive charge is detonated a distance
away from the sheet, the shock waves forcing
the material into the die cavity.
Relationship of force to bend radius in simple benidng
In simple bending the force required is inversely
proportional to the bend radius
